Studies have shown that aerobic scope (oxygen availability for metabolism) of marine fishes and invertebrates, and its interactions with temperature, food availability and ocean chemistry, is a major factor determining life history, growth and distribution (Pauly, 2010; Pörtner, 2010). Fishes and invertebrates generally have an optimal environmental temperature that allows maximum aerobic scope, which is reduced as temperature increases or decreases from the optimum. Changes in ocean chemistry, particularly ocean acidification and reduced oxygen level, can also reduce the aerobic scope of the organisms (Pörtner, 2010; Cheung et al., 2011a). Changes in food availability, especially in early life stages, can affect survival and productivity of the exploited populations. These physiological mechanisms have direct implications for the distribution and abundance of fishery resources.
As distributions of marine organisms are generally dependent on optimal environmental conditions (e.g. temperature, oxygen, food availability), long-term changes in temperature and/or other ocean conditions often coincide with observed changes in distribution and fisheries (Sumaila et al. 2011). In an analysis of 50 abundant species in the waters around UK and Ireland, 70% of the species demonstrably responded to warming in the region by changing distribution and abundance (Simpson et al., 2011). Specifically, warm-water species with smaller maximum body size have increased in abundance while cold-water, large-bodied species have decreased in abundance. Such changes are significant and related to changes in water temperature (Simpson et al., 2011) Recent analyses of Scottish and English commercial catch data spanning the period 1913–2007 has revealed that the peak catches of target species such as cod, haddock (Melanogrammus aeglefinus), plaice (Pleuronectes platessa) and sole (Solea solea) shifted latitudinally, albeit not in a consistent way (Engelhard et al., 2011). For example, cod distribution seems to have shifted steadily north-eastward and towards deeper water in the North Sea. Sole seems to have retreated away from the Dutch coast, southwards towards the eastern Channel whereas plaice distribution has moved north-westwards (van Keeken et al., 2007; Engelhard et al., 2011) while southern species such as sea bass (Decentrarchus labra) and red mullet (Mullus barbatus) may be expanding their northern distribution boundary into UK waters, in particular the English Channel and southern North Sea. These changes in commercial catches are likely to be indicative of an underlying climate-driven distribution shift in the exploited population, as shown from the significant correlation between environmental temperature and distribution metrics of these species based on fishery-independent survey data (Perry et al., 2005; Dulvy et al., 2008; Hiddink and ter Hofstede, 2008; Engelhard et al., 2011). However, differential exploitation patterns and ecosystem production trends may also contribute to such observations. International fisheries landings data for the whole north-east Atlantic region (from around 36–80oN and 42oW–68.5oE) demonstrate both northerly and southerly shifts over recent decades. Many more species shifted southwards than northwards between the 1970s and 1980s (a relatively cool period) and vice versa in the 1980s and 1990s (a relatively warm period) (Heath, 2005), clearly illustrating that changes in underlying fish distribution patterns manifest themselves as changes in the distribution of commercial fishery catches, with resulting impacts on profits. These changes also have implications for fisheries management (see discussion below).
Changes in productivity and variability of fisheries resources
Recruitment variability is a key measure of the productivity of a stock, and is defined by the number of juvenile fish that have survived from the annual egg production and become available to the fishery. There are a number of hypotheses proposed to explain the large reduction observed in numbers remaining from the egg to the adult stage in marine fish, but most of them focus on physical processes and the necessity for adequate environmental conditions, e.g. optimal environmental window (Cury and Roy, 1989); Triad hypothesis (Bakun, 1996); oscillating control hypothesis (Hunt et al., 2002), or the provision of adequate food at the right time (e.g. match-mismatch hypothesis Cushing, 1990; Durant et al., 2007). Empirical data on exploited populations and fisheries often show strong relationships between recruitment success, fisheries catches and climatic variables in a number of fish and shellfish stocks that are important to the UK and Ireland. These strong relationships have been demonstrated, for example, for cod, whiting and haddock (Brander and Mohn, 2004; Cook and Heath, 2005), plaice (van der Veer and Witte, 1999), herring (Nash and Dickey-Collas, 2005), Atlantic mackerel (Jansen and Gislason, 2011), sea bass (Pawson, 1992), and scallops (Shephard et al., 2010).
