Scripps Oceanography experts weigh in on how ocean life is impacted by the climate event
By Lauren Fimbres Wood, Scripps Institution of Oceanography
El Niño is a climate pattern that is declared when monthly sea surface temperatures in the central and eastern tropical Pacific Ocean are measured above normal, for at least five consecutive overlapping three-month periods.
This ocean warming has impacts on global temperature, weather patterns, and the life that lives in the ocean. In fact, the name El Niño, Spanish for the boy, was coined in the 1600s by Peruvian fisherman, who recognized the phenomenon when changing ocean currents and warmer waters caused a drop in their catch.
Additionally, this El Niño comes at a time when the West Coast has already been experiencing an extended marine heat wave that has resulted in sea bird deaths along the coast.
Below experts from Scripps Institution of Oceanography at the University of California San Diego explain how marine ecosystems are impacted by El Niño and how they track these impacts.
For additional information on El Niño, visit the El Niño Resource Hub.
How does El Niño impact marine ecosystems?
Mark Ohman (Distinguished Professor of the Graduate Division, Biological Oceanography): There are different effects for different organisms, but El Niño often reduces coastal upwelling in the eastern Pacific, which is an important source of nutrients for the plankton at the base of the food web. So, the broadest impact is that overall biological productivity in the eastern Pacific tends to decrease and contract closer to shore.
Deeper waters that are upwelled toward the ocean surface also carry low oxygen and more acidic water. So relaxation of upwelling leads dissolved oxygen to increase and ocean pH (a measure of acid-base balance) to increase slightly during El Niño.
At the same time, the warmer waters off the coast of North America can also lead to an influx of subtropical and tropical species of plankton and fish. In San Diego, we sometimes see major influxes of swimming red crabs (Grimothea planipes) that normally breed off the coast of Baja. When I first came to Scripps, a marine technician told me about a guano index for El Niño. If the guano stains from the seabirds on the Scripps Pier turned from white to pink to red, this indicated there was a strong El Niño, because the birds were feasting on the red crabs that had come north from Baja waters. (However, not every El Niño is accompanied by red crabs.)
In general, the zooplankton — the free-drifting animals of the sea — change dramatically during El Niño. For example, temperate latitude varieties of krill decrease in abundance, while subtropical krill species increase. Since some predators (fish, mammals, seabirds) depend on particular prey species for their feeding and breeding success, this change in zooplankton species affects their consumers.
Seabirds and marine mammals may also alter the timing of their migrations if their major food sources diminish because of reduced upwelling. Blue whales, for instance, usually migrate into Southern California waters between May and September and could delay their arrival if reduced upwelling means less of the right kind of krill are available.
Colleen Petrik (Assistant Professor of Biological Oceanography): Warmer waters in the eastern Pacific can allow for big open ocean fish like tuna to expand their range closer to the California coast. This range expansion also occurs vertically in the water column. Normally, deeper waters that are high in nutrients can be low in oxygen. But when upwelling is reduced by El Niño, oxygen levels in the middle depths can increase, which allows species like tuna (that need a lot of oxygen) to move into those depths and find more food. Tuna often do quite well during El Niño years because they can expand their range horizontally into the eastern Pacific and vertically into these deeper waters that normally don’t have enough oxygen.
However, the reduced upwelling noted above often has negative effects on many marine species, especially predators that feed on the large plankton fueled by upwelled nutrients or the fish that eat those plankton. In addition to the reduced energy to sustain food webs, the upwelling that does occur is reduced in its spatial extent. This can lead to animal habitat contractions and compressions, promoting more interactions between predators and prey, competitors, and humans.
What happens to kelp forests during El Nino?
Ed Parnell (Associate Researcher in Integrative Oceanography Division): El Niño is devastating for giant kelp (Macrocystis pyrifera) forests in Southern and Baja California. Kelps need cool, nutrient-rich waters to thrive but are exposed to warmer, nutrient-poor conditions during El Niños. Storm tracks also change during El Niños, thus exposing the forests to more damaging winter swells because storms are typically closer and come from a direction less sheltered by our offshore islands.
