Recently, global warming attributed to climate change is causing rapid environmental changes including high fluctuation in water temperature, resulting in mass mortality and disease outbreaks in aquaculture practices. Fish meal production is predicted to decrease over time due to reduction in capture production of fishes for use of fish meal in association with climate change. Regardless the fact that fish meal is an important ingredient for aquafeed, there is no choice but to reduce fish meal content in aquafeed for sustainable aquaculture. Therefore. The current study was conducted to understand what relationship between manipulation of fish meal content and physiological responses under chronic and acute temperature stress exists in juvenile olive flounder (Paralichthys olivaceus), a commercially important aquaculture fish species in Asia.
Four hundred eighty juveniles averaging 13.6±0.02g (mean±SEM) were randomly distributed into each of the 24 rectangular tanks (20 fish per tank). Each set of 12 tanks was assigned to either constant water temperature group reared at 19.5°C(named as non-stressed group: NS) or gradually increased water temperature group reared at from 19.5°C to 30°C (1.5°C increment/week) (named as thermal-stressed group: TS) throughout the trial. Each NS and TS group consisted of four fish meal replacement treatments including 60, 40, 20, 0% fish meal content in diet (2 × 4 factorial arrangement; N = 3 per treatment). Following the 8-week trial, fish from the NS group were exposed to acute thermal stress (2-h heat shock at 30°C and 2-h recovery at 19.5°C). Growth performance and stress- and immune-related measurements for stressed and non-stressed juveniles were investigated.
Results from the chronic exposure experiment showed that the TS group had significantly lower final body weight (FBW), weight gain (WG), specific growth rate (SGR), feed efficiency (FE), condition factor, and viscerosomatic index than the NS group (P < 0.05). There was also a significant main effect of the fish meal replacement in both NS and TS groups showing that FBW, WG, SGR, and FE generally decreased with increasing fish meal replacement level. A significant temperature stress main effect on plasma cortisol, glucose, triglyceride, total protein, and glutamic oxaloacetic transaminase levels was observed. A significant fish meal replacement main effect on plasma total cholesterol, triglyceride, and glutathione peroxidase levels was found.
Results from the acute exposure experiment show that the overall pattern of changes in the plasmatic enzymes and metabolites were, to some extent, incomparable to those from the chronic exposure experiment.
Expression levels of genes, involved in temperature stress responses in various tissues will be discussed later.