Effects of Dietary Gamma-Aminobutyric Acid (GABA) Inclusion on Acute Temperature Stress Responses in Juvenile Olive Flounder (Paralichthys olivaceus)

Simple Summary: The rapid warming of South Korean waters due to climate change presents a significant threat to marine aquaculture. As temperature stress becomes an increasing concern, nutritional strategies play a vital role in mitigating its impact on farmed species. While dietary gamma-aminobutyric acid (GABA) has been shown to mitigate stress in various species, research on its effects in mitigating temperature stress in olive flounder, a key aquaculture species in the Republic of Korea, remains limited. This study aims to assess the effects of dietary GABA inclusion on acute temperature stress in olive flounder (Paralichthys olivaceus). Therefore, we conducted analyses at various biological levels of organization after raising olive flounder juveniles on diets supplemented with different levels of GABA (control (GABA74), 174 ppm of GABA (GABA174), 275 ppm of GABA (GABA275), 396 ppm of GABA (GABA396), 476 ppm of GABA (GABA476), and 516 ppm of GABA (GABA516)) for 8 weeks. Although our observations suggest that GABA alone may not effectively mitigate acute temperature stress in this species, we cannot entirely rule out its efficacy, as several other factors may converge to influence the effect of GABA. Understanding these factors will be crucial in guiding future research on GABA in various aquaculture species, particularly those that may appear unresponsive to GABA inclusion under certain stress conditions. This study investigated the potential of dietary gamma-aminobutyric acid (GABA) inclusion to mitigate acute temperature stress impacting the physiological resilience of juvenile olive flounder (Paralichthys olivaceus). A total of 360 juvenile fish, with an average initial weight of 12.97 +/- 0.1 g (mean +/- SEM), were randomly assigned in triplicate to 18 tanks (20 fish per tank) and reared at 19.5 degrees C for 8 weeks, with bi-monthly collection of growth performance data. The fish were fed one of six experimental diets: control (GABA74), 174 ppm of GABA (GABA174), 275 ppm of GABA (GABA275), 396 ppm of GABA (GABA396), 476 ppm of GABA (GABA476), and 516 ppm of GABA (GABA516). At the end of the trial, one group of fish was subjected to lethal temperature stress (31 degrees C) for 48 h, while another was exposed to acute temperature stress (29 degrees C) for 6 h. Growth performance remained relatively stable across all inclusion levels (p > 0.05), with the final body weight (FBW) ranging from 48.2 +/- 0.3 g (GABA174) to 50.3 +/- 0.6 g (GABA516) and the feed conversion ratio (FCR) varying between 2.06 +/- 0.07 (GABA396) and 2.35 +/- 0.07 (control). There were no significant differences in average whole-body composition across all dietary treatments, with moisture content ranging from 74.8 to 75.0%, crude protein from 17.8 to 18.2%, crude lipid from 2.89 to 3.15%, and crude ash from 3.62 to 3.80%. Similarly, there were no significant differences in cumulative survival rates during lethal temperature exposure between the GABA-supplemented groups and the control group, with an average of 28.5 +/- 4.6%. Additionally, GABA inclusion did not significantly alter plasma-free amino acid profiles, antioxidant enzyme activities, or immune functions (p > 0.05). However, temperature significantly reduced the levels of superoxide dismutase (SOD) from 3.34 +/- 0.17 to 2.29 +/- 0.36 mu g/mL and increased the levels of glutamate oxaloacetate transaminase (GOT) from 17.1 +/- 0.8 to 46.3 +/- 6.2 U/L, glutamate pyruvate transaminase (GPT) from 14.4 +/- 0.6 to 30.2 +/- 2. 1 U/L, glucose (GLU) from 13.3 +/- 0.5 to 68.7 +/- 7.7 mg/dL, total protein (TP) from 2.94 +/- 0.00 to 3.21 +/- 0.1 g/dL, and cortisol from 5001 +/- 147 to 6395 +/- 194 ng/mL. Furthermore, no significant changes were observed in the expression of key stress-related genes, including heat shock proteins (hsp60, hsp70, and hsp90) and the warm water acclimation-related gene wap65. This study establishes the safety of GABA as a dietary inclusion for olive flounder and highlights its potential to enhance stress resilience in aquaculture. However, the effectiveness of GABA-based interventions could depend on critical factors such as dosage, stress duration, and species-specific responses. Our findings highlight the need for further research to optimize GABA inclusion strategies, particularly with consideration for long-term physiological impacts.