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Albatross-Bigelow survey data calibration for American lobsters
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    "The Research Vessel Henry B. Bigelow (including a new survey bottom trawl and survey protocols) replaced the NOAA Research Vessel Albatross IV during 2009 for use in conducting Northeast Fisheries Science Center (NEFSC) spring and fall bottom trawl surveys. Paired tow calibration studies were carried out using both vessels during 2008. The vessel calibration experiment data and a beta-binomial model were used to estimate length based calibration factors that convert American lobster catches by the Bigelow into equivalent catches by the Albatross. Calibration factors for lobster were estimated so that the Bigelow catch for a particular size and station divided by the calibration factor estimates the equivalent catch by the Albatross. A total of 2,314 lobsters were taken by one or both research vessels at 201 out of 636 experimental stations. The Bigelow caught 64% more (N=1,481) lobsters than the Albatross (N= 833). Lobsters were taken by both vessels at 87 stations lobsters, the Albatross caught lobsters but the Bigelow did not at 36 stations, and the Bigelow caught lobsters but the Albatross did not at 78 stations. The Bigelow consistently caught higher numbers of lobsters <10 cm carapace length (CL) and higher proportions of total catch for lobsters <9 cm CL. At least 30 lobster s were taken in each 1 cm size group between 3 and 16 cm CL but sample sizes were lower at smaller and larger sizes. Calibration factors were equation1> 5 for lobsters smaller than about 5.5 cm CL and ranged from about 1.2 to 4.8 for larger lobsters. CVs for calibration coefficients were less than 0.2 for lobsters 5-17 cm CL but increased to 0.48 at the smallest size and to 0.62 at the largest size. Considering the magnitude and variance of the estimates, calibration coefficients for lobsters 5+ cm (the size range currently used in stock assessments) are precise enough for use in providing management advice. A lookup table with calibration factors and CVs for lobsters in Bigelow bottom trawl survey catches is provided"--Abstract.
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