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For a predator at the margin of maintenance ... the difference between annual growth and annual decline is one fish. Though this seems obvious it begs the question. What is the difference between good growth and mortality? What I have learned from my study of LMB energetics is that the difference was much less than I earlier thought.

Everyday all day and all night LMB are burning energy and they need 1.33% of their body weight in BG each day (on average) to meet this metabolic demand. This equates to ~4.84 lbs of BG per year for every pound the LMB weighs. And so there is need to consume this quantity of prey to ensure there is energy for metabolic processes, to avoid predators, and to fight off bacterial and fungal infections. If they don't consume enough, they will decline and are at greater risk of mortality. The balance must be tenuous because natural mortality where harvest plays no role in mortality is thought to average 30% of the adult population every year.

For an 18" LMB at standard weight ... approximately 536 BG at the optimum length of 3.33" are required for maintenance each year. To be sure only a small number are almost precisely optimum but since the distribution is equal in number on both sides of the optimum length we might reasonably assume that bigger prey balance the smaller prey to result in an average that is close to equivalent to optimum. So an interesting question is how many more BG are needed to grow 1 lb? Below is spreadsheet using the energetics model. The red squares are output and the green are inputs which determine the outcome of the simulation. Growth is assumed over 180 days and then for 185 days it is assumed that the LMB only gets maintenance.

[Linked Image from i.imgur.com]

So there are few things I would like to bring to attention.

First the FCR isn't that great. 20.9 lbs of BG to grow 1 lb in 1 year when the LMB starts at 3.34 lbs. As it turns out, When any LMB grows by 30.7% in 1 year the FCR is ~20.9. So where did we get the number 10 lbs BG to gain 1lb ? An FCR of ~10 applies when an LMB consumes enough to double its weight. So when a 1/2 lb LMB grows to a pound the FCR is very close to 10. When a 1 lb LMB grows to 2 lbs the FCR is very close to 10. The reason the FCR isn't that great for the 18" LMB growing 1 lb is because most of the consumption went to maintenance. The maintenance need for the year is 19.8 lbs of BG and it only takes 1.47 lbs of BG (once the maintenance is met) to grow the LMB by 1 lb. Growth tells one dang near exactly how much of the consumption was above maintenance and it tells one an approximate number of prey the LMB consumed to grow and to be maintained.

Second, the immediate consequence of the first point is that it probably doesn't take very many additional BG to grow the 18" LMB by 1 lb. As it turns out, it takes 43 additional optimum length BG to grow the 18" LMB by 1 lb. Or roughly an 8% increase in the number of BG eaten during the 180 day growing season results in a 30.7% increase in weight. So it seems remarkably easy to gain in times of surplus. But if you look in the lower left hand corner you will see that growth comes at a price and that price is an increased need for maintenance consumption. For the growing season the consumption is 10.6 lbs total and the LMB grew 1 lb in so doing but to maintain that fish now weighing 4.35 lbs for the next 6 months takes as much total consumption as it did to grow it 1 lb. LMB need to grow but if every LMB in the pond is growing the pond has to produce ever increasing forage. If forage production is finite and limited, then obviously some LMB must decline if others are to grow. This brings us to the next point.

Third, the immediate consequence of the second point is that it doesn't take a whole lot of missed meals (that is ... missed fish needed for maintenance) to result in significant decline and possible mortality. If the LMB is missing the same number of BG (43) for a year (below the maintenance need) the LMB will decline around 23% which for an LMB starting at standard weight would result in a RW of 77. Its a fine line that LMB walk to grow and survive. Just missing (below that needed for maintenance) 1 fish meal every 8 days on average can result in very poor condition and threaten survival. Missing 1 fish meal on average every 4 days almost assuredly spells disaster and premature death (what we call natural mortality).

So all of this is a mix of good news and bad. On the good news front is that LMB can swiftly grow into any modest void left by mortality. It is also good news that mortality will make room for survivors to grow, Finally, it is good news that even small increases in the number of fish consumed can lead to great gains. On the bad news front ... LMB can swiftly grow to a standing weight that cannot be maintained (the evidence of which is natural mortality of young or middle aged adults and individual fish of poor RW). More bad news is that we cannot grow fish by increasing forage without also increasing the amount of forage required to maintain them ... unless of course ... some of the fish die.

There is no stasis situation. Good condition of fish is evidence of the past and yet tells us nothing about the future. So the key is to plan from the beginning to transition from a glut initial year class to a progression of recruits that can yield desired results. The sooner a plan is enacted the smoother and more successful the transition will be. Mortality has to play a role in planning as it is unavoidable. Without mortality ... fish will not grow, whether we cull or not, nature will cull with natural mortality. Nature is a cruel enforcer of mortality and it is also inefficient. Culling can reduce the consumption of what is otherwise a "lost cause" fish making more forage available to other fish ... that is ... more forage availability than letting nature do the culling (which allows the fish to consume and compete over a longer period of time). Sooner is better when it needs to be done and nature will be doing it anyway. Growing supplemental forage can make huge immediate difference but it should be in the context of a plan that isn't just about immediate growth/condition. Rather it should be used in the context of maintenance that can support modest additional growth.

I am very interested in understanding what is the most appropriate mortality rate to use in planning. For example, is the 30% annual mortality that on average naturally occurs most appropriate? The other piece I am interested in is what percentage of this mortality can be harvest without affecting the total mortality? One approach that would help with this goal is to concentrate on lower RW fish as they are most vulnerable to natural mortality something we already know to do. But all the same it would good to know if harvest would increase total mortality in order to appropriately plan. Culling is possibly the most effective means to more efficiently utilize forage. Consider the fish in the 3rd point of an 18" LMB consuming 493 BG/year but declining. If it can be harvested 3 months before mother nature kills it ... then this could free as many as 123 BG for remaining LMB. This has the potential of adding ~1 lb to 3 of its similar length peers.

Last edited by jpsdad; 09/09/23 06:12 AM.

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I once heard that the best bass “raiser” was a bluegill farmer. Lusk told me that about 100 years ago.

