Abstract
An ecological niche modelling (ENM) approach was used to predict the potential feeding and spawning habitats of small (5–25kg, only feeding) and large (>25kg) Atlantic bluefin tuna (ABFT), Thunnus thynnus, in the Mediterranean Sea, the North Atlantic and the Gulf of Mexico. The ENM was built bridging knowledge on ecological traits of ABFT (e.g. temperature tolerance, mobility, feeding and spawning strategy) with patterns of selected environmental variables (chlorophyll-a fronts and concentration, sea surface current and temperature, sea surface height anomaly) that were identified using an extensive set of precisely geo-located presence data. The results highlight a wider temperature tolerance for larger fish allowing them to feed in the northern – high chlorophyll levels – latitudes up to the Norwegian Sea in the eastern Atlantic and to the Gulf of Saint Lawrence in the western basin. Permanent suitable feeding habitat for small ABFT was predicted to be mostly located in temperate latitudes in the North Atlantic and in the Mediterranean Sea, as well as in subtropical waters off north-west Africa, while summer potential habitat in the Gulf of Mexico was found to be unsuitable for both small and large ABFTs. Potential spawning grounds were found to occur in the Gulf of Mexico from March–April in the south-east to April–May in the north, while favourable conditions evolve in the Mediterranean Sea from mid-May in the eastern to mid-July in the western basin. Other secondary potential spawning grounds not supported by observations were predicted in the Azores area and off Morocco to Senegal during July and August when extrapolating the model settings from the Gulf of Mexico into the North Atlantic. The presence of large ABFT off Florida and the Bahamas in spring was not explained by the model as is, however the environmental variables other than the sea surface height anomaly appeared to be favourable for spawning in part of this area. Defining key spatial and temporal habitats should further help in building spatially-explicit stock assessment models, thus improving the spatial management of bluefin tuna fisheries.
Highlights
The methodological approach used in our ENM is essentially composed of four main steps (Fig. 1), namely: (1) identify the main behaviours and ecological traits of ABFT based on literature; (2) collect and process the ABFT presence data and environmental covariates by geographical area; (3) derive a cluster analysis to identify a suite of relevant thresholds of environmental variables related to the ABFT ecology that describe the feeding and spawning habitat characteristics and (4) develop a habitat model to classify on a daily basis the degree to which each portion of the study area is either suitable or unsuitable for each habitat
The observations were relatively well distributed by area and season allowing the modelling of both the feeding and spawning habitats with more than 90% of data related to large ABFT from January to June in the Gulf of Mexico and from May to September in the Mediterranean Sea
We substituted the satellite-derived sea surface temperature (SST) with sea surface temperatures (SST) from ocean circulation models because satellite SST was found to be highly variable from day to day and may not represent the mixed layer temperature well
Summary
Thunnus thynnus (Linnaeus, 1785), is a highly migratory species able to tolerate wide ranges of environmental conditions (Arrizabalaga et al, 2015) in tropical and temperate waters of the Atlantic Ocean and Mediterranean Sea. The relatively recent introduction of remote sensing and access to relevant data from the scientific community allowed for incorporating environmental data into distribution and abundance analyses This has confirmed that bluefin tuna distribution is significantly affected by spatial and temporal variations of environmental conditions (see e.g. Fromentin et al, 2014). An attempt to associate presence data with several plausible factors affecting bluefin tuna distribution and abundance was recently undertaken by Druon et al (2011) providing potential feeding and spawning habitats in the Mediterranean Sea. In this paper, we link the ecological traits of small and large ABFTs to environmental variables (Ecological Niche Model approach, hereafter ENM) and investigate the respective feeding and spawning requirements. The seasonal and decadal habitat variability and spatial extent were discussed with respect to their potential impact on east and west ABFT stocks dynamics, as well as the utility on assessment and management
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