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Population Assessment

New article with pics for your evaluation. Note the annual morts and size distribution of the fish.

Author intro --Length analysis is a common practice in fish stock assessment. The length distribution of a fish population at any point in time reflects the interactions of recruitment, growth, and mortality, which, in turn, manifest links with the biotic and abiotic environments. Thus, length data provide insight into the dynamics of fish populations and can assist in identifying population difficulties such as inconsistent year-class strength, slow growth, excessive mortality, and inadequate food or habitat.


Transactions of the American Fisheries Society 136:409–415, 2007
Copyright by the American Fisheries Society 2007

Approximate Sample Sizes Required to
Estimate Length Distributions
L. E. MIRANDA*
U.S. Geological Survey, Mississippi Cooperative Fish and Wildlife Research Unit,
Post Office Box 9691, Mississippi State, Mississippi 39762, USA

Abstract.—The sample sizes required to estimate fish length were determined by bootstrapping from
reference length distributions. Depending on population characteristics and species-specific maximum
lengths, 1-cm length-frequency histograms required 375–1,200 fish to estimate within 10% with 80%
confidence, 2.5-cm histograms required 150–425 fish, proportional stock density required 75–140 fish, and
mean length required 75–160 fish. In general, smaller species, smaller populations, populations with higher
mortality, and simpler length statistics required fewer samples. Indices that require low sample sizes may be
suitable for monitoring population status, and when large changes in length are evident, additional sampling
effort may be allocated to more precisely define length status with more informative estimators.

Interesting, Eric. We don't normally think about populations where a predominance of small size is due to the fish not living long enough to get big. But 60% annual mortality would be a bite - less than 10% of a population makes it through 3 years.

OTOH 10% is a lot more than the most generous fry survival rates - I assume the mortality rates above must start after fish reach adulthood or at least intermediate size (???).

I'd love to hear the professorial take on this from Dr. Dave or Dr. A or Dr C.

Me to . Compare the PSD for each one - BG , Crappie and LMB. You can see the dramatic pronounced pattern (population swing) in crappie.

OK, Theo, I'll try.

First of all, Steve Miranda is one of our best fisheries scientists. He is both a quantitative scientist, but also very practical in his approach. That’s a very nice combination for the profession.

Second, it’s interesting that you guys picked up on the “tangent” from the paper, which is the modeling exercise. From a practicing biologist’s perspective, the sample size estimates are what will catch most folks’ attention. For example, the sample size needed for 1-cm length-frequency histograms are larger than for 2.5-cm (1 inch) length frequency histograms, and needed sample sizes are lower yet for PSD calculations. Why is that? Because PSD is just a coarse length frequency! For largemouth bass the length categories are 8-12 inches, and 12 inches and longer (contrast that with a length frequency histogram where fish are grouped into centimeter length groups). So, Dr. Miranda’s paper will be highly cited for those sample size estimates.

Now, the exercise in Figure 1 seemed to interest everyone the most. Well, these are just simple size structure estimated from growth rates and mortality rates. I call these simple “Anderson” models, because Dick used this process to come up with expected PSD values once you know expected growth rates and expected mortality rates. The effect of mortality on fish population size structure is the first example I show in my fisheries management class when we discuss these “Anderson” models.

Now, I think your questions revolve more around the mortality rates. Mortality rates can be both easy and difficult to obtain. Accurate rates are quite difficult to attain. I know we’ve often discussed on the Forum that fast growth tends to lead to shorter life span. Remember that Steve is using total annual mortality here. This can be from both natural mortality and angling mortality. So, does increasing angler harvest affect size structure of fish populations? Yes, if the harvest is sufficient.

Thanks Dave. In this study actual angling morts should be minimal (much lower than most ponds or small lakes). It was done on Barnett Resv. (33,000+ acre river fed impoundment) with some but not a lot of fishing pressure and creel limits. Can you give us a easy to understand explanation of what we are seeing in the figure above on the average (not focusing on % morts) populations. I am trying to understand what the charts tell us other than your explanation of assumed % morts.. This is a fairly balanced fishery with good growth in WC , LMB , TShad , BG , RES , HSB (and parentals) , cats as well as rough fish ( carp , bowfin , gar , chain pickerel ,ect). Not a lot of yearly water elevation change 8 ft max.(from flood to low pool) and not a deep lake (my guess 75 % of water area between 4 and 20 ft).

OK, I think I know what you are asking Ewest.

