VARIATION IN LIFE-HISTORY CHARACTERISTICS AND MORPHOLOGY OF SOCKEYE-SALMON IN THE KVICHAK RIVER SYSTEM, BRISTOL BAY, ALASKA

Sockeye salmon Oncorhynchus nerka spawn in many streams and along lake beaches of the Kvichak River system in Alaska, but fry from the disti nct spawning areas reside in a common nursery habitat, Iliamna Lake. I n addition, Kvichak River subpopulations have similar dates of adult e ntry into fresh water, similar migration distances, and similar spawni ng dates. These similarities in rearing environments and migratory tim ing enabled us to test the hypothesis that differences in spawning and incubation habitat alone can promote differentiation in traits associ ated with reproductive success. River-spawning sockeye salmon tended t o be larger at age and older than those spawning along island beaches. Females from rivers were more fecund but had smaller eggs than the be ach-spawning females. Males from beaches were deeper-bodied and (in on e comparison) had relatively longer lower jaws than males from rivers. The tendency of river-spawning females to mature later than beach spa wners may be related to a higher marine growth mte and greater increas e in fecundity with length. Differences in male morphology may reflect the countervailing pressures of natural and sexual selection. We conc lude that these patterns of variation reflect, in part, adaptations to spawning and incubation conditions of the populations.