INFLUENCE OF STOCKING DENSITIES ON WALLEYE FRY VIABILITY IN EXPERIMENTAL AND PRODUCTION TANKS

In 1992 at Rathbun State Fish Hatchery, experiments were conducted to evaluate the effects of stocking density on performance of larval wall eyes (Stizostedion vitreum) in small (278-L) cylindrical tanks, and to compare fry performance in production-scale (679L) and small tanks. H atched fry were reared for 21 d and fed Fry Feed Kyowa B400 and B700 i n all experiments. Densities of 20, 30, and 40 fry/L were evaluated in two trials, and densities of 20, 50, and 60 fry/L were evaluated in a nother trial. Fry performance did not differ significantly among densi ty treatments of 20, 30, or 40 fry/L. In the other trial, however, sur vival was significantly greater for fish stocked at 20 fry/L than for fish stocked at 50 or 60 fry/L, but the number of fry produced (yield) was 216% greater in the tanks stocked at 60 fry/L. Growth rate was no t affected by density in any trial. For comparisons between 278-L and 679-L tanks, three 679-L tanks were stocked at 20 fry/L in each of the three trials. Survival and viability differences between tank sizes w ere inconsistent, but the highest survival was an average of 74.9% to 21 d posthatch in the three 679-L tanks in the second trial. Cannibali sm and gas bladder inflation were more common in the small tanks. Inci dence of cannibals was as high as 7.5% of the original number stocked in the small tanks, but did not exceed 3.5% in the large tanks. Over a ll three trials, from 98.5 to 100% of the survivors in the 278-L tanks , and from 88 to 96.3% in the 679-L tanks, had inflated gas bladders. The outstanding rate of gas bladder inflation is attributed to the use of a surface spray, which seems to clear the water surface of oil and debris. These experiments demonstrate the feasibility of mass culture of larval walleyes in production-scale tanks.