Comparative fecundity and egg survival in two stocks of goitred coho salmon (Oncorhynchus kisutch Walbaum) from Lake Erie

The reproductive success of two stocks of Lake Erie coho salmon (Oncorhynchus kisutch Walbaum) was examined in an attempt to determine if the low embryonic survival of one of the stocks could be related to the epizootic of thyroid hyperplasia exhibited by that stock. In one stock (derived from Lake Michigan and introduced annually into Trout Run, Fairview, Pennsylvania, by the Pennsylvania Fish Commission) the mortality of the embryos and yolk sac fry was significantly higher and the weight of yolk sac fry and parr was smaller than in a second self-reproducing stock which spawns in Young and Fishers creeks, Ontario. The gonadosomatic indices and number of eggs per unit weight of female were larger in the Pennsylvania stock, although carcass weights, weights of unfertilized eggs, and number of eggs per female were similar in the two stocks; secondary sexual characteristics were poorly exhibited in both stocks. There were no differences in plasma thyroid hormone levels or the degree of thyroid hyperplasia in the two stocks, suggesting that the low survival of the embryos and poor growth of the yolk sac fry in the Pennsylvania stock cannot be attributed directly to thyroid dysfunction, as had been proposed previously. Plasma thyroid hormone levels fell from relatively high levels in sexually immature salmon collected in summer to near zero levels in the pre-ovulatory to post-ovulatory salmon collected in the fall. Moreover, there was a significant decrease in plasma triiodo-L-thyronine (T3) levels in males during the period between early October and early December. No such decline was seen in females, which had low plasma T3 levels by the first (early October) fall collection.