This stuff is mostly about the environment.


The best cattleman is a grass farmer with fertile soil.

Want big, healthy, deer? Do a soil sample of your land and plant a fertilized winter wheat patch.

Does a predator eat from hunger or also opportunity? I dunno.

Now what to do with extra 20 pounds that I’m carrying around my butt and gut.

Last edited by Dave Davidson1; 09/10/23 09:55 AM.

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Originally Posted by Dave Davidson1
Does a predator eat from hunger or also opportunity? I dunno.

Dave, I am not so sure its "or". I would say a predator eats from hunger and also opportunity. Without opportunity, for example, how else could a predator eat? Even so, I think hunger is a pretty strong motivator. I think I would equate the need for maintenance with hunger. IOWs, we might consider a predator hungry if it hasn't eaten enough today to maintain its body weight. Hunger must strongly motivate anyone (including any creature or any fish) to eat. Just as a survival mechanism if nothing else.

Dave, for BG prey at the optimum length, it takes less than 2 BG per day throughout the growing season (about 3 every two days) for maintenance. But if just 2 are consumed daily the growth is remarkable with an 80% increase in weight. With 3 consumed everyday the growth almost defies reason with a 438% gain. Imagine an 18" standard weight LMB consuming 3 optimum length BG every day for 180 days growing to 17.96 lbs. Yes, you read that right. smile 540 optimum length BG consumed over 180 days at the rate of 3 per day will grow an 18" standard weight LMB from 3.33 lb to almost 18 lbs.

Now I will try to make the case that hunger is "probably" a stronger motivator to consume (to eat) than is the opportunity for growth (a round belly). 1 optimum length BG isn't enough for an LMB to meet its metabolic demand and it must eat more than one every day (on average) in order to prevent itself from eventually dying. So there is a strong motivation to consume 2 every day if it can. If it can, it will grow remarkably (80%) just being motivated to meet its maintenance every day. So for the 18 standard weight LMB that is a gain of 2.66 lbs growing from 3.33 lbs to 5.99 lbs.

OTOH, how motivated are LMB to consume 3 optimum length BG every day? I think this is great question. To be sure, an LMB can do this if two conditions are met. First, it must have the opportunity ... there must be optimum length BG available in sufficient density to provide the opportunity. Second, it must have the will to consume the third BG after the second BG has satisfied his need for maintenance. IOWs the consumption of the 3 optimum length BG is determined by the LMBs will to consume beyond what is essential to survive. Dave, 3 optimum length BG isn't a whole lot of food. It represents only 2.72% of an LMB's body weight. An 18" LMB can manage this amount of consumption if it has the will to consume that much. Now if we think about it, it is very uncommon to see this level of consumption for LMB that are 12" or longer. I can't reference a case where a 1 lb grew to 4.5 lbs in 180 days. Now that said, it only takes 3 optimum length bluegill per day to register that growth ... so if it doesn't happen ... either the will is missing or the opportunity is missing (or probably both in most ponds). But lets say we supplied a 1 lb LMB with abundant optimum length BG providing it all the opportunity required to consume 3 daily... the amount of BG consumed daily above 2 per day would be a measure of its propensity to consume above the maintenance motivator.

I don't know for sure ... but for LMB over 12" ... I think most of the growth comes from the consumption of prey that is motivated by the need for maintenance (hunger). Because an LMB cannot consume a partial BG, the consumption of the BG that takes it over the daily maintenance requirement also provides a surplus of energy that can be assimilated into the body of the LMB as gain. It is sufficient to provide remarkable growth that it is only observed when populations are sufficiently limited to support that growth. In other words, I think most of growth of LMB over 12" does not come from being gluttonous ... but from consuming prey that was consumed to meet their daily energy needs.

I would like to mention that it may behoove adult LMB to limit consumption to the number that meets their metabolic demands (which of course will have a small surplus that allows for growth). For the purpose of survival, it is more efficient use of energy. If every LMB were gluttonous, they would allocate to growth prey they could have later put to use for maintenance. So the prey that are consumed in excess of the maintenance need will not be available to support LMB when they may need them later to survive. To complicate matters, gluttonous consumption would more rapidly increase their weight which would of course more rapidly increase the maintenance needed to support them. So to ensure the survival of the greatest number of adult LMB ... they should only consume as much as they need to survive. It would behoove nature to genetically imbue adult LMB with this conservative behavior as a survival strategy.

Dave, I would like to mention that little LMB are gluttonous creatures. So one may ask what gives? But if one thinks along the same line of thinking ... (What strategy ensures the best survival?) .... we swiftly come to a reasonable understanding. The little LMB consume at much greater rates of consumption so that they can grow to a size that takes them off the menu of their parents. IOWs, it behooves their survival to grow fast (just the opposite of adults). When one looks at the whole picture ... it just makes perfect sense.

Regarding BG farming, I agree with you. Producing substantial crops of BG is important to growing LMB. But I think this only 1/2 of the picture. I think population management is no less important. I will try to put this in the context of cattle ranching. Having great productive pasture doesn't guarantee that cattle will grow well on it. One must understand that there is a limit and that the number of cattle on the pasture in combination with the production of grass forage will determine how well the cattle gain. If an excessive amount of cattle are on the pasture and left there too long ... they will instead decline even though the pasture is in and of itself a great producer of grass forage.

Last edited by jpsdad; 09/10/23 01:44 PM.

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LMB are interesting. I had stocked fatheads and small sunfish of various types. I later bought some bass fry to put in my spring fed creek. I acclimated and then gently released them. I was watching 2 of them, side by side. Then, there was only one with the tail of the other sticking out of its mouth. I never saw the move.


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Dave, while a teenager I kept an aquarium of native fishes. Had a LMB, a couple of BG, a GSF, a LES, and a very large White River Crayfish. Was a hoot. I would feed them earthworms and FHM. The little LMB would catch FHM and the tails would hang out just like you mention above.