In the case of cod, there is a well-established relationship between recruitment and sea temperature (O'Brien et al., 2000; Beaugrand et al., 2003; Clark et al., 2003), but this relationship differs between the different cod stocks that inhabit the North Atlantic (Planque and Fredou, 1999). For stocks at the northern extremes, warming leads to enhancement of recruitment, while in the North Sea, close to the southern limits of the range, warmer conditions lead to weaker than average year classes. During the late 1960s and early 1970s, cold conditions were correlated with a sequence of positive recruitment years in cod, haddock and whiting (Brander and Mohn, 2004; Cook and Heath, 2005) and subsequently high fisheries catches for a number of years thereafter (Heath and Brander, 2001). For Atlantic mackerel, currently forming the most valuable fishery in Ireland, increases in sea surface temperature are known to affect the timing and magnitude of growth, recruitment and migration with subsequent impacts on permissible levels of exploitation (Jansen and Gislason, 2011). Similarly recruitment in blue whiting (Micromesistius poutassou) also seems to be dependent on prevailing climatic conditions. Hátún et al. (2009) demonstrated that the position of the North Atlantic subpolar gyre west of the British Isles seems to regulate spawning distribution of blue whiting and hence future recruitment patterns in this species. For scallops, data collected from the scallop fishery in the Isle of Man showed that numbers of young scallops each year were, on average, positively related to seawater temperature in the spring when the animals were spawned. The gonads of adult scallops were also larger in warmer years, indicating higher egg production (Shephard et al., 2010). Thus, ocean warming may have opposite effects on the productivity of different exploited fishes and invertebrates, depending on their temperature preference and current environmental conditions.
Ocean acidification may have direct and indirect impacts on the recruitment, growth and survival of exploited species (Fabry et al., 2008) and some species may become more vulnerable to ocean acidification with increases in temperature (Wood et al., 2010; Hale et al., 2011). The impacts are suggested to be particularly apparent for animals with calcium carbonate shells and skeletons such as molluscs, some crustaceans, and echinoderms (Gazeau et al., 2007; Cooley and Doney, 2009; Kroeker et al., 2010), but research shows large variations between and within taxonomic groups. While invertebrates including Nephrops and lobsters are highly valuable fisheries in the UK and Ireland, these species are also shown to be potentially vulnerable to impacts from ocean acidification. For example, under a CO2 concentration that is equivalent to the high emission scenario in the IPCC AR4 assessment (SRES A1F, 1200 ppm by 2100; IPCC, 2007), European lobster (Homarus gammarus) did not show significant change in carapace length or development, but carapace mass was significantly lower during the final developmental stage of the larvae (Arnold et al., 2009). Also, experiments on larvae of oysters (Crassostrea virginica and C. ariakensis) have shown that pCO2 within the range projected by the IPCC (up to 1200 ppm by 2100) could lead to 16% decrease in shell area and 42% decrease in calcium carbonate content relative to those kept under the pre-industrial CO2 level (280 ppm) (Miller et al., 2009). Current evidence on direct impacts of ocean acidification on commercial finfish, however, is limited. It is possible that the physiological functions of some finfish may be impaired (Munday et al., 2010), the development and survival of their early life history stages may be impacted (Munday et al., 2010; Frommel et al., 2012) and the invertebrate food sources of fishes may be affected by ocean acidification (Fabry et al., 2008; Hale et al., 2011). On the other hand, a meta-analysis of vulnerability in various marine taxonomic groups suggests that many marine organisms may be more resistant to ocean acidification than previously thought, and their long-term adaptive capacity is poorly understood (Hendriks et al., 2009).
There is strong consensus and robust evidence in the literature that acidification of the ocean will continue in this century as atmospheric CO2 concentrations increase. So far, impacts from ocean acidification on UK and Irish fisheries have yet to be detected. Little modelling has yet taken place to scale up from laboratory experiments to populations and to the consequences for fishermen and fleets. A preliminary economic assessment estimated the extent of possible economic losses to the UK shellfish industry under ocean acidification (Pinnegar et al., 2012). Four of the ten most valuable marine fishery species in the UK are calcifying shellfish and the analyses suggested that losses in the mollusc fisheries alone could amount to £55–379 million per year by 2080 depending on the CO2 emission scenario chosen. In addition, a further £59.8–124.6 million might be lost from the shellfish aquaculture industry assuming future CO2 concentrations increase from the current level of ~380 ppm to ~740 ppm (pH 7.9–8.0). Thus, there is a clear economic reason to improve our understanding and techniques for modelling the up-scale implications of physiological and behavioural responses to ocean acidification. Such analyses are necessary if we are to understand the magnitude of impacts from ocean acidification on fishing fleets, which would help refine estimates of impacts on the UK and Irish national economies (Le Quesne and Pinnegar, 2011).
