Lower Columbia River Alternative Commercial Fishing Gear Mortality Study: 2011 and 2012

This study was designed to estimate the survival associated with the release of fish from commercial fishing gear fished in the Columbia River below Bonneville Dam in the months of August, September, and October. This report includes results from the first two years, 2011 and 2012, of a three year study.

Steelhead, coho salmon, and fall Chinook salmon, both tules and brights, were the focal species for this study. Survival was estimated using the Ricker two-release method which estimates survival of each treatment group based on recapture probabilities of treatment fish relative to control fish. Treatment fish were captured in beach and purse seines, tagged with passive integrated transponders (PIT) and released between river miles 129 and 144. Control fish were captured at the Bonneville Adult Fish Facility (AFF), PIT tagged, and released into the Columbia River near Skamania Landing (river mile 139). Survival was estimated as a cumulative measure and broken into short-term and long-term. Cumulative survival was from release to the final detection point, short-term survival was from the release point to Bonneville Dam, and long-term survival was from Bonneville Dam to a final detection point. Re-detections were based on array observations, fish trap or spawning ground recaptures, and fishery mortalities as reported in the PIT Tag Integration System (PTAGIS) database.

For steelhead, cumulative survival for these two years ranged from 89% to 92% for the beach seine and 97% to 98% for the purse seine releases. Long-term survival ranged from 92% to 94% for the beach seine and 98% to 99% for the purse seine releases.

For bright fall Chinook salmon, cumulative survival ranged from 56% to 75% for the beach seine and 74% to 78% for the purse seine releases. Long-term survival ranged from 82% to 88% for the beach seine and 88% to 94% for the purse seine releases.

For tule fall Chinook salmon, cumulative survival ranged from 69% to 90% for the beach seine and 64% to 70% for the purse seine releases. Long-term survival ranged from 90% to 92% for the beach seine and 67% to 76% for the purse seine releases.

For coho salmon, cumulative survival ranged from 50% to 62% for the beach seine and 59% to 77% for the purse seine releases. Long-term survival ranged from 86% to 93% for the beach seine and 77% to 91% for the purse seine releases.

The survival estimates produced from this study for these two years may underestimate the survival following release from commercial fishing gear if fish in the treatment groups were a mix of fish destined for spawning localities above Bonneville Dam and those intending to spawn in the lower river below Bonneville Dam. This explanation warrants further study. Pending results from the third year of the study in 2013, the estimates between Bonneville Dam and the final detection point may be considered a maximum survival estimate (or minimum mortality) associated with the commercial fishing gear handled in 2011 and 2012.