In papers I have read on consumption they remark about how small LMB tend to take larger proportioned prey than larger LMB do. They attributed that to the growth strategy for survival. IOWs the risk of choking is weighed against the risk of not growing fast enough. They thought the greater risk of larger prey was balanced by increased growth and improved chances for survival.

Was a lot of work and $(FHM) to keep that aquarium clean and the fish fed. The crayfish in prep for molting tried to block off the entrances under his rock with gravel. It was a flat rock supported by 3 stones. But this didn't save him. When he molted, the BG turned on their sides swimming under the rock and picking him apart. I didn't see it myself but a friend who slept over did. After losing the crayfish I decided to release them all and at my Dad's prompting undertook raising guppies as a diversion.


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Interesting thread guys.
Total mortality = Natural mortality + harvest.
At small size natural morts are very high and reduce as the fish get larger. At mid to large size natural morts are lower but harvest should be higher.
Big fish require more to maintain bodily function - it takes more energy for a 5 lb fish to move than a 3 lb fish. Same for other bodily functions - so more food to maintain statis. Good news is a larger fish can eat bigger food items. Also, a bigger fish can eat his/her smaller kin who are not getting enough to eat. Large LMB eat a lot of 8-12 inch LMB. Hard part is to manage all of this.
















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Very good thoughtful information. I like it. What reference was used for the food conversion model?

Last edited by Bill Cody; 09/11/23 07:35 PM.

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Thank you Bill and Eric for dropping in. Was hoping to see both of you and ask some questions. Before the questions ... Bill the reference is noted in an earlier thread of mine titled "LMB are remarkable converters of fish". There is a link in the first sentence that will take you there. The model formulation isn't directly discussed in that paper but was originally proposed by Swingle (though not in units of energy) for fish. The principles of feeding are much older than Swingle however and I do discuss these principles in the forum on feeding. I think that thread is called "Feeding and Conversion". The reference I noted above contained sufficient detail to resolve parameters of the functions for wet forage.

I am interested in your thoughts on mortality.

First, what is an appropriate assumption of natural mortality for planning? For example, 30% a number thought to be average?

Second, should we consider harvest mortality to be additive to natural mortality when the harvest is less in number than what we expect natural mortality to be? Or possibly somewhere in between, for example, in someway proportionately additive? Above I made the case that natural mortality of young and middle aged adults may be primarily caused by competitive pressures where some members of the population meet maintenance and grow ... while others are not able to meet maintenance causing them to be vulnerable to disease and other causes of mortality. If this is true, then taking a fish which is not competing well and is in relatively poor condition may not increase the total mortality. IOWs, if it would have died anyway, removing it by fishing may not increase the expected total mortality. Check out, the SS below. In this scenario we are just looking at the population of trophy recruits with an expected natural mortality of 30% annually. Each year 3 fish are selected for trophy path and in this scenario mother nature is allowed to do all the mortality. The mortality essentially forms a population pyramid that looks a whole lot like a trophic pyramid where each year of growth seems to lead to fewer larger fish supported by the food chain. By year 8 only 1 fish in that year class is expected to remain in the 5 acre scenario.

[Linked Image from i.imgur.com]

This scenario seeks to maintain a population of between 6 and 7 >15" fish per acre as recommended by Willis for a trophy pond. The number per acre before the annual mortalities occur can be as great as 9 to 10 per acre. Restricting tagged recruits to 3 per acre in the 12"-15" and harvesting any other 12" to 15" or any other untagged >15" fish would seek to maintain this population structure. Presumably, enough 8-12" fish grow into the 12" - 15" size each year to select recruits each year.

So is the natural mortality estimate too high?

Also look in the lower left of the image. The natural mortality is estimated at 2.9 per acre per year. Could a person remove up to 2 tagged LMB >15" per acre per year if the RW was significantly lower than peers without increasing the total mortality?

Last edited by jpsdad; 09/11/23 08:57 PM.

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Once LMB get to about 8 inches the main mortality cause is predation upon that size fish. There is some starving to death especially in under managed ponds. Same for disease - small % in over 8 inch LMB. This assumes normal circumstances (no pandemic , DO crash or major stress disaster) The more stress the higher the mortality % so average #s are hard to guess. In addition, life expectancy varies substantially between nLMB and fLMB and upon latitude +-. Harvest is the management tool and is additive to natural morts - maybe some minor adjustment is warranted.
Not sure this helps any with your questions.

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Morts LMB TPWD 210.png
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First, what is an appropriate assumption of natural mortality for planning? For example, 30% a number thought to be average?

Second, should we consider harvest mortality to be additive to natural mortality when the harvest is less in number than what we expect natural mortality to be?

If one is wanting an accurate estimate of mortality I think it is difficult to use a consistent percentage for natural mortality due to numerous natural factors that can contribute to deaths among widely different pond habitats. Predation losses can vary widely among waters depending on type and size of predators. Often in the literature 30% mortality loss is used. I am not sure how accurate that number is in different types of situations. It probably all depends, but when dealing with mortality loss it is probably a good starting percentage. The long term morality study summary presented above by ewest provides a good starting basis from an actual detailed study that measured natural mortality for LMB.

Second - When measuring mortality as a total mortality, I think one has to include harvest mortality. Any form of fish loss from a community should be included in total mortality. Any form of loss is loss from the ecosystem. If one's goal is to subdivide mortality into different categories then harvest mortality can be one form or a portion of the total mortality.


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Isn't total mortality 100%? No fish lives forever. Just sayin'.


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Originally Posted by ewest
Once LMB get to about 8 inches the main mortality cause is predation upon that size fish. There is some starving to death especially in under managed ponds. Same for disease - small % in over 8 inch LMB. This assumes normal circumstances (no pandemic , DO crash or major stress disaster) The more stress the higher the mortality % so average #s are hard to guess. In addition, life expectancy varies substantially between nLMB and fLMB and upon latitude +-. Harvest is the management tool and is additive to natural morts - maybe some minor adjustment is warranted.
Not sure this helps any with your questions.

