Warming Seas Are Rewiring the Base of the Seafood Chain, Major Climate Review Warns
A major new climate science assessment finds Irish and UK waters are seeing a fundamental shift in plankton communities, with smaller, less nutritious species replacing the large copepods and diatoms that have historically fed herring, mackerel, sandeel, Nephrops and whitefish stocks.
A comprehensive new assessment of climate change impacts on plankton and pelagic habitats around the UK and Ireland has found that warming, stratification and shifting ocean currents are already restructuring the tiny organisms that underpin every fish stock fishermen depend on, with consequences for herring, mackerel, sandeel and whitefish recruitment likely to deepen in the decades ahead.
The report, published by the Marine Climate Change Impacts Partnership (MCCIP) and led by researchers at the University of Plymouth, Plymouth Marine Laboratory, Cefas and colleagues across the UK marine science community, synthesises more than one hundred studies published since 2019. It updates a 2020 assessment and offers the most detailed picture yet of how plankton communities across the North Sea, Celtic Seas, Irish Sea, English Channel and North-East Atlantic are responding to a warming ocean.
At the heart of the findings is a shift in size. Long-term monitoring, including the Continuous Plankton Recorder Survey that has sampled UK and Irish waters since the 1930s, shows large phytoplankton and zooplankton declining in oceanic waters beyond the continental shelf, while smaller plankton and microbial species increasingly dominate warmer, more stratified shelf seas. That matters because smaller organisms pass energy up the food chain far less efficiently than large diatoms and copepods. Every fish that depends on a healthy plankton base, from herring larvae to sandeel to whiting, ultimately feels the effect.
“Warming has advanced egg hatching for Nephrops norvegicus by around seventeen to nineteen days, with little corresponding change in the pelagic larval stage.”
The Irish Sea features directly in the report’s regional findings. Researchers note that warming and intensifying seasonal stratification have reorganised plankton communities there, with bacterioplankton in warm, nutrient-poor surface waters becoming less diverse and the genus Synechococcus making up an increasing share of the community, particularly in late summer. Separately, a study cited in the review found that warming has advanced egg hatching for Nephrops norvegicus, the langoustine that underpins one of Ireland’s most valuable fisheries, by around seventeen to nineteen days when comparing the 1980s and 1990s with the 2000s and 2010s, with little corresponding change in the duration of the pelagic larval stage. That timing shift carries real implications for recruitment to the Nephrops fishery.
Elsewhere around Irish and UK coasts, the picture is similarly mixed but consistently pointing toward disruption. The spring plankton bloom, the annual pulse of primary production that fish larvae time their hatching around, now occurs up to a month earlier than in the late 1990s in parts of the North Sea. Where fish spawning has not shifted in step, the mismatch risks leaving larvae without the food they need at the critical early-life stage, a dynamic long known to science as the match-mismatch hypothesis. The report links this to reduced sandeel prey availability and knock-on effects for seabird breeding success, and flags similar risk for herring recruitment.
Gelatinous organisms, including jellyfish and salps, are increasing in some regions, most notably the Western Channel and Celtic Sea, where salps have become established off Cornwall and Devon since 2019 and now form dense summer blooms in some years. These filter feeders compete directly with fish larvae for the same food supply and can prey on fish eggs, while contributing little back to the traditional food web that supports commercial stocks.
Looking ahead, the report’s modelling work paints a sobering picture for fisheries productivity. Warming under high-emission scenarios is projected to reduce primary productivity by intensifying stratification and depleting surface nutrients, while increasing the dominance of small phytoplankton adapted to nutrient-poor conditions. One modelling study cited finds that the cold-water copepod Calanus finmarchicus and cod are projected to decline further under strong warming, while hake and mackerel are expected to expand northward to track both thermal niches and reduced zooplankton prey further south. Herring larvae may require up to thirty-five per cent more prey to survive as warming increases their metabolic demands, raising recruitment risk if prey fields continue to decline or shift.
Carbon export is also expected to weaken as the shift toward smaller, slower-sinking plankton reduces the rate at which Irish and UK seas draw carbon out of the atmosphere and into the deep ocean, a service with significance well beyond fisheries management.
Further offshore, in the Rockall Trough and Atlantic North-West Approaches that matter to Ireland’s pelagic fleet, the report notes strong plankton-fish connectivity. Blue whiting recruitment, including a notably strong year class in 2020, has been driven by elevated chlorophyll under stable, stratified conditions with low mixing. A weakening European Slope Current since the late 1990s has altered water masses flowing into these grounds, with knock-on consequences for nutrient supply that remain an area of active research. The report’s authors describe this as a data-poor region generally, with in-situ monitoring limited to a small number of fixed stations, leaving much of the water column that Irish trawlers and pelagic vessels actually fish under-sampled.
The authors are careful to flag where confidence remains limited. Agreement among studies on the future magnitude and regional pattern of change is described as moderate at best, with major gaps in understanding of microbial and gelatinous plankton and in monitoring coverage across western and offshore Scottish waters, where important fisheries are concentrated. The report also highlights a growing pressure that has received little attention until now: the rapid expansion of offshore wind infrastructure, which introduces new hard substrate that boosts filter-feeder grazing on plankton and can alter local stratification and nutrient supply through installation-scale turbulence and mixing.
For an industry already contending with tightening quotas, effort restrictions and the aftershocks of Brexit-era access negotiations, the report’s central message is a stark one. The productivity of the seas fishermen work is being reshaped from the bottom up, and the changes are running well ahead of the policy indicators currently used to track them. The authors call for broader taxonomic monitoring, increased sampling frequency, and better integration of lower trophic levels into the stock assessment models that ultimately set quotas, warning that without this, fisheries management will continue working from an incomplete picture of what is actually happening beneath the waves.
Source: MCCIP Science Review 2026: Impacts of Climate Change on Plankton and Pelagic Habitats around the UK and Ireland (Holland et al., 2026)
