Natural Resources, School of


Date of this Version

Winter 11-30-2012

Document Type



Spirk, P.J. 2012. Effects of length limits on sexually size dimorphic fishes. MS Thesis. University of Nebraska-Lincoln.


A THESIS Presented to the Faculty of The Graduate College at the University of Nebraska In partial Fulfillment of Requirements For the Degree of Master of Science, Major: Natural Resource Sciences, Under the Supervision of Professor Kevin L. Pope. Lincoln, Nebraska: December 2012

Copyright (c) 2012 Peter J. Spirk


Length limits are used by fishery managers as a method to alter size structure of fish populations. Unfortunately, biological differences between fish sexes (i.e., sexual-size dimorphism) may lead to sex-specific rates of recruitment, growth, and mortality. The addition of angler harvest to most aquatic systems likely accentuates differences in sex-specific rates by selectively harvesting the fastest-growing and largest fish from a population. The first objective of this study was to document the extent of sexual-size dimorphism for white bass and walleye at a Nebraska reservoir. Growth rates were similar between male and female white bass although male white bass were consistently shorter than their female counterparts at a given age. Male walleye grew slower and were consistently shorter than their female counterparts at a given age. The second objective was to document the size, sex, and age of white crappie, white bass and walleye harvested in two Nebraska reservoirs. Harvest was female biased for both white crappie and white bass, whereas harvest was similar for both male and female walleye. The third objective was to determine if size-, sex- or age-selective harvest was occurring for white bass and walleye at a Nebraska reservoir. Anglers harvested female white bass at a greater proportion than was sampled during NGPC annual population surveys. Anglers at Sherman Reservoir did selectively harvest walleye based on size, although in contrast to the white bass population, sex-selective harvest was not apparent for walleye. The final objective was to provide a model that predicts possible outcomes from using different length limits for sexually size dimorphic fishes. Although there was a noticeable difference in the number of fish in a population for each length limit, the pressure applied to the population by catch-and-release mortality kept the sex ratio close to a 1:1.

Advisor: Kevin L. Pope