It does help. I don't fully understand the environment of the test ponds ... but particularly recruitment along with survival and in particular growth and/or decline of the original survivors would be something I would like to dedicate time to study. If the entire population was known yearly both the control group and recruits, what a wealth of understanding that could be obtained from how population densities affected growth. Based on their findings, I have to question other sources but would like to know more. With regard to the risk of survival, per the study you referenced Eric, I think LMB must be quite a bit tougher than I thought and less prone to mortality than I presented earlier. IOWs LMB must be able to withstand substantial decline and survive anyway ... more than what I thought anyway. (I was thinking that backtracking 35% from a time at standard weight would spell mortality for an LMB ... but this may not be true). Remember the photo of the 5 lb LMB with a 3 lb head?

My original hunch was that mortality caused by decline would be somewhere in the neighborhood of what I had read average mortality was. If mortality were 30% of adults, then surviving adults (and recruits reaching adulthood) could grow into this freed carrying capacity by averaging in the neighborhood of 30% growth in the subsequent year. Actually if survivors grew 30% by weight the gain would be about 2/3 of the loss to mortality where the other 1/3 would be filled by recruits. So imagine 30 lbs of mortality for each 100 lbs of adults being replaced by 21 lbs of growth of pre-existing adults and 9 lbs of adult recruits (adult recruits being LMB growing into the 12-15" class). To be sure, how the mortality weight of the carrying capacity gets distributed to the growth of peer adults would be sensitive to the extent new adults are recruited.

If mortality is below 5% for years 5 through 11, it can almost be neglected altogether for these ages. That would sure make selection easier as it would take far less recruitment to meet the 6-7 LMB >15" rule of Willis. Also ... long lived fish that just keep on ticking risk over-representation of the >15" class. Fishing mortality at some level may have to be enacted to ensure room for young recruits. An approach to address aged fish is to harvest after some minimum time in the pond after selection. For example, reducing the recruitment rate to 1 LMB > 15" per acre-year and removing any fish that were tagged 7-8 years ago would prevent over accumulation for the circumstance where there is practically no mortality of >15" fish.

Thank you for this reply, you have given me food for thought. Could a link be provided to the paper?


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Last edited by ewest; 09/13/23 11:08 AM.

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Originally Posted by Bill Cody
Quote
First, what is an appropriate assumption of natural mortality for planning? For example, 30% a number thought to be average?

Second, should we consider harvest mortality to be additive to natural mortality when the harvest is less in number than what we expect natural mortality to be?

If one is wanting an accurate estimate of mortality I think it is difficult to use a consistent percentage for natural mortality due to numerous natural factors that can contribute to deaths among widely different pond habitats. Predation losses can vary widely among waters depending on type and size of predators. Often in the literature 30% mortality loss is used. I am not sure how accurate that number is in different types of situations. It probably all depends, but when dealing with mortality loss it is probably a good starting percentage. The long term morality study summary presented above by ewest provides a good starting basis from an actual detailed study that measured natural mortality for LMB.

Second - When measuring mortality as a total mortality, I think one has to include harvest mortality. Any form of fish loss from a community should be included in total mortality. Any form of loss is loss from the ecosystem. If one's goal is to subdivide mortality into different categories then harvest mortality can be one form or a portion of the total mortality.

Bill, thank you. I am only beginning to study natural mortality and it does indeed confound me as a variable that may not be very predictable. To be sure ... yes I understand that harvest mortality is part of the total but was just asking whether harvest of individuals which appear at risk of natural mortality would increase the total. IOWs, if a fish would have died from natural causes anyway perhaps taking the fish by fishing would not increase total mortality (because a fish that would have been lost naturally is harvested instead). Also fishing mortality tends to increase growth and condition of survivors and I wondered if improved condition could lead to lower mortality of the survivors than might otherwise take place. These sound like reasonable arguments but if the natural mortality is very low, fishing mortality could exceed natural mortality and certainly in that case would have to add to the total lost. Eric's reference suggest we should expect mortality to be low for middle aged fish.

As I mention above, I am only now beginning to study mortality. So lots of questions. Two interesting articles that got me looking deeper are one by Willis with comments on SMB.
and this article pertaining to LMB.


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...I (jpsdad) understand harvest mortality is part of the total but was just asking whether harvest of individuals which appear at risk of natural mortality would increase the total. IOWs, if a fish would have died from natural causes anyway perhaps taking the fish by fishing would not increase total mortality (because a fish that would have been lost naturally is harvested instead).


Cody - My understanding and concept of mortality is harvest removals contribute to total mortality.
jpsdad - ““” IOWs, if a fish would have died from natural causes anyway perhaps taking the fish by fishing would not increase total mortality (because a fish that would have been lost naturally is harvested instead).”””

cody- This is IMO confusing or combining harvest and harvest mortality which is why they are or can be considered two types of mortality.

Cody – Harvesting the fish or it dying naturally contributes to total mortality. I think harvesting would not increase the final total mortality but it would contribute to final total mortality. I think this topic is subdividing total mortality into harvest mortality and natural mortality. It is my understanding that fish mortality (harvest & natural) by basic definition is/was established or proposed to primarily measure all basic losses from the community and not established to account for any resultant biomass gains nor predict future gains to the fishery. Mortality, as I understand it,, is just a measure of all numbers lost from the population or from the community as a whole. Mortality is a measure or an estimate of the basic numbers of losses. Repercussions from the losses to my limited knowledge of the literature has not been discussed as common knowledge nor explored with detailed research. If it is well known or researched,, how or where was it explored or proven? I could be easily wrong about this.

“””Also fishing mortality tends to increase growth and condition of survivors and I(jpsdsd) wondered if improved condition could lead to lower mortality of the survivors than might otherwise take place.”””

