Asilvestramiento de los peces introducidos por la acuicultura y sus efectos ecológicos potencialesel caso de la lubina, "Dicentrarchus labrax" (Linnaeus, 1758), escapada en las Islas Canarias

Resumen

Fish farming in floating cages is a constantly growing economic activity worldwide. The Canary Islands have not been outside this, and since the early 90's production has grown, becoming one of the most important regions of Spain in aquaculture production. This rapid expansion has been accompanied by increased environmental monitoring of pollutants, nonetheless escapees have been traditionally relegated to the background, receiving no consideration as a threat to the sustainability of aquaculture. Escape events can release a large number of fish occasionally (massive escape) or few fish recurrently (chronic or leaking escape). The massive escapes occur mainly through technical failure or breakage of the materials the facilities are composed of, triggered by unfavourable conditions in the sea (sea storms). Moreover leaking escapes often occur due to the presence of holes in the nets as well as during harvesting. Taking into account potential negative effects caused by escapees, information concerning the ecological role of these species in shallow coastal habitats of the Canaries is scarce. This thesis addresses the problem by focusing on the process of feralization of sea bass (Dicentrarchus labrax L.) once they escape from floating cages, since this species is considered non-native for the central and western islands of the archipelago. The spatial distribution patterns of the bass and its relation to escape events are also explored. All the foreseeable ecological effects of escapees are discussed and measures for monitoring and mitigation are proposed. After the introductory chapter, which sets the study framework, chapter two is a preliminary study on escaped fish off Tenerife. Therein, it is shown that abundance of escaped sea bass depends mainly on the distance from the aquaculture facility. Habitat features also affect their distribution. Preliminary data on the diet of fugitives is also provided, it is demonstrated that escapees actively predate in natural environments, and their diet is similar to that of wild sea bass in their natural range. Farmed sea bass feralization is further analysed in chapter 3. There the biological changes sea bass undergo once they have escaped are explored. For this the morphometric traits of escaped and farmed fish that have spent all their life-cycle in culture conditions are compared. Differences in growth profiles measured in scales and condition indexes are also studied. Results show that the escaped sea bass have a more streamlined body-shape than those in culture. The growth profiles show that sea bass escapees' somatic growth rates are similar to those of farmed fish. Additionally, the general loss of fats causes a decrease in Fulton and hepatosomatic indices to levels similar to wild counterparts. This is the result of a combination of individual phenotypic plasticity and natural selection, the former allows individuals to adapt themselves in order to survive, and the latter makes the fittest individuals have higher survival rates in the wild. This suggests that those escapees that survive long enough show good performance in the wild. Gonadal maturation and potential for reproduction of the sea bass escapees are analysed in chapter 4. This paper contains the first record of male and female escapees' simultaneous maturation in the wild. The males' gonad parasite Sphaerospora testicularis is also cited for the first time in the Canaries. Possibly it was introduced together with fry brought from mainland. The implications of these results regarding the possibility of self-sustaining populations and the spreading of parasites by escaped fish are discussed. Chapters 5 and 6 are dedicated to studying the mass escape that occurred off La Palma in early 2010. This is the largest sea bass escape event officially reported to date worldwide, during which around a million and a half individuals were released into the wild. Abundances of escaped fish at different spatiotemporal scales, before and after the escape event, are analysed in Chapter 5. It is also examined whether the La Palma Marine Reserve, near the escape point, has any influence on the number of escaped fish observed. 'First- sale' data of the two fishermen's associations on the island are collated to detect changes in local artisanal fisheries. Escaped sea bass abundances in the wild suggest temporal patterns in which during the winter there is a higher number of escapes due to sea storms. Meanwhile, during summer, bass abundance is lower due to a lower escape rate and possibly increased fishing mortality. La Palma Marine Reserve showed no special patterns of escapee abundance, thus it was equally affected by the escape event. The island's artisanal fisheries were clearly altered by the massive escape, and some months, escaped fish amounted up to 40% of bentho-demersal fish catches. A year after the escape event there were substantial sea bass captures off the west coast of the island. In Chapter 6 the changes in trophic structure of fish assemblages after the massive escape are explored. As the trophic level of escaped fish is medium-high (3.3-3.8), the mean trophic level of the fish assemblage would be expected to increase due to the presence of escapees. The data support the latter, but the MPA showed some resilience to these changes due to a better structured fish community. This assemblage has a biomass of herbivores that prevents increments in mean trophic level such as those in unprotected areas. However, the presence of escapees did induce a significant increase in mean trophic level, even in the marine reserve. The consequences of these changes are highly unpredictable because of the complex interactions that occur in the marine environment. Furthermore the diet of individuals caught off La Palma MPA six months after the massive escape and that of single individuals fished at different sites off Tenerife and La Palma is compared. Stomach contents suggest that the resources exploited by escaped sea bass depend on the size of the latter, but also on how long ago they escaped. This is consistent with wild sea bass foraging behaviour, which is known to suffer ontogenetic changes throughout their life cycle, and confirms that feral sea bass diet is very similar to that of wild counterparts, crustaceans and fishes being the most important prey. The results of this thesis show the potential impacts of escapees in the Canaries, which may generate undesirable consequences if escape events are not managed properly. These interactions range from food webs modification to public health issues due to the lack of traceability of escapees, and include fisheries interactions. The effects of escaped fish are difficult to model in a complex ecosystem such as coastal waters and, in most cases, they have barely been identified. This generates a high degree of uncertainty, in an environment where restoring habitats after negative impacts is costly. All these features lead to the need to manage escapees within the paradigm of risk evaluation and management. Therefore mitigation, contingency and restoration plans, jointly with constant monitoring and research should be encouraged. These tools, and a precautionary approach would enable a sustainable and environmentally-compatible management of aquaculture in the archipelago, and are clearly relevant to its future development on an international scale.