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Sexual maturation in sea- and freshwater phases of Pacific salmonids and investigations of reproductive hormone involvement in osmoregulatory adaptation Smith, Jack Lloyd


The changes in reproductive parameters and hormones, and blood sodium during the final weeks of sexual maturation in wild coastal salmon, chinook, Oncorhynchus tshawytscha, and steelhead, O. mykiss, were documented for stocks spawning in two similar river systems on the west coast of Vancouver Island, British Columbia, Canada. Chinook were blood-sampled and sacrificed at intervals during migration from the ocean into brackish Nitinat Lake, through the completion of final maturation in Nitinat River, and at the post-spawning stage. Gonadosomatic index (GSI), hepatosomatic index (HSI), and oocyte germinal vesicle stage and hydration level were measured in sacrificed chinook and steelhead. Stamp River steelhead were serially sampled for blood during the month preceding and the month following final maturation. Blood samples were analysed by specific radioimmunoassay for gonadotropin (GtH), estradiol-17β (E₂), testosterone (T), 11-ketotestosterone (11-KT), and 17α,20β-dihydroxyprogesterone (17,20-P). Blood sodium was measured by flame photometry. GSI and hydration level of oocytes increased in females of both species between the earliest stages of maturation and ovulation. Patterns of hormone concentration were similar in chinook and steelhead, but the peaks in concentration of the steroids at each stage were much lower in steelhead. Concentrations of E₂ and T declined from initial levels to ovulation in females, while GtH and 17,20-P increased. In chinook males, T concentration peaked and declined before spermiation, while 11-KT was highest at spermiation, and declined thereafter. The concentration of 11-KT was higher than that of T in steelhead males at spermiation. Little change in GtH and 17,20-P concentration occurred in steelhead males, but these hormones increased in relation to onset of spermiation in chinook. Fertility was compared between chinook and chum salmon, O. keta, maturing in brackish and fresh water. Fertility was low in both brackish and freshwater chinook females at the beginning of the chinook migration, when salinity was high in Nitinat Lake. Blood sodium was higher in brackish water females, but there was no correlation between egg mortality and blood sodium at ovulation. Later in the season, as salinity in net pens decreased, chinook fertility increased. At the beginning of the chum migration season, when salinity was lowest, chinook fertility was equal to that of chum maturing at the same location. The relationship between plasma reproductive hormone concentration and the ability to osmoregulate in sea water was compared in post-spawning steelhead and coho salmon, O. kisutch, transferred from fresh water to brackish holding facilities. All hormones declined to low levels in steelhead while still in fresh water, within 3 weeks of spawning, but persisted at spawning levels for at least 6 weeks in coho. Steelhead blood sodium was stable in the range indicating osmoregulatory adaptation as salinity was increased to full sea water shortly after transfer. Coho blood sodium was higher in 2/3 sea water than that of steelhead in full sea water. High levels of reproductive hormones may interfere with the ability of salmonids to osmoregulate in sea water.

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