Water temperature is one of the most significant abiotic factors influencing fish energy partitioning and digestive capacity, which determines the amount of energy available for processes such as growth (Papoutsoglou & Lyndon, 1995; Nytrø et al., 2014). Knowing the optimal water temperature for the rearing of a given fish species is highly important from the productivity, nutrition and animal welfare points of view. The present work aimed to identify the optimal water temperature for the culture of the thicklip grey mullet Chelon labrosus, as well as to assess the effects of said temperatures on the activity and gene expression of its digestive enzymes.
Materials and methods
The fish (C. labrosus, N = 168, initial length 13.93 ± 0.96 cm, initial weight 32.09 ± 7.11 g) were distributed into 12 recirculating tanks (100 L), separated in four water temperature treatments (18, 22, 26 and 30 °C) with three replicates each. Fish were fed twice a day on a commercial feed (Gemma Diamond 1 mm, Skretting) containing 57 % protein and 15 % lipids for 90 days. Biometrical measurements (fork length and total weight) were recorded at days 0 (T0), 30 (T1), 60 (T2) and 90 (T3). At T1, 4 fish of each group were sacrificed and liver, muscle and gastrointestinal tract samples collected for biochemical composition, digestive enzyme activity and gene expression analyses. The same was done with the rest of the fish at T3.
Results and discussion
The best growth results were achieved at 22 °C, with a downwards trend at either side of the temperature spectrum (Table 1), although the only significant weight differences were observed at T3, between the 22 and 30 °C groups. This accurately reflects the dome-shaped growth curves typically caused by water temperature (Nytrø et al., 2014), due to the interaction between the acceleration of biochemical reactions, increases in feed intake (Davis & Hardy, 2022) and digestibility (Papoutsoglou & Lyndon, 1995), and the raise of basal metabolic demands (Kaushik & Schrama, 2022). In general, exposure to different temperatures did not produce changes in performance at the different samplings except the 30 °C group, as its SGR significantly improved at T3 when compared to T1. Even though this shows the ability of the fish to somewhat acclimate to this extreme temperature, it is still outperformed by the other groups. Our results suggest that the temperatures selected for the trial are at the upper part of the theoretical curve (except 30 °C), with the peak being close to 22 °C.
The group with the most energy reserves at both samplings, in term of carbohydrate and lipid contents (Table 1), was the 18 °C group. The fact that these fish did not grow as much as others despite their high energy availability indicates that at this temperature the metabolism of the fish is slower than at the other temperatures used, which causes a low energy expenditure and, concomitantly, growth. As already mentioned, the 22 °C group performed the best in terms of growth, which is reflected in containing lower energy reserves than the 18 °C group, suggesting that this group is expending much more energy, and directing it towards growth. This shows that this expenditure is not a sign of stress (Wendelaar Bonga, 1997). Energy reserves in the 26 °C group are similar or lower than in the 22 °C group, but with a slightly lower growth performance. It has to be taken into account that, in general, feed intake and digestibility increase alongside water temperature until a certain point (Papoutsoglou & Lyndon, 1995; Davis & Hardy, 2022), so groups 26 and 30 °C could have a higher energy and nutrient intake than the others. The fact that the 26 °C group did not reach the growth values of the 22 °C group, coupled with the theoretical higher availability of energy and nutrients, could indicate that 26 °C are stressful for the fish (Wendelaar Bonga, 1997), although not critically. In the 30 °C group, the lowest energy reserves and growth values were found, especially at T1, and even though they slightly improved at T3, it can be said that this temperature is out of the optimal range, and it is stressful for the fish (Wendelaar Bonga, 1997). Anyway, it is clear that raising the temperature above 26 °C could be dangerous for the well-being of the fish. The measurement of digestive enzyme gene expression and activity will help to clarify our observations. Nevertheless, those analyses have not been fully performed, and they will be key to shed some more light to the results obtained and draw conclusions that are more robust.
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