Effect of a Mammal Excluder Device on Catch Rates of Pelagic Fishes

The National Oceanic and Atmospheric Administration (NOAA) Northwest and Southwest Fisheries Science Centers conducted gear comparison trials with a new marine mammal excluder device installed in a small pelagic rope trawl. This trawl is used by NOAA Fisheries for studies of juvenile salmon and other small pelagic fishes along the West Coast from California to southeast Alaska. We present an analysis of both size bias and overall catch rate differences between the two nets, based on gear comparison trials (trawls with and without the excluder installed) conducted during off the Washington coast in September 2009, May 2011, July 2011, and June-July 2014. We conclude that the excluder reduces catch rates of most small pelagic fishes. Reductions are severe for some species: coho, chum, and sockeye salmon, northern anchovy, and Pacific herring. Thus, using the excluder as designed has serious consequences for surveys of small pelagic fishes, both compromising CPUE time series and increasing the survey effort needed to collect adequate number of fish for laboratory work. These problems may be reduced with re-design or changes in operation, which are being addressed in ongoing work.