Dietary lipid utilisation under different thermal conditions in flathead grey mullet (Mugil cephalus)

Abstract

Water temperature and dietary lipid levels are key factors influencing fish growth and metabolic homeostasis in aquaculture. This study aimed to understand the interactive effects on growth performance, metabolic response, gut microbiota, and gut evacuation dynamics in flathead grey mullet ( Mugil cephalus ). A 100-day feeding trial was carried out to assess the effects of two dietary lipid levels (10%, L10; 15%, L15) at 22 °C (LT) and 28 °C (HT). Specific growth rate (% day -1 ) was higher ( p < 0.05) at HT (0.90 ± 0.01 for both diets) than at LT (0.70 and 0.62 for L10 and L15, respectively). Feed intake at L15 diet was lower at both temperatures ( p < 0.05) while FCR was reduced only by HT and not by lipid levels. Serum metabolic profiling indicated that higher temperatures accelerated energy turnover, as evidenced by elevated serum glucose and lactate at HT across both dietary groups ( p < 0.05). Accordingly, fish at HT in both dietary treatments exhibited lower total protein and cholesterol ( p < 0.05), suggesting a more efficient utilisation of protein reserves and lipid processing to meet increased metabolic demands. Gut evacuation dynamics were described here for the first time in this species by sampling multiple intestinal sections at different time points (1, 3, 5, 7, 9, and 12 hours post-prandial). The intestine was almost completely empty by approximately 8 h post-prandial, with the notable exception of fish fed the HL diet at LT, where a significant delay in evacuation (18 h) was observed. This delay may be attributed to the combined effects of higher dietary lipid levels and lower temperatures, which both prolong digestion and intestinal transit times. Gut microbiota composition was primarily shaped by temperature, with a shift toward a specialised profile dominated by Cetobacterium at HT in both dietary groups. Overall, the results highlight that the combination of a 15% lipid diet and 28 °C was associated with the most positive outcomes among the tested conditions, providing further insight into how dietary composition and temperature modulate digestive physiology in flathead grey mullet.

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