Implications for resource availability
Model simulations suggest that distributions of exploited species will continue to shift in the next five decades both globally and in the north-east Atlantic specifically (Cheung et al., 2009, 2010, 2011a; Lindegren et al., 2010). For example, an approach called the Dynamic Bioclimate Envelope Models (DBEMs) has been developed to project future distribution and resource availability under scenarios of climate change (Cheung et al. 2008, 2009, 2010, 2011a). The approach is based on observed species distribution patterns, population dynamics and includes empirical scaling of primary production. An analysis by Cheung et al. (2010) included all major exploited species in the world (1066 species of fish and invertebrates). This study estimated future changes in maximum potential catch (a proxy of maximum sustainable yield) as exploited species shift their distribution and marine primary productivity changes under scenarios of climate and ocean chemistry (SRES A1B,) projected by the NOAA GFDL coupled climate model CM2.1. Cheung et al. (2010) suggest that climate change may lead to large-scale redistribution of global maximum catch potential, with an average of 30–70% increase in yield of high-latitude regions (>50o N in the northern hemisphere), but a drop of up to 40% in the tropics. Northern European countries such as the UK and Ireland are projected to gain slightly in maximum potential catch (but not as much as countries such as Norway and Iceland). A small gain (<5%) is projected in the North Sea if potential effects of ocean acidification are ignored (Figure 3).
Figure 3. Projected changes in maximum catch potential by 2050 relative to 2005 (10-year average) under the SRES A1B scenario with assumption of no sensitivity (black bar) and high sensitivity (open bar) to ocean acidification. Projections were produced from the dynamic bioclimatic envelope model with physical and biogeochemical outputs from the NOAA's GFDL Earth System Model (TOPAZ) (redrawn from Cheung et al., 2011a).
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Subsequently, Cheung et al. (2011a) explicitly accounted for the potential impacts of ocean acidification, low oxygen and changes in size-structure of phytoplankton into the DBEM to predict maximum catch potential in 2050. The results suggest that these additional factors may cause a reduction in the maximum catch potential by up to 30% in the North Sea and other European seas. Any scenario that would result in a 10–25% loss of shellfish landings as a result of ocean acidification will have dramatic impacts on the UK fishing industry in the order of £26–66 million per year and around 1000–3100 potential job losses. In contrast, using a size-structured food web model (Blanchard et al., 2009) to investigate how future temperature and primary production could modulate future fisheries potential in the UK, a 24% increase in potential catch was predicted (Blanchard et al., 2010). Possible reasons for the differences in the projected changes from DBEM (Cheung et al., 2011a) and Blanchard et al. (2010) can be because of the consideration of the assumed negative effects of ocean acidification (admittedly still uncertain) on maximum potential catch by the DBEM that is not included in the Blanchard et al. (2010) analysis. Moreover, the biogeochemical models that were used by the two studies are substantially different. A detailed numerical comparison could be carried out to gain a better understanding of the differences in the projected catch changes from the two models. Overall, these modelling approaches have their own sets of assumptions and uncertainties, including those arising from the biological and ecological components in the impact assessment model and the physical and chemical components of the Earth System Models (Stock et al., 2010). Given these model assumptions and uncertainties, interpretation of the model projections should focus on the general trend instead of the detailed numerical outputs.
Climate change is expected to result in reduction in the potential catch of species with northern distributions, such as cod and plaice because of a shift in distribution and changes in stock productivity. On the other hand, some evidence suggests that warm-water species are moving into UK and Irish seas and offer new fishing opportunities, although detailed scientific analysis is lacking. Notable examples include new and/or expanding fisheries for sea bass, red mullet, John dory (Zeus faber), anchovy (Engraulis encrasicolus) and squid. Biomass estimates for sea bass in the eastern Channel have quadrupled from around 500 t in 1985, to in excess of 2100 t in 2004/2005, with populations also increasing rapidly in the western Channel, North and Irish Seas (Pawson et al., 2007). This has resulted in a dramatic expansion of sea bass fisheries (rising to 2500 vessels in 2004), both within the commercial fisheries sector, but also in the recreational fishing sector, for which sea bass is a key target species. Sea bass are caught by angling on the Scottish east coast, but the northern limit of the commercial bass fishery is around Yorkshire, where trawl-caught fish are increasingly being landed into Scarborough and Whitby. In 2009, 7000 t of sea bass were landed in the UK, compared with 460 t in the mid-1980s. Red mullet is a non-quota species of moderate, but increasing, importance to UK fisheries. From 1990 onwards, international landings from the English Channel in the UK increased strongly, and so have the landings from the North Sea (Engelhard et al., 2010). France is the main country benefiting from this species (contributing around 70% of the landings in France in 2009). However, UK commercial catches have also increased significantly, from only 26 t in 1980 to 392 t in 2009 (Engelhard et al., 2010). Beare et al. (2004) demonstrated that red mullet is one of many species that have become prevalent in North Sea bottom trawl surveys in recent years, rising from near-absence during surveys between 1925 and 1990, to around 0.1–4 fish per hour of trawling between 1994 and 2004. Commercial catches of anchovy increased dramatically around UK coasts in 2007 rising to around 939 t, with the result that several pelagic fishing boats switched to actively targeting this species for the first time. The apparent increase coincided with a gradual spread of anchovy (as indicated by research surveys) northward into the western Channel, southern North Sea and Irish Sea over the past decade and observations of large populations of juveniles in the Thames estuary and along the Dutch coast. A recent ICES report (ICES, 2008) confirmed that the species is now widely distributed over almost 80% of the North Sea, even though only occasional records of anchovy had been made off Britain and in the Skagerrak in the period between 1977 and 1989. Squid numbers are highly uncertain around UK coasts, but there are strong indications that cephalopods (squid, octopus, cuttlefish) generally are also becoming more abundant, possibly in response to a change in environmental conditions (Hastie et al., 2009a). Cephalopod populations are suggested to be responsive to climate change (Hastie et al., 2009a); growth in squid availability in the North Sea is generating considerable interest among policymakers and marine ecologists and has led to the establishment of a new fishery off the Aberdeen coast (Hastie et al., 2009b). Off north-east Scotland, where most of the squid is found, more boats are now trawling for squid than the region's traditional target species, such as haddock and cod (Hastie et al., 2009b).
