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

Abstract

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.

Pacific Lamprey and Western River Lamprey marine ecology: Insight from new ocean collections

“…Especially notable were higher catches near the mouth of the Columbia River, which is not surprising given that Western River Lampreys are often caught in the Columbia River estuary during summer (Weitkamp et al 2015). These catches likely reflect high effort near the Columbia River mouth rather than reflecting high abundance, as both multi-day gear trials with surface trawls (Wainwright et al 2019) and nearshore purse seining (Weitkamp, unpublished data) caught nearly one-quarter (23 of 72) of the Western River Lampreys in our records. In contrast, systematic sampling across a grid of stations by the juvenile salmon survey caught Western River Lampreys from Cape Flattery, Washington, to Newport, Oregon, with similar (but not higher) catches on the Columbia River transect.…”

“…As part of this larger study, two smaller projects caught lampreys that were included in the data set. For the first project, gear trials using the Nordic 264 trawl towed at the surface were conducted near the mouth of the Columbia River during the summers of 2011, 2014, and 2015 (Wainwright et al 2019). The other project deployed fine-mesh purse seines (228-304 m long × 12-18 m deep, with 1-cm mesh) in shallow water (4-35 m deep) near the mouth of the Columbia River in July-September 2012 (L. A. Weitkamp, unpublished data).…”

Evidence of Temperature‐Driven Shifts in Market SquidDoryteuthis opalescensDensities and Distribution in the California Current Ecosystem

“…In paired trawls with and without the MMED in different positions, squid catches declined by 12% when the MMED was in an upward position and by 52% when the MMED was in a downward position (Wainwright et al. 2019). Therefore, we adjusted the total squid catches a priori for years with the MMED by using the catch ratios for the upward and downward positions (12% and 52%, respectively) as reported by Wainwright et al.…”

“…Therefore, we adjusted the total squid catches a priori for years with the MMED by using the catch ratios for the upward and downward positions (12% and 52%, respectively) as reported by Wainwright et al. (2019).…”