Ideal versus practical ecosystem boundaries for reporting ecosystem indicators in the International Commission for the Conservation of Atlantic Tunas (ICCAT) Convention Area

Biogeographic classification, as a tool for identifying meaningful ecoregions and setting appropriate spatial scales for management, is increasingly gaining importance in fisheries policy and becoming a necessary element of the Ecosystem Approach to Fisheries Management (EAFM). Some international and national fisheries organizations, pioneers in operationalizing an EAFM, have developed their own ecoregions to guide their management and ecosystem advice. The International Commission for the Conservation of Atlantic Tunas (ICCAT), in charge of managing tuna and tuna-like species and associated ecosystems in the Atlantic Ocean, is currently exploring approaches to facilitate the implementation of an EAFM within its Convention Area. Yet it has not established ecoregions or area-based fisheries management to inform its ecosystem research and advice.

The main aim of this thesis was to review existing marine biogeographic classifications of the pelagic ocean and examine their relevance to inform the selection of ecoregions or area-based fisheries management units to facilitate the implementation of an EAFM in ICCAT. Therefore, first I conducted a literature review of existing marine biogeographic classifications and their relevance to inform the choice of ecoregions to support the management of tuna and tuna-like species in line with an EAFM. Second, I examined the current fisheries management units in place in ICCAT to discuss their suitability to support an EAFM. Last, I analyzed the spatial patterns of catches for the main commercial species of tunas and billfishes and assessed how species compositions vary geographically and their associations with the most relevant biogeographic classification of the pelagic ocean.

I found that the current fisheries management in ICCAT focuses on single species and stock management. In order to apply an EAFM, ICCAT would need to recognize that stocks are part of food webs, that species interact, and co-occur in space and compete for resources, and hence move into a framework of multispecies assessment and advice in line with an ecosystem approach. Despite tunas and billfishes having a broad tolerance for a wide range of environmental conditions, I found they form distinct communities across the Atlantic Ocean. Of all the biogeographic classifications reviewed, I found that spatial scales of the Spalding’s Pelagic Provinces of the World (PPOW) aligned better with the spatial distributions of catches of tunas and billfishes, suggesting that environmental conditions captured by the PPOW might be controlling to some extent the spatial distribution and co-occurrence of these species. Finally, I proposed five potential ecoregions with relatively distinct communities of tuna and billfishes across the Atlantic Ocean, which could be a starting point to foster fruitful debates in the ICCAT scientific community to inform the potential choice of ecoregions that are both ecologically meaningful as well as practical from the management side to support the operationalization of an EAFM in ICCAT.