Giant kelp resilience depends on recovery conditions between El Niños such as those present during La Niñas when the coastal shelf is exposed to cooler, nutrient-rich waters. However, El Niños are getting warmer because they are superimposed on a trend of regional ocean warming. The result is that kelps are not exposed to favorable recovery conditions for as long as they once were decades ago. They do not recover as fully due to a combination of factors including their shortened lifespans, resulting in stands of smaller plants that are not as resilient or reproductive.
The El Niño that is developing along the equator and that is forecasted to affect southern California this fall will further stress kelp forests that have only partially recovered since the last El Niño and the severe stress of the 2014-15 marine heat wave. Such repetitive stress favors the growth and cover of smaller subsurface canopy algal species that can prevent recovery of our iconic surface giant kelp forest. And since giant kelp is much more productive than these understory kelps, its loss means that there will be less food and habitat for species that live within the forests and nearby habitats such as beaches, submarine canyons and soft-bottom shelf communities.
I am greatly concerned that with continued ocean warming and the increased likelihood of severe and possibly more frequent El Niños, much of Southern California’s kelp canopy could disappear within my kids’ lifetime.
Will California see more sharks with an El Niño?
Jack Elstner (PhD Candidate – Biological Oceanography): Beaches throughout Southern California serve as critically important habitats for several species of sharks. This includes juvenile white sharks, which aggregate seasonally in nursery hotspots along the Southern California coast. Under normal conditions, juvenile white sharks are seen in their highest densities in the summer and fall, then migrate south down to Baja California when water temperatures drop in the winter. During the last strong El Niño, however, juvenile white sharks were observed at aggregation sites in much higher densities than under normal conditions. This is a pattern that we expect to see this year as well. If warm conditions persist, we also expect to see sharks staying in Southern California later into the season. During strong El Niño events in the past,Southern California has also hosted more tropical shark species not typically encountered in the region, such as hammerhead sharks, which follow warm water northward.
What happens to seabirds during an El Niño?
Tammy Russell (Marine Ornithologist and Postdoctoral Scholar): During El Niño events, the general trend off California is that cool-water species become less abundant, and warm-water species more abundant, including rare tropical species moving north, like what we’ve documented for several booby (Sula) species. During warm water events, including marine heatwaves and El Niño, seabird mortality events also occur, and the longer these warm periods persist, the more mortality we see (see examples here and here). The warm waters and increased stratification of the water lessen nutrient availability and decrease production at the base of the food web, which impacts the entire ecosystem. Due to less food availability, marine predators, like seabirds, can’t find enough food which leads to decreased body condition, reproductive success, and increased mortality. The decrease in ecosystem production will disproportionately affect species with constrained diets and/or distributions. For example, those that are unable to shift their diet to other prey or those that only use coastal ecosystems and don’t travel far to forage. These species will likely be hit the hardest as they are not able to be flexible in their diet or travel further or to new habitats to find food.
How do marine mammals respond to El Niño?
Simone Baumann-Pickering (Professor of Biological Oceanography): There is no single response shared by all marine mammals. Whales, dolphins, seals and sea lions generally respond less to the temperature itself than to the way El Niño reorganizes the marine food web. Changes in upwelling and ocean circulation can alter where plankton, fish and squid occur, and marine mammals may consequently shift their distribution, migration timing or foraging behavior. Long-term studies off Southern California have found that the occurrence of several cetacean species varies with El Niño conditions and ocean temperature, most likely because their prey is being redistributed.
The consequences also depend on the species. Whales and dolphins can travel considerable distances to locate suitable feeding habitat. Breeding sea lions are more constrained because mothers must repeatedly return to their pups. During previous warm-water events, mothers traveled farther to find food, and many pups became undernourished or stranded. Some warm-water species may move northward and become more common off California, while species associated with cooler water may become less common.
Some effects can extend far beyond California. Eastern North Pacific gray whales depend heavily on productive summer feeding grounds in the Bering and Chukchi seas to build the energy reserves needed for their migration to Mexico and back. Their recent multi-year population decline, poor body condition and exceptionally low calf production have been associated with reduced prey availability and broader ecosystem changes in Arctic and sub-Arctic feeding areas. This illustrates how changing feeding conditions in one part of a whale’s range can have consequences that become visible thousands of miles away along the California coast where strandings have become more frequent.
