Research Document - 2005/065

Physical oceanographic conditions in NAFO Division 3P during 2005 - possible influences on the distribution and abundance of Atlantic cod (Gadus morhua)

By Colbourne, E.B., and E.F. Murphy

Abstract

Oceanographic data from NAFO Division 3P during the spring of 2005 are examined and compared to the previous year and the long-term (1971-2000) average. Temperature measurements on St. Pierre Bank show anomalous cold periods in the mid-1970s and from the mid-1980s to mid-1990s. Beginning in 1996 however, temperatures moderated, decreased again during the spring of 1997 and returned to more normal like values during 1998. During 1999 and 2000 temperatures continued to increase, reaching the highest values observed since the late 1970s in some regions. During 2001-2003 however, temperatures cooled significantly to values observed during the mid-1990s with the average temperature during the spring of 2003 the coldest in about 13 years. Temperatures during both 2004 and 2005 warmed considerably over 2003 values to 1°C above normal in some areas. The areal extent of <0°C bottom water during 2003 increased to the highest in about 13 years but decreased during 2004 and 2005 to <10%, the lowest since 1988. The areal extent of bottom water with temperatures >3°C has remained relatively constant at about 50% of the 3P area during the past decade. On St. Pierre Bank bottom water with temperatures <0°C essentially disappeared during the warm years of 1999 and 2000, reappeared again during 2001 to 2003 and disappeared again during 2004 and 2005. In general, temperatures during the past two years increased significantly over values observed during 2001-2003. The most evident trend in the numbers of cod caught per set during the multi-species surveys was the high number of zero catches in the <0°C water on St. Pierre Bank and regions to the east of the Bank, mainly from 1985 to 1998 but also from 2001 to 2003. During 1999 and 2000 larger catches became more wide spread over St. Pierre Bank as cold (<0°C) water disappeared from the area. In general, during most surveys the larger catches occurred in the warmer waters (2°-6°C) along the slopes and areas to the west of St. Pierre Bank. In 2004 there was no observed shift in the distribution of cod over St. Pierre Bank and there were many low or zero catches in the warm deeper waters off the banks compared to most years. During the spring of 2005 however there was an increase in the number of non-zero catches on St. Pierre Bank and an apparent increase in the size of the catches in deeper waters with temperatures >2°C. Finally, variations in the estimated abundance and biomass of cod from the RV surveys in strata with water depths <92 m are significantly correlated with bottom temperatures for that depth range.

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