Research Document - 2011/037
Can poor fish condition explain the elevated natural mortality of cod and other marine fish in the southern Gulf of St. Lawrence?
By D.P. Swain, H.P. Benoît, L. Savoie, and T. Surette
Abstract
Natural mortality (M) of older (5+ yr) southern Gulf of St. Lawrence cod has been elevated throughout the 1990s and 2000s. In the northern Gulf cod population, increased natural mortality has been hypothesized to reflect increased starvation due to poor fish condition, resulting from harsh (cold) environmental conditions. We examined whether poor condition may explain the elevated natural mortality of cod and other marine fish in the southern Gulf. Temperature of the Cold Intermediate water Layer (CIL) in the Gulf of St. Lawrence was below normal in the early to mid 1990s, but then increased and has been above average throughout the 2000s. There has been no decrease in M of 5+ southern Gulf cod in response to the warming of the CIL. In contrast to CIL temperature, the ambient temperature of cod (i.e., bottom water temperature in the areas occupied by cod) in September during the feeding season was near average levels throughout the 1990s and 2000s. Ambient temperature of southern Gulf cod was lowest in the early to mid 1980s, when M of 5+ cod was estimated to be substantially lower than in the 1990s and 2000s. Cod in the southern Gulf exhibit a marked seasonal cycle in condition, with lowest condition in the spring. Southern Gulf cod are in better condition in the spring than reported for the northern Gulf stock. It has been suggested that cod are at increased risk of mortality when their condition factor K is less than 0.7. In the early 1990s, the proportion of cod with K<0.7 was much lower in the southern Gulf stock than observed in the northern Gulf stock. This proportion has since dropped to even lower levels. Overwinter mortality of fish in poor condition could result in a truncation in the distribution of K at low values. There is no indication of such truncation in the distribution of cod condition in spring. Condition of southern Gulf cod in spring improved substantially from the 1990s to the 2000s, but this was not reflected in any decline in M. Over the longer 1971-2009 period, the strongest changes in condition occurred in the late 1970s and early 1980s. Condition in September was at the lowest levels observed in this 40-yr period in the early to mid 1980s. Estimated 5+ M was substantially lower then than in the 1990s and 2000s. At the community level, there is no association between patterns in condition and trends in abundance or mortality. Some species with elevated natural mortality show little seasonal variation in condition (e.g., winter skate); others with strong seasonal cycles in condition do not appear to have elevated natural mortality (e.g., Greenland halibut, herring). Similar interannual patterns in condition are shared between species that have declined in abundance and have elevated mortality and those with stable abundance. For species with estimated trends in M, periods of low condition are generally not coincident with periods of increasing or high M. Increased starvation due to poor condition does not appear to be an important cause of the current high levels of M for 5+ cod and other large demersal fish in the southern Gulf of St. Lawrence.
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