Research Document - 2015/009

Environmental conditions in the southern Gulf of St. Lawrence relevant to snow crab

By J. Chassé, P.S. Galbraith, N. Lambert, M. Moriyasu, E. Wade, J.Marcil and R.G. Pettipas

Abstract

Near-bottom temperatures and salinities in the southern Gulf of St. Lawrence (sGSL; Magdalen Shallows) were examined using data primarily collected during the snow crab and multi-species surveys. Annual data are presented relative to normal conditions (average of 1971-2000). The cold intermediate layer (CIL) volume time series clearly shows a greater quantity of cold water in the 1990’s in the sGSL with a return to more normal conditions towards the end of the time series (2010). The analysis shows that the inter-annual variability of September bottom temperatures is larger in shallow waters (e.g. Northumberland Strait) and along steeper slopes (e.g. Laurentian Channel) than over the relatively flat bottom of the Magdalen Shallows. The five-year running means of bottom temperature anomalies for snow crab fishing areas 12, 19, 12E and 12F show a continuously decreasing temperature from the last maxima in the early 1980s to the minima in the early 1990s followed by a general warming trend since that time. The area of the bottom of the Magdalen Shallows covered by waters between -1°C and 3°C during the September multi-species survey has shown a general decrease since the beginning of the 90’s. Although there is a lot of inter-annual variability, the annual mean temperature within the snow crab habitat area increased significantly over the same period. The value in 2010 is significantly higher than the long-term mean and represents the highest value since 1982 when the maximum (1.31°C) of the time series was reached. The sGSL is covered with salinities < 34 psu with low inter-annual variability. The analysis of the commercial snow crab thermal distribution revealed that the occurrence of commercial crab in areas with temperatures above 3°C is very rare, with only 5.2% of the catches in bottom waters above 3°C. The results suggest that commercial male crab favoured habitat between -1.2 to 1.2°C. The depth distribution suggested that commercial crab favoured habitat between 65 to 100 m. The analysis of the temperature coverage of the old and new polygons used for the estimation of the biomass reveals that the new polygon has a better coverage of the snow crab thermal habitat than the previous polygon. The appropriateness of the polygon coverage also increases when bottom temperature thresholds of 3ºC to 5°C are considered. Since 95% of the crabs are caught below 3°C, the new polygon covers the commercial snow crab habitat relatively well.

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