Redistribution of whales and their prey can also increase interactions with people. During recent marine heat waves, warm offshore waters compressed productive, cool-water habitat closer to the California coast. Anchovies and feeding humpback whales became concentrated in some of the same areas where Dungeness crab gear was deployed, sharply increasing the risk of whale entanglement. These changing patterns have required fishers and managers to adjust the timing and location of the fishery to reduce risk to whales. This habitat compression has also led to an increase in ship strikes. An El Niño can therefore affect not only marine mammals, but also where and when human activities can safely occur.
From the perspective of our research, we would not simply look for an overall increase or decrease in marine mammals. We would look for species-specific changes in when animals arrive, how long they remain, where they forage, what their body conditions are, and how consistently they use particular habitats. Long-term autonomous acoustic records are especially valuable because they can reveal whether these changes are brief responses to a single event or part of a longer-term shift in the ecosystem.
How do scientists track impacts to marine ecosystems during an El Niño?
Mark Gold (Director of the California Cooperative Fisheries Investigation (CalCOFI): El Niño events in the California Current ecosystem are characterized by a suite of linked physical, chemical, and biological changes. The formation of El Niños at the equator are tracked by buoys and satellites. The biological impacts to marine life are tracked by myriad ocean observations. To contextualize the strength of an El Niño, it is necessary to compare current events to those in the past.
Because CalCOFI has one of the longest continuous marine ecosystem time series in the world (since 1949), scientists can compare El Niño years against decades of baseline variability and identify ecosystem responses that are similar during El Niños. During this time California has experienced seven strong El Niño events with 1997-1998 and 2015-2016, being amongst the strongest ever recorded. CalCOFI has been tracking the distribution and abundance of hundreds of fishes and planktonic organisms in the California Current Ecosystem since 1951. By doing this we have been able to detect and quantify the changes to fish and krill during several major El Niños in the past. A common pattern observed is the northward and eastward migration of tropical species into southern California waters. We will see how this one compares to major El Niños from 1957-1958 to the most recent one in 2023-2024. Additionally, researchers continuously evaluate the reproductive success, diet, and spatial distribution of top predators like seabirds and marine mammals to assess how these bottom-up environmental perturbations cascade through the upper trophic levels of the food web.
CalCOFI is also the reason we know that El Nino is a basin-wide phenomenon. It was at a CalCOFI meeting in 1958 that scientists discovered that the climate pattern named in Peru actually affected the entire Pacific Ocean basin and the entire world. Since then NOAA officially declares an El Niño event when sea surface temperatures during the preceding 3 months are 0.5°C above the historical average in the east-central equatorial Pacific Ocean.
Simone Baumann-Pickering (Professor of Biological Oceanography): No single measurement can capture how an entire marine ecosystem responds to El Niño. Scientists combine satellite observations of sea-surface temperature, ocean color and productivity with measurements from ships, moorings, gliders and autonomous floats. Biological surveys track plankton, fish, squid, seabirds and marine mammals, using tools that include net sampling, eDNA, visual surveys, animal tags, photographs, stranding records and acoustic instruments.
Long-term ocean observatories are especially valuable because they can measure several parts of the ecosystem at the same place and time. Moorings can record physical and chemical conditions such as temperature, salinity, oxygen, currents, pH, and fluorescence throughout the water column. Active acoustic instruments transmit sound and analyze the returning echoes to measure the distribution, movement and relative abundance of zooplankton, fish, and squid. Passive acoustic recorders listen for naturally occurring sounds, including crustacean and fish choruses, and the calls and echolocation clicks of marine mammals. These complementary measurements allow scientists to examine the ecosystem from its physical environment through several levels of the food web.
Our group contributes by developing and using these integrated, long-term acoustic and oceanographic observations. Passive acoustic monitoring allows us to examine when marine mammals and other sound-producing animals are present, how consistently they use particular habitats and whether their seasonal or foraging activity changes, comparing before, during and after an El Niño. Active acoustics provide information about the prey communities beneath them, and concurrent oceanographic measurements help identify the environmental conditions that may be driving those possible changes.
The long baseline is critical. Ocean conditions and biological communities already vary with season, location and from year to year, so observations spanning many years are needed to distinguish an El Niño response from normal variability or longer-term change. Each method also has limitations: passive acoustics measure vocal presence and activity rather than automatically providing a count of animals, and active acoustics do not always identify organisms to species. The strongest understanding therefore comes from combining acoustics with oceanographic measurements, biological sampling, environmental DNA, and visual observations to track changes across the ecosystem as a whole.
Is El Niño always bad for the ecosystem?
Rasmus Swalethorp (Director of Ship Operations for CalCOFI and Associate Project Scientist): Not necessarily. There will always be different species that thrive in different conditions (“winners” & “losers”). One thing we have learned from studying the ocean during these past 77 years is that El Ninos share commonalities in how they impact the ocean, but there are nuances to the ocean and atmospheric physics that may serve to worsen or lessen those impacts, or in some cases even bring about some positive changes to biological communities that we care about like we experienced during the 2014-2016 combined El Nino and marine heatwave. During this time, a warm water anomaly (2014-2015) preceded the last big El Nino from 2015-2016 and induced many positive changes to the California Current Ecosystem. For example, anchovy increased from record low to record highs during this El Nino. This has provided a robust food source for many species that eat anchovy including sea lions, large fish like tuna, and sea birds. In addition, rockfish did really well during this last big El Nino, which has helped them further recover from overfishing over the past 20 years.
Is there a likelihood that harmful algal blooms could increase during El Niño?
Clarissa Anderson (Biological Oceanographer and Director of the Southern California Coastal Ocean Observing System): Harmful algal blooms (HABs) occur in California in response to enhanced nutrients in surface waters, which usually occurs in spring and summer during the height of coastal “upwelling,” a natural process that brings deep nutrients to the surface. In the Southern California Bight, nutrient enrichment from urban runoff can also stimulate blooms and enhance the toxicity of toxin-producing algae. La Niña periods tend to heighten the amount of nutrients “pumped up” to the surface during coastal upwelling episodes. El Niño episodes generally suppress nutrient levels in coastal California via changes to water column temperature and density structure.
There is still much debate on what water warming means for coastal HAB formation in California, which makes prediction during any given event difficult. Forty years of data on HABs indicate that they are only weakly correlated with El Niño events. For instance, the spate of large marine mammal mortality events from HABs over the last four years (between 2022 and 2025) occurred during La Niña or ENSO-neutral periods. Prior to that, these toxic domoic acid (DA) events that harm animals and contaminate shellfish occurred periodically year to year and were often more associated with La Niña years than El Niño years. Marine mammal mortality events from DA toxicosis, however, have been shown to be linked to El Niño years prior to 2012; any such connections are perhaps amplified by the low plankton productivity (i.e. fish food) that leads to malnourishment of young and nursing California sea lions. During the marine heatwave, a.k.a. “The Blob,” that lasted from late 2014 to 2016 and was amplified by a very large 2016 El Niño, there was a long and dramatic DA event that impacted marine mammal populations from Santa Barbara to Alaska. While the warm water was a possible cause for the bloom, the upwelling of cold water close to shore was also a factor in stimulating algae growth. Not all marine heatwaves, however, lead to HABs.
Given the complex relationship between water temperature and other factors, such as the amount of nutrients available to microscopic algae/plankton in the ocean, it will be hard to say what this year’s very strong El Niño will bring. What I can say is that over the last decade, blooms of the Pseudo-nitzschia group, the plankton that produces DA, now occur almost year-round in the same way the wildfire season has expanded from its traditional boundaries on the U.S. West Coast. The real mystery lies in whether El Niño conditions will stimulate the ocean to produce toxins, thus forming a HAB that threatens human and wildlife health.
The California coast has already been experiencing a marine heat wave. What could an El Niño mean for ecosystems on top of already high ocean temperatures?
Simone Baumann-Pickering (Professor of Biological Oceanography): The risk is that the existing marine heat wave transitions directly into El Niño-driven warming, extending the duration of unusually warm conditions and affecting a greater depth of the water column. NOAA scientists report that the present coastal heat wave has persisted since 2025 and is not yet caused by El Niño. El Niño’s influence is expected to become more important during fall 2026 and winter 2026–27. Prolonged warming can change the effectiveness and nutrient content of coastal upwelling, redistribute prey and compress the remaining cool, productive habitat into smaller areas. Marine mammals may need to travel farther or change where they forage. Habitat compression can also concentrate whales, prey, and fishing activity in the same coastal areas, potentially increasing risks such as whale entanglement and ship strikes. Warm conditions may additionally favor harmful algal blooms and increase food-web stress affecting sea lions and other predators. However, the outcome is not predetermined. Local winds and upwelling can sometimes maintain productive coastal conditions even when offshore waters are unusually warm, and El Niño events do not all develop in the same way. That is why sustained monitoring throughout the water column and across the food web is essential.
