Fish age, nematode (Philonema oncorhynchi) infection, and development of sexual dimorphism by the adult male sockeye salmon, Oncorhynchus nerka, in western Alaska

Of 255 mature male sockeye salmon (Oncorhynchus nerka) from two Alaskan lakes (Aleknagik and Iliamna), 254 (99.6%) were infected with the nematode Philonema oncorhynchi with a mean wet mass of 3.1 g (SD = 3.4 g) of nematodes. Fish that had spent 1 year in the lake prior to seaward migration had significantly lower parasite masses than those that had spent 2 years in the lake (2.7 vs. 4.1 g). However, there was no significant difference in parasite masses between lakes or among spawning populations within the lakes for fish of a given age or of all ages combined, even though the populations differed in age composition. The variation in relative parasite mass (mass of parasite (g) /mass of fish (kg)) was mainly explained by the significant variation in total mass of the parasites and the sea age of the fish. Thus, total parasite mass was correlated with the duration of residence in fresh water, while the relative parasite mass was correlated with fish size, and hence with the duration of residence in the sea. In spite of heavy infections (up to 28 g of nematodes) in many salmon, there were no corresponding reductions in mass/length or in the development of two sexually dimorphic features, body depth and snout length. Thus, parasite infection did not appear to affect traits under sexual selection, as hypothesized by Hamilton and Zuk.