We investigated the cause of the low survival to hatch of embryos of coho salmon (Oncorhynchus kisutch) from the Fairview, Pennsylvania, stock in Lake Erie. In 1988, survival to hatch of this stock was only 42%, whereas another Great Lakes coho salmon stock of similar genetic origin had an 84% survival to hatch. Laboratory cross-fertilization studies between the Fairview stock and a reference Lake Erie stock from Simcoe, Ontario, showed that eggs from the Fairview stock were the probable source of the low fertility. The presence of overripe eggs in Fairview females was associated with poor fertilization and low survival to hatch. Plasma gonadotropin II levels were similar in preovulatory females taken from the Fairview and Simcoe stocks, but testosterone and 17α, 20β-dihydroxy-4-pregnen-3-one levels were significantly lower in the Fairview females. Increasing the triiodo-L-thyronine (T3) content of the eggs by the administration of T3 to the preovulatory females did not enhance egg fertility. We propose that the low survival to hatch of the Fairview embryos is due to delayed oocyte maturation and ovulation and vent maturation, which may have been caused by exposure of the Fairview salmon to warmer water during the period of late ovarian maturation and migration.