Interacciones entre el cultivo de peces en jaulas en mar abierto y la pesca artesanal

Resumen

The anthropogenic pressure over the coastal zone has dramatically increased around the world in the last 50 years. One of the main industries called to “conquer the coastal zone” is sea cage fish farming, which, after the stagnation and future constraints of agriculture production, is predicted to increase the production over the future decades to feed a global raising population. In light of the predicted scenario, where fish farming will be one of the main sources of human seafood, further increase on coastal pressure will occur throughout the Mediterranean Sea. On the wake of the development of industrial sea cage fish farming in the Mediterranean Sea, which started 30 years ago, different influences on the ecosystem and coastal users (specially coastal fisheries) have been noticed (Chapter 1). In the early stages of sea cage fish farming development, damage on benthic protected species as marine phanerogams like Posidonia oceanica were noticed, as well as anoxia episodes in soft bottoms underneath sea cages due to excess of organic matter. However, it is in the last 10 years when the influences of fish farming on coastal artisanal fisheries in terms of wild fish aggregation, excess feed and fish escapes, have been brought up. The present doctoral thesis aims to shed light on the aforementioned aspects, addressing, in first term, how the excess feed is assimilated by the assemblage of aggregated ichthyofauna from a holistic perspective including different trophic gilds, from planctivorous to top predator species (Chapter 2). Secondly, despite the benthic influence of sea cage fish farming has been extensively studied in regards with the geochemistry, there is almost no research on its influence on benthic macro-fauna, either fish or crustaceans. Therefore, the high valued species caramote prawn (Melicertus kerathurus) was studied, since it is a key species for the sustainability of fishing fleets exploiting this fishing resource within the Mediterranean Sea (Chapter 3). Furthermore, as a third step on the understanding of the influence of sea cage fish farming on the behaviour and habitat use of commercially exploited fish, the predator species bluefish (Pomatomus saltatrix) was studied (Chapter 4). Eventually, fish farms export biomass towards the environment not only in shape excess feed but also through fish escapes. The abundances of escaped fish in the wild are shaped by coastal fisheries but either the studies on the incidence of escaped fish on fisheries landings or the ability to capture escapees by different fishing gears are scarce. Therefore, a simple methodology to differentiate between escaped and wild individuals of Sparus aurata was developed and the incidence of escaped gilthead sea beam in fisheries landings was assessed (Chapter 5). In the chapter 6 of this PhD, the obtained results are discussed in terms of management of the coastal zone. Additionally, a set of guidelines in regards with feeding optimization and recapture of escaped fish are outlined to provide decision makers with scientific data to be implemented in management measures for the sustainable activity in the coastal zone. Throughout Chapter 2, it was demonstrated that all the analysed fish species with commercial interest (Sardinella aurita, Caranx rhonchus, Mullus barbatus), including bluefish (P.saltatrix), resulted influenced by terrestrial fatty acids contained in fish feed. The excess feed in shape of fish biomass is also accumulated in higher trophic levels of the food chain via predation on fish which do exploit excess feed as a trophic resource. The presence of terrestrial lipids on aggregated fish, points towards these fish acting both as a biofilter at pelagic and benthic levels. The incidence of fish influenced by excess feed was also noticed in fisheries landings at a scale of tens of kilometers, with all the implications it may have for the management of coastal areas. Throughout Chapter 3, the influence of excess feed on benthic species was assessed through stable isotopes analyses. The peanaeid caramote prawn (Melicertus kerathurus) was used as a study model. In comparison to prawn muscle, fish food was significantly depleted in both 13C and 15N but had a much higher C:N ratio. Prawns collected near fish cages were slightly depleted in 13C and 15N compared to baseline values, but the depletion was small and only significant for 13Ccorr. The laboratory study showed that the species can readily feed on fish pellets, so it is likely that prawns near fish cages ingest some fish food directly. Prawn appear to become 15N depleted as they grow, possibly due to the allocation of energy reserves (lipids) in gonad development before spawning. Opposite to nitrogen, the carbon isotopic values of wild prawn showed a small but significant depletion near fish cages, as it would be expected if prawn were feeding on carbon-depleted fish food. Prawn near to farms were approximately 8% larger and 31% heavier than those sampled further away, most likely due to the farm buffer area acting as a no-take zone around