Monitoring welfare indicators of rainbow trout (Oncorhynchus mykiss) in a commercial organic farm: Effects of an innovative diet and accelerometer tag implantation

To meet the growing demand for aquatic products, the aquaculture sector is expanding, including organic aquaculture. One of the main challenges in this industry is the development of suitable fish feeds. EU regulations have made recommendations, including the incorporation of fishmeal from organic aquaculture trimmings. However, the use of such fish meal may pose issues due to lower protein content and elevated phosphorus concentrations that could conflict with national environmental regulations. Thus, there is a pressing need for innovative research to formulate environmentally friendly feeds for organic aquaculture. This study aims to evaluate the feasibility of utilizing an innovative organic feed derived from trimmings (substitution of 64.3% of dietary fish meal trimmings with fishmeal protein concentrate from trimmings) for rainbow trout (Oncorhynchus mykiss). This study assessed the impact of this innovative diet on rainbow trout welfare over a two-month period in real-farming conditions, using: (i) growth parameters, (ii) external morphological damages, (iii) swimming activity, used as a proxy for energy expenditure, and (iv) blood and liver health and welfare indicators. Given that fish swimming activity was evaluated using surgically implanted accelerometer tags, we also investigated the potential adverse effects of implanting these tags on fish welfare. Our findings indicate that accelerometer implantation did not significantly affect fish welfare, but a 18.6% reduction in specific growth rate was shown in tagged fish compared to their non-tagged counterparts. Importantly, the innovative diet did not impair health and welfare indicators of blood and liver or lead to external morphological abnormalities. An increased energy expenditure in fish fed the innovative diet compared to those on a control diet was, however, shown, but there was no observable impact on growth performance. The use of this innovative diet may offer a viable solution to reduce the release of soluble phosphorus into the environment while maintaining appropriate standards for fish growth and welfare in organic conditions. Nevertheless, long-term consequences should be explored.