Currently soybean meal (SBM) is used to partially replace fishmeal in diets of carnivorous fish to reduce costs and the extraction of fishery resources. However, a high SBM content causes enteritis, the non-contagious subacute inflammation of the intestine, which also result in immune and physiological changes that in turn alter the absorption of nutrients and growth of some fish, such as totoaba. The totoaba is an endangered endemic fish from the Gulf of Baja California with a high potential for aquaculture.
On the other hand, prebiotics can prevent enteritis caused by SBM. These ingredients have the characteristic to be resistant to digestion and stimulate gut microbiota. Macroalgae contain polysaccharides that meet the characteristics of prebiotics, however, their effects are species-specific, so the aim of this work was to evaluate the effects of macroalgae in SBM diets on the growth performance of juvenile totoaba, feed utilization, digestibility of the diets, body composition, enzyme activities (trypsin, lipase, amylase), immune responses (respiratory burst and leukocyte count), histological changes and gene expression (il8, aqp8, IgM). Four isoproteic (48%) and isolipidic (13%) diets were tested. Two commercial diets with local macroalgae meal (Alg1 and 2) and two experimental diets, both with 26% of SBM and one including a macroalgae prebiotic mix (SBM and SBM+Pre).
At the end of the trial fish fed with SBM and SBM+Pre diets resulted a significant higher growth and utilization of nutrients compared to the diets with macroalgae. Also, significant differences were found between the treatments in the histological samples of the distal intestine. There was expression of genes related to the immune system such Igm and il8, and cell structure such as aqp8, but only SMB+Pre had a lower level than the other treatments.
These results reveal that the commercial diets negatively affected the gut health of totoabas even more than diets that included SBM, however more research is needed to elucidate which compounds of these diets altered the gut and make clear if there is a prebiotic effect of the macroalgae on other species.