Effect of a Mammal Excluder Device on Trawl Catches of Salmon and Other Pelagic Animals

Concern about bycatch of marine mammals by fishery research gear has led to the use of mammal excluder devices (MEDs) in some surveys. We measured the effect of a rigid-grate MED on trawl fishing effectiveness for juvenile Pacific salmon Oncorhynchus spp. and other upper-pelagic species in the northern California Current. Catches of 11 taxa were compared among sets of a pelagic rope trawl without an MED and with an MED in two orientations: angled upward or angled downward. We estimated differences in overall catch ratios and size selectivity for each MED orientation compared to the unmodified net. The MED orientation had a species-specific influence on both catch ratio and size selectivity, with median ratios ranging from 0.0 to 1.8 for the upward orientation and from 0.4 to 2.7 for the downward orientation. Video observations helped to explain performance differences and may help to improve MED design. Use of an MED in trawl surveys may substantially increase or decrease the catch of various taxa, contributing further uncertainty to the analysis of survey time series for either abundance or catch composition.