BILATERAL ASYMMETRY, SEXUAL DIMORPHISM, AND NEMATODE PARASITES IN MATURE MALE SOCKEYE-SALMON ONCORHYNCHUS-NERKA

The extent of bilateral asymmetry of 310 mature male sockeye salmon (O ncorhynchus nerka) from 6 Alaskan populations was determined to invest igate the relationship between fluctuating asymmetry in paired morphol ogical characters and physical traits (body size and shape) that seem to signal male quality. There was marked variation within and among po pulations in body size and morphology (e.g., body depth), but no diffe rences in the overall level of asymmetry were detected among the popul ations, nor between the two systems investigated (lakes Aleknagik and Iliamna). The counts or measurements of a number of the baits evaluate d for asymmetry varied among populations, but most differences were at tributable to population-specific variation in body size. The six popu lations exhibited marked similarities in the patterns of asymmetry. Th ree paired characters (length of pectoral fins, number of branchiosteg als, and number of teeth) were directionally asymmetric, indicating a genetic basis for asymmetry rather than an environmental influence. Tw o other paired characters (ventral fin length and number of enlarged t eeth) were bilaterally variable but not correlated with any attribute of male quality. Based on the low variation between the different popu lations in level of asymmetry, we conclude that either stress does not influence asymmetry levels in sockeye salmon or that these population s are under similarly low levels of stress. We also explored the relat ionship between biomass of a nematode parasite (Philonema oncorhynchi) and asymmetry. The weight of parasites was related to fluctuating asy mmetry in the ventral fin length but was not related to fluctuating as ymmetry in number of large teeth.