Hotter seas, fewer fish: what El Niño means for us

ILOILO CITY — The combination of El Niño and rising ocean temperatures could produce longer-lasting and more intense marine heatwaves, threatening fisheries and food security in the Philippines, an oceanographer warned.

Charina Lyn Amedo-Repollo, an assistant professor at the University of the Philippines Marine Science Institute, said the more intense and prolonged warming of the ocean has a significant ripple effect on marine biodiversity.

“The greatest impact of El Niño and the increase in ocean heat is on fisheries because the concentration of phytoplankton decreases due to ocean warming. Phytoplankton are the base of the marine food web. If there is no food, there are no fish, so it affects our fisheries,” Amedo-Repollo said.

She said organisms that cannot move away from warmer waters are particularly vulnerable to marine heatwaves, including coral reefs, oysters, and green mussels.

For organisms that can move, she said warming waters could force them to relocate to colder areas, potentially changing biodiversity and fish catch in particular areas.

“The fish that are more common in an area may need to move to colder areas because of the increasing heat. So the catch of fishermen changes because of the increasing warming, resulting in a shift in biodiversity,” she said.

Amedo-Repollo said a prolonged increase in marine heatwaves could affect not only fisheries but also aquaculture, tourism, cultural and supporting ecosystem services, and food security.

‘A new normal’

Amedo-Repollo said the country needs to adjust its preparations because ocean temperatures are no longer following the conditions of the past.

Institute data showed the longest-lasting and most intense marine heatwave was recorded in 2024, followed by 2020, 1998, 2021, and 2023.

The 1998 data coincides with the 1997-98 El Niño, one of the strongest on record.

“You can see that these years are our recent years. Whatever preparations we had before, we have to adapt because our normal is different now. The situation is different, and so are the implications for humans,” she said.

She noted that the highest sea-surface temperature anomaly in the Philippines occurred in September 2020 at 0.97 degrees Celsius, followed by 0.95 degrees Celsius in August 1998, during a strong El Niño.

Amedo-Repollo said the 2020 record was particularly alarming because it occurred during a La Niña period, when Pacific waters are typically cooler than average.

“That means the warming of the ocean is not only affected by El Niño but also by La Niña,” she said.

She noted that 2020 was also the year Typhoons Rolly and Ulysses devastated the country in November.

“The ocean is really warming up, and that is the new normal now. On top of that, we have El Niño, so it becomes stronger because of this background temperature and becomes more destructive if we are not prepared,” she said.

Marine heatwaves persist

A marine heatwave occurs when sea-surface temperatures in a specific region exceed a local historical threshold, typically the 90th percentile, for at least five consecutive days.

El Niño is a climate pattern marked by warmer-than-average sea-surface temperatures in the central and eastern tropical Pacific.

Amedo-Repollo said institute data showed the Philippines experienced two major peaks in marine heatwaves during the 2023-24 period.

The first peak occurred from November 2023 to January 2024.

The highest peak was recorded from April to August 2024, after the El Niño phase had already ended.

High marine heatwave conditions persisted until December 2024.

The areas most affected were in the western Philippines, including Iloilo and the rest of Panay Island.

She said the extreme warming toward the end of the El Niño phase suggests marine heat can persist longer than expected because of the combined effects of El Niño, monsoon dynamics, and long-term climate change.

“Water has the highest heat capacity, and once it absorbs heat, there is a way for the ocean to release that heat, and that is El Niño,” Amedo-Repollo said.

For Iloilo, her data showed an average marine heatwave temperature anomaly of 0.6316 degrees Celsius from 2021 to 2026.

The highest recorded value was 1.7860 degrees Celsius in January 2024.

“With the El Niño right now, we would expect that by January or February, the ocean temperature in Iloilo would peak at its highest,” she said.

The institute has already developed a local marine heatwave tracker, although Amedo-Repollo said it is not yet publicly available.

Lack of data

Amedo-Repollo said local governments need longer and more consistent datasets to determine how marine heatwaves are affecting their areas and to develop appropriate adaptation measures.

“The takeaway for marine heatwaves, for you to be able to prepare for the warming of the ocean, is that it should be site-specific because one place cannot have the same response as another in terms of how to adapt and become resilient to ocean warming,” she said.

She encouraged local government units to establish and sustain monitoring systems because the lack of local data remains a major problem.

“It is also important for LGUs to see the changes and impacts. You need longer datasets or observations. I really encourage LGUs because the primary problem that we see is the lack of data,” she said.

“How can we identify the problem if we do not have data to go back to? If you have historical data, you know how to move forward. I think it also requires political will from the government to make the effort and provide funding to sustain the monitoring system of cities or LGUs,” she added.

She also pointed to the lack of studies documenting marine organism mortality caused by extreme marine heatwaves in the Philippines.

Other countries have documented such mortality since 2023, she said.

She said more research and mapping are needed to identify areas and ecosystems most vulnerable to marine heatwaves.

“Other countries have already mapped that. We have to ramp up our research to provide more information because this will also provide a roadmap for our government on where to put the effort and where to put the funding,” she said.

 

Reducing local stressors

While El Niño and marine heatwaves cannot be prevented, Amedo-Repollo said governments can reduce local pressures that make marine ecosystems more vulnerable to extreme warming.

She recommended measures such as establishing closed fishing seasons, strengthening marine protected areas, and properly managing commercial and municipal fishing.

She said pollution from rivers and human waste should also be addressed.

She said reclamation and dredging activities should be carefully managed because they can add stress to marine environments.

She also called for investment in natural carbon sinks, including mangroves and seagrass ecosystems, and the identification of resilient marine organisms that should be prioritized for conservation and proper management.

“We cannot prevent El Niño or marine heatwave (MHW), but we can prevent climate hazards from becoming disasters by anticipating them, reducing local stressors, and acting before thresholds are crossed,” she said.