Temperature and primary production shaping SHEVs spatial distribution
Based on the concept of the ecological niche sensu Hutchinson (1978), our models rely on the ecological requirements of each species, mean Sea Bottom Temperature (SBT), SBT range -a proxy for temperature seasonality - and primary production being the variables that best reproduced the contemporary spatial distribution of SHEVs. Temperature is a key factor for the life cycle of all aquatic animals, particularly for ectotherms, and especially for SHEVs that have a pelagic larval/recruitment phases27. Our results show that the spatial distribution of SHEVs is strongly explained by variability in seasonal SBT. Coastal Mediterranean fish abundance, including the eight species we considered here, can be influenced by temperature seasonality in coastal28 and deep zones29. The effects of temperature variations on species depend upon the timing of life cycle, the intensity and duration of exposure, as well as the speed at which changes in temperature occur. Acute short-term variations of temperature might have drastic, often detrimental, effects on fish physiology, whereas long-term gradual variations can lead to potential acclimation, through variations in metabolic and feeding behavior30.
Primary production sustains the whole marine food chain and provides most of the endosomatic energy needed for heterotrophic species: previous studies have shown that 8% of the worldwide aquatic primary production (but ~ 25% for shelf ecosystems31) is required to sustain fisheries at the global scale32. Although obtained from values integrated within the upper water column (0–30 meters depth), our simulations show that the inclusion of a proxy for food concentration is important to assess the distributional range of SHEVs. While the eight SHEVs we modelled are carnivores with trophic levels ranging from 3.2 (surmullet33) to 4.4 (European hake34), the indirect relationship between primary production and fish stocks has already been thoroughly documented35. The relationship between primary production and upper trophic levels is also strongly influenced by factors related to the trophic processes that define the movement of endosomatic energy along the food chain, but also other physical factors such as chlorophyll a36. Incorporating trophodynamic in species distribution models - such as the direct/indirect biotic and/or trophic interactions (e.g., prey/predator relationships) - is needed to infer their relative contribution to community structure and dynamics, or to quantify the regulating effects of upper (or lower) trophic levels by bottom-up (or top-down) forces37. However, their integration in modelling frameworks is still a methodological challenge8.
South to North SHEVs range shift distribution
Climate-induced changes in the distribution of fish communities have been described in several marine ecosystems38. Our simulations show a future range shift, from the Mediterranean Sea to the North European coasts, in the distribution of the eight SHEVs for all levels of warming. In the Mediterranean Sea, a high decline in the ESI of the SHEVs is expected by the end of the century under a pronounced warming (RCP8.5), while a potential temperature-induced limitation is expected to slow down the decline rate in ESI values by at least 20% under scenarios RCP 2.6 or RCP 4.5. The predicted decline in ESI in the Mediterranean Sea is likely to be accompanied by an increase in ESI along the North European coasts. From a fishery management point of view, our results reveal that the Mediterranean countries catches of all SHEVs will drastically decrease if the temperature keep increasing by the end of the century (up to + 3.2°C under scenario RCP 8.5), while North European countries will likely benefit of stable or increasing SHEVs catch. The magnitude of range shifts in SHEVs distributions in the Mediterranean Sea may deeply affect ecosystem functioning and economic activities related to fishing39. Similarly, the spatial range of SHEVs is expected to shrink whatever the scenario and the future time period. Fisheries management adaptations to climate change should urgently consider these predictions, as the rapid decrease in covered area (period 2030–2039, e.g., for the anglerfishes, the gilthead seabream or the European seabass) may induce a rapid and non-reversible change in fisheries resources. Mid- and long-term projections highlight that the loss in the spatial extent of species is higher when the warming becomes severe (RCP8.5 versus RCP2.6/RCP4.5). This is in phase with40 which quantify the benefits to marine fisheries - and related economic outcomes41 - of limiting global warming to 1.5°C above preindustrial level.
Fisheries and aquaculture management implications
Here, we highlight the importance of simulating long-term changes in fisheries under several climate change scenarios, especially in the context of uncertain future outcomes for food and nutritional security42. Developing and implementing climate-adaptive strategies that can help address shifts in species distribution can be of interest to help adapting fisheries and aquaculture to climate change, in particular through change in commercially-targeted species, spatial reallocation of fishing effort, improvement of fishing techniques and engines, or the implementation of fishing rights based on historical stock distribution43 (Lindegren and Brander, 2018). Transformative adaptation of current fisheries and aquaculture, as well as their management, is urgently needed, especially for the most vulnerable countries such as northern African countries where socio-economic exposure, vulnerability and risk to climate change are high in comparison to European countries6. While aquaculture has been suggested as an alternative to the dramatic decline in Mediterranean and Black Sea fisheries44, our simulations detect that the two most farmed Mediterranean fish - i.e., the European seabass and the gilthead seabream – may also be impacted by warming by the mid/end of the 21st century, with a reduction in the potential for offshore aquaculture suitable sites. Assessment of the impact of climate change on Mediterranean offshore aquaculture is yet to be developed, however, to consider a large panel of abiotic45 and biotic factors including the risk of increasing disease outbreaks46, as well as regional economic peculiarities such as heterogeneity in national economies, national food self-sufficiency and human habits for foods.
Because of the high sensitivity and exposure of Mediterranean fisheries to climate change6, coordinated actions and mitigation activities must be undertaken to stem the repercussions of the ongoing decline in marine resources. As a way of adaptation, changes to the food commodity market and/or its diversification, through the commercialization of lower economic value and/or non-indigenous fish species must be considered47: in the eastern and central Mediterranean Sea, respectively, marbled rabbitfish Siganus rivulatus and the blue crab Callinectes sapidus are now commercially exploited48.
To conclude, our study predicted the potential decline in SHEVs stocks in the Mediterranean Sea and their potential reallocation in the North European coasts, under different RCP scenarios and three time period. Whatever the future warming conditions in the upcoming decades, an adaptation of the fisheries and aquaculture strategies is urgently needed, for all countries, and mostly, the most vulnerable ones. We therefore support further initiatives aiming to predict the ecological and economic consequences of climate change on the fisheries and aquaculture, at the Mediterranean and European Seas scale.