Patrick Lehodey, CLS Space Oceanography Division, Department of Marine Ecosystems, France, is an invited speaker in S10: Forecasting climate change impacts on fish populations and fisheries.
High seas fisheries target a limited number of tuna and associated species (e.g. swordfish and marlins) that share this oceanic habitat with other large predators, including sharks, marine mammals, turtles, and sea birds. Indeed, rather than a single habitat, these species have overlapping vertical and horizontal habitats defined by their preferences and tolerances developed over the evolution for several key physical and biological variables. The short-living skipjack tuna is the most tropical and productive species with its core habitat in equatorial and tropical waters. Bluefin and albacore are long-living temperate species, moving far in high latitudes searching for rich foraging grounds but returning to warm waters for spawning, leading to seasonal migrations and complex population dynamics mechanisms interacting with several environmental variables.
Habitat modeling and their projection in the future using Intergovernmental Panel on Climate Change (IPCC) scenarios have been useful to investigate the potential changes in distribution of pelagic species. They mainly suggest poleward latitudinal shifts of present distributions which seem corroborated by recent observations, e.g. with Atlantic bluefin migrating further north in summer following the warming of water masses (MacKenzie et al. 2015) or comparisons with past warm events (e.g. El Niño phases). Questions about future changes in abundance and appropriate fishing management measures are more complex to answer. They require combining the fishery sciences based on population dynamics equations and robust statistical approaches to estimate population and fisheries parameters, with the ecological sciences describing species distributions and interactions with their physical and biological environment. Then, the coming challenge will be to provide answers to fisheries managers and regional organizations on the most plausible changes in abundance and distribution for the coming decades of the main exploited species, integrating combined impacts of climate change, natural variability, and fishing mortality.
At least two examples of modeling approaches developed following these criteria have provided the first projections of the impacts of climate change on two species of tuna using close AR4-A2 and AR5-RCP8.5 IPCC scenarios. For skipjack (Lehodey et al. 2013; Dueri et al. 2014) both models (SEAPODYM and APECOSM-E) forecasted an improvement of skipjack habitat in the eastern Pacific Ocean together with an extension to higher latitudes, while the habitat suitability is predicted to decrease in the too warm and less productive western equatorial warm pool. For albacore (Lehodey et al. 2014), the study predicted a decrease in biomass stabilizing after 2035 and then increasing after 2080 due to the emergence of a new spawning ground in the north of Tasman Sea.
Such key results for tuna stock management could not be revealed without models describing the whole population dynamics in relation to environmental conditions. These simulations also indicate that fishing remains the major driver controlling the future of these populations. Nevertheless, to be of interest for management issues, a primary condition will be for this generation of models to convince that they have encapsulated the main drivers of the system and the target populations, that is they can simulate, a minima, their past history and population structures at least as good as existing standard stock assessment models.
Predicted changes in larvae distributions of (left) skipjack and (right) south Pacific albacore tuna between first and last decades of 21st century under IPCC A2 scenario. (Redrawn from Lehodey et al. 2013; 2015) Click on the image for a larger version.