Pond Boss Forums MANAGING AN EXISTING POND Evaluating and adjusting fish populations Population Assessment info

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.