Most of the thermal tolerance studies on fish have been performed on juveniles and adults, whereas limited information is available for larvae, a stage which may have a particularly narrow range in tolerable temperatures. Moreover, previous studies on thermal limits for marine and freshwater fish larvae (53 studies reviewed here) applied a wide range of methodologies (e.g. the static or dynamic method, different exposure times), making it challenging to compare across taxa. We measured the Critical Thermal Maximum (CTmax) of Atlantic herring (Clupea harengus) and European seabass (Dicentrarchus labrax) larvae using the dynamic method (ramping assay) and assessed the effect of warming rate (0.5 to 9°C h-1) and acclimation temperature. The larvae of herring had a lower CTmax (lowest and highest values among 222 individual larvae, 13.1–27.0°C) than seabass (lowest and highest values among 90 individual larvae, 24.2–34.3°C). At faster rates of warming, larval CTmax significantly increased in herring, whereas no effect was observed in seabass. Higher acclimation temperatures led to higher CTmax in herring larvae (2.7 ± 0.9°C increase) with increases more pronounced at lower warming rates. Pre-trials testing the effects of warming rate are recommended. Our results for these two temperate marine fishes suggest using a warming rate of 3–6°C h-1: CTmax is highest in trials of relatively short duration, as has been suggested for larger fish. Additionally, time-dependent thermal tolerance was observed in herring larvae, where a difference of up to 8°C was observed in the upper thermal limit between a 0.5- or 24-h exposure to temperatures >18°C. The present study constitutes a first step towards a standard protocol for measuring thermal tolerance in larval fish.

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The sulphur amino acids methionine and cysteine and their derivative taurine are metabolically active molecules with interlinked roles in nutritional requirements. Deficiencies in these nutrients are linked to poor growth and health; however, the impacts of these deficiencies on organ structure and function are largely unknown. This study examined the effects of dietary methionine, cysteine, and taurine fed at different levels on yellowtail kingfish liver histology and surface colour, plasma biochemistry, and posterior intestine histology. Samples were collected from two dose-response feeding trials that quantified (1) the taurine requirement and sparing effect of methionine by feeding yellowtail kingfish diets containing one of seven levels of taurine at one of two levels of methionine, and (2) the methionine requirement and sparing effect of cysteine by feeding yellowtail kingfish diets containing one of five levels of methionine at one of two levels of cysteine. Yellowtail kingfish fed inadequate levels of dietary methionine, cysteine, and taurine exhibited thicker bile ducts, less red livers, more intestinal acidic goblet cell mucus and supranuclear vacuoles, and less posterior intestinal absorptive surface area. Further, thicker bile ducts correlated with less red livers (a*, R), whereas increased hepatic fat correlated with a liver yellowing (b*). Our results indicate a shift toward histological properties and functions indicative of improved intrahepatic biliary condition, posterior intestinal nutrient absorption and homeostasis of yellowtail kingfish fed adequate amounts of methionine, cysteine, and taurine. These findings may assist in formulating aquafeed for optimized gastrointestinal and liver functions and maintaining good health in yellowtail kingfish.