Cody – Numbers losses depending on type could contribute to later total mortality. Mortality as number losses from a population or from the community does have affects on the remaining fishes probably mostly as beneficial factors and at times maybe some negative factors. I don’t think the actual affects of mortality for increased growth and condition of remaining fish has been tested or has actually been proven based on numbers previously lost. How would it be proven with testing? What would the control group look like compared to the test group? I think this concept at this point is basically theory or as a modeling concept for the resultant benefits of mortality. Someone somewhere has probably already discussed this topic in some form. I think a lot of this is conjecture which is good thinking about this fishery mortality topic.

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jpsdad, You reference an article by Dave Willis in which he states the oft quoted theory that,

“In general, fishery biologists expect to see shorter-lived fish at southern latitudes and longer-lived fish at northern latitudes… Why the big difference? Well, much apparently has to do with growth rates. In northern Canada, walleyes may only grow an inch a year. In those circumstances, they tend to live much longer. In a Kansas reservoir, an age-3 walleye typically is 18 inches long! When growth is that fast, then mortality rates are much higher.”

Does this apply only to the differences between Northern and Southern ponds or could this be true of LMB at any given latitude? In other words, is there a secondary effect whereby efforts to increase the growth rate of a given class also serves to shorten their lifespan and ultimately impact their mortality in later years?

Just a thought.

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Growth rates and longevity are not directly tied together. In some cases, there could be a secondary cause and effect. They are 2 different measures which can be tangentially related.
















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Fast growth has been related to shorter life spans although not always. It depends.


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jpsdad,

I hate to do this to your meticulously constructed spreadsheets, but I suspect natural pond mortality is not a fixed percentage of unknow quantity, but rather a variable that is closely tied to fish stressors in the pond.

A pond could go three straight years mostly remaining at full pool and having somewhat regular rain episodes to exchange water and avoid low DO events. Then in Year 4, the pond is in a big drought and drops 5' below normal pool. Further, the hottest days of the year occur when the pond is at its lowest level, so a low DO event is triggered in the pond.

That year would be a huge stressor on the fish. You might have more mortality in one summer than over the previous three years.

Obviously, the same would be true of the year that a northern pond gets early ice cover and then heavy snowfalls to initiate a winter fish kill.


Perhaps you could throw in a "high fish mortality" event in different years of your spreadsheets and do a sensitivity analysis to see the effects of a Year 2 event versus a Year 5 event?

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IMO those spread sheets were created as just an average basis assuming "fairly" consistent pond conditions from one year to the next. It would be very difficult IMO to build into the spreadsheet the widely variable pond conditions. How would one be able to account for all the variabilities possible of pond conditions throughout the country? Considering just the possible wide differences of our current crazy weather events would be a very big challenge to account for all or many of those variables. Then we have the variables of changing forage densities, maybe new species added or some species lost, water chemistry changes, predatory fluctuations, etc. etc. Way too many variables to consider.

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Originally Posted by Bill Cody
Cody – Harvesting the fish or it dying naturally contributes to total mortality. I think harvesting would not increase the final total mortality but it would contribute to final total mortality. I think this topic is subdividing total mortality into harvest mortality and natural mortality. It is my understanding that fish mortality (harvest & natural) by basic definition is/was established or proposed to primarily measure all basic losses from the community and not established to account for any resultant biomass gains nor predict future gains to the fishery. Mortality, as I understand it,, is just a measure of all numbers lost from the population or from the community as a whole. Mortality is a measure or an estimate of the basic numbers of losses. Repercussions from the losses to my limited knowledge of the literature has not been discussed as common knowledge nor explored with detailed research. If it is well known or researched,, how or where was it explored or proven? I could be easily wrong about this.
quote]

I think we are in agreement. Fishing or harvest mortality takes place by humans and not nature. To be sure, because harvested fish are not able to die of natural causes ... it also makes that sense that to some extent harvest mortality is in lieu of natural mortality. The first time I was presented with this idea was pertaining to hunting while taking a hunter safety course. One instructor was a warden and the other a biologist. The biologist argued that hunting mortality, in general, was too low to impact populations because winter caused conditions where food was in short supply and mortality reduced populations to winter carrying capacity. He argued (don't know how many would argue this today ... some 40 years later) that 20% of the bobwhite population would survive to nest in the spring whether we hunted them or not. He also argued that winter range was what limited Colorado's Elk population. The Elk herd expands each year to more than the winter habitat can carry and that hunting just prevents some natural winter mortality (not on an individual basis but an overall herd basis)

The link at the In-Fisherman seemed to suggest less of link between each type ... talking of them being separate and additive. To be sure, if we are looking at the past, the numbers are definitely separable and also additive in hindsight. But if we look at the next year, it would seem that to some extent we have some control through harvest planning (particularly in private water). In other words, If we anticipate natural mortality, we might reduce the number of natural morts by killing them ourselves, potentially only losing the total number we would have lost anyway.

For public waters, the risk is often over-exploitation. But not always. As catch and release fishing has gained favor, we are beginning to see DOW outreach to encourage the harvest of small adults that over-recruit and prevent older fish (and themselves) from reaching trophy size.


[quote]“””Also fishing mortality tends to increase growth and condition of survivors and I(jpsdsd) wondered if improved condition could lead to lower mortality of the survivors than might otherwise take place.”””

Cody – Numbers losses depending on type could contribute to later total mortality. Mortality as number losses from a population or from the community does have affects on the remaining fishes probably mostly as beneficial factors and at times maybe some negative factors. I don’t think the actual affects of mortality for increased growth and condition of remaining fish has been tested or has actually been proven based on numbers previously lost. How would it be proven with testing? What would the control group look like compared to the test group? I think this concept at this point is basically theory or as a modeling concept for the resultant benefits of mortality. Someone somewhere has probably already discussed this topic in some form. I think a lot of this is conjecture which is good thinking about this fishery mortality topic.[/

Bill, I think it can be argued that its isn't just mortality that determines individual growth of survivors. Other factors like competition from recruits could also have major impact. When we think about it, this is why BG cannot grow as well without a predator paring their numbers. We commonly consider water to have the ability to carry a given weight of a fish species. To be sure, we do have ample evidence that it is the number that are living that determines growth and ultimate weight for any particular carrying capacity. This is why we pay close attention to stocking numbers. You make a very good point about the lack of experimental control that is needed to understand the difference a particular action makes or how to separate that effect from others.

