Estimating Chinook and Sockeye Salmon Escapement on the Copper River, Alaska, Using Mark-Recapture with External PIT-tags
Keith van den Broek, Jason J. Smith, and Guy Wade
Abstract.—A feasibility study was begun in 2005 to obtain annual escapement information for sockeye salmon Oncorhynchus nerka over a minimum 3 years, using fish wheels and mark–recapture techniques already employed for Chinook salmon Oncorhynchus tshawytscha since 2001. Failed trials using both traditional spaghetti tags and injected PIT tags led to development of a new type of dorsal tag which which encapsulates a 134.2 KHz PIT tag in the marker of a 70 mm dual-anchor T-Bar tag. This tag was first employed in 2007 to estimate the inriver abundance of Chinook and sockeye salmon returning to the Copper river. For the first sample event, up to three live-capture fish wheels were operated at Baird Canyon for a total of 4,495 h from 18 May to 6 August. During this period, 4,456 adult Chinook salmon and 11,027 adult sockeye salmon were marked. For the second sample event, up to two fish wheels were operated at Canyon Creek near the lower end of Wood Canyon for 3,717 h from 28 May to 19 August. A total of 4,192 Chinook salmon and 56,5511 sockeye salmon were examined for marks. Of these, 459 Chinook salmon and 521 sockeye salmon were recaptures. Using a temporally stratified Darroch estimator, abundance of Chinok salmon measuring 500 mm FL or greater than migrated upstream of Baird Canyon from 18 May to 6 August was 46,349 (SE = 3,283). Using a similar estimator, estimated abundance of sockeye salmon that migrated upstream of Baird Canyon from 18 May to 6 August was 1,290,591 (SE = 92,590). This was the first ever defensible escapement estimate derived for sockeye salmon on the Copper River, and the fifth straight year for Chinook salmon with similar data quality to previous years using traditional spaghetti tags.