Natural Selection and Reproductive Success of Female Coho Salmon (oncorhynchus Kisutch): a Study in Female Competition

This thesis looks at how reproductive behavior and life history traits evolve under female-female competition, in the coho salmon (Oncorhynchus kisutch). Previous studies of reproductive success in female fishes have ignored the contribution of behavior to reproductive success, and thus to natural selection. Reproductive success of individual females was studied for two years in a wild population in Washington State, from the first spawning in November until fry emergence in April. Mature adults were tagged and followed until death. The position, depth, completion date, and eventual fate of each nest was known for most females in the population. Territory quality was quantified by examining survival of eggs to fry. The age and growth patterns are known from analyses of adult scales. My data are supplemented with additional sources on fecundity, and on post-emergent freshwater, ocean, and fishing mortalities. Nest depth was strongly correlated with female body size (r=.778, P<.OOl), as was female breeding life (r=.65, P<.001) which ranged from 3 to 22 days. Females spent almost all of their breeding life guarding their territory to prevent other females from re-using it and thereby digging up the eggs. Variance in nest survival was due to competitive differences in guarding time, fighting ability, and nest depth among females of different body sizes. A total of 24% of all nests in the stream were lost.