BIOLOGICAL MECHANISMS ENABLING SYMPATRY BETWEEN SALMONIDS WITH SPECIAL REFERENCE TO SOCKEYE AND CHUM SALMON IN OCEANIC WATERS

Interspecific relationships between salmonids in oceanic waters were examined with special reference to sockeye salmon Oncorhynchus nerka and chum salmon Oncorhynchus keta, which are sympatric species in the Bering Sea and the North Pacific Ocean and have ocean migrations of similar duration. Although there certainly exist similarities or overlaps between the two species, several conspicuous differences in ecological, physiological and morphological characteristics were also found. Sockeye are not distributed as widely as chum, which is consistent with the narrower range of optimum water temperatures for sockeye than for chum. Sockeye exhibit clearer feeding rhytms and ingest more nutritious food organisms than chum, although the two species have similar diel feeding patterns in general. Chum, which are known to be more opportunistic feeders, are superior to sockeye in digestion ability, and this may enable chum to utilize peculiar or poorly nutritious food organisms such as jellyfish, on which other salmonids seldom feed. Specific differences in environmental adaptabilities and feeding habits may help reduce interspecific competition and allow sympatry during the oceanic life of these two species.