Research Document - 2009/087

Temperature conditions in the southern Gulf of St. Lawrence during 2008 relevant to snow crab

By J. Chassé and R.G. Pettipas

Abstract

Near-bottom temperatures in the southern Gulf of St. Lawrence (Magdalen Shallows) during 2008 were examined primarily from data collected during the snow crab and multi-species surveys. The snow crab survey was conducted in July-September, while the multi-species survey was in September only. Data from the multi-species surveys, which are available for a much longer period than those from the snow crab survey, were compared to their normal conditions (1971-2000). Additional temperature data from other fisheries surveys and oceanographic studies in these same areas were also examined. In the southern Gulf of St. Lawrence during 2008, conditions were variable but tended to have cooled throughout much of the region compared to the previous year. Near bottom temperatures over a large portion of the deeper parts of the southern Gulf were below (colder) the long-term (1971-2000) average, while the shallower parts exhibited warmer conditions. The cooler coastal water is consistent with a significant increase in the Gulf wide snow crab habitat index (area of the bottom covered by water temperatures between –1 and 3°C) relative to 2007 and it is around the long-term average. The mean temperature within the area of -1°C to 3°C is below normal and fell considerably compared to last year. This is a typical situation as the two time series are negatively correlated. The core temperature was the fourth lowest of the time series and comparable to the value observed in the 1990s cold period. This represents a continuous decline of the core temperature since 2006 when the highest value of the previous 23 years was observed. The crabs caught during the annual snow crab surveys were found in cooler waters in 2008 than in 2006 and 2007, which is believed to reflect in large part the higher availability of warmer temperatures during 2006 and 2007. Typically, the catch analysis shows that the adult snow crabs have a preference for cooler water, but it is not the case for 2008 when the percentage of crab caught in the cooler portion of the habitat was lower than usual. This is the third year in a row that this phenomenon is observed. The new habitat index, based on preferred temperatures, considerably increased from 2007 for both the males and females. The index is now above the time series averages with a value of 0.54 for the males and 0.48 for the females.

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