First of all, I assume you realize that Figure 1 has nothing to do with Ross Barnett Reservoir? I looked through Steve’s methods, and he used summary papers based on broad geographic ranges for the largemouth bass, white crappie, and bluegill growth rates to develop those hypothetical length frequencies in Figure 1. The references for the three sources of growth data are in his citations.

Figure 4 in that paper has the actual length-frequency data for Ross Barnett Reservoir. I could not find the sampling time, but I’m sure it was either fall or spring, so let’s progress with that thought. Let’s start with LMB, which had a PSD of 54. Whether this is a fall or spring sample, the fall age-0 or spring age-1 LMB look to average about 8 inches long (20 cm). Does that sound right to you for established populations? I’d say yes. When I look at this length frequency from a northern biologist’s perspective, it looks darn good to me. We like a mid-range PSD, and there are many, many fish that exceed 15 inches (38 cm; Dick Anderson’s RSD-P). In fact, some of the bass exceed 25 inches, which is trophy stuff to me! I don’t see many peaks and valleys in the length information, meaning that reproduction and year-class strength must be pretty consistent. Growth must be moderate to high by the standards of your region, or the fish would not reach those large sizes.

Next, let’s look at the crappies. The PSD is 79, which borders on too high. However, too high typically means low recruitment, and there a nice year class of fall age-0 or spring age-1 crappies right at 4 inches. I also like to see such a good number of fish over 12 inches (30 cm); again, growth probably is pretty good for that locale. I have a sneaking suspicion that you can see the three younger age groups in this length frequency. If this is a spring sample, the age 1 fish are 4 inches, the age 2 fish are 8-9 inches, and the age 3 fish are that bump in lengths near 13 inches. There could be a missing year class between 4 and 9 inches, but I sort of doubt it. I’ve heard that southern crappies grow fast and die young; if my guess is right about the year classes, then that story would hold true.

What about bluegills? First of all, we’ve got some sampling bias probably happening. A rookie might think that the past year’s reproduction and recruitment was poor, as there are too few small fish in this sample. However, we typically underestimate the very small bluegills. I see no reason to suspect a weak year class of fall age 0 or spring age 1 fish, but could be wrong. I really would only know by working there long term and following the catches by age group over time in my annual samples with the trap nets. Bluegill size structure is ok, but not great. Would that sum up the fishery down there? In the upper Midwest, we often don’t see the quality of bluegills in larger reservoirs that we see in ponds and small impoundments. Here at Ross Barnett, there is a moderate PSD (good), and a few fish make 8 inches (20 cm). So, I’d rank it ok, but not great. In these large waters with lots of shad (you mentioned the threadfin; are gizzards also present?), we typically don’t see bluegill quality as good as the small waters. However, I’ve never been to this reservoir, and I’m not experienced in southern waters. Just making educated guesses.

I’d like to fish for both crappies and largemouth bass in this reservoir. It is a quality fishery!!

To more fully interpret the population samples for largemouth bass, bluegill, and white crappie, I’d sure like to see more information: catch per hour of electrofishing or per trap net night; growth; and age frequency. However, we of ten have to make suppositions based on just the length frequency, just as I did in the last paragraph.

Here’s a final thought on “balance.” The balance concepts “work” much better in ponds and small impoundments. In a 33,000 acre reservoir, the fishery is often driven by recruitment (i.e., year-class strength in the various fish species). However, I can see why you say “balance” for this reservoir – guessing from the length-frequency information in Figure 4, recruitment has been pretty darn consistent for largemouth bass, white crappie, and bluegill.

Dave thanks. I have quickly read but not studied the paper yet. I missed the part about the hypothetical lengths and only saw the actual samples from Barnett. I also did not catch the differences from one set of charts to the other. I guess I almost backed into the answer to my question.

I was looking for broad based PSD , year class and growth info for management purposes. Those were in the first set of charts as I thought but for the wrong reason. Even a blind hog can sometimes find an acorn.

My question is far more basic than that. What type , if any , of conclusions/info can we draw from the first set of charts using average mort rates wrt age , length and year class size of populations.

Ahh, good question. If you know typical growth and mortality rates, then you can develop a good picture of what you might expect for a length frequency at any particular location. If you then sample a population, and it differs from that expected pattern, you can ask the right questions. If there are primarily small fish in a population sample, then you ask where are the big fish? Typical answers are 1) the anglers harveted them and we need protection (regulation), or 2) the population density is too high, growth is slow, and the fish never reach those larger sizes (need to reduce population abundance). The correct management action will then vary depending on the suspected (or preferably "known" ) interpretation of the population sample.

Ewest - what about posting figure 4 here?