Estimating Overwinter and Smolt to Adult Survival for Coho Salmon in Olympic Peninsula Streams: Evidence of Multiple Life History Strategies for Juvenile and Adult Coho

The downstream movement of coho salmon parr to the marine environment has been well documented, but it has long been assumed that these were “surplus” and did not survive to return as adults. Over the last 8 years we have PIT tagged more than 25,000 juvenile coho salmon in two streams of the Olympic Peninsula to determine their movement, survival and the contribution of various juvenile life histories to the adult spawning population. We have documented two peaks of juvenile emigration – one in the fall as age 0 parr and one in the spring as age 1 smolts. The size at tagging in late summer and location in the watershed appear to be the primary determinants of whether fish emigrate to sea in the fall as age 0 parr or in the spring as age 1 smolts. Returns of PIT tagged adults reveal that juveniles from the fall emigration do in fact survive to return as adults and can contribute up to 35 percent of the total adult return. Furthermore, these adults tended to spend only one winter at sea and return at 2 year old as compared to the more traditional spring emigrants that return at 3 years of age. Our results indicate that traditional methods of spring smolt enumeration may largely underestimate juvenile survival and total smolt production and also lead to overestimation of smolt to adult survival. This is an important consideration for coho salmon life cycle models that assume juvenile and adult coho salmon have a fixed life history or use traditional parr to smolt and smolt to adult survival rates.