Impacts of Climate Variability on Fisheries in the Northwest Atlantic

K.F. Drinkwater
Bedford Institute of oceanography
Dartmouth, N.S. CANADA
K_drinkwater@BIONET.BIO.DFO.CA

Fish and shellfish respond directly to climate fluctuations. The responses are species dependent and can include noticeable changes in growth and reproduction of individual fish, as well as the distribution and abundance of fish populations. A few examples are provided below.

Mean bottom temperatures account for 90% of the observed (10-fold) difference in growth rates between different Atlantic cod (Gadus morhua) stocks in the North Atlantic (Brander, 1994). Warmer temperatures lead to faster growth rates. In the Northwest Atlantic, the largest cod are found on Georges Bank with a 4-year old fish being, on average, five times bigger than one off Labrador and Newfoundland. Temperature not only accounts for differences in growth rates between cod stocks but also year-to-year changes in growth rates within a stock. Sea temperature declines since the mid-1980s are responsible for approximately 50% of the recent observed decrease in size-at-age of Atlantic cod on the north-eastern Scotian Shelf (Campana et al., l995) and off Newfoundland (de Cárdenas, 1996; Shelton et al., 1996). This is particularly important given that 50-75% of the declines in the spawning stock biomass of the Newfoundland, Gulf of St. Lawrence and north-eastern Scotian Shelf cod stocks during this period were caused by reduced weight-at-age (Sinclair, 1996). Reduced growth rates at lower temperatures are, in large part, due to changes in feeding rates (McKenzie, 1934) although they may also arise through delayed spawning, initially causing a short growing season, and subsequently smaller size latter in life.

Temperature also affects the reproductive cycle of fish and shellfish. Atlantic cod off Labrador and the northern Grand Banks mature at age 7 and in the northern Gulf of St. Lawrence and the eastern Scotian Shelf at age 6 while in the warmer waters off southwest Nova Scotia and on Georges Bank they mature at 3.5 and 2 years, respectively (Myers and Mertz, 1997). While cold temperatures typically result in delayed spawning through slow gonad development, as shown for Atlantic cod on the northern Grand Bank (Hutchings and Myers, 1994), the relationship between temperature at the spawning site and time of spawning depends on local hydrography and fish distribution. For example, in contrast to the northern Grand Banks, cold temperatures off southern Newfoundland lead to earlier spawning of cod (Hutchings and Myers, 1994). These fish reside in warm offshore waters and move onto St. Pierre Bank prior to spawning. In very cold years on the Bank, they appear to delay migration onto the Bank thereby remaining in the warm offshore waters longer, resulting in faster gonad development and an earlier readiness to spawn.

Temperature is one of the primary factors, together with food availability and suitable spawning grounds, in determining the large-scale distribution pattern of fish and shellfish. Because most fish species or stocks tend to prefer a specific temperature range (Coutant, 1977; Scott, 1982), long-term changes in temperature can lead to expansion or contraction of the distribution range of certain species. These are generally most evident near their northern or southern boundaries; warming results in a distribution shift northward and cooling draws species southward.

Capelin, a cold-water pelagic species and the major food source of Atlantic cod off Newfoundland and Labrador, spread southward as far as the Bay of Fundy when temperatures declined south of Newfoundland in the mid-1960s and retracted northward as temperatures rose in the 1970s (Frank et al., 1996). During cooling in the later half of the 1980s and into the 1990s, capelin again extended their range, eastward to Flemish Cap and southward onto the north-eastern Scotian Shelf off Nova Scotia (Frank et al., 1996; Nakashima, 1996). This recent shift appears to be part of a larger scale ecosystem change. Arctic cod (Boreogadus saida), another small coldwater pelagic fish whose primary grounds have traditionally been the Labrador Shelf to northern Newfoundland, recently pushed southward to the Grand Banks and into the Gulf of St. Lawrence in large numbers (Lilly et al., 1994, Gomes et al. 1995). Southward shifts in the distribution of groundfish specles off Newfoundland and Labrador (Atlantic cod: deYoung and Rose 1993, Taggart et al. 1994; Greenland halibut (Reinhardtius hippoglossoides), American plaice and several non-commercial species: Gomes et al. 1995) and snow crab on the Scotian Shelf (Tremblay, 1997) have also been documented during this recent period of cold water.

Recruitment is a measure of how many young survive long enough to potentially enter the fishery. Understanding recruitment variability has been the number one issue in fisheries science this century. Evidence of changes in fish abundance in the absence of fishing suggests the likelihood of environmental causes. Since the advent of intensive fishing, it has become increasingly difficult to sort out the relative importance of fishíng versus environment as the cause of recruitment variability. Still, recruitment levels have frequently been associated with variations in temperature during the first years of life of the fish (Drinkwater and Myers, 1987). The recruitment levels of Atlantic cod off West Greenland, Labrador and Newfoundland have generally been high when ocean temperatures are warm and decrease when temperatures are cold (Taggart et al., 1994), but the warm periods were also those in which the spawning stock biomass were high and thus temperature as the main cause of recruitment decline can not be confirmed. During the mid-1980s to the mid-1990s, extremely cold temperatures were observed in the northern regions and recruitment from Labrador to the Grand Bank was poor. At the same time, as previously mentioned, cod moved further southward. A hypothesis suggests these two features are related; in cold years, spawning tends to occur at more southerly locations where larval retention and hence survival is poor (deYoung and Rose, 1993). Other studies have suggested that the collapse of the cod stocks was not caused by a low larval survival index (recruitment/spawning stock biomass) and attributes the poor state of the fish stocks to overfishing (Myers et al., 1996).

Atlantic salmon, unlike their Pacific cousins, are multi-year spawners. Most of those that spawn in the rivers of eastern Canada in summer, later migrate to the Labrador Sea where they overwinter (Reddin and Shearer, 1987). The young salmon, or "smolts", also travel to the Labrador Sea where they reside until ready to return to the rivers. There is large variability in the numbers of salmon returning to the rivers of eastern Canada each year. The similarity in the interannual variability from different rivers over widely separated regions suggests that the numbers of returning salmon are most likely determined in the marine environment. A wintertime index of the areal extent of sea surface temperatures (4°-8°C) in the Labrador Sea conducive for salmon has been developed which shows a high positive correlation with the number of salmon returning to North America during the following spring and summer (Friedland et al., 1993; Reddin and Friedland, 1993). This winter index has been used to predict prefishery abundance of salmon entering the rivers during the following late spring or summer.

While the few examples provided above indicate certain affects of environment and environmental variability on fish stocks, taking the step to predict the response of local marine organisms to possible future climate change scenarios becomes a highly speculative exercise. First, our knowledge of linkages between environment and fish is at best limited. Secondly, fish also respond to other forcing such as commerical fishing and biological factors, including predation and competition. Separating out these effects from environmently-induced changes in the fish stocks is often difficult. However, continuing research is gradually improving our ability to detect climate impacts on fish.

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