Climate change effects on marine fish

• The previously documented climate change impacts on fish include changes to species distributions, body size, phenology, recruitment and fish biomass. 
• General trends of ocean warming impacts include distributional shifts towards northern ranges of fish distributions; larger species decreasing in body size while smaller fish may more readily adapt to the environment; earlier spawning; and changes in fish biomass which may be reflected through geographic changes in catch.
• Climate change impacts act in tandem with other factors (density dependence, trophic interactions) and are difficult to disentangle from these. 
• Most research on the impacts of climate change on fish has focused on ocean warming. The impacts of other environmental changes associated with emissions of greenhouse gases (e.g., ocean acidification, sea ice changes) will be revealed as further research is done. 

The climate-associated changes in oceans projected by global models include rising sea temperatures and changes to primary production. Warming in the North Atlantic is expected to continue, with the greatest warming predicted for the Atlantic-Arctic including the Barents Sea and Greenland Seas (Peck and Pinnegar, 2018). Models also indicate an overall decline in primary productivity for much of the North-East Atlantic, but a moderate increase in the Atlantic-Arctic (Peck and Pinnegar, 2018). Both of these developments have been linked to changes in the distribution of fish species. The research on temperature-related shifts in marine fish has been active for more than a decade (Perry et al., 2005; Poloczanska et al., 2013). Recent assessments of the long-term distributional shifts of key commercial fish stocks in Europe find distributional shifts for all examined species, the main drivers of which are environmental conditions, principally temperature (Baudron et al., 2020; ICES 2016). Species identified as ‘big movers’ with distributional shifts associated with global warming included anchovy (expansion of population in North Sea); white anglerfish (increase in Iceland, increase abundance in northern North Sea); cod (northward expansion in Barents Sea, northward movement in North Sea); megrim (L. boscii northward shift in North Sea, L. whiffiagonis negatively correlated to temperature in Spain); haddock (decreased occurrence in southern North Sea); hake (increased abundance in northern North Sea); and plaice (abundance decline in bay of Biscay due to temperature effect on juvenile habitat) (ICES 2016). The westward and north-westward distributional changes of the North-east Atlantic mackerel (Scomber scombrus) have been linked to ambient temperature and mesozooplankton density (Olafsdottir et al., 2019). 

The role of temperature in distributional changes is difficult to disentangle from other factors such as the role of stock size. Temperature may affect the expansion or contraction of a species due to changes in habitat suitability or recruitment rate (due to effects on physiology, survival, or impacts on planktonic life stages or planktonic prey), and stock size may enhance or counteract these effects through its role in density-dependent habitat use (Baudron et al., 2020). Distributional shifts are not only reflected geographically but also in terms of water depth. In European shelf seas, where bottom temperatures have experienced 1,6°C increases between 1980 and 2004, a deepening response of demersal fish such as cod, megrim and anglerfish has been noted (Dulvy et al., 2008) and the availability of suitable habitat at increasing depth may limit species range shifts under future climate scenarios (Rutterford et al., 2015). Food web interactions also play a role in species range shifts. Predictions of species range shifts have the potential to overestimate range shifts if food web interactions and allometry are ignored. Models show slower range shifts for species experiencing dynamic trophic interactions than for single species (Tekwa et al., 2022). 

The average body size of marine organisms is projected to shrink under ocean warming as a result of direct impact, and as an adaptive response, with small-bodied pelagic fish being possibly more resilient to climate change (Lefort et al., 2015). Indeed, in fish assemblages in the western English Channel, larger species (rays, hake and anglerfish) have decreased significantly in both mean body size and abundance over the last century, while smaller fish species (mainly flatfish and gobies) have increased in abundance (Genner et al., 2010). Similar trends have been observed in the same species in the Celtic Seas (Pinnegar et al., 2002). Within species, changes in size-at-age across lifespan are expected to differ, with juvenile fish growing more rapidly due to the ecophysiological effects of temperature and adult maximum sizes being reduced for metabolic reasons (Ikpewe et al., 2021). In general, long-term data series have shown how smaller fish species might be able to quickly respond to the thermal environment due to their life history traits, while larger fish species’ observed decline in abundance and size could be caused by co-occurring warming and size-selective overharvesting (Genner et al., 2010; Pecuchet et al., 2017; Beukhof et al., 2019). Conversely, under reduced fishing pressure, increases in the abundance of long-lived species with slower life-history strategies have been observed in the southern North Sea, despite increasing temperatures in recent decades (Murgier et al., 2021).

Herring populations in the Celtic Seas and north-west of Ireland have seen a steady decline in size-at-age over the last few decades. Growth of this pelagic fish has been shown to be impacted by warming conditions, and it is hypothesised that at the southern limit of the distribution range, herring might be more susceptible to climate warming compared with northerly populations (Lyashevska et al., 2020). The opposite  has been observed for herring populations located at the northern edge of the distribution range. Norwegian spring-spawning herring’s recruitment, body size and spawning stock biomass (SSB) are positively correlated with temperature (Graham and Harrod, 2009).  For other fish, such as cod and haddock, at the northern distributional range the warmer years are leading to earlier spawning, and with high levels of food available juveniles reach a larger size earlier in the year, increasing the survival rate during the subsequent winter months (Ottersen, 2000). Cold-water species, like arctic and boreal fish, are predicted to be particularly impacted (Poloczanska et al., 2013) by the warming climate. The wolffish Anarhichas lupus in the North-East Atlantic, caught as by-catch in mixed fisheries in the North Sea, has been declining in abundance and has also been shown to be experiencing a change in size distribution (Bluemel et al., 2022). 

