Given the predicted increase in average ocean temperatures, and in the frequency and severity of heat waves, cold events and hypoxic episodes, due to climate change there is significant focus on determining the temperature and oxygen (hypoxia) limits of many fish species. Methods used to determine acute temperature (CTMax and CTMin) and hypoxia tolerance vary in the literature, and there is concern about how lab-based measures relate to those in free- swimming fish. Further, longer term (incremental temperature, ITMax) tests have now been conducted on a number of species as they better reflect the temporal nature of seasonal changes in water temperature in temperate regions (including at aquaculture cage-sites), but there is limited data on how CTMax and ITMax values differ.
To investigate how different methods of determining acute upper temperature and hypoxia tolerance affect the values obtained, three widely used protocols that utilize heart rate-temperature relationships and other indices to estimate a species’ tolerances were compared using adult (~ 800 g) Atlantic salmon from the same population. Salmon were: i) implanted with data loggers, given a month to recover, and tested (2°C h-1; an ecologically-relevant rate of heating) while free-swimming in a tank with conspecifics; ii) fitted with Doppler flow probes and exposed to a 2°C h-1 temperature increase in a respirometry chamber until CTMax; and iii) anaesthetized, implanted with a data logger, and pharmacologically stimulated prior to a temperature increase of 10°C h-1 (i.e., using the ‘ramp protocol’). Fish in respirometers and those free-swimming in the tank were also exposed to a stepwise decrease in water oxygen level (from 100 to 30% air. sat.) and the oxygen level at which bradycardia occurred was determined.
Resting heart rate (fH) was significantly lower for the free-swimming fish as compared to those in respirometers (~49 vs. 69 min-1), and this was reflected in their scope for fH (~70 vs. 105 min-1) and their CTMax values (27.7 vs. 25.9°C). Further, the Arrhenius breakpoint temperatures and the temperature at maximum fH for the free-swimming fish were much greater than for the other two groups, respectively (~18.4, 18.1 and 14.6°C; 26.5, 23.2 and 20°C). Finally, the oxygen level at which bradycardia occurred was significantly higher in free-swimming salmon as compared to those in respirometers (62 vs. 52% air sat.). Interestingly, the ITMax for salmon (determined at 0.2°C day-1) was 25.2°C, ~3-4°C lower than their CTMax (28.5°C; both measured in free-swimming fish), and this difference was much greater than previously determined in cod (21.8 vs. 22.5°C).
Overall, these studies question the relevance of CTMax measurements (particularly those not performed on free-swimming fish or at ecologically relevant rates of heating) with regard to making predictions of how climate-related changes in water temperature and oxygen levels will impact salmon (fish) physiology and survival in culture.