Atlantic cod, Gadus morhua Background

Transcription

1 Atlantic cod, Gadus morhua Background The Atlantic cod is an arctic to temperate marine species of the eastern and western North Atlantic. On the western side it ranges from 69º10 N ( to 35º10 N (Scott and Scott 1988). The biology of Atlantic cod was reviewed by Scott and Scott (1988), and more recently by Klein- MacPhee (2002). The species is demersal though it makes pelagic forays for feeding and spawning. Cod can be found from the surface to 600 m in brackish to marine water (Froese and Pauly 2005), associated with various substrates when near the bottom. Larger fish tend to stay closer to the bottom and to be found in deeper water. Food consists of a wide variety of organisms, principally fishes, decapod crustaceans, and squids. These prolific spawners reproduce in various seasons in different parts of the western Atlantic range, covering all months of the year (except August and September on the Scotian Shelf - Markle and Frost 1985). There is a general pattern of reduced spawning in more northern waters in winter and in more southern waters in summer (Hardy 1978, Table 7). Eggs, larvae, and early juveniles are planktonic for up to several months before descent to the bottom. During the first year of life many are associated with the bottom in very shallow water, but also on the offshore banks. Cod are migratory in the extremes of their western Atlantic range, but less so in between. In Labrador waters cod are present in summer and early fall, but move to deeper water or south for the winter and spring. In Canadian waters south of southern Grand Bank cod are basically nonmigratory, though they make localized movements. Southern fish move north and east to cooler water in summer and fall, returning southward for winter and spring. Though not a schooling species, cod form aggregations during feeding. The current view is that there are four populations of Atlantic cod in Canadian waters: Arctic, Newfoundland and Labrador, Laurentian North (northern Gulf of St. Lawrence and off the south coast of Newfoundland), and Maritimes (southern Gulf of St. Lawrence, Scotian Shelf, and Gulf of Maine) (COSEWIC 2003). Atlantic cod in waters further south likely form additional populations. Historically Atlantic cod has been an extremely important commercial species in the western Atlantic. Though still important, declining stocks have caused great reductions in quotas or fishery closures in most waters (see for a summary of the history of the cod fishery and its management in Canada). Cod has been fished commercially from 63 o N ( to 35º54 N ( Recreational fishing is important in U.S. waters (Klein-MacPhee 2002), but is under tight control in Canadian waters. In May 2003 the Newfoundland and Labrador cod population was designated as endangered by the Committee for the Status of Endangered Wildlife in Canada; the Laurentian North (northern Gulf of St. Lawrence and St. Pierre Bank) population was designated threatened and 1

2 the Maritimes and Arctic populations special concern. No populations are listed under Canada s Species at Risk Act. Temperature limits, critical thresholds, vulnerability, and barriers to adaptation Sea surface temperatyures in the current distribution of the Atlantic cod range from a February minimum of -2.1ºC to an August maximum of 28.2ºC. Atlantic cod prefer cooler waters, but are influenced by time of year, geographic location, and fish size; biological conditions, e.g. capelin concentration may take precedence over temperature preference within limits (Rose et al. 1994, Rose and Leggett 1989, Scott and Scott 1988). Hardy (1978) and Klein-MacPhee (2002) summarized cod temperature preferences. Their overall temperature range is -2 to 20ºC, both of which may be lethal, with optima from 0-6ºC and usually less than 10ºC. In the Gulf of Maine cod inhabit waters of 0-13ºC (0-10ºC on the Scotian Shelf -Page et al. 1994). Eggs in Newfoundland and Labrador waters persist at temperatures down to -1.0 to 0ºC (Thompson and Riley 1981), and Newfoundland and Labrador eggs and larvae must experience temperatures down to -1.35ºC (Sundby 2000). Temperatures remaining below 4ºC during the larval phase prevent survival to juveniles (Laurence 1978). Larval cod freeze at ºC (Valerio et al. 1992); juveniles freeze at approximately -1.55ºC, and adults at about -1.2 o C (King et al. 1989). Though large cod tend to avoid temperatures warmer than 10 o C, they are at times abundant on Nantucket Shoals in waters up to 15ºC. Jobling (1988) estimated the final temperature preferendum of cod at 13.5ºC. Growth and feed conversion in Iceland and European Atlantic cod is optimal at temperatures approximating 10 o C, though performance is better in small juveniles at higher temperatures (Björnsson and Steinarrson 2002, Björnsson et al. 2001, Pörtner et al. 2001). Lower sensitivity of small cod to higher temperatures explains their occupation of shallow waters in summer. Earlier stages of cod also appear to be less sensitive to higher temperatures. Experiments with eggs to recently transformed juveniles have shown mortality to increase dramatically above approximately 12ºC -15ºC (Iversen and Danielssen 1984, Yin and Blaxter 1987). Experiments by Steinarsson and Björnsson (2000), Otterlei et al. (1994) and Otterlei et al. (1999) led to estimates for optimum growth in larvae varying from ºC to 14-16ºC, with optimum survival at ºC, and optimum growth in juveniles at 13.7ºC; growth in juveniles decreases rapidly above 16ºC. Recruitment is highest for cod stocks in waters of 5-8ºC (Brander 2000, Planque & Frédou 1999). Atlantic cod ranked as one of the least sensitive species to global warming examined in this study. The most critical stage with respect to temperature in Atlantic cod life history may be the planktonic larval stage in which survival requires temperatures rising to at least 4ºC, optimum survival occurs at only ºC, and mortality increases dramatically above 12ºC -15ºC (see above). 2