Climate change is expected to have an important impact on fish biomass, with the global catch redistributed by 2055. Models predict an increase of maximum catch potential (the maximum exploitable catch of a species assuming the geographic range and fisheries selectivity do not change) in high latitude regions and poleward tips of the continental shelf margin, and declines within the tropics and at the southern margins of semi-enclosed bodies of water (Cheung et al., 2010). Under this scenario, the Arctic and sub-Arctic region would see an increase in maximum catch potential, while north-west European shelf areas (Celtic Seas and North Sea) and the Bay of Biscay would experience a decrease (Cheung et al., 2010). Nevertheless, the increased ice melting rate and freshwater input in the boreal region will also increase stratification, which in turn will hinder the nutrient supply from the deeper layers, hence limiting primary production (Lefort et al., 2015). 

Traditionally, stock assessment approaches assume that abiotic conditions are constant. However, climate change has been identified as a stressor that has already effectively reduced MSY for many important stocks globally (Bryndum-Buchholz et al., 2021). Accordingly, different OSPAR Regions are expected to be impacted in different ways: the North-East Atlantic has already seen dramatic changes in sea surface temperature which have caused changes not just in distribution, but also in abundance. Fish biomass in the Arctic is expected to change due to fast warming and freshening (Drinkwater, 2005). These new conditions may reduce reproductive capacity, thus making certain stocks vulnerable to fishing levels that were previously considered sustainable (Brander and Mohn, 2004), but in other cases a positive effect on fish biomass may be observed (Arctic Council, 2005). 

The response of the Atlantic cod Gadus morhua to climate change has been the subject of many investigations due to its commercial importance. The recruitment of different Atlantic cod stocks has been impacted by climate (Graham and Harrod, 2009), particularly at the edge of its geographical range (positive in cold-water stocks and negative in warm-water stocks) (Planque and Frédou, 1999). In general, all cod stocks will be impacted, albeit differently, by climate change, especially by temperature during the spawning season, with negative effects in the populations at the warmer edges of the distribution, and positive ones in the lower thermal range (Mantzouni and MacKenzie, 2010; Núnez-Riboni et al., 2019). The potential for incorporating association with sea temperatures during spawning season into stock forecasts has recently been demonstrated for gadoid stocks (cod Gadus morhua and whiting merlangus merlangius) in the Irish Sea region (Bentley et al., 2021) and incorporated into the ICES advice scenarios for cod Gadus morhua in the Irish Sea in 2022 (i.e., via F_ECO reference points; ICES, 2022).

Other climate change impacts and concluding statements

The most frequently studied climate change impacts are discussed above, but the suite of climate associated changes is broader (e.g., including changes in sea ice, salinity, and oxygen; coastal erosion; and extreme events) and the list of impacts on fish has the potential to expand as more research is done. The impacts of ocean acidification on fish are less often studied but may impact early life stages (Pimentel et al., 2016). Biogeographic shifts in zooplankton in response to warming are documented but other factors such as changing salinity may also impact zooplankton abundance or community composition (Wells et al., 2022). Impacts may also be cumulative, as when environmental impacts (changes in temperature, salinity, pH, oxygen) interact with other biological changes/interactions (e.g., density dependence, food web interactions). Ocean changes that lead to altered timing of primary production (Hjerne et al., 2019; Mészáros et al., 2021) may act in tandem with the impact of ocean warming on fish phenology (i.e., change in timing of spawning, larval hatching or migration), leading to mismatches between first-feeding fish larvae and their prey (Neuhemier et al., 2018). For example, research from the North Sea reveals that temperatures affect the match between the hatch time of the sandeel (Ammodytes marinus) and the egg production of its copepod prey. The specific cause is the rate of seasonal temperature decline between autumn and winter that is related to sandeel gonad and egg development, while February temperatures relate to the timing of copepod presence. Thus, the interplay of these two temperature relationships affects the trophic match between sandeel and their prey (Regnier et al., 2019). In Atlantic cod, another North Sea species with temperature-dependent gonadal development, research suggests that rising sea temperature has led to a shift in spawning phenology (McQueen and Marshall, 2017). With environmental conditions expected to change drastically in the next two decades (IPCC, 2022), impacting fish in various different ways, estimates of fishing reference points under average, constant climate conditions might become obsolete (Travers-Trolet et al., 2020) and novel approaches will be required in order to incorporate ecosystem-driven changes in stock biomass into current fisheries advice frameworks (Howell et al., 2021; Bentley et al., 2021). 

Fish populations will also be expected to experience indirect impacts of climate change, through impacts on fisheries. In addition to the obvious changes to fish and fisheries outlined above, future efforts to reduce greenhouse gas emissions might cause fishery displacement as a consequence of changes to the gear or technology used. Good fisheries management will support resilience in exploited fish species. Hence, robust, science-based management plans will be crucial to ensuring the adaptive capacity of fish throughout the OSPAR Maritime Area.