Critical thermal maxima and minima of Sardinops sagax caeruleus Girard 1854 and the analyses of behavioral responses to establish adequate endpoints

Endpoint responses identifying critical thermal maximum (CTMax) and minimum (CTMin) temperatures of the Pacific sardine Sardinops sagax caeruleus Girard 1854 were estimated, after acclimation to a summer (SC) and a winter (WC) thermal cycle, to establish a precise endpoint for this species. The initial response observed during critical thermal limits was the onset of muscle spasms (OS) which was followed by loss of equilibrium (LE). Significant differences were found between SC and WC acclimated sardines when LE was used as the CTM endpoint. No differences were observed when the OS endpoint was used, owing to the high variance of OS values. Sardines had a CTMax of 30.4 and 31.8 °C for fish that underwent the WC and SC, respectively, while the CTMin was 5.2 and 6.5 °C for WC and SC sardines, respectively. Sardines showed a marked tolerance to low temperatures and some capacity to modify their critical limits when acclimated to different cycling thermal regimes. The thermal limits can be useful to compare sub-populations of this species inhabiting the North Pacific Ocean. Moreover, loss of equilibrium (LE) was a more consistent response than onset of spasms (OS); thus, it can be used as a reliable endpoint in further studies.