FISH AGE, NEMATODE (PHILONEMA-ONCORHYNCHI) INFECTION, AND DEVELOPMENTOF SEXUAL DIMORPHISM BY THE ADULT MALE SOCKEYE-SALMON, ONCORHYNCHUS-NERKA, IN WESTERN ALASKA

Of 255 mature male sockeye salmon (Oncorhynchus nerka) from two Alaska n lakes (Aleknagik and Iliamna), 254 (99.6%) were infected with the ne matode 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 seawar d migration had significantly lower parasite masses than those that ha d spent 2 years in the lake (2.7 vs. 4.1 g). However, there was no sig nificant difference in parasite masses between lakes or among spawning populations within the lakes for fish of a given age or of all ages c ombined, even though the populations differed in age composition. The variation in relative parasite mass (mass of parasite (g) /mass of fis h (kg)) was mainly explained by the significant variation in total mas s of the parasites and the sea age of the fish. Thus, total parasite m ass was correlated with the duration of residence in fresh water, whil e the relative parasite mass was correlated with fish size, and hence with the duration of residence in the sea. In spite of heavy infection s (up to 28 g of nematodes) in many salmon, there were no correspondin g reductions in mass/length or in the development of two sexually dimo rphic features, body depth and snout length. Thus, parasite infection did not appear to affect traits under sexual selection, as hypothesize d by Hamilton and Zuk.