Soybean protein concentrate as feed for Salmo salar

Fish meal is an expensive and unsustainable source of protein for fin fish diets in aquaculture. Many manufacturers are incorporating plant materials as a substitute, and soybean is a popular choice due to its availability and cheap cost. However carnivorous fish haven’t evolved the mechanisms for effectively digesting carbohydrates and anti-nutritional factors found in plant meals. As a result previous studies have it to cause inflammation of the intestine, impeding digestion and growth. This inflammation is caused by alcohol-removable components of the soybean. However even with these components removed to give soybean protein concentrate (SPC), the intestinal microbiota community is still affected, which likely exacerbates any inflammation.

This study looks at four Salmo salar diets containing different levels of SPC, fish meal, and the prebiotic mannan-ogiosaccharide (MOS). It’s also based in open sea pens at a commercial sea farm in Tasmania, Australia. This is unlike previous studies conducted in closed and stable environments. These previous studies also found that in late summer SPC fed fish had higher mortality, lower growth and apatite, and vomiting as well as diarrhoeic effects, termed summer gut syndrome (SGS).

The results found that bacteria increased in the directional tract in the late summer. Feeding SPC to salmon increased diversity of bacteria in the intestinal tract and the presence of two that have not been associated with marine fish before – Escherichia sp and Propionibacterium acnes. These two have only previously observed in fresh water fish. These two were also associated with SGS in S. salar.

Temporal changes also occurred and dominance of micro-organisms switched over the experiment. In contrast to past studies individual bacterial communities were found to be different between fish, though this is likely as the experiment was not in a closed environment. After comparisons with similar studies the author also found that the bacteria that colonise S. salar intestines include Betaand Gamma-Proteobacteria, Actinobacteria, Bacteroidetes, Firmicutes and Verrucomicrobia. Bacterial profiles couldn’t be compared between fish due to the large individual variation mentioned earlier; however what was detected was consistent with findings in similar studies of fish and crustaceans.

The Escherichia sp identified was most similar to four undifferentiated Shigella sp, and symptoms from diets without MOS infer that they caused vomiting and diarrhoeic conditions found in the late summer. The experiment also showed that though the cause of the disorder wasn’t identified, MOS does prevent the adverse affects and recommends further studies for its inclusion on soybean diets.

The use of antibiotics as preventative measures has hard restrictions in the EU due to developed resistance in bacteria, and does not seem to be a good recommendation. As well as this as the full function of MOS on the S. salar intestinal community is not known, and could suppress beneficial bacteria, and affecting S. salar health. A similar experiment with SPC in more temperate waters may not initiate SGS, and could need no prebiotic measures. As well as this branches of further research should include other preventative means such as probiotics and a wider sample group for comparison of bacterial community profiles.

- Green, T. J., Smullen, R., & Barnes, A. C. (2013). Dietary soybean protein concentrate-induced intestinal disorder in marine farmed Atlantic salmon, Salmo salar is associated with alterations in gut microbiota. Veterinary microbiology, 166(1), 286-292.