B

1. Thermal sensitivity of aerobic capacity and swimming performance in the Atlantic cod (Gadus morhua) DOMINIQUE LAPOINTE1, HELGA GUDERLEY1 AND JEAN-DENIS DUTIL2 1 Département de biologie, Université Laval, Québec, Québec, G1K 7P4, Canada. 2 Ministère des Pêches et des Océans, Institut Maurice-Lamontagne, C.P. 1000, Mont-Joli, Québec, G5H 3Z4, Canada. CONTEXT Environmental changes that have a significant impact upon feeding, maturation, reproduction and migration will influence the energy budget of fish and could affect stock productivity (Lambert and Dutil, 1997). Between 1993 and 1996, Atlantic cod from the southern Gulf of St. Lawrence occupied, on average, waters with temperatures of 5°C in winter (Cabot Strait) and 2°C in summer (Magdalen Shallows) (Swain et al., 1998). To gain insight into the effects of life at such cold temperatures, we studied the thermal sensitivity of Atlantic cod by measuring physiological responses during short-term exposure to cold (3°C) and warm (11°C) temperatures. DESCRIPTION Atlantic cod used in this experiment were taken from a laboratory stock, at the Maurice-Lamontagne Institute in Mont-Joli. These had been captured by trawling in the Gulf of St. Lawrence near Grande-Rivière in July 2002 (4T and 4Vn NAFO divisions). 11 cod were acclimated to 7.1 ± 0.4°C, a salinity of 27.8 ± 0.9 psu and natural day lengths for our latitude for 6 months in two circular tanks (1.5m diameter). Cod were tested at 3.3 ± 0.7°C and 11.0 ± 0.1°C. We measured standard metabolic rate (SMR), active metabolic rate (AMR), critical swimming speed (Ucrit), time to exhaustion, post-exhaustion metabolic rate (EMR) and morphological parameters. RESULTS • Cod swimming performance 1. General characteristics of experimental cod Note. Data are shown as mean ± SD, n=11. Stars indicate statistical differences (paired t-test, P<0.05). 2. Aerobic capacity A B Note. Date are shown as mean ± SD, n=11. Stars indicate statistical differences (paired t-test, P<0.05). Ucrit, critical swimming speed. • Are performance hierarchies maintained at different temperatures? Figure 1. Aerobic capacity of cod at 3 and 11°C. In A, total metabolism and in B, mass specific metabolism. Data are shown as mean, n=11. SMR, standard metabolic rate; 15 to 80, swimming speed in cm s-1 during critical swimming speed test, illustrating the active metabolic rate (AMR) and EMR, post-exhaustion metabolic rate. Dashed lines represent speed at the start of repeated burst-coast movements (red = 3°C and green = 11°C). • Effect of temperature on cod metabolism * P<0.05 ** P<0.001 A B * REFERENCES Claireaux, G., Webber, D.M., Lagardère, J.-P. and Kerr, S.R. 2000. Influence of water temperature and oxygenation on the aerobic metabolic scope of Atlantic cod (Gadus morhua). Journal of sea research. 44: 257-265. Lambert, Y. and Dutil, J.-D. 1997. Condition and energy reserves of Atlantic cod (Gadusmorhua) during the collapse of the northern Gulf of St. Lawrence stock. Canadian Journal of Fisheries and Aquatic Sciences. 54: 2388-2400. Swain, D.P., Chouinard, G.A., Morin, R. and Drinkwater, K.F. 1998. Seasonal variation in the habitat associations of Atlantic cod (Gadus morhua) and American plaice (Hippoglossoidesplatessoides) from the southern Gulf of St. Lawrence. Canadian Journal of Fisheries and Aquatic Sciences. 55: 2548-2561. * * Figure 2. Metabolism of cod measured at 3 and 11°C. In A, total metabolism and in B, mass specific metabolism. Data are shown as mean ± SD, n=11. The asterisks over bars indicate statistical significant differences between the two temperatures (paired t-test or Wilcoxon signed rank test, p<0.05). SMR, standard metabolic rate; AMR, active metabolic rate; Scope, aerobic scope; EMR, post-exhaustion metabolic rate. CONCLUSIONS Cold temperatures affected some aspects of cod metabolism predictably. Standard metabolic rate decreased at low temperature, suggesting that in cold environments, cod decrease the intensity of their basal metabolic functions. However, in contrast to earlier studies (Claireaux et al., 2000), other parameters were unaffected by temperature. Despite the decrease of SMR, cod conserve an equivalent metabolic scope at cold (3°C) and warm (11°C) temperatures. Surprisingly, the number of burst-coast movements performed during a test was significantly higher at 3 than at 11°C, suggesting that cod rely more upon white fibres at colder temperatures. Interestingly, cod turned out to be better sprinters in cold than in warm waters. The similarity of time to exhaustion at 3 and 11°C suggests that performance of white fibres was little affected by temperature. Photo: Richard Larocque, Pêches et Océans Canada