Resumen de Fouling management in cage aquaculture. New strategies, effects on nets, fish health and economic impact
There is an open debate in the aquaculture industry regarding fouling management strategies. The aim of this study was to contrast a newly implemented technology, the on-site cleaning, to the existing and widely used copper based antifouling paints to more efficiently control an issue that affects fish farming worldwide, the fouling production. The experimental work was planned and conducted in real working conditions simulating part of a sea bream growth cycle. Despite knowing that working in field conditions might represent some difficulties, it was thought that the obtained results would much better explain what happens in the real life. As the study was set within an industrial PhD framework, a wide scope investigation was considered, and therefore, a wide range of parameters were analyzed. Nets are the strongbox that protect the most important asset of pisciculture companies, the biomass. For this reason, the effects of the tested strategies on netting integrity were assessed. Afterwards, those aspects resulting from the different rearing conditions generated by one or the other fouling management strategy and that were thought to have an effect on fish health and performance were studied. Growth was analyzed, as one of the main goals of the aquaculture industry, but also the presence of parasites and the skin microbiota, since the proliferation of these organisms is often associated with nets hygienic status. A histological study in gills was performed to determine the magnitude of the effects of the different maneuvers on this tissue, which is essential for the survival of fish due to its multifunctionality. The content of copper in gills, liver and muscle was analyzed, not only to see if it could pose a danger to animals, but also because of the concern that heavy metal accumulation generates among fish consumers. We also wanted to see whether the two strategies tested had different effects on the animals in terms of welfare impairment and stress response. Therefore, some of the main indicators of the primary and secondary stress responses were analyzed. We also wondered if the different strategies caused differences at molecular level, so we analyzed the modulation of the gene expression of those genes involved in the stress response in liver, gills and spleen, as organs mainly involved in homeostasis and the immune response of animals, or in the case of gills because they are specially exposed to the effects of rearing conditions. Finally, a basic approach to the economic impact that the new methodology might have in the production cost compared to the existing one was done, since one of the main motivations of the industry is to reduce the operational costs involved in fouling management. The results suggested that in terms of welfare impairment and stress response, the on-site cleaning strategy did not suppose any advantage or disadvantage compared to the old strategy of net replacing. However, negative consequences as netting damage, parasite infestation, fish gill damage or poor growth were detected. This is due to the current use of the on-site cleaning technology. The economic approach revealed that the high cleaning frequency required to keep nets in fairly good conditions originates elevated costs that counteracts the initial advantages of this strategy pursued by the industry. The on-site cleaning technology might be very useful in specific aquaculture environments like the massive offshore cages currently endeavored. But the bottleneck that the high cleaning frequency represents, and its consequences must be solved to make this alternative a reliable future solution. As a result of this investigation, an alternative midway among the two tested scenarios, emerges as a promising option which, under our point of view deserves further attention and research.
