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High walleye density delays spawning.

Select Science--Computer-based models have been developed for a variety of sport and commercial fishes that allow fishery managers to predict the effects of various harvest scenarios. These models rely on input data of demographic variables such as growth rate, natural mortality rate, size and age at sexual maturity, and fecundity--the number of eggs produced by mature females.

The accuracy and reliability of these models is, of course, only as good as the input data. Biologists and knowledgeable In-Fisherman readers know that growth of fish in a population often slows as density increases because the limited food is shared among more fish, and the possible effects of density is a wild card in most population models.

A study by Wisconsin and Minnesota fishery researchers in Big Crooked Lake, Wisconsin, demonstrated that age at maturity but not fecundity of walleye increased as density increased.* Because of the energetic demands of developing eggs and sperm, fish size determines sexual maturity. Thus, higher density causes slower growth and later maturity. By the same reasoning, the lack of an effect on fecundity is surprising but also good news. When energy is limited in dense populations, some fish reduce egg production or may not spawn every year. This doesn't appear to be the case for walleye, at least not in Big Crooked Lake.

Population size estimates are increasingly needed to allocate harvest among different recreational, commercial, and First Nation fisheries. These estimates are expensive and labor intensive. The precise relationship between age at maturity and population density may provide an economical tool for estimating population size. But there is another message here for anglers and biologists. The results suggest to me that there is a limit to walleye density in natural populations in that delayed maturity at higher densities may curtail reproductive output of the population.

* Schueller, A. M., M. J. Hansen, S. P. Newman, and C. J. Edwards. 2005. Density dependence of walleye maturity and fecundity in Big Crooked Lake, Wisconsin, 1997-2003. N. Am. J. Fish. Mgmt. 25:841-847.

Caption: PHOTO//ERIC ENGBRETSON

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Title Annotation:Bits & Pieces: Blending Fishery Science with Everyday Fishing
Author:Schramm, Hal
Publication:In-Fisherman
Date:Mar 1, 2017
Words:338
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