My interest lately has been trying to make sense of adult natural mortality as it was portrayed in the two links I provided. Willis stated that 30% to 35% is ordinary natural mortality of adult SMB in Midwest ponds. The In-Fisherman article stated that 30% natural mortality is average for adult LMB (but could vary from 15% to 60%). Now we are talking adults which are not as frequently preyed upon so these numbers do seem very high yet they are described as normal. I was reminded of the lessons of hunting harvest in that class some 40 odd years ago. The same idea seemed to apply. Over winter the production of YOY ceases and maybe the water just can't carry the fall standing weight through winter. Some have to go so to speak. It seems very reasonable to me ... but such a hunch could be wrong.

I think we can agree that once a population reaches carrying capacity ... that (without mortality) growth will cease because forage production will then only support the maintenance of the existing population. In as much as we can agree that mortality frees forage for consumption by surviving peers and larger fish, I think we can agree that (except in cases where recruitment is excessive or forage production declines) that surviving fish will consume more than they otherwise would if the morts had instead survived. I know you mentioned this before and it is something I agree with. 1 lb of growth per annum per individual is good but not excessive growth. When I use this assumption across all of the adult sizes in the SS, the growth ranges from 100% to 11% ... but of interest ... the average is 31% year over year growth. So from the stand point of members of the adult population growing by 31% over a year where the pond is already at carrying capacity ... mortality of 31% of the adult biomass provides the needed forage (provided recruits are not over-accumulating). IOWs this level of mortality can support the kind of growth we would like to see. For it to be sustainable, we must only recruit into the adult class the numbers we are losing to total mortality.

What we haven't answered is whether 30% is good number for natural mortality. Think of that movie with Bruce Willis ... Die Hard. Maybe they just wont die naturally at that rate after becoming adults. It does seem very high for adults to die at that rate without some explanation for why. Even so, the number enables good growth where recruitment is stable. So if nature will not thin at that rate, increasing harvest mortality to achieve a total mortality of 30% should presumably produce similar (possibly better) results to a 30% total mortality comprised solely of natural mortality.

Last edited by jpsdad; 09/15/23 05:03 AM.

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Eric,

I appreciate the correspondence and will read the paper as soon as I can. Sometimes like to chew on things but will share my thoughts after doing so. Thanks again.


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Originally Posted by FishinRod
jpsdad,

I hate to do this to your meticulously constructed spreadsheets, but I suspect natural pond mortality is not a fixed percentage of unknow quantity, but rather a variable that is closely tied to fish stressors in the pond.

A pond could go three straight years mostly remaining at full pool and having somewhat regular rain episodes to exchange water and avoid low DO events. Then in Year 4, the pond is in a big drought and drops 5' below normal pool. Further, the hottest days of the year occur when the pond is at its lowest level, so a low DO event is triggered in the pond.

That year would be a huge stressor on the fish. You might have more mortality in one summer than over the previous three years.

Obviously, the same would be true of the year that a northern pond gets early ice cover and then heavy snowfalls to initiate a winter fish kill.


Perhaps you could throw in a "high fish mortality" event in different years of your spreadsheets and do a sensitivity analysis to see the effects of a Year 2 event versus a Year 5 event?

Rod, I think for infrequent events like what you mention ... a person has to take stock and adjust to the situation at hand. It doesn't make sense to plan for that event happen but one should give thought to how he would deal with it should it happen and be prepared to react to the adverse circumstances when they occur.

In a case where the acreage is dramatically reduced ... one must expect at least proportionate decline in carrying capacity. The harvest mortality should reflect the decline in carrying capacity and one needs to be prepared to adjust also after refilling and get back on track.

The SS accommodates line by line population mortality but it is the summation of average expectations that is modeled to be a snap shot of the end of the growing season (in any given year). As a management tool, it is intended to estimate the forage requirement of adults >15" and its sub-recruiting class of 12"-15" to test whether expected forage production could support the population. Growth of each year class is also a goal ... IOWs we are not seeking a stasis situation with regard to individual fish but to have progression of the year classes in weight through sustainable recruitment and mortality of adults. Anderson's population structure requires substantial mortality of the 12"-15" class if they are to grow as we would like for them. For example if a 12" standard weight LMB grows 1 lb in 1 year... it becomes a member of the 15" class and we would grow the >15" population number by close to 200 %. Obviously, that is too many in the > 15" class and so we have to make a choice for sustainable trophy production. With Anderson, the more frequent catches of >15" fish tells one they need to thin them. But what if the growth of 12"-15" is too slow to grow out of the class in a year? Well then wouldn't this limit the potential of the water to produce trophies? BG size structure benefits from slow growing 12"-15" fish but each year that an LMB spends in this class is a year it doesn't spend >15" It seems prudent to thin the number of fish in the 12"-15" class down to the number one needs for recruitment into the >15" at least by the end of growing season. Then depend on 8" to 12" fish growing into this size class for next year.

Last edited by jpsdad; 09/15/23 05:11 AM.

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Pretty scholarly stuff here. And without “Yeah, but”, life would get boring. We know that carnivores are also opportunists but only at times. I once caught a coyote in a leg hold trap that was absolutely fat. That’s not normal for the song dogs. I also recall a pack of coyotes howling close to the house when we were butchering a deer. That was a couple of years ago

Logic? Animals don’t have it but make survival type decisions. Seems that deer can go nocturnal during hunting season without a shot being fired.

I have a pic somewhere of a big PVC tube type bird feeder 11 coons were either on it or around it. They actually removed the top cap that I hammered on and have to use a hammer to tap the top off. And, had to be somehow sitting on it to do it. How? Once again, dunno.

We’ve all heard “ match the hatch”. Nothing in the water looks like a purple or red rubber worm which speaks to opportunist feeding.

