Solid-State Fermentation of Plant Feedstuff Mixture Affected the Physiological Responses of European Seabass (Dicentrarchus labrax) Reared at Different Temperatures and Subjected to Salinity Oscillation

SIMPLE SUMMARY: Climate change is a growing challenge to the aquaculture industry by causing suboptimal farming conditions that reduce fish performance and increase disease outbreaks. Climate change may also limit access to marine and terrestrial animal feed ingredients for aquafeed production, thus creating a need for novel alternatives. Plant feedstuffs in their crude forms have several antinutritional factors that limit their incorporation level in aquaculture feed. However, when these plant ingredients are subjected to fermentation processes, digestibility and bioactive capacity increase, enabling an increase in their inclusion level in the aquafeed formulation. Considering this, we evaluated the effects of a plant feedstuff mixture (PFM: soybean meal, rapeseed meal, sunflower seed, and rice bran) fermented with Aspergillus niger on the growth performance and physiological responses of European seabass subjected to environmental stress. Our results demonstrated an interaction effect between the environmental conditions and the novel feed on immune and antioxidant responses. However, the novel feed was unable to improve the performance indicators in seabass subjected to stressful environmental conditions. In fact, the fermented feed caused an inhibition of growth performance, which was not observed in fish fed the non-fermented feed. ABSTRACT: This study aimed to evaluate the effects of dietary inclusion of plant feedstuff mixture (PFM) pre-treated by solid-state fermentation (SSF) on the physiological responses of European seabass. For that purpose, two diets were formulated to contain: 20% inclusion level of non-fermented plant ingredients mixture (20Mix) and 20Mix fermented by A. niger in SSF conditions (20Mix-SSF). Seabass juveniles (initial body weight: 20.9 ± 3.3 g) were fed the experimental diets, reared at two different temperatures (21 and 26 °C) and subjected to weekly salinity oscillations for six weeks. Growth performance, digestive enzyme activities, humoral immune parameters, and oxidative stress indicators were evaluated. A reduction in weight gain, feed intake, and thermal growth coefficient was observed in fish fed the fermented diet (20Mix-SSF). Salinity oscillation led to an increase in weight gain, feed efficiency, daily growth index, and thermal growth coefficient, regardless of dietary treatment. Higher rearing temperatures also increased daily growth index. No dietary effect was observed on digestive enzymes activities, whereas rearing temperature and salinity oscillation modulated digestive enzyme activities. Oxidative stress responses were significantly affected by experimental diets, temperature, and salinity conditions. Catalase and glutathione peroxidase activities showed an interactive effect. Fish reared at 21 °C showed higher enzymatic activity when fed the 20Mix-SSF. Conversely, fish reared at 26 °C showed higher GPx activity when fed the 20Mix diet. Fish reared at 26 °C showed reduced peroxidase and lysozyme activities, while salinity fluctuation led to increased lysozyme activity and decreased ACH50 activity. ACH50 activity increased in fish fed the 20Mix-SSF. Overall, the dietary inclusion of PFM fermented by A. niger was unable to mitigate the impact of environmental stress on physiological performance in European seabass. In fact, fermented feed caused an inhibition of growth performances and an alteration of some physiological stress indicators.