farms. The latter might facilitate growth through excess feed ingestion and protect prawns from fishermen. As observed in previous chapters, fish farms may have an attraction effect on marine species including predators. Therefore, the habitat use of bluefish (P.saltatrix) in a coastal area, where fish farming exists, was analysed in Chapter 4. In the case of P.saltatrix, the fish farm affinity seems to be driven not by the farm structure per se, acting as a traditional FAD (fish aggregation device), but by the presence of aggregated fish acting as preys which, indeed, are attracted by excess feed. It was observed that the aforementioned attraction effect might influence the coastal distribution of bluefish, especially before the water temperature peaks in summer. It was in spring when bluefish was mostly distributed, and moving back and forth, between farms in deeper areas. However, it was in summer time when the species showed the widest distribution over the study area compared to spring and autumn, when it was circumscribed to farming areas. The landing statistics of the species mirrored the results obtained by the telemetry data, pointing towards a winter migration when sea surface temperatures decreases from 20º. The aggregation effect on bluefish observed in farming areas might render adjacent fishing areas vulnerable to increased predation rates on other exploited species. Additionally, the variability among the fishing gear used by artisanal fisheries, together with the presence of fish farming, might rise synergistic or antagonistic effects on commercially exploited species, and it should be further investigated. The last experimental contribution of this doctoral thesis put into numbers the incidence of escaped gilthead sea bream in fisheries landings for the first time in the Mediterranean Sea (Chapter 5). Moreover, a new methodology to distinguish between farmed and wild individuals was developed. Overall, the ability of artisanal fisheries to recapture escaped fish was validated. The overall incidence of escaped gilthead sea bream resulted 14.9% although values up to 20.8% were attained. The escapees were captured in areas no further than 20 km off the farms, being most of the escapees captured in nearby fishing areas, few km away the fish farms. Fishing gears as trammel-nets and gillnets captured most of the fish compared to longlines. The latter could indicate a higher fishing effort of these gears compared to long lines, however the proportion of captured fish did not differed much. Escapees were detected within the oldest size classes of the captures, pointing towards a potential admixture between farmed and wild genotypes. The highest landed biomass corresponded to large-sized escapees, as well as the derived revenues, which accounted for two thirds of the total income derived from the recaptured fish. The last contribution of this doctoral thesis (Chapter 6) summarises the potential consequences of the obtained results from a management point of view. Additionally, a set of guidelines to be used by decision makers in order to design future regulations for sea cage aquaculture management are provided. The extent of the effects of excess feed on landed fish should be further investigated in terms of quality, weight, price, p and consumer’s perception among others. Despite there is no negative influence of fish farming on the high valued species Melicertus kerathurus, other commercially valued species, which are present throughout the whole year in the coastal zone, might be affected and further research should be addressed. In light of the future coexistence of fish farming and artisanal fisheries in the coastal zone, more research should be addressed towards unveiling potential synergies between fish farming and coastal fisheries (e.g. fish farms acting as small MPAs). Long term monitoring programs on the influence of fish farms in regards with the habitat use of migrant predators should be set, since the continuous presence of fish preys might render these facilities as areas were sub-populations might remain all year long. The incidence of escapees in fisheries landings should be used as an indicator of good practice at farms, therefore, monitoring of the catches of cultured species should be established in landing ports. Additionally, periodic genetic analysis to control de farmed genotype introgression in the wild gene pool are suggested. The ability of artisanal fisheries to recapture escaped fish should be used to design contingency plans to recapture escaped fish in the future, as it has occurred in other countries where a more strict fish farming regulations exist. Before the establishment of a sea cage facility is carried out, the Environmental Impact Assessment (EIA) should include a SWOT Analysis (Strengths, Weaknesses Opportunities, and Threats) of the activity in relation to coastal fisheries. All the aforementioned research should be conducted to better assess the influence of fish farming on coastal fisheries in the forthcoming era of marine fish farming. Only in this way, the sustainable integration of sea cage aquaculture in the coastal zone will be achieved.