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Factors influencing reproductive success of male sockeye salmon Hoysak, Drew J.

Abstract

In this thesis I studied factors that influence reproductive success in two age classes of male sockeye salmon (Oncorhynchus nerka). In my first experiment I tested for the presence of frequency-dependent reproductive success of male age classes in single spawning events and examined the influence of the number of males present on frequency-dependence. I placed groups of males in an enclosure with a nesting female, allowed them to spawn, and determined paternity of the resulting offspring. In spawning trials with three males, frequency-dependent reproductive success was apparent. That is, both age classes had greater mean reproductive success when they were in the minority. However, frequency-dependent reproductive success was not present in groups with six males and the two age classes did not differ in mean reproductive success. My second study was a detailed examination of factors that may have influenced individual male reproductive success in the previous experiment. Alpha male positions were usually held by five-year-old (5Y) males characterized by large body size and large secondary sex traits. Among satellite males, three-year-old (3Y) males held positions closer to the female than did 5Y males, and distance from the female was a good predictor of whether or not a male would participate in spawning and of the order in which males began spawning. Overall, alpha males fertilized more eggs than did 3Y satellite males, and 3Y satellite males fertilized more than 5Y satellite males. Behaviour during spawning appeared to influence male success. Path analysis suggested that male success was directly affected by the time spent in spawning posture which, in turn, was influenced by the time that a male began to spawn. I found no evidence of a relationship between male fertilization success and distance from the female during spawning. In the next experiment, I compared male age classes in their success in sperm competition. I fertilized eggs in vitro with mixtures of milt from pairs of males that differed in age. The milt mixtures contained equal numbers of sperm from each male. Paternity analysis of the resulting offspring revealed that success in sperm competition did not depend on male age. However, success in these competitive fertilizations did not conform to the fair raffle model of sperm competition. Paternity in most of the egg batches was biased in favour of one male. I did not detect any morphological or genetic correlates of success in sperm competition. In my final set of experiments I used in vitro fertilization to examine some aspects of sockeye salmon fertilization dynamics. Sperm maintained high fertility for 10 s after being exposed to water but fertility dropped sharply after that. The association between eggs and sperm occurs very rapidly. 80% fertilization success was achieved with five s of sperm-egg mixing and c. 25% success was achieved with < 0.5 s of sperm-egg mixing. Sperm from 3Y and 5Y males differed little in motility and did not differ at all in their propensity to fertilize eggs.

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