Ecophysiology of juvenile California halibut Paralichthys californicus in relation to body size, water temperature and salinity

Food consumption, metabolism, growth, conversion efficiencies (food assimilation, gross and net growth) and whole-body water content of small (118 to 172 mm TL) and large (237 to 310 mm TL) juvenile California halibut Paralichthys californicus exposed to various combinations of water temperatures (14, 20, 25 and 28 degree C) and salinities (8, 17 and 34 ppt) were quantified in laboratory experiments. Small juvenile halibut were able to grow and maintain water balance over almost the entire ranges of water temperatures and salinities tested, except at 14 degree C and 8 ppt, where they lost weight but gained about 2% body water. Large juvenile halibut were far less tolerant of variations in water temperature and salinity. Regardless of salinity, large juvenile halibut exposed to 14 and 25 degree C lost weight due to greatly reduced energy intake, and experienced >90% mortality at 28 degree C; in diluted seawater at 14 and 25 degree C they also experienced water balance problems. Only at 20 degree C did surplus energy and lack of water balance problems allow large juvenile halibut to grow across all salinities. Differences in energetic and water balance responses of small and large juvenile halibut correspond to the habitat preferences of each size group. Small juvenile halibut are estuarine, and their ability to tolerate wide variations in water temperatures and salinities allows them to exploit estuaries and coastal lagoons with abundant small prey (gobies), warm temperatures, and few predators. However, in winter, with the increased probability of estuarine or lagoon mouth closures, water temperatures and salinity can rapidly drop to levels unfavorable for growth of small juvenile halibut. Large juvenile halibut, with their reduced tolerance for varying temperatures and salinities, must migrate from lagoons and estuaries into open-coast environments, where they also benefit from abundant large prey. Closure of river mouths likely pose the greatest risk to large juvenile halibut, should they become trapped in cold, hyposaline coastal wetlands.