Fish cultured in intensive farming conditions may develop severe skeletal deformities that affect both their external morphology and welfare. In the case of commercially important fish species, such as the gilthead seabream (Sparus aurata), this is a matter of great concern and a major factor that affects production costs, downgrades hatcheries’ production and fish market value. Among the solutions proposed to improve the skeletal deformities of aquaculture fish, the supplementation of their diet with natural compounds or extracts that stimulate skeletogenesis is increasingly seen as an economically sound approach to improve the competitiveness of the aquaculture industry and fish health.
In this regard, we supplemented a commercial fish diet with two ethanol extracts prepared from Skeletonema sp. and Tetraselmis sp. and containing bone anabolic compounds, and used them to feed gilthead seabream (Sparus aurata) juveniles of 4.8±0.7 g until they tripled their weight. Fish were housed at the Estação Piloto de Piscicultura de Olhão (EPPO/IPMA) in 250-L rectangular tanks with water renewal of approximately 350 L per hour and a temperature of 25.3±1.0ºC, and fed ad libitum 3 to 4 times a day.
Considering fish growth performance indicators, dietary treatments had no impact on feed conversion rate (FCR), although a slight increase was observed for fish fed with the diet supplemented with Skeletonema sp. extract. The total length was not significantly altered in fish fed supplemented versus control diets, but a positive effect was observed on the weight of the fish fed the diet supplemented with Tetraselmis sp., which consequently show a moderate increased condition factor. The analysis of several haematological parameters revealed that food supplementation with microalgae extracts does not influence fish physiological status or stress levels. In terms of bone parameters, the vertebrae of fish fed with the diet containing Skeletonema sp. extract showed a higher phosphorous content, although the optimal balance between calcium and phosphorous was maintained in the vertebrae of fish fed supplemented diets. The analysis of the type and severity of the skeletal deformities revealed that most of them are associated with the caudal fin complex, independently of the treatment, but that the incidence is reduced when fish are fed with the diets containing extract of Skeletonema sp. (deformity charge (df) = 1) or Tetraselmis sp. (df = 1.17) when compared to the control diet (df = 1.36). The expression of several bone marker genes was also altered upon dietary treatment, as it is the case of the osteoblast markers sp7 and col1a1, that are, respectively, up and downregulated for fish treated with Skeletonema sp., and osteoclast marker acp5, that is upregulated for fish fed with Tetraselmis sp. Interestingly, the expression of the glutathione peroxidase gpx1 was increased in the vertebrae of fish treated with both extracts, suggesting an activation of antioxidant mechanisms by both experimental diets. Overall, the data gathered in this study points out to a general improvement of fish skeletal status upon dietary treatment with microalgae extracts containing bone anabolic compounds, without compromising, and possibly improving, fish growth performance indicators.
Funding
This study was funded by the Portuguese Foundation for Science and Technology (FCT) through the project UIDB/04326/2020 and by the European Maritime and Fisheries Fund (EMFF/FEAMP) through the National Operational Programme MAR2020 and project OSTEOMAR MAR-02.01.01-FEAMP-0057.