Effects of Elevated Water Temperatures on Hemato-Biochemical and Histological Changes in the Juvenile Red Spotted Grouper (Epinephelus akaara)

The increase in water temperature attributed to global warming poses a significant concern for aquaculture and fisheries experts. Consequently, the current study explored the impacts of elevated temperatures on the hemato-biochemical and histological aspects in the juvenile red spotted groupers (Epinephelus akaara). Juveniles with a length (TL) of 9.4± 0.12cm and body weight (BW) of 12.89± 0.61g were subjected to thermal exposure at 24 (Control), 28, 32 and 36°C for 4 weeks, following a two-week acclimation period at 24°C. A total of 180 juvenile fish were sacrificed at three different time points (1, 14, and 28 days) for blood and tissue samples. Following 1 day of exposure, the thermal stress resulted in a significant increase in hematocrit [Ht], red blood cell [RBC] count, cortisol, glucose, glutamic oxaloacetic transaminase [GOT], and glutamic pyruvic transaminase [GPT] levels at 36°C, in comparison with the other temperature groups (24, 28, and 32°C) (P< 0.05). After 14 days, no noticeable changes were observed. Interestingly, after an exposure of 28 days, a significant rise in Ht and cortisol levels was reported in the group exposed at 32°C. Severe histological damage such as epithelial necrosis [EN] and shortening of secondary gill lamellae [SSL] in the gills, and cytoplasmic vacuolization [CV], shrinkage, and coalesce of hepatocytes [CHP] in the liver, was observed in the 36°C group after 1 day. Furthermore, secondary lamellar hyperplasia [HSL] in the gills and swollen hepatocytes [SHP] in the liver became evident at 32°C following the exposure of 28 days. In summary, our findings indicate that 36°C is a lethal temperature for the red spotted grouper, and prolonged exposure to 32°C can induce some degree of physiological stress in juvenile individuals.