Research Document - 2014/116

Thermal habitat of the Atlantic mackerel: Depth of 8 °C isotherm in the southern Gulf of St. Lawrence between 1960-2014

By Peter S. Galbraith and François Grégoire

Abstract

Atlantic mackerel (Scomber scombrus L.) is a pelagic fish that, in Canadian waters, reproduces principally in the southern Gulf of St. Lawrence in June and July. It is sensitive to water temperature, compensating the variability by changes in its distribution that allow the species to remain within its range of temperature preference. While the latter is between 7°C and 16°C, field observations also suggest that the species does not tolerate water temperatures lower than 5-6°C or higher than 15-16°C. Spring migration along the United States American coast has been associated with warming temperatures, and in particular with the timing of water temperatures reaching 8°C. In recent years, industry catches in the Baie des Chaleurs have been occurring in deeper waters. Our objective is to verify whether the depth of the preferred temperature habitat, associated with the 8°C isotherm, has increased in recent years, which could explain the observed distribution changes. Therefore, we investigated the interannual variability of the Magdalen Shallows habitat with temperatures higher than 8°C in mid-June as well as in September, between 1960 and 2014. In general, in June, the western part of the Magdalen Shallows is warmer with deeper 8°C isotherms than the eastern part. A trend with increasing depth of the mid-June 8°C isotherm was observed in both regions, but higher on the western side. The 8°C June isotherm depth has increased from about 7-8 m during the 1960s (with averages as low as 2 m in some years) to about 12 m since 2010. In the extreme warm case of 2012, the 8°C isotherm reached an average of 15 m in both regions while exceeding that value over large areas. However, the interannual variability of the depth of the 8°C September isotherm does not show any long term trend or an east-west disparity.

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