© 2003 by ICES/CIEM International Council for the Exploration of the Sea/Conseil International pour l'Exploration de la Mer
Influence of improved performance monitoring on the consistency of a bottom trawl survey
Alaska Fisheries Science Center, Resource Assessment and Conservation Engineering Division, National Marine Fisheries Service NOAA, 7600 Sand Point Way NE Building 4, Seattle, WA 98115, USA
*Correspondence to M. Zimmermann; tel: +1 206 526 4119; fax: +1 206 526 6723. e-mail: mark.zimmermann{at}noaa.gov.
Numerous trawl hauls, made during the triennial bottom trawl surveys (1977–1998) conducted by the National Marine Fisheries Service off the U.S. West Coast, had unusually small catch rates of benthic fish and invertebrates (cpueB), probably because the trawl failed to contact the seabed (off-bottom). Technological advances in the equipment used to monitor trawl performance since 1986 have increased our ability to recognize off-bottom tows, and cpueB has risen. As direct trawl performance measurements were not available in earlier surveys, a minimum cpueB derived from the survey with the best monitoring of bottom contact of the time-series (1998) was used as a criterion to eliminate trawls with poor bottom contact from earlier surveys. The truncated data sets produce significantly larger biomass indices, especially in 1980, with increases of 43, 45, and 56% for Dover sole, petrale sole, and Pacific sanddab, respectively. The analysis suggests that changes in cpueB over the time-series may be related more to changing survey fishing methods than to changes in abundance. Other bottom trawl surveys, which have also added trawl monitoring equipment during their time-series, may have experienced similar changes in trawl performance.
Keywords: assessment surveys, benthic, bottom trawl, net monitoring, off-bottom, operational data, water hauls
Received 10 October 2002; accepted 27 February 2003.