THE EFFECT OF SOYBEAN MEAL LEVEL IN DIETS FOR TOTOABA (Totoaba macdonaldi): INDUCTION OF ENTERITIS

The high cost of fishmeal decreases the sustainability of aquaculture and has prompted the search for new alternative protein sources such as vegetable proteins. Defatted soybean meal (SBM) is considered a viable alternative to replace part of FM in marine fish diets due to high availability, adequate protein content (40-48% crude protein) with a constant amino acid profile and a low cost However, the increase in SBM inclusion in fish diet is related to the occurrence of enteritis, which is defined as non-infectious inflammation of distal intestine. Totoaba macdonaldi under culture conditions can reach a weight of 6 kg in 24 months and is considered an excellent candidate for commercial aquaculture in the Baja California region of México. Therefore, in this work we evaluated the effect of soybean meal in diets for totoaba juveniles using growth performance, intestinal integrity, digestive capacity and gene expression as response variables.

Four isonitrogenous (485 g CP kg^-1 diet) and isolipidic (86 g CL kg^-1 diet) diets were formulated to replace fishmeal protein at 0, 25, 50 and 75% replacement levels. A mixture of soybean meal and soy protein concentrate (SBMix) was used at a proportion of 4:1. The soy mixture inclusion levels resulted in 0, 22, 44, 65% of the diet. All diets included 1 g kg^-1 of krill oil as attractant and 10 g kg^-1 of taurine. Three tanks per treatment with 4 fish per tank were used and fed for 56 days to apparent satiation (three times a day). At the end of experiment the specific growth rate (SGR), feed conversion ratio (FCR), condition factor (CF), protein efficiency ratio (PER) and feed intake (FI) were assessed. Organosomatic index of the fish was estimated. The distal intestine and liver of one fish per tank (n=3) at 28 and 56 days were dissected and histologically evaluated. The relative expression levels of interleukin (il8) were quantified by qPCR. Enzymatic activity was determined in the pyloric caeca (CP), intestine and stomach (n = 3) at the end of the experiment. Growth of juvenile totoaba was significantly affected by dietary SBM inclusion levels (P<0.05). Gut and liver histology revealed a relative severe damage resulting in a dose-dependent effect as the SBM dietary inclusion levels increased. At 28 days, a progressive increase in the number of GC, MF coalescence, LP thickness were observed. In addition, the connective tissue of MS with a few infiltrations of EG, as well as a reduction in MF length and number of SNV were observed. These intestinal morphology changes are typically associated with an inflammatory process of the gut. At the end of the experiment treatments with 44 and 64% SBMix inclusion levels resulted in a marked tissue disruption of MF. The liver showed an increased vacuolization with nuclear displacement at 44 and 64% SBMix inclusion levels. Activity of trypsin, chymotrypsin, amylase, L-aminopeptidase and total alkaline proteases in CP and intestine was reduced with increasing inclusion levels of SBMix in the diet with the exception of lipase activity. Additionally, expression levels of il8 were indicative of a chronic stress reaction regardless of the inclusion level. These results indicate a significant detrimental effect of the intestinal integrity of totoaba fed SBM-based diets at the higher inclusion levels, suggesting that SBM should be cautiously used in totoaba feed formulation