In Ireland, new fisheries have recently opened up for boarfish Capros aper, a small, previously unimportant species that is converted to fish meal for aquaculture. Landings have grown rapidly from less than 120 t in 2001, to more than 139 000 t in 2010 (ICES, 2011). Both Irish and Danish fishermen have invested in new technologies to successfully catch and land this species, and in addition, Irish fishermen also invested in scientific research to increase knowledge of the biology and dynamics of this resource (White et al., 2011). This fishery is now worth more than €4 million year-1, yet the fishery only became commercially viable following a sudden expansion of the stock in the early 1990s. Boarfish became increasingly prevalent in French and UK survey catches after 1991 (Pinnegar et al., 2002), and this phenomenon has been reported as occurring simultaneously elsewhere in the North Atlantic including the Bay of Biscay (Farina et al., 1997; Blanchard and Vandermeirsch, 2005), the Gulf of Lion (inside the Mediterranean; Abad and Giráldez, 1990) and on offshore seamounts (Fock et al., 2002). In the past boarfish outbreaks had been linked to storms and variability in offshore climate (Cooper, 1952). Their appearance after 1991 across the whole North Atlantic basin may be linked to a series of strong positive anomalies in the North Atlantic Oscillation (NAO) since positive phases of the NAO tend to be associated with above-average temperatures across northern Europe as well as strong westerly winds.
The impact of changes in resource availability on fisheries and fishing fleets will vary around the UK and Ireland depending on the make-up of the fleet in each fishing port, the species being targeted and distance/proximity to resources in the future. For example, the UK pelagic fleet (mainly targeting herring and mackerel) primarily operates out of Peterhead and Fraserburgh in NE Scotland, in Lerwick (Shetland Islands), as well as Ardglass in Northern Ireland. The fleet may benefit from the increases in abundance of sardine and anchovy in the North Sea that are associated with warming, although it may suffer from a projected decline in mackerel catch potential (Pierre et al., 2012). It should be noted that pelagic fisheries are known to respond dramatically to environmental fluctuations (Barange et al., 2009), leading to temporal closures during periods of low abundance (Arnason, 2006; Barange and Perry, 2009). By contrast, UK beam-trawlers are primarily concentrated in south-west England (Brixham, Plymouth and Newlyn), targeting sole, plaice and cuttlefish while the demersal ‘otter-trawl’ fleet which targets species such as haddock, cod and whiting, is more widely distributed but landings are highest in Peterhead, Aberdeen and Hull. The fishing ports that have probably benefited most from species migrating into UK waters are those in the English Channel, where sea bass, anchovy, red mullet, and John dory have expanded the most. However, the ‘western approaches’, English Channel and southern North Sea are also the regions that have witnessed the greatest declines of ‘traditional’ (often cold-water) target species such as cod, and where future prospects for such species look the most bleak (Drinkwater, 2005). In the UK, pots and trap fisheries comprise 1672 vessels and land high quality shellfish mainly for export to EU countries (12% of total income from European lobster, edible crab and whelk) (Anderson and Guillen, 2009). In Ireland, it is thought that a similar number of inshore vessels use these types of fishing gear although the exact numbers are not known. The valuable crustacean species targeted by this fishing sector are potentially most vulnerable to ocean acidification (although the sensitivity is somewhat unclear), consequently the long-term sustainability of this sector remains highly uncertain.