Pond Boss Forums STOCKING A NEW POND Raising Forage and Bait What is the difference between growth and decline

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.



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.