| Abstract: | Thyroid hormones, 3,5,3′,5′- tetraiodothyronine (T4) and 3,5,3′- triiodothyronine (T3) have been found in the eggs of several teleost species and are potential regulators of larval development, growth, and survival. The purpose of the present study was to determine whether natural variation of T3 and T4 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 effect on larval performance in mass culture. Immersion studies were conducted with samples of larvae from four of the same stocks at exposure concentrations of 0.01, 0.05, and 0.1 ppm of T3 and T4. Natural egg concentrations of T3 (range from 0.70 to 1.5 ng/g wet weight of egg) were not significantly different among stocks. Although means of T4 concentration among stocks were significantly different (range 0.53–9.27 ng/g), the difference was caused by the exceptionally high concentration for the Wisconsin stock (9.27 ng/g ± 2.20). Performance measures of the Wisconsin stock (Mississippi River), were not related to T4 concentration in that stock. In spite of similarity in concentrations of T4 and T3 in the eggs, there were significant performance differences among the stocks (survival to 21 d, gas bladder inflation, viability, cannibalism, and growth). The immersion studies, however, revealed a significant difference in incidence of cannibalism and temperature units (TU) to 50% mortality (i.e., survival) between the control groups and treatment groups exposed to 0.01 to 0.1 ppm T3 and T4. Survival was extended more than 2-fold longer in larvae immersed in T3 compared with T4. The immersion study indicated that thyroid hormones are potentially regulators of walleye development, but further investigations are needed to determine reasons for differences in larval performance based on natural concentrations in the egg and artificial exposure (immersion). |
