Atlantic bluefin tuna: population dynamics, ecology, fisheries and management

Abstract

Both old and new information on the biology and ecology of Atlantic bluefin tuna have confronted scientists with research challenges: research needs to be connected to current stock‐assessment and management issues. We review recent studies on habitat, migrations and population structure, stressing the importance of electronic tagging results in the modification of our perception of bluefin tuna population dynamics and behaviour. Additionally, we question, from both scientific and management perspectives, the usefulness of the classical stock concept and suggest other approaches, such as Clark's contingent and metapopulation theories. Current biological information confirms that a substantial amount of uncertainty still exists in the understanding of reproduction and growth. In particular, we focus on intriguing issues such as the difference in age‐at‐maturity between West Atlantic and Mediterranean bluefin tuna. Our description of Atlantic bluefin tuna fisheries places today's fishing patterns within the two millennium history of exploitation of this species: we discuss trap fisheries that existed between the 17th and the early 20th centuries; Atlantic fisheries during the 1950s and 1960s; and the consequences of the recent development of the sushi–sashimi market. Finally, we evaluate stock status and management issues since the early 1970s. While important uncertainties remain, when the fisheries history is confronted with evidence from biological and stock‐assessment studies, results indicate that Atlantic bluefin tuna has been undergoing heavy overfishing for a decade. We conclude that the current exploitation of bluefin tuna has many biological and economic traits that have led several fish stocks to extreme depletion in the past.

Fisheries-induced disruptive selection

“…Fishing policies can be selective for both size and maturity stage of individuals in the stock: size selectivity results from mesh-size and gear regulation or from sizespecific incentives (Hart and Reynolds, 2002;Fromentin and Powers, 2005), while maturity selectivity may arise when a 12 stock's juveniles and adults are spatially segregated during spawning (Sinclair, 1992;Swain and Wade, 1993;Engelhard and Heino, 2004;Opdal, 2010). Harvest is readily adaptive, because fishers constantly tune their effort and selectivity for 14 maximum profit, targeting stock components that are most profitable to harvest.…”

Aggregations of wild Atlantic Bluefin Tuna (Thunnus thynnus L.) at Mediterranean offshore fish farm sites: Environmental and management considerations

“…The starting stock of farmed ABT originates from catches made mostly by purse seiners that capture live individuals at different life cycle stages that are destined for farming purposes. As the activity is very lucrative, an increased catch effort has been noted during the last ten years or so (Relini, 2003;Fromentin and Powers, 2005;Ottolenghi, 2008). As a result of overexploitation of the species, ABT has recently been listed as an 'endangered' species (criterion A2bd, IUCN Red List), with the status of its wild populations being deemed 'critical' (ICCAT, 2012).…”

Otolith shape variation provides a marker of stock origin for north Atlantic bluefin tuna (Thunnus thynnus)

“…The relative contributions of the two stocks to mixed assemblages vary spatially, temporally and ontogenetically (Taylor et al 2011). Exploitation in mixed fisheries is likely to have a disproportionate effect on the western stock because its productivity is estimated to be one-tenth that of the eastern stock (Fromentin and Powers 2005). Stock simulation results indicate that the incorporation of mixing rates into stock assessment within a mixed-stock modelling framework is essential for sound evaluation of quotas and effective stock-rebuilding plans, particularly for west Atlantic bluefin tuna (Taylor et al 2011).…”

“…Atlantic bluefin tuna are managed as two separate stocks by the International Commission for the Conservation of Atlantic Tunas (ICCAT): the western stock spawns in the Gulf of Mexico, whereas the eastern stock spawns in the Mediterranean Sea (Fromentin and Powers 2005). Severe overfishing in recent decades led to marked declines in abundance (Safina and Klinger 2008;MacKenzie et al 2009), prompting fishing restrictions and stock-rebuilding plans (Fromentin et al 2014a;ICCAT 2014).…”