Benthic communities' response to different trawling impact levels: generalization towards developing a Mediterranean model = Resposta de les comunitats bentòniques a diferents nivells d’impacte de pesca d’arrossegament: generalització per al desenvolupament d’un model mediterrani

Abstract

Fisheries management has definitively embraced the Ecosystem Approach to Fisheries (EAF) philosophy and new management policies are incorporating integrative measures that take into account not only target species but also other ecosystem and socioeconomic elements involved in the fisheries system. In this context, the main aim of this thesis is to assess benthic community (infauna and epifauna) response to commercial trawling in the Mediterranean continental shelf in order to increase the scientific knowledge necessary for an EAF implementation. This aim is addressed within an integrated framework, considering different fisheries’ aspects discussed in each chapter, which constitute the specific objectives of this thesis.

As a first objective, the trawling fleet activity was described and the level of fleet disturbance, which depends on fishing effort, was assessed at the benthic community level. Fishing effort was estimated using different approaches: fisheries data, Vessel Monitoring System (VMS) data and Side Scan Sonar (SSS) data. From this study we obtained an accurate estimation of trawl fishing effort in our communities of interest, which is the basis for the following studies. Then, as a second objective, the effects of trawling on soft bottom habitats’ functionality was assessed by evaluating trawling impact on ecological units called Ecosystem Service Providers (ESP) and on benthic communities’ functional redundancy. ESPs were described as a group of species that perform an ecosystem function that will ultimately deliver ecosystem services and that are linked to key functions of soft-bottom habitats (e.g., nutrient cycling, carbon sequestration, production, benthopelagic coupling and habitat provision). The study revealed that these ESPs were favored by particular sediment types, and trawling effects overlapped this natural variability. On the other hand, functional redundancy on these communities was also assessed using two complementary approaches: redundancy achieved through trait abundance (i.e. large amounts of a trait), that was called “common traits”, and redundancy achieved through trait richness (i.e. large numbers of distinct taxa exhibiting the same trait), that was called “widespread traits”. Both measures were found to be affected by trawling, although the former differentiated more clearly between heavily fished and moderately fished areas. The role of rare species in both measures was also analyzed and it was found that generally they exhibited the same traits as the most abundant species. The subsequent objective was to assess the potential effects of the observed benthic communities’ functional changes on target species. As a case study, red mullet (Mullus barbatus) was chosen because of its commercial importance in Mediterranean fisheries and its close relationship with benthic communities. The study highlighted that functional changes caused by trawling on benthic habitats might result in an overall negative effect on the red mullets’ stock.

A second part of this thesis focuses on the importance of communication among fisheries’ stakeholders and provides tools to share scientific knowledge in an understandable way. In this context, a DPSIR (Drivers–Pressures–State Change–Impact–Response) framework was adapted to the trawling fisheries system in the study area in order to integrate the complex relationships between human uses, ecosystem components and the demand for ecosystem services in trawling grounds. In line with the philosophy of making scientific knowledge understandable, this thesis developed a platform with a user-friendly interface that represents trawl impacts on the seabed in our study sites and includes the possibility of adding new study sites. The platform also includes a simulation model that allows the user to change fishing effort on the study sites and visualize the potential changes in the benthic community structure caused by this change. This platform is meant to be a deliberation support tool for fisheries’ decision actors. Finally, by integrating all the acquired knowledge on trawl driven changes on benthic communities as well as literature information, this thesis recommends several management measures, indicators and tools to be used in an EAF management plan for the Catalan trawl fleet, which might be also applied to other Mediterranean trawl fisheries.

L’objectiu principal d’aquesta tesi és estudiar de manera integrada la resposta de la comunitat bentònica (infauna i epifauna) a la pesca d’arrossegament en els fons tous de la plataforma continental del Mediterrani per tal d’incrementar el coneixement necessari per a implementar una gestió de les pesqueries basada en l’ecosistema.

Així doncs, el primer pas fou estimar de manera acurada la dinàmica de la flota de les àrees d’estudi i l’esforç pesquer a què estaven sotmeses les comunitats bentòniques. A partir d’aquesta estimació es va avaluar l’impacte de la pesca en la funcionalitat d’aquestes comunitats, estudiant-ne els possibles efectes sobre els proveïdors de serveis ecosistèmics i sobre la redundància funcional. És a dir, els efectes es van estudiar sobre les unitats ecològiques que duen a terme funcions ecosistèmiques i que en última instància determinaran la provisió dels serveis ecosistèmics. També es van avaluar els efectes potencials d’aquests canvis a nivell de les comunitats bentòniques en una important espècie comercial en el Mediterrani com és el moll de fang (Mullus barbatus).

Un segon bloc de la tesi se centra en la importància de la comunicació entre els diferents actors involucrats en les pesqueries i per això calen eines fàcilment comprensibles per a compartir informació entre tots els estaments involucrats. En aquest context, aquesta tesi inclou una representació del sistema pesquer seguint la filosofia del DPSIR (Drivers – Pressures – State Change – Impact– Response: Activadors – Pressions - Canvi d’estat – Impacte - Resposta) amb la finalitat de fer més comprensibles les relacions entre els diferents elements del sistema pesquer. En aquesta mateixa línia s’ha desenvolupat una plataforma de coneixement que recopila informació sobre l’impacte de la pesca d’arrossegament en les zones d’estudi així com un model de simulació que permet a l’usuari simular un canvi d’esforç en una d’aquestes comunitats i visualitzar l’impacte d’aquest canvi en l’estructura de la comunitat bentònica.

Tenint en compte els resultats obtinguts, com a última finalitat de la tesi es presenten una sèrie de recomanacions per a la implementació d’un pla de gestió basat en l’ecosistema per a la flota d’arrossegament catalana, que podrien ser extensibles a qualsevol flota d’arrossegament mediterrània.