Calibration of bottom trawl survey vessels : re-analysis of comparative fishing experiment between the CCGS Alfred Needler and CCGS Teleost on the Scotian Shelf and Bay of Fundy in 2005

Abstract

Bottom-trawl surveys provide key inputs to stock assessments for groundfish stocks and other taxa, for ecosystem monitoring and reporting, and for research. These surveys can produce annual indices of abundance that are proportional to stock size, provided that the proportionality constant, typically called catchability, does not change over time. This is typically achieved through the use of standardized survey design and procedures. In the Maritimes Region, Fisheries and Oceans Canada (DFO) conducted an annual Summer Ecosystem Research Vessel Survey of the Scotian Shelf and Bay of Fundy employing the Canadian Coast Guard Ship (CCGS) Alfred Needler from 1982 to 2023 but due to repairs, the survey was completed by the CCGS Teleost instead in 2007, 2018, 2022 and 2023. The two vessels used the same protocol, but there were differences in vessel characteristics. In order to test potential differences between vessel catchabilities for a large number of taxa of interest, a comparative fishing experiment was conducted in the summer of 2005 between the two vessels. Data analysis was conducted in 2009, but ultimately considered unreliable for most species by researchers and assessors in the Region. Upon vessel replacement by the new CCGS Captain Jacques Cartier and CCGS John Cabot, a second comparative fishing experiment was conducted between Teleost and the new vessels. There was a growing interest in a review of both experiments and a re-analysis of the first experiment, such that time series of catch data from the survey across the past four decades employing four different vessels could be consistently maintained and integrated. This document briefly describes the experiment in 2005 and the resulting data, followed by detailed analyses for 60 fish and invertebrate taxa routinely sampled during the survey for which there were sufficient data from the experiment. Recommendations for vessel calibrations of catch numbers based on the results of the analyses were provided for these taxa, where eight taxa had length-dependent conversion factors using length-disaggregated analysis, five taxa had length-independent conversion factors using length-disaggregated analysis, four taxa had length-independent conversion factors using length-aggregated analysis, and calibration of 36 taxa were recommended as not necessary. Results were also provided results from length-aggregated analysis of catch weights to supplement the length-aggregated analysis for catch numbers for 19 taxa.