Fishmeal, plant protein, and fish oil substitution with single-cell ingredients in organic feeds for European sea bass (Dicentrarchus labrax).

Single-cell ingredients (SCI) are considered promising nutrient sources which are produced using environmentally friendly biotechnological processes. The aim of the current study was to evaluate the replacement of fishmeal, plant protein sources, and fish oil with SCI in organic feeds for European sea bass (Dicentrarchus labrax). Bacterial protein, yeast protein, and microalgae were used to replace fishmeal trimmings, soya bean meal, and fish oil from trimmings. Triplicate groups (30 fish per replicate) of European sea bass (14.4 ± 2.4 g) were fed the experimental diets for 71 days. The results showed that the incorporation of SCI at all levels of inclusion significantly enhanced nutrient digestibility. Additionally, growth performance parameters were not affected by SCI inclusion, exhibiting similar or improved values. Moreover, a tendency for improved anterior and posterior gut structure was observed and a significant increase of lysozyme activity at the two highest inclusion levels of SCI was determined. Overall, the results showed that the inclusion of SCI at 15% (bacterial: yeast: algae-9.4: 4.7: 1) is possible without compromising any of the parameters evaluated. According to these findings, a higher substitution of fishmeal trimmings, plant protein sources, and fish oil from trimmings with SCI in organic diets for European sea bass (D. labrax) can be further evaluated in future studies.

Effects of a natural parasitical infection (Lernanthropus kroyeri) on the immune system of European sea bass, Dicentrarchus labrax L.

The immune response of European sea bass, Dicentrarchus labrax, to a natural infection by the copepod parasite Lernanthropus kroyeri was evaluated for the first time in vivo. The results clearly demonstrated the triggering of the fish immune system by the parasite. Lysozyme activity and alternative complement pathway were involved in the early action against the parasitical infection, whilst classical complement and respiratory burst (RB) activity took over in the later stages of infection. It was hypothesized that the levels of alternative and classical complement and RB stimulation indexes may determine the resistance capacity of the fish to the parasite. It is not clear how parasites can survive despite the strong immunological arsenal deployed by the fish. The continual increase of prevalence and severity of parasite infection suggested that the parasite's mechanism of evasion of the immune system was extremely successful. The contrasting decrease in the negative effects of parasites on the fish health (such as gills anaemia) suggested that an equilibrium between the parasites and their hosts was reached in chronic infection. These dynamic interactions between parasites and fish hosts were probably the main determinant of host specificity.