Climate change and fisheries

• 1. Introduction

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  Climate change and fisheries are locked in a relationship that touches the food supply of billions of people. A projection drawn from modelling suggests that global fish community biomass could fall by as much as 30 percent by the year 2100. That single number sits at the centre of a story involving ocean chemistry, shifting currents, displaced fishing grounds, and communities in places like Bangladesh, rural Alaska, and the Pacific islands whose entire way of life depends on what the water yields.

  The questions worth sitting with are these: which parts of the ocean are hit hardest, and why? What happens to the hundreds of millions of people whose protein comes almost entirely from fish? And is there any path toward resilience, or are the changes already moving faster than any fish, or any fishing village, can follow?

• 2. What Warming Oceans Do

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  Rising ocean temperatures and ocean acidification are direct products of higher concentrations of greenhouse gases in the atmosphere. The chemistry shift makes calcification harder for marine organisms: shrimp, oysters, corals, and the zooplankton that form the base of the marine food chain all build calcium shells, and the process becomes more difficult as acidity rises. Scientists describe the result as "cracks in the food chain."

  Copepods offer a precise illustration of how the disruption cascades upward. Cool-water copepod assemblages have shifted northward as surface waters warm. The warm-water copepods that replaced them carry a lower biomass and run to smaller individual sizes. Atlantic cod depend on large copepods for feeding, and as those prey animals have moved toward the poles, cod recruitment has plummeted and mortality rates have climbed.

  Larger lakes face a parallel problem. As surface temperatures rise, large predatory fish that require cool water may disappear from smaller lakes entirely. Their absence can trigger blooms of nuisance algae that degrade water quality and create potential health hazards for the communities drawing water from those lakes.

• 3. Catch Potential and Geography

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  Fish catch across the global ocean is projected to decline by 6 percent by 2100, with tropical zones bearing a steeper loss of around 11 percent. By 2050, diverse models suggest total global catch potential could shift by less than 10 percent depending on the trajectory of greenhouse gas emissions, but the geographical spread within that average is extreme.

  The South Pacific regions face the biggest reductions in maximum catch potential. Skipjack tuna and bigeye tuna populations are expected to move further east as ocean temperatures and currents shift, relocating fishing grounds toward the Pacific islands and away from the primary fishing territory of Melanesia. That shift threatens western Pacific canneries and could redistribute tuna production in ways that leave food security in the region uncertain.

  Decreases in both marine and terrestrial production are predicted for almost 85 percent of coastal countries examined, with national capacity to adapt varying widely. Species already under pressure from over-fishing, such as variants of Atlantic cod, carry heightened vulnerability because over-fished populations have reduced size, genetic diversity, and age structure, leaving less biological buffer against environmental stress. Atlantic cod in the Baltic Sea, already operating close to their upper thermal limits, are particularly exposed.

• 4. Communities at the Edge

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  Fish provides essential nutrition for 3 billion people globally. For at least 400 million people from the poorest countries, it supplies at least 50 percent of their animal protein and minerals. Over 500 million people in developing countries depend directly or indirectly on fisheries and aquaculture for their livelihoods.

  In Bangladesh, Cambodia, Gambia, Ghana, Sierra Leone, and Sri Lanka, fish accounts for more than 50 percent of protein intake. Fishing communities in Bangladesh face not only sea-level rise but also increased flooding and more frequent typhoons. Communities along the Mekong river produce over 1 million tons of basa fish annually; saltwater intrusion from rising sea levels, combined with the effects of dams, threatens both livelihoods and that production base.

  In rural Alaska, residents of Noatak and Selawik struggle with unpredictable weather, shifts in fish abundance and movement, and changes in boat access. Low-lying nations such as the Maldives and Tuvalu face an existential version of this pressure; entire communities there may become among the first climate refugees. Worldwide food security as an aggregate may not shift dramatically, but rural and poor populations face disproportionate harm because they lack the resources to rebuild infrastructure quickly.

• 5. Fishing's Own Emissions

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  About 0.5 percent of total global greenhouse gas emissions in 2012 came from fishing vessels, including inland vessels, representing 172.3 million tonnes. Aquaculture's footprint is larger: an estimated 385 million tonnes of equivalent were emitted in 2010, which equals roughly 7 percent of all emissions from agriculture.

  Those numbers make the fishing sector a small but non-trivial contributor, and they also point toward a lever. Removing fuel subsidies for fishing fleets could carry a double benefit: cutting both emissions and the economic incentive for over-fishing. The World Development Report 2010 calculated that reducing overcapacity in fishing fleets and rebuilding fish stocks could increase economic returns from marine capture fisheries by US$50 billion per year while simultaneously lowering greenhouse gas output from those fleets.

• 6. Over-fishing as an Accelerant

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  Over-fishing does not simply run parallel to climate change; it amplifies it. A population thinned by over-fishing has less size, less genetic diversity, and a younger age structure than a healthy one. Each of those deficits reduces the population's ability to absorb environmental stress, including temperature shifts and oxygen depletion.

  Studies show that in areas where fisheries have not yet collapsed, unsustainable levels of fishing are already having a significant impact on the industry. Destructive fishing practices reduce biodiversity, and lower biodiversity in turn reduces ocean resilience to the stresses that climate change imposes. Minimising over-fishing is therefore both a fisheries management goal and a climate adaptation strategy.

• 7. Adaptation Pathways

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  A 2025 systematic review of small-scale marine fisheries found that 67.7 percent of documented responses to climate- and resource-related shocks were short-term coping strategies; only 32.3 percent were longer-term adaptive strategies. The temperature change and the accompanying drop in dissolved oxygen are expected to occur too quickly for many affected species to adapt biologically. Fish can migrate to cooler water, but suitable spawning sites are not always available at higher latitudes or greater depths.

  The World Bank and the Food and Agriculture Organization run programmes to build policy resilience through risk assessments, early warning systems, and institutional strengthening. Algal biofuels represent one economic diversification option for coastal communities: marine algae can produce 15-300 times more oil per acre than conventional crops such as rapeseed, soybeans, or jatropha, and they do not compete for scarce freshwater.

  Restoring mangrove forests captures several benefits at once: shoreline protection, breeding grounds for fish, and carbon sequestration. Six Pacific countries have formally committed to protecting the reefs of the Coral Triangle, a biodiversity hotspot, through GEF-funded research programmes. Investment in sustainable aquaculture can buffer water demands in agriculture while also diversifying economic activity for communities whose fishing livelihoods are contracting.