Influence of short-term estuarine rearing on the ocean survival and size at return of coho salmon in southeastern Alaska

Hatchery smolts of coho salmon Oncorhynchus kisutch, cultured for 2-5 weeks in estuarine net-pens, were compared with those released directly from fre shwater to determine whether short-term seawater rearing would influence th eir ocean survival or size at return. Samples of each experimental group we re marked with coded wire tags before release, and ocean survival and size at return were estimated from tag recoveries. Releases of all experimental groups were timed to coincide with the peak emigration of wild smolts from local streams. After accounting for year-class variation, the duration of e stuarine net-pen rearing had a significant positive effect on the rate (P = 0.012) and size (P < 0.001) of returning adults. Smolt size at release did not significantly affect either survival or size at return (P greater than or equal to 0.34). The results suggest that short-term estuarine rearing b efore release may ameliorate the effects of osmotic stress on growth and su rvival during the critical period of seawater adaptation. This approach may have utility for enhancement pro.-rams that have access to marine sites be cause it can provide for increased smolt production and higher ocean surviv al, resulting in a possibly greater biomass of returning adults.