The acute and incremental thermal tolerance of Atlantic cod (Gadus morhua) families under normoxia and mild hypoxia.

Given climate change projections, the limited ability of fish reared in sea-cages to behaviourally thermoregulate, and that thermal tolerance may be heritable, studies that examine family-related differences in upper thermal tolerance are quite relevant to the aquaculture industry. Thus, we investigated the upper thermal tolerance of 15 Atlantic cod (Gadus morhua L.) families by challenging them with acute (2 °C h) and incremental (1 °C every 4 days) temperature increases (CT and IT tests, respectively) under normoxia (~ 100% air saturation) and mild hypoxia (~ 75% air sat.). The cod's CT was 22.5 ± 0.1 °C (mean ± S.E.) during normoxia and 21.8 ± 0.1 °C during hypoxia (P < 0.001); and these two CT values were significantly correlated across families. In both the normoxic and hypoxic IT tests, feed intake fell by ~50% between 17 and 18 °C, and stopped entirely by 21 °C. No mortalities were observed under 20 °C in the normoxic and hypoxic IT tests, and the IT value was ~21.7 °C in both groups. Differences in the upper thermal tolerance between families were only observed in the CT experiment. No correlation was found between the specific growth rate and the CT of the families. Further, no correlation existed between CT and IT. This study is the first to compare the thermal tolerance of fish families to both CT and IT challenges, and the data: 1) suggest that the Atlantic cod is quite tolerant of acute (i.e., hours) or short-term (i.e., weeks) exposure to high water temperatures (i.e., up to 20 °C); 2) indicate that it might be difficult to select fish with higher IT values; and 3) question the relevance of CT for selecting fish that are destined for sea-cages where temperatures slowly warm over the summer.