Have been spending a bit of time down at the beach lately and thinking about how our oceans are changing and what we can possibly do about it. Here's a bit of a rundown.
The Oceans have always been the world's biggest carbon sink - absorbing 90% of the CO2 produced on land, but Climate Change is reshaping them faster than many scientists predicted. Marine heatwaves, acidification, shifting currents, and migrating species are already disrupting ecosystems and coastal communities. While mitigation remains essential, countries are also investing heavily in adaptation — practical steps to cope with the changes already underway.
Marine Heat Waves
Marine heatwaves are becoming more frequent, intense, and longer-lasting. One of the more obvious effects is coral bleaching and degradation of coral reefs. To learn more and see what this looks like at scale, click here. Corals can usually recover from a 1°C temperature rise, but at 2°C or with repeated warm spells, they generally do not.
The Impact of Warming on Coral Reefs
Coral reefs aren’t just beautiful to look at — they’re nurseries for marine biodiversity, supporting around 9,000 species, including many of the fish we eat. They protect coastlines from storms, preserve genetic diversity, and generate millions in tourism revenue.
This makes it especially tragic when places like Australia's Great Barrier Reef (GBR), once described as the 8th Wonder of the World, become degraded and die. Since the 1980s, over half the GBR has been lost, with some areas suffering even greater declines. While pollution, crown-of-thorns starfish outbreaks, and cyclones have also caused damage, global warming is now widely acknowledged as the primary cause of reef decline.
Many of the world’s 153 reefs are similarly affected with gorgonians –a type of branching coral in the Ligurian Sea off Italy's North West Coast, experiencing almost 100% mortality during 4 to 6 o C above average warming periods in 1999 and 2022. France has also observed the same phenomena on its Mediterranean Coast.
Adaptation Strategies for Coral Reefs
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Assisted evolution & selective breeding: Developing heat-tolerant corals.
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Coral nurseries: Growing fragments in controlled conditions before replanting.
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Cloud brightening trials (Australia): Spraying seawater into the air near corals to create brighter, cooling clouds. Experimental but promising.
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Cool water injection: Pumping cold water from depths onto corals to cool them. Energy-intensive and used only for small, high-value sites
Mapping and monitoring: Early-warning systems using satellites predict bleaching events. Australia’s Scientific Research Organisation (CSIRO) offers a 3-month ocean heat stress outlook. It also predicts fish kills and algal blooms which we will be discussing next.
Europe’s Horizon Virtual Marine Platform has digitally mapped oceans to predict the effects of events such as oil spills, the best location for the release of sea turtle hatchlings or the impact on fish stock which wind farms or other interventions will have.
Algal Blooms and Fish Kills
Another of the more obvious manifestations of Climate Change and warming ocean temperatures are algal blooms and fish kills, such as the one which is still ongoing in South Australia. Since it began in March 2025, citizen scientists have variously counted between 57,000 - 90,000 dead marine creatures including whales, dolphins, sharks, sea lions and sting rays, with huge impacts on fishing, aquaculture and tourism over an area stretching from Spencer Gulf to Gulf St. Vincent. Marine scientists regard this as a major climate‑driven crisis rather than an occasional nuisance, and are responding with a three‑pronged strategy that includes immediate emergency measures, medium‑term mitigation efforts, and long‑term adaptation initiatives.
Emergency Response Measures
- Information and response hubs: South Australia established a public website and hotline for real-time updates, coordinating state and federal agencies.
- Fishing restrictions: Temporary bans on certain species began November 2025 to allow recovery.
- Monitoring and mapping: Continuous tracking of bloom extent, water temperature, oxygen levels, and species deaths. The bloom covers 4,400 km² and reaches 30 meters deep.
- Rapid necropsies and toxin testing: Scientists test dead animals to identify causes such as toxins, heat stress, or oxygen depletion.
- Reducing nutrient runoff: Excess nutrients from agriculture and wastewater fuel blooms. States are tightening fertilizer controls, upgrading wastewater treatment, and restoring wetlands that filter runoff naturally.
- Improving coastal water circulation: Calm, warm, stagnant water encourages blooms. Some regions are trialling engineering solutions like artificial mixing and aeration in bays and estuaries.
- Protecting Seagrass and Kelp Forests
Healthy marine vegetation like seagrass and kelp absorbs excess nutrients and stabilises ecosystems. Restoration projects are underway in several Australian states, helping buffer against algal blooms and support marine life.
- Better Early-Warning Systems
Satellite monitoring, ocean buoys, and predictive models are expanding, enabling authorities to act before blooms peak and reduce their impacts.