Hunger isn’t the only fish motivator. We’ve all changed lures and changed our catch rate. Why can hungry fish be picky eaters? Again, I dunno. Maybe that’s why I have way too many tackle boxes.

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Dave, I appreciate your comments regarding the song dogs. I enjoy the sounds of the dusk and the song dog's part of it. Lot's great memories of the end of a day's hunt where the howls were the ear candy that told me it was time to go home.


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Originally Posted by Retired on 40
jpsdad, You reference an article by Dave Willis in which he states the oft quoted theory that,

“In general, fishery biologists expect to see shorter-lived fish at southern latitudes and longer-lived fish at northern latitudes… Why the big difference? Well, much apparently has to do with growth rates. In northern Canada, walleyes may only grow an inch a year. In those circumstances, they tend to live much longer. In a Kansas reservoir, an age-3 walleye typically is 18 inches long! When growth is that fast, then mortality rates are much higher.”

Does this apply only to the differences between Northern and Southern ponds or could this be true of LMB at any given latitude? In other words, is there a secondary effect whereby efforts to increase the growth rate of a given class also serves to shorten their lifespan and ultimately impact their mortality in later years?

Just a thought.

RO40,

It think growing fast carries some level of risk for an LMB. They are taking the chance that enough forage will filter through smaller predators to support them to. The forage that supports larger LMB is in shorter supply and so if they outgrow their competition they will be more susceptible to decline.

The paper Eric sent me seems to support this. It was really a study of Florida LMB growth rather than a study of natural mortality but its evidence supports the idea that mortality is low when fish are growing. IOWs when there is plenty of forage to go around ... mortality is very low. The low mortality (I think) arose mostly because they were fed sufficiently to grow year over year. This involved removing YOY each year ... and restocking various combinations of prey fish to include additional add stocks of forage if conditions warranted. I think if the fish had been left to grow on in pond production of forage (without yearly draining, intervention, and restocking) and if the YOY were not removed each year that the mortality of adult fish would have reflected mortality more in line with 30% annual mortality.

The paper can be found here. Mortality was highest when growth was exceptionally good or when growth was exceptionally poor. I think this has to do with the timing of mortality. The metrics were obtained in October and if mortality occurred late in the year (like middle to late summer) it would make sense that mortality was associated with very poor growth. When mortality was high in the period just following the October census this should reduce competition for forage and result in very good growth in the next census. When there was high mortality associated with low growth ... higher growth was observed the following year but especially so when their was high mortality in the following year. There seemed to be bottlenecks of forage in the latter part of one year carrying over into the next year that produced the highest growth in the next year's census.

But to return to your question:

Quote
In other words, is there a secondary effect whereby efforts to increase the growth rate of a given class also serves to shorten their lifespan and ultimately impact their mortality in later years?

This is such an insightful question that I appreciate greatly.

I think it depends ... I think as long as there is plenty of forage in each subsequent year AND the forage required isn't running the pond to hot then I think natural mortality will be low for both fast growers and slow growers. IOWs I think large fish are the most susceptible to disruption of the food chain and forage shortages. Apex predators are always the most susceptible to food chain disruption.

I think the hard part is the second piece of the conditional "it depends" above. Providing enough forage each subsequent year without running too hot. I think a sustainable system is one that doesn't require more forage next year than it did this year. IOWs a sustainable biome is one where forage production is consistent from year to year and the biomass supported by it is also consistent from year to year. I am of the opinion that a sustainable system must have mortality in order to maintain biomass and allow for growth with a stable production of forage. If the mortality is fishing mortality that balances forage supply with predator biomass ... then I think one should expect natural mortality to be low.

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jpsdad - For all those not fully up to date on fishery jargon, would you please explain or describe your terms "forage running too hot" and "running the pond too hot"? Thank you.

Last edited by Bill Cody; 09/16/23 07:31 PM.

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Did you take issue with it? All I mean is that a there are limits. I meant so much fertility that the risks of mortality arise from something other than not having enough forage. Things like too much plant respiration, algal die-off, the aerator breaking down, the electric going out at the wrong time, a week of cloudy weather. Generally speaking, water quality parameters are more stressful in more fertile environments. It is just a risk that is different than not having enough forage that can also cause fish mortality.

RO40 originally asked whether pushing fish to grow would make them die faster. I think fast growing larger fish are no more susceptible to natural mortality when they are getting enough to eat and water quality is good.

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No issues with the wording. Just wanted novices and inexperienced to understand the terminology and meaning of "hot"..


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"Hot" is a rather vague word and to be vague about those other risks that can cause mortality was something I wanted to do. Mostly, because I don't know how much is too much. "Running hot" is how Snipe described conditions in his pond leading into the time where the crayfish were crawling onto the ice. Certainly some water can handle more of it than others.

I am reminded of the Virginia Female Only pond Solitude is managing. They reported growing Northern LMB from 1.2 lb to a few ounces shy of 10 lbs in 3 years. They initially stocked the LMB at 67 females to the acre (and also 67 HSB to the acre). I presume the HSB were relying mostly on feed but I could be wrong about that. The LMB are fed supplemental forage. We can estimate the forage required each year to make that ultimate weight in 3 years which is in the SS below. The SS shows the initially stocking at each anniversary and year 3 is coincident with their report. Note how with no mortality the forage requirement grows exponentially. The 3rd year FCR is 13.43 to 1. They reported worse FCR (20-30) but this could be explained if they stocked more forage than was consumed ... or forage larger than optimum ... or if some of the forage wasn't consumed because of preference for feed by HSB. IOWs Some of the forage supplement wasn't eaten for one reason or another. The growth was amazing so it would seem that they were consuming as much as they cared to.

[Linked Image from i.imgur.com]

For the 4th year the consumption to support maintenance and growth is 3666 llbs of BG per acre. So maybe this is hot? Of interest the consumption dry weight is ~ 733 lbs of dried BG. Imagine feeding a feed of 100% BG Meal to the LMB at this annual rate. This would be a feed of approximately 70% protein. This Virginia pond has the advantage of a good Spring to carry waste from additional forage consumption and the wastes of feeding resident BG down stream.

