Habitats and nekto-benthic communities of the bottom-trawl sishery developed on the continental shelf of the Balearic Islands

Résumé

Introduction The presence of biogenic benthic habitats in the continental shelf of the Balearic Islands is well known. In the circalittoral, outstanding due to their presence, the following can be found: the bottoms of calcareous red algae of the Corallinaceae family (known as maërl beds), considered particularly sensitive to the anthropogenic impacts; the crinoids beds, considered an essential habitat for the vital cycle of the fishing resources; or the Peyssonnelia beds, characterized by high biomasses of soft red algae, mainly of the Peyssonnelia genus. The bottom trawl fishery, developed in the continental shelf of the Archipelago takes place in these types of bottoms. In spite of this, and the negative effects of bottom trawling on benthic habitats, in the Balearic Islands, there is little information on the relationships between these habitats and the exploited species, whose biology and exploitation state is also unknown for the majority. Research contents This doctoral thesis studies the benthic habitats and the nekto-benthic communities exploited by the bottom trawl fishery developed on the continental shelf of the Balearic Islands. The main objectives include determining: the relationship between habitats and fishing resources, the main factors affecting the distribution of both, and how the presence of the main biogenic habitats affects the physiological condition of the associated fish species characterizing them. For the first time, the biology and exploitation state of some of the fishing resources characterizing these bottoms is studied, as well as the effect of changing the mesh geometry in the codend of the bottom trawl gears, from diamond to square, on the selectivity of this fishery. The results highlight that the presence of maërl and Peyssonnelia beds in the bottom trawl fishing grounds of the shallow shelf, and the crinoid beds in those of the deep shelf, is an important factor for the distribution of nekto-benthic fishing resources. Hydrodynamics influence the distribution of both habitats and fish. In this sense, fish size appears as a key trait for their distribution across gradients of change of both habitat and hydrodynamic regime. The study of the biology of characteristic fish species of the shallow shelf indicates that the most structurally complex biogenic habitats, showing the highest algae biomasses, such as maërl and Peyssonnelia beds, allow the individuals inhabiting them to have a better physiological condition than those distributed in habitats showing lower structural complexity and benthic biomass, such as the sandy bottoms. The main by-catch species of the bottom trawl fishery developed in the continental shelf of the Balearic Archipelago show a similar, or even more pronounced, level of overexploitation than the main target species of this fishery, which are the striped red mullet (Mullus surmuletus) and the hake (Merluccius merluccius). The use of square mesh in the codend substantially improves the selectivity of the bottom trawl gears, reducing the discarded catch and increasing the size at first capture of most species. However, these sizes are still smaller than the size of sexual maturity of most species, and in some cases, even smaller than the minimum landing sizes. Conclusion The results highlight the importance of the presence of the biogenic benthic habitats for the populations of nekto-benthic organisms. Some of these habitats can play the role of oases where fish could face the energetic requirements of their vital cycle in a better physiological condition. In the Balearic Sea, an area standing out by its oligotrophy and where the most important commercial species show signs of overexploitation, the sustainability of fishing resources will depend on the ability to implement management measures aiming beyond the improvement of the current exploitation regime. They will have to take into account key aspects of the vital cycle of these species and the conservation of the habitats supporting them.