3 Impacts A 4ºC rise in global temperature will impact the future distribution of Atlantic cod in the western Atlantic. Results from all models and scenarios are generally similar and show a potential loss of habitat in waters from southern New England southward, which will eliminate the recreational fishing there. The CCSR model also shows loss of habitat in Labrador waters where the fishery already is closed. No northward gain of habitat is predicted in our study, which does not extend to the northern limit of Atlantic cod. All other waters generally will remain suitable for Atlantic cod. The impact of global warming on Atlantic cod stocks will be mediated in a complex manner through abiotic and biotic ecosystem processes; simple relationships are unlikely to be of major import (Daan 1994). In a major review of climate induced temperature effects on Atlantic cod, Pörtner et al. (2001) predicted As a preliminary picture for Atlantic cod, global warming will lead to a northward shift of populations. (Atlantic cod) will experience increased growth performance and fecundity at more northern latitudes as water temperatures rise. The expected shift in energy budgets suggests that this stimulating effect of warming on growth in Northern populations may be larger than expected from the typical rate effect of temperature on biochemical reactions (the Q10 effect) alone. However, if polar water temperatures remain the same as of today the limitation of growth performance and fecundity by cold temperatures in the North together with a more restricted range of geographical distribution will lead to a reduced overall population size of the species. Individuals of this species may be adaptable to increasing water temperatures resulting from global warming by slowly shifting their distribution to remain in suitable temperatures. This appears possible because of their life history, mobility, and the presence of appropriate habitat where water temperatures will remain suitable. Also, studies off Newfoundland (Rose et al. 1994) and in the Gulf of St. Lawrence (Castonguay et al. 1999) found cod to be distributed further northward in years with warmer water temperatures. Impacts of shifting cod populations are speculative. Grand Banks cod have genetically based higher rates of growth in larvae and food conversion in juveniles compared to Gulf of Maine cod (Purchase and Brown 2000). Thus, if warming temperatures and a northward shift in cod distribution results in greater population overlap, individuals from southern populations may be out-competed by individuals of more northern populations. Any resulting increase in adult cod (as a predator) density may force juvenile cod (prey) into sub-optimal areas with lower temperature, reducing their growth and survival (Kristiansen et al. 2001). Clearly, predicting such inter-population dynamics and consequences resulting from shifting distributions in response to global warming is beyond the scope of this project. 3

4 If Atlantic cod successfully shift their distribution in response to global warming to maintain environmental temperature while accommodating other ecosystem factors, they will see no increase in growth rate and will reproduce successfully. But if warmer temperatures are experienced they will stimulate an increase in the rates of growth (e.g. Brander 1994, Purchase and Brown 2001) and other functions where temperatures do not become excessive. Moderately higher temperatures (8-10ºC) will induce faster development of eggs and larvae, thus lowering mortality by decreasing the duration of these vulnerable stages (Bradbury et al. 2001, Otterlei et al. 1999, Ottersen and Loeng 2000, Sundby 2000). However, though egg development will be faster with increasing temperature, the size of resulting larvae is equivocal. They may be smaller and perhaps of poorer quality (Pryor and Brown 1999), or larger (Pepin et al. 1997), and better able to survive (Kjesbu et al.1996). The vulnerable larvae and early juveniles of Atlantic cod are found in the plankton in all months of the year. Projected warming of Canadian Atlantic waters in the winter and early spring will have no detrimental effect on planktonic stages of cod. Should global warming result in cooling of waters off Labrador as predicted by model CCSR, they likely will remain below 4ºC for many months, preventing survival of cod larvae. The effect of projected warming above 12º -15ºC in the summer and early fall on mortality should be ameliorated by the tendency of reduced spawning in the southern portion of cod s range at that time. Higher temperatures also will directly influence zooplankton biomass (prey availability), thus increasing growth and survival of larval cod (Ottersen and Loeng 2000). Prey availability also may affect recruitment. In recent decades cod stocks in colder waters have shown an increase in recruitment with increasing temperature (Planque and Frédou 1999), perhaps explained by increased availability of Calanus finmarchicus, the dominant prey species of pelagic cod larvae and early juveniles, through changing oceanic advection (Sundby 2000). Global warming may produce the same result if the relationship of oceanic advection to increasing temperature remains unchanged. Sealworm, or codworm (Pseudoterranova decipiens species complex), is widely distributed in cod spatially and temporally from Labrador south to the lower Bay of Fundy and northeastern Gulf of Maine, with greatest infestation in the more southern waters (McClelland et al. 1983, McClelland et al. 1985, Measures 1996). Sealworm abundance in fish and in grey seals (Halichoerus grypus), the principal definitive host, has been predicted to increase should water temperatures increase in the Gulf of St. Lawrence (Marcogliese et al. 1996). This prediction likely should apply to all Canadian Atlantic waters. References: COSEWIC COSEWIC assessment and update status report on the Atlantic cod Gadus morhua in Canada. Committee on the Status of Endangered Wildlife in Canada. Ottawa. xi + 76 pp. 4

5 Jobling M A review of the physiological and nutritional energetics of cod, Gadus morhua L., with particular reference to growth under farmed conditions. Aquaculture 70:1-19. Laurence, G.C Comparative growth, respiration and delayed feeding abilities of larval cod (Gadus morhua) and haddock (Melanogrammus aegelfinus) as influenced by temperature during laboratory studies. Mar. Biol. 50: 1 7. Valerio PF, Goddard SV, Kao MH, Fletcher GL Survival of northern Atlantic cod (Gadus morhua) eggs and larvae when exposed to ice and low temperatures. Canadian Journal of Fisheries and Aquatic Sciences 49: Yin MC, Blaxter JHS Temperature, salinity tolerance, and buoyancy during early development and starvation of Clyde and North Sea herring, cod and flounder. Journal of Experimental Marine Biology and Ecology 107: