Elastic Coho: Phenotypic Changes in Response to Size-Selective Fisheries

Size-selective fisheries are expected to affect traits such as individual growth rate, size-at-maturation, and fecundity either directly or as correlated traits.  The direction and magnitude of the response should depend on the heritability of the trait, selection differential and fishery exploitation rate.  We analyzed size-at-age and fecundity data representing four populations of hatchery Coho salmon (Oncorhynchus kisutch) from Puget Sound and coastal Washington state, USA, which had experienced 10 or more years of moderate to high fishery exploitation by both troll and terminal gillnet fisheries with varying levels of size-selectivity.  This was followed by 10 years of low exploitation by primarily terminal gillnet fisheries. We estimated temporal trends in adult (age 3) size-at-maturation (growth rate) and fecundity and found that they were correlated with total exploitation rates and selection differentials on body length.  When fishing exploitation rates were dramatically relaxed, populations responded quickly returning back to initial body sizes over the next 10 years. Our results strongly suggest that size-selective fishing can affect growth rates and fecundity in hatchery Coho salmon, that the direction and magnitude of change is determined by selection differentials and exploitation rates, and that after relaxation of exploitation rates, mean body size and fecundity returned to values comparable to those at the beginning of the sampling period.