A study of thyroid hormones in eggs and larvae of walleye, Stizostedion vitreum, and rainbow trout, Oncorhynchus mykiss

Eggs from many species of marine teleosts contain the thyroid hormones (THs), thyroxine (T[subscript]4) and 3,5,3[superscript]\u27 triiodothyronine (T[subscript]3). Elevating striped bass egg T[subscript]3 concentration by maternal injection increases larval growth and survival; T[subscript]3 may act during the critical period when larvae begin feeding. We investigated egg T[subscript]3 levels in walleye, whose life history is similar to striped bass, and compared walleye egg TH developmental changes with rainbow trout, a species with larger eggs, slower development and no critical period. Samples of eggs and larvae were frozen for hormone determination or fixed for thyroid histology at intervals up to 14 and 49 days posthatch (walleye and trout). THs were extracted and their concentrations determined by radioimmunoassay. Both walleye and trout eggs contained T[subscript]4 and T[subscript]3. Walleye egg T[subscript]4 concentration varied between hatcheries and individual females. Trout eggs contained more total THs than walleye, but TH concentrations were similar. Thyroid follicles appeared in both species at 5 days posthatch and are present during the walleye critical period. Neither species\u27 larval hormone stores were depleted during development, but whether THs are present in sufficient amounts to optimize walleye survival during its critical period is not clear;Walleye have a life history similar to striped bass and both are difficult to culture. Therefore we investigated whether naturally occurring levels of these hormones in walleye eggs would relate to several parameters of larval performance in culture. Egg concentrations of T[subscript]3 correlated positively with gas bladder inflation rates and incidence of cannibalism in fry from six hatchery stocks. T[subscript]3 content per individual egg correlated positively with larval size at hatch, and T[subscript]4 concentration correlated positively with energy content of the egg;Immersing walleye fry in T[subscript]3 and T[subscript]4 (0,.01,.05,.10 ppm) under starving conditions increased both incidence of cannibalism and survival (time to 50 percent survival) in a dose related manner. T[subscript]3 was more effective than T[subscript]4 in these experiments;Thyroid hormones are potentially important regulators of walleye development and understanding their function may be important to achieving optimum culture conditions