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Shellfish Assessment & Groundfish Assessment Programs

Estimating The Catching Efficiency of the Eastern Bering Sea Survey Trawl For Snow Crab

Following the 2009 eastern Bering Sea (EBS) bottom trawl survey, the AFSC Groundfish Assessment Program, in collaboration with the Bering Sea Fisheries Research Foundation (BSFRF), conducted a cooperative trawling experiment to estimate the catching efficiency (i.e., the proportion of crab within the trawl path actually captured) of the standard NMFS survey trawl for snow crab.

The project sought to improve upon the findings of a previous (1997) study by extending the sampling area to cover a wider region of the species range and sampling a broader size distribution, particularly smaller crab. The results of the 2009 study will provide the basis for developing sex-specific selectivity functions along with estimating other selectivity parameters for the purpose of improving the snow crab stock assessment model.

The experiment, conducted on the Bering Sea snow crab grounds from 20 to 24 July, consisted of side-by-side comparative towing involving three vessels each using a different gear type: the NMFS chartered F/V Arcturus towing the standard 83-112 Eastern survey trawl using standard survey protocols, the NMFS chartered F/V Aldebaran towing a modified version of the survey trawl, and the BSFRF chartered F/V American Eagle towing a trawl designed for the European Norway lobster (Nephrops norvegicus) fishery.

The NMFS modified trawl and the BSFRF trawl were intended to provide absolute density estimates to be used as the basis to compute the catching efficiency of the survey trawl. NMFS trawl modifications designed to increase crab catch of all sizes included the addition of a small mesh liner throughout the entire lower panel of the net, additional weight strung across the center of the trawl footrope, and the addition of a tickler chain designed to stir up snow crab embedded in the bottom just forward of the trawl footrope.

Following field tests of the new equipment in snow crab habitat, it was decided to remove the tickler chain and to shorten the experimental tow duration to 5 minutes because catch rates of epibenthos and retention of mud by the trawl gear jeopardized the completion of the experiment.

The NMFS-modified trawl, with the ad hoc changes in design and fishing protocols, was used in 11 comparative tows with the NMFS standard survey trawl alone and 24 triplicate side-by-side tows involving all three gear types in which all snow crabs were sexed and then measured to the nearest mm (carapace width) in order to determine length-specific snow crab catch efficiency.

Preliminary results from this study have been presented at the NPFMC Crab Plan Team Meeting, held at the AFSC on 14 September. T-test results showed crab catch densities from the BSFRF trawl were significantly higher than the NMFS modified survey trawl for the five common size categories reported in the NMFS Annual Crab Report to Industry (i.e., market males >102 mm, pre-recruit males 78-101 mm, small males <78 mm, large females >50 mm and small females <50 mm).

It was therefore concluded that the BSFRF trawl catch rates would be used as the basis for calculating the efficiency of the standard survey trawl (CPUE of the standard survey trawl/CPUE of the BSFRF trawl) with the assumption that the BSFRF trawl caught 100% of the crab in its trawl path.

Preliminary results show that more escapement of snow crab under the footrope of the EBS survey trawl occurs than previously estimated. Specifically, only 35% of the large males, 27% of the pre-recruit males, 13% of the small males, 25% of the large females, and just 3% of the small female snow crab in the path of the survey trawl are captured.

The analyses for this study are ongoing. Future steps involve the calculation of selectivity functions by sex and exploring the effects these functions may have on the current stock assessment model output.

By Ken Weinberg

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