Long-Term Climate Adaptation
This is the toughest — and most crucial part. Scientific consensus points to marine heatwaves as the main driver of extreme bloom events. The South Australian bloom was linked to water temperatures 2.5°C above normal during a prolonged heatwave.
- Climate mitigation policies remain essential to reduce greenhouse gas emissions and slow warming.
- Resilient fisheries management involves adjusting quotas, closing fisheries when needed, and supporting aquaculture operators hit by mass mortality events.
- Research into bloom-forming species helps predict future events and plan responses better.
- Community and industry adaptation supports tourism, fishing, and aquaculture sectors facing more frequent marine mortality events.
Australia is taking action, but not fast enough to outrun the problem. It is also not the only country facing this dilemma.
A Global Problem
The USA - In the Pacific North West there is a heat anomaly which can run from Kamchatka in Russia all the way along the coast of the USA from Alaska to California. Known as "The Blob" it has caused massive fish kills and algal blooms. After first appearing in late 2013–2014 it has since re‑emerged multiple times, including a major return in August 2025 when sea-surface temperatures rose to 20°C (68°F) -the warmest on record.
With the 2015 event shutting down crab and shellfish fisheries and causing widespread marine mammal strandings, marine biologists are tracking kelp loss, fish distribution changes, and stressed marine mammals, all linked to prolonged warm-water exposure.
How Warming Contributes to Fish Kills
It should be noted that warming may not be the only contributor to to algal blooms and fish kills. Eutrophication – lack of oxygen which encourages blooms, can also be due to too much pollution or nutrient run -off from farms or sewerage works, and algal blooms themselves use up available oxygen in the water in their decay. The effects can last long after temperatures return to normal and have a widespread impact right across the marine ecosystem.
Impacts recorded following previous Blob events include:
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Zooplankton decline reducing food for salmon, seabirds, and whales.
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Mass die-offs of seabirds and sea lions have also been documented during past Blob years.
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Shifts in species ranges, with warm-water species moving northward.
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Kelp forest decline, especially in British Columbia and Alaska, due to heat stress and increased grazing by warm-water-tolerant species.
Acidification
Another cause of fishkills and especially shellfish, crustaceans and corals, is because the ocean is becoming more acidic as it absorbs excess carbon dioxide from the atmosphere, forming carbonic acid. This reduces the carbonate minerals which oysters, mussels, crabs and other shell builders need to grow their shells. It also makes them more vulnerable to predators and diseases. Some examples follow.
- United States (Pacific Northwest)
Oyster hatcheries nearly collapsed when ocean acidification prevented larvae from forming shells — one of the earliest and clearest signs of climate‑driven marine disruption.
- On North Atlantic Coasts including Scandinavia, the UK, Canada, and the northeastern USA, acidification and warming have affected shell formation and growth rates of the Blue Mussel (Mytilus edulis) which supports a significant aquaculture industry.
- USA (Atlantic and Pacific Coasts), Canada, Japan, China: Ocean warming and acidification are affecting larval survival and the shell growth of scallops. Disease outbreaks are also a concern.
Adaptation Strategies for Acidification
Researchers and Aquaculture operators are experimenting with several strategies. For example, breeding acid-tolerant oyster and mussel strains has shown promise in the U.S. Pacific Northwest and France. Trials using crushed minerals to enhance alkalinity are underway and networks monitoring pH changes provide aquaculture operators with early warnings of acidic upwelling events, to help them take timely action. You can read more here.
Global Warming and Changing Ocean Currents
Less visible but no less insidious is the fact that global warming also alters Ocean Currents. As the atmosphere warms, the ocean absorbs about 90% of the excess heat trapped by greenhouse gases. This warms the surface layer, making it lighter and less likely to sink. This matters because the global “conveyor belt” of currents relies on cold, dense water sinking in the polar regions to pull the whole system along. When surface water is too warm, that sinking slows.
Meanwhile, melting glaciers and ice sheets add huge volumes of freshwater to the ocean. Freshwater is less dense than salty seawater, so it too forms a lighter layer on top. This further prevents the sinking motion needed to maintain ocean circulation.
Studies show that the Atlantic Meridional Overturning Circulation (AMOC) - the current which brings warm nutrient - rich water up from the Caribbean to the North Atlantic, may have slowed by around 15% since the 1950s, and is now at its weakest in over a thousand years.
