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Rising ocean temperatures spell 'real threat' to fish species in Irish waters

There are many warning signs that Earth’s oceans are heating up due to climate change.

WARMER OCEAN TEMPERATURES due to climate change could attract invasive species that threaten native marine life, according to an expert, with knock-on impacts for fishing and biodiversity. 

A heatwave in Irish waters this week came in the wake of a startling report by the Marine Institute last month that identified that Irish sea temperatures are rising, and as the government develops legislation to try to protect vulnerable ocean ecosystems before it is too late.

There are many warning signs that Earth’s oceans are heating up. Global ocean temperatures last month were the warmest of any May in modern records, according to the Copernicus Climate Change Service. 

A recent report found that average sea surface temperatures in the North Atlantic are the hottest on record, with warming rates more than three times the global average occurring in the eastern Mediterranean Sea, the Baltic and Black Seas and the southern Arctic.

In the North Atlantic near Ireland and the UK, a marine heatwave saw water temperatures up to four degrees higher last week than average for this time of year. 

The ocean has a high threshold for heat absorption and can absorb large amounts of excess heat, Fair Seas Communications Officer Jack O’Donovan explained to The Journal. However, rising greenhouse gas emissions and global temperature increases are pushing it to capacity.

“When we think about our warming planet, the ocean has been a huge buffer up until this point, helping us to protect the land by drawing in all of this excess heat into the ocean,” O’Donovan said.

“What’s happening now is because we’re reaching higher temperatures, the ocean’s heat capacity is getting pushed and that’s why we’re seeing these kinds of heatwaves. The ocean has the ability to protect us but only up to a point until we push it too far.”

As temperature patterns change, the profile of marine life that can and can’t survive in Irish waters will also likely shift.

Fish and plants that can currently thrive around Ireland may move northwards in search of cooler temperatures, while species currently found closer to the equator could migrate to Irish latitudes.

“The Atlantic stretches from Antarctica to the Arctic and across the equator, so it covers every temperature band. There are animals living in the ocean now that are adapted to living near the equator and there’s ones that are adapted to living in the Arctic, but across the world, the ocean is getting warmer,” O’Donovan said.

“We may see reductions in species and then we also may see increases in other species that are currently living in waters south of Ireland’s territorial waters.”

‘We could be seeing more species that come and cause disruption’

A major problem is the risk of invasive species that threaten the delicate balance of Ireland’s ocean ecosystems.

“We have a well-balanced ecosystem here that has been in tune with itself for thousands of years, but as the temperatures change, more invasive species might start to arise that could cause big disruption,” O’Donovan said.

“If you get an invasive species that is a danger to a particular, say, crab, and all of a sudden it starts to affect the native crab population, then we have issues from an economic front and the fishing point of view as well,” he said.

Already, an invasive species of seaweed called sargassum has taken root on Irish shores. It was first seen in Ireland in 1995, then again in 2001, and has since spread to almost all coastal areas around the island.

Sargassum grows densely and creates a dark canopy, disrupting native kelp forests.

brown-alga-japanese-wireweed-sargassum-muticum-close-up-underwater-in-the-ocean-eastern-atlantic-spain Alamy Stock Photo Alamy Stock Photo

“A species like that creates a real threat. Our kelp forests are extremely important for storing carbon and they’re nursery grounds for crabs, lobsters, and even fish like cod and herring. If our kelp forests start to become uninhabitable, then you have a massive knock-on effect in fisheries and in food for all our other biodiversity and wildlife,” O’Donovan warned.

“With warmer waters, potentially we could be seeing more species like that coming in and causing disruption.”

How humans interact with the ocean can either help the climate or harm it.


One powerful way that oceans can limit greenhouse gas emissions is by storing carbon on the seafloor in the remnants of marine animals and plants – but human activities can dredge that carbon up and release it into the atmosphere as carbon dioxide, a greenhouse gas.

“Reducing emissions is key to mitigating climate breakdown,” O’Donovan said.

“The ocean stores more carbon than anything else on the planet. The ocean stores so much carbon through biodiversity. Skeletons and shells, as they grow, they’re using carbon dioxide and creating physical carbon. When that sinks to the bottom, it gets locked away in the sea floor and out of the atmosphere.

“But if you comb over that bottom, if you’re fishing with bottom trawlers and if you’re dragging a net across the bottom, you’re stirring up the bottom of the sea and releasing that carbon back into the water column and then back into the atmosphere.”

An Irish Ocean Climate and Ecosystem Status Report released by the Marine Institute last month identified changes in Ireland’s waters.

The average surface-level temperature off Ireland’s north coast has risen by half a degree over the last decade and the ocean off the southwest coast is expected to become warmer and less salty by 2035.

Marine Institute Chief Executive Paul Connolly explained that changes in the ocean affect “seafood, transport and biodiversity”.

“The oceans provide 50% of the oxygen we breathe. They are a critical element of the global climate system in their role to regulate atmospheric processes and for distributing heat, salt and organisms.”

He said the Institute’s research showed that “the impact of climate change is already evident in Irish marine waters with patterns of harmful algal blooms changing”.

Protected areas

Part of Ireland’s approach to defending the sea will be the creation of special Marine Protected Areas (MPAs).

The government published a proposed bill in 2022 outlining its plan to identify areas of the sea in particular need of protection.

“Marine Protected Areas use the best available science to identify key places for biodiversity or potentially storing carbon in the ocean. Then what you do is you work in consultation with the stakeholders involved, so you talk to the local fishing communities, you talk to businesses in the area, you talk to people who have an interest in the ocean in a particular area, and work together to develop a management plan that allows for the best outcomes for biodiversity,” O’Donovan explained.

He said that processes allows biodiversity to recover from overexploitation.

“Marine Protected Areas will help to increase fish stocks. You allow fish to have a safe haven where they can grow larger and when the fish grow larger, they produce more young. It means you’re increasing the fisheries around the coast because it’s a reproductive hotspot.

“We want really good legislation that will allow us to be able to manage and monitor the Marine Protected Areas properly, not that they’re just paper parks. Currently, we have special areas of conservation through EU law, but they’re only protected on paper. The new legislation has to be watertight.”

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