Below I recreate the same initial stocking but also employ fishing mortality in order to keep the forage requirement stable and in the neighborhood of the first year's requirement.

[Linked Image from i.imgur.com]

Is 850 lbs of forage per acre-year running too hot? In some ponds probably so, in others, probably not. Each person will have to be the judge of that for his own water. Anyways, given 850 lbs consumption per acre year one could grow some hefty female northern bass in 3 years. Is year 4 possible? I don't know but provided a person harvests 3 LMB by the middle of year 4 AND 821 lbs of BG forage are consumed ... it is in theory possible. The unknown is just how much forage will be consumed by survivors. It would require the consumption of 634 optimum length BG by each of the remaining 9 LMB (a little less than 2 per day)or on per acre basis the consumption of 5433 optimum length BG per acre ... plus ... whatever the 3 harvested LMB consumed. If the 3 LMB were harvested at the close of year 3, then just the 5433 optimum length BG would need to be consumed. At the start of year 4, an optimum length BG is ~4.53 inches at the end the optimum length is just under 5" in length. Can a pond produce this many optimum length BG for nine 9.2 lb bass to consume? Dang I don't know but if it can't produce a similar quantity that averages the weight of optimum length BG ... then they ain't going to be growing as much as modeled.

Last edited by jpsdad; 09/23/23 06:00 AM.

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Good analysis of the "hot" definition.
My note regarding the VA Female LMB project. Keep in mind that not all the 67 female bass grew to a few ounces less than 10 lbs in 3 years. My guess would be smaller ones were 7lbs and most others were 7 to about 10 lbs. There are always jumpers as most aggressive individuals and a few slow growers in every stocking or year class. Regardless this all female project demonstrated excellent LMB growth that is possible when placed into a "hot" forage community. .

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Originally Posted by Bill Cody
My note regarding the VA Female LMB project. Keep in mind that not all the 67 female bass grew to a few ounces less than 10 lbs in 3 years. My guess would be smaller ones were 7lbs and most others were 7 to about 10 lbs. There are always jumpers as most aggressive individuals and a few slow growers in every stocking or year class.

Good point Bill. I did adjust lower than the maximum but still wondered if I should have adjusted more. The SS is intended to be an average for a selection year class where some members of the class outperform while others underperform.

I do want to also mention that in the examples I provided ... we have to consider them highly fertile environments. It's worth asking, "Do we need that much fertilty to grow big fish?" Consider a 1 acre swimming BOW where relatively clear water is desired. Could a person still grow trophy LMB? See the SS below where 112 lbs of BG consumption per acre is sufficient consumption to grow female LMB from 1.25 lbs to over 8 lbs in 4 years. The standing number of LMB in this scenario is only expected to be 4 per acre. 1 fish is harvested in its 4th year for each acre. Each Fall this 4th year LMB is replaced with one 1.25 lb female LMB.

[Linked Image from i.imgur.com]

This is a very modest approach to growing trophy fish. Not many but an assembly line of one 8 lb LMB each year. There may occasionally be mortality of the 4th year class by natural means. When that happens, there will be no harvest in a 1 acre pond. Restricting harvest to 4th year fish ensures each gets their opportunity to complete the growout. In cases where the pond is unable to produce 112 lbs of lepomis forage ... supplemental forage like TP, RBT, or crayfish could fill the gap. The weight equivalents in terms of energy in wet weight are below.

.89 lbs of TP equals 1 lb BG
.68 lbs of Northern Crayfish equals1 lb. BG
.69 lbs of RBT equals 1 lb BG

Given the small numbers of LMB mouths to feed, a lepomis with less reproduction may be more ideal (eg RES). It would take around 2350 optimum length prey per acre to fill out the 112 lb forage requirement so the production number doesn't have to be large. I've even wondered if something like a BG-RES hybrid might work in combo with 3 lbs/AC of TP brooders. Still I think would prefer RES. Although RES couldn't support a trophy LMB fishery if the LMB were recruiting, where numbers of female LMB are restricted to balance RES forage production there could be balance for a handful of large LMB per acre.

The expected standing weight fluctuates between 17 and 26 lbs of LMB per acre.

Last edited by jpsdad; 09/23/23 06:02 AM.

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IMO Good estimates of "normal" production of LMB. I like the examples. Examples do show that pond productivity as water clarity does definitely determine fish poundage.


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jpsdad Offline OP
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Originally Posted by Bill Cody
IMO Good estimates of "normal" production of LMB. I like the examples. Examples do show that pond productivity as water clarity does definitely determine fish poundage.

Bill, I am not so sure any of the examples above were normal although it is very "kind" of you to say that. We "normally" have LMB recruitment to contend with and the standing weight of LMB less than 15" usually consume >50% of the yearly prey consumption by weight. Because each LMB consumes a comparable number of proportionately sized BG, the number of BG reaching the optimum sizes for >15" LMB are much fewer than being consumed by <15" bass. The struggle is growing enough 3" to 5" BG for the >15" LMB to eat. Willis thought 6 LMB >15" was a practical limit for most Midwestern ponds where trophy LMB were a goal. He of course meant working with the native fertility and so ponds can be made to support more by supplementing forage or feed.

I think female only environments are out of reach for most pond owners. There is really no industry supporting it for the masses. All the same, understanding what a succession of growing females in memorable sizes need in terms of forage consumption is useful. Most BOW owners with an LMB trophy bent would be pleased to have 4 LMB per acre averaging over 5 lbs in their BOW. Regardless of whose pond, what its fertility is, or what forage is added, it still "only" takes 112 lbs of appropriate length forage consumed to maintain and grow 4 of them so long as 1 is harvested near the end of 4 years and 1 recruit replaces it. The SS examples are not so much examples of what a person can do ... they are examples of the forage consumption it takes for the LMB modeled in it. They are examples of the prey consumption that balances the maintenance and growth of a hypothetical population (or subset of one).

Last edited by jpsdad; 09/23/23 05:36 AM.

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