Predator-recognition training of hatchery-reared walleye (Stizostedion vitreum) and a field test of a training method using yellow perch (Perca flavescens)

Fishes reared in captivity are predator-naïve and suffer large predation mortality when stocked into lakes with a full complement of predators. We tested the potential of predator training to enhance post-stocking survival of hatchery-reared walleye (Stizostedion vitreum). In the first part of the study, we found that walleye (i) use chemical cues for assessing predation risk, (ii) do not have innate recognition of the odor of northern pike (Esox lucius) as an indicator of predation, and (iii) associate predation risk with pike odor after a single simultaneous encounter with pike odor and chemical alarm cues from walleye skin. In the second part of the study, we attempted to mass-train yellow perch, Perca flavescens (as a surrogate for walleye), to fear pike odor. Perch response to pike odor was not changed by placing sponge blocks containing pike odor and perch alarm cues around the perimeter of a pond. On pre- and post-training assays, perch avoided traps scented with perch alarm cues, but did not avoid traps labeled with pike odor or water. We conclude that recognition training offers potential as a management tool for walleye, but significant logistic challenges must be solved before it can be implemented.

Does the value of newly accessible spawning habitat for walleye (Stizostedion vitreum) depend on its location relative to nursery habitats?

The benefit accrued from habitat restoration actions may depend not only on the quality and quantity of habitat restored, but also on its location relative to other critical habitats. For example, walleye (Stizostedion vitreum) larvae need to be transported by river currents from spawning to nursery habitats soon after emergence. We developed a model of river transport survival for larval walleye to evaluate potential recruitment from riverine spawning habitats to lacustrine nursery areas. Published relationships were used to model transport survival as a function of temperature and velocity. At greater distances from nursery areas, mortality risk increases, particularly as a result of starvation at relatively high river temperatures. We applied the model to the Sandusky River, a tributary to western Lake Erie, and found potential larval production from spawning habitat above an existing dam to exceed the potential of presently accessible spawning areas by a factor of 8. We used a generalized version of the model to show that for transport distances of less than 100 km, transport survival is much more sensitive to variations in river flows, whereas at greater distances, temperature becomes increasingly important.

Sexual size dimorphism of walleye (Stizostedion vitreum vitreum)

Sexual size dimorphism of walleye (Stizostedion vitreum vitreum) is achieved by females having higher growth rates than males before and after maturation, resulting in females with greater asymptotic sizes. Mercury (Hg) concentrations in epaxial muscle by age and weight for 31 populations of walleye were used to evaluate the relative importance of differences in consumption and activity in generating sexual size dimorphism. Growth efficiency by sex, age, and maturity is estimated by a ratio of annual increments in weight (g) to annual increments of Hg (mg), using the pooled changes in weight and Hg loadings of males and females from all lakes. The higher growth rates of females arise from greater consumption and higher growth efficiency. Growth efficiency of both sexes is similar before maturity, but the growth efficiency of mature males is substantially lower than that of either immature males or mature females. We propose that the inferior growth efficiency of males is a function of the greater activity of males, particularly during the spawning season when scramble competition for fertilization is likely to produce substantial increases in male fitness as a result of increased efforts to find and spawn with females.