Mark Gold (Director of CalCOFI): If it follows the same pattern as 2014-2016, and so far it is looking very similar, we can expect continued high anchovy and rockfish abundance, low sardine abundance, harmful algal blooms, increased availability of large fish such as tuna, yellowfin and dorado to US anglers, and kelp die offs due to decreased nutrient availability. Early observations in winter, spring and early summer show continued high anchovy abundance and overall high species diversity.
We might also expect a higher diversity of fish and invertebrate species diversity if it follows the same patterns observed during the 2014-2016 marine heatwave and El Niño events, due to a northward advection of tropical species into Southern California if areas of cooler and more productive habitat remains to support resident temperate species. In addition, we might experience further northward expansion of tropical seabird species, and more southern species of marine mammals to make their entrance.
We also expect further reduction in the introduction of nutrients from the deep ocean to the surface due to further deepening of the thermocline during the El Niño than what occurs from the marine heatwave alone. The anomalous warm upper ocean limits upwelling of colder deep nutrient rich waters to the surface where light can penetrate and fuel much of the phytoplankton and zooplankton production. Therefore, southern California may likely experience a reduction in plankton production, which can impact the specific species and communities’ feeding off of them. Production may also be compressed closer to shore and in turn compress the foraging range and limit the food to many plankton consumers such as several species of baleen whales and seabirds.
The extent of the temperature anomalies, the dampened upwelling and the biological impacts are hard to predict as they depend on the complex interactions of the physical forces at play. But based on what we experienced during the 2015-2016 El Niño event, the duration of the event and the prevailing wind direction and strength may be important. Strong localized equatorward winds, that were very anomalous to a typical El Niño, increased coastal upwelling and dampened the negative impact to plankton productivity and likely created areas of suitable habitat to resident temperate species. We do not know whether such a situation may reoccur. Although we have not yet seen the full suite of major El Niño impacts off Central and Southern California, we expect these impacts may become more pronounced in the coming months as El Niño strengthens likely during fall and winter 2026 into 2027.
Tammy Russell (Marine Ornithologist and Postdoctoral Scholar): Prolonged warm, less productive periods make it difficult for the ecosystem to recover and over time wear away at an animal’s fitness and ability to reproduce and survive. Although we recently experienced a large die-off event of seabirds due to the marine heatwaves, many seabirds were able to shift their distributions north (e.g., brown pelicans), or may have changed their foraging strategies to stay alive during those warm conditions and find pockets of food. Even if seabirds are able to find food during these warm conditions, it may be less nutrient-rich, as there is also less and/or less nutrient-rich food available for fishes, krill, and other common prey of seabirds. For many seabirds, the extra energy spent travelling further coupled with decreases in food quality, means many of them are not in prime condition and we saw that many did not breed this year— there were many empty seabird colonies off Southern California and Baja California, Mexico. Our fear is that with El Niño comes continued warm water, less productive conditions that will continue to challenge seabirds and other marine predators, and their ability to find food, survive, and reproduce. We’ll be continuing our beach surveys to keep that long time-series going, in addition to increased surveys of seabird roosts and breeding colonies over the next year.
What can I do if I see an unusual fish?
Ben Frable (Manager of the Marine Vertebrate Collection): Unusual fish are a common occurrence during El Niño as the warmer water brings more tropical and unusual fish towards California. During the 2014-2018 blob years, the Scripps Marine Vertebrate Collection team recorded 36 new and rarely seen species in California waters. For this event, we’re hoping to crowdsource images from the community of anglers, divers, surfers, beachcombers, photographers, or anyone who happens to find a fish they haven’t seen locally before. Photos, videos, and sightings of unusual or unexpected fishes from California can be submitted to this El Niño Fish Records Form.
For additional information on El Niño, visit the El Niño Resource Hub.
