Impacts of climate change on fish

The story so far

The shelf seas surrounding the British Isles have warmed four times faster than the global average over the last 30 years. Recent warm conditions are unlike anything in the last 20,000 years, and warming is highly likely due to human activities linked to the global carbon cycle.

Recent warming has caused some cold-water demersal (bottom-dwelling) species to move northwards and into deeper water (e.g. cod, whiting, monkfish), and has caused some warm-water demersal species to become more common or “invade” new areas (e.g. John dory, red mullet).

Pelagic (blue-water) species are showing distributional shifts, with mackerel now extending into Icelandic and Faroe Island waters (with consequences for management), sardines and anchovies invading Irish and North Sea environments, and anchovies establishing breeding populations in the southern North Sea. Teasing apart the relative influences of the North Atlantic Oscillation, the Atlantic Multidecadal Oscillation and Global Warming as drivers for these changes is an important challenge.

When demersal fish communities are assessed at local scales across the region, 36 of the 50 most common species show a response to warming, with 75% of these increasing in abundance, leading to reorganization of local communities. International commercial landings of species identified as warm-adapted (e.g. grey gurnard, red mullet, hake) have increased 250% in the last 30 years while landings of cold-adapted species (e.g. cod, haddock, whiting) have halved.

In warm years, summer spawning fish (e.g. mackerel, horse mackerel) are spawning earlier and further north on the Porcupine Bank. Conversely, spring spawning fish (dab, whiting, lemon sole) are spawning earlier in southern England following cold winters due to females moving further offshore into less preferred but warmer waters.

Body size distributions of fish, both within species and across ecosystems, have been affected by climate change as well as fishing. Warm, lower-oxygen conditions favour smaller individuals, and by 2050 weights of fish could be reduced by 14-24%.

Declines in salmon are strongly correlated with rising temperatures in oceanic foraging areas, with temperature affecting growth, survival and maturation of salmon at sea. Freshwater temperatures have also increased significantly in the last four decades, with implications for survival of juvenile diadromous fish, including both anadromous (river spawning: salmon, trout, shad) and catadromous (sea spawning: eels and flounder) species. For eels, climatic changes in the spawning areas of the Sargasso Sea are likely impacting reproduction and larval survival.

Future predictions: ocean acidification

One-third of all anthropogenic CO₂ has been absorbed by the oceans, mitigating warming but decreasing the pH. Ocean acidification is occurring faster than any time in the last 300 million years, and is expected to continue through the 21^st Century leading to a drop of 0.3 to 0.4 pH units.

In addition to compromising calcifying animals, from unicellular algae (e.g. coccolithophores) through to large urchins and bivalve molluscs, ocean acidification is now known to affect fish, with impacts on growth, neurological function, physiology, behaviour and cognitive processing.

More work, combining lab studies, long-term multigenerational studies, studies of natural CO₂ hotspots and of temperate water fish, and mechanistic modelling is needed to predict the full impacts of ocean acidification on fish.

Future predictions: developments in modelling

Ocean temperatures are predicted to rise a further 2 to 4 ºC during the 21^st Century. Predicting impacts of warming in fish communities is complex; and consortia are now using multiple-modelling approaches (ranging from statistical to mechanism-based) that are forced using an ensemble of climate scenarios to predict the range of likely outcomes for fish around the British Isles.

Models generally predict poleward (northward) movement of species ranges, leading to substantial losses in availability of traditionally harvested species (e.g. horse mackerel, sole, haddock). Models also predict changes to primary production throughout the British Isles, with southern regions (e.g. Celtic Sea, English Channel) becoming up to 10% more productive, while northern regions (e.g. central and northern North Sea) becoming up to 20% less productive, with obvious implications for the fisheries underpinned by these plankton communities.