ESTIMATION OF SELECTION DIFFERENTIALS FROM FISH SCALES - A STEP TOWARDS EVALUATING GENETIC ALTERATION OF FISH SIZE IN EXPLOITED POPULATIONS

We developed a model to predict genetic response to selection imposed by a size-selective fishery. Use of our model provides estimates of selection as a function of heritability and overall selection differential imposed against a size-related trait, length of a scale radius from the focus to a given annulus (radius n). Selection differentials were estimated using data from the walleye (Stizostedion vitreum) fishery in Red Lakes, Minnesota. Overall selection differentials, incorporating selection differentials estimated by age and sex, and weighted by the contribution to spawning, were negative for the 1955 spawning population and positive for that of 1958. Directions of selection differentials, estimated by age and sex, generally corresponded to expected directions of selection based on the shape of the gillnet selectivity curve for walleye, suggesting that scale radii are useful for estimating selection against size traits. Results indicated that selective effects of gear may be surprisingly complex and population characteristics must be well understood before changes in fishery management are considered. We suggest that measurement of overall selection differentials alone may provide a useful management tool for indicating genetic risks of selective fisheries.