THYROID-HORMONES AND THEIR INFLUENCES ON LARVAL PERFORMANCE AND INCIDENCE OF CANNIBALISM IN WALLEYE STIZOSTEDION-VITREUM

Thyroid hormones, 3,5,3',5'-tetraiodothyronine (T-4) and 3,5,3'-triiod othyronine (T-3) have been found in the eggs of several teleost specie s and are potential regulators of larval development, growth, and surv ival. The purpose of the present study was to determine whether natura l variation of T-3 and T-4 in the eggs of six stocks of walleye, five wild stocks from Kansas, Iowa, Wisconsin, Minnesota, and North Dakota, and a semi-domesticated stock from an Ohio fish hatchery, have an eff ect on larval performance in mass culture. Immersion studies were cond ucted with samples of larvae from four of the same stocks at exposure concentrations of 0.01, 0.05, and 0.1 ppm of T-3 and T-4. Natural egg concentrations of T-3 (range from 0.70 to 1.5 ng/g wet weight of egg) were not significantly different among stocks. Although means of T-4 c oncentration among stocks were significantly different (range 0.53-9.2 7 ng/g), the difference was caused by the exceptionally high concentra tion for the Wisconsin stock (9.27 ng/g +/- 2.20). Performance measure s of the Wisconsin stock (Mississippi River), were not related to T-4 concentration in that stock. In spite of similarity in concentrations of T-4 and T-3 in the eggs, there were significant performance differe nces among the stocks (survival to 21 d, gas bladder inflation, viabil ity, cannibalism, and growth). The immersion studies, however, reveale d a significant difference in incidence of cannibalism and temperature units (TU) to 50% mortality (i.e., survival) between the control grou ps and treatment groups exposed to 0.01 to 0.1 ppm T-3 and T-4. Surviv al was extended more than 2-fold longer in larvae immersed in T-3 comp ared with T-4. The immersion study indicated that thyroid hormones are potentially regulators of walleye development, but further investigat ions are needed to determine reasons for differences in larval perform ance based on natural concentrations in the egg and artificial exposur e (immersion).