Meanwhile currents like the East Australian Current are strengthening and moving further South. Indeed, Southern waters appear to be experiencing the fastest warming of all. The Agulhas Current which runs along the South East coast of Africa is also weakening. Their upwellings which bring nutrients from the deep ocean are thus also changing.
Changing currents cause most mobile marine species to shift their ranges poleward, usually much faster than those which live on land. Such changes disrupt larval dispersal, nutrient distribution, migration routes, and ecosystem structure. They also bring new diseases and predators.
It has been estimated that fisheries in some regions may decline by as much as 20–30% by mid-century, with enormous consequences for the fishing industry, aquaculture, coastal communities and economies, as well as the food security of some 2 -3 billion people who depend on seafood for their protein.
Impact on Aquaculture and Fisheries
Shellfish
Globally, Shrimp farming is one of the fastest growing sectors in the aquaculture industry, but it and wild fisheries are facing disease outbreaks linked to warming waters and habitat degradation from coastal development.
Pacific Oysters are among the most widely farmed shellfish globally, critical to coastal economies. As ocean temperatures have warmed there have been catastrophic losses in the industry due to Pacific Oyster Mortality Disease (POMS).
First detected in France in around 2008. it reached Australia’s East Coast in 2010. In 2013 it killed 10 million oysters in 3 days. By 2016, it was found to be the cause of mass oyster die - offs in Southern Tasmania. Fortunately, the disease is not harmful to humans, but nevertheless represents a huge economic loss to oyster farmers and the economy. Similar die -offs during extreme temperatures have also occurred in other countries including New Zealand, the Pacific Northwest (USA, Canada), France, Japan, and China.
Mussels too are feeling the heat. Within 2 months of a prolonged Marine Heatwave in 2022, Mussel beds completely disappeared from parts of the Mediterranean around Italy and Spain and along the Adriatic Coast. Both the wild fishery and mussel farms were affected. A long term study published in France at the end of last year, showed mortality rates in mussels to be around 40% in temperatures above 25°C and that Pacific Oysters were also affected. They not only lost condition, after heatwaves but the effects of heat stress were likely to persist into the next generation.
Other Commercially Valuable Fish
Chile, the world's second biggest producer of farmed Atlantic Salmon, has also experienced repeated mass mortality events linked to rising sea temperatures. In late 2023, Sernapesca reported the removal of 1,500 tonnes of dead salmon from the Reloncavà Estuary after a harmful algal bloom. Industry data from 2024 shows mortality rates rising across all major farmed species including Coho Salmon and Rainbow Trout.
Pacific Salmon, vital for commercial, recreational, and indigenous
fisheries in the USA, Canada, Russia, and Japan, face disrupted spawning
and migration patterns from rising river and ocean temperatures.
Acidification also impacts their prey, and they too are experiencing
significant warming-related mortality.
NOAA reports increased pre-spawn deaths, migration failures, and thermal stress in North American populations, while recent peer-reviewed studies show similar declines in Japanese Chum Salmon as marine heatwaves intensify. Japan has the largest commercial fishery of this species.
Other affected species include the Atlantic Cod populations in North America and Europe. These have declined due to overfishing and warming waters that push their ranges northward.
Tuna - all species—including skipjack, yellowfin, bigeye, albacore and bluefin—are shifting poleward in the Atlantic, Pacific and Indian Oceans, driven by reduced prey availability, altered migration routes and declining spawning success.
Adaptations to a Warming Climate and Shifting Ocean Currents
These include dynamic fisheries management which adjusts quotas and zones as species migrate, high-resolution ocean modelling, and climate-ready marine spatial planning.
To make shellfish more resilient, the aquaculture industry is adopting more sustainable practices such as not moving spat between locations of higher risk and lower risk, disinfecting equipment and placing shellfish such as oysters higher in the water column. More generally there are efforts to reduce environmental impacts of farm run -off and conducting restoration projects for wild habitats such as oyster reefs and mangrove swamps, as well as seagrass and kelp beds to improve ecosystem health. Some of the more innovative approaches include the following
Gene Editing and Genomic Tools
- Environmental DNA (eDNA) Monitoring
Managing Species Migration, Invasives & Marine Diseases
With warm‑water species moving south or north, invasive species are expanding and marine diseases are appearing in new regions. New diseases too, leaving researchers scrambling to catch up.
Adaptation strategies
- Targeted culling (lionfish in the Caribbean, crown‑of‑thorns starfish on the GBR).
- Stricter biosecurity for ballast water and port inspections.
- AI‑based disease monitoring in aquaculture (Norway, Chile, Australia).
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