Research Document - 2008/069

Review of the survey and analytical protocols used for estimating abundance indices of southern Gulf of St. Lawrence snow crab from 1988 to 2006

By M. Moriyasu, E. Wade, M. Hébert, and M. Biron

Abstract

In the past two decades of the survey history, some methodological adjustments and new equipment have been introduced to improve the procedure and accuracy of the net swept calculations but the sampling protocol has remained since the beginning of the survey. The addition of new and more performing technologies such as temperature-depth sensor, DGPS positioning system and the Netmind® net monitoring system has led to better estimate of the effective tow length and trawl width. Given the changes that have occurred during the snow crab trawl survey, a standard method is necessary in order to compare the time series data from 1988 to 2007. The comparative study amongst three different trawl boats used for the survey was not amenable. It is assumed that the catchability of the trawl has been equal to 100% for all years regardless of the boat used and that sensor measurements are true measurements of the trawl behavior. The 2006 survey served as the reference since most of the methodologies recommended during the recent Snow Crab Kriging Methodology Workshop were adapted. Therefore, the standardization of the survey data since 1988 was done using the KED technique along with the kriging polygon of 37,518 km² used during the latest years of the survey, and with the 3 year average variogram.

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