A northern elephant seal (Mirounga angustirostris) at the Año Nuevo Reserve, on the coast near Pescadero, California. Photo: Daniel Costa, UC Santa Cruz.
American researchers reconstruct fish populations in the difficult-to-study mesopelagic zone and find that fish numbers fluctuate sharply every three to five years.
Northern elephant seals are revealing how much fish is present in the twilight zone – a difficult-to-study region at depths of 200 to 1,000 metres in the open ocean. Using this information, American researchers have estimated how fish population sizes have fluctuated over the past 45 years (Science, 14 February). This is possible because the foraging success of the elephant seals turns out to correlate strongly with local air pressure, water temperature, and water circulation – parameters for which data from recent decades are available. ‘Estimates of fish populations in the twilight zone are currently not measurable by any other means’, the biologists write.
Cascades
It is well established from terrestrial systems that variations in available food sources can trigger cascades that reverberate throughout the food web, sometimes causing large fluctuations in animal populations. For marine systems, such relationships are harder to quantify, and this is especially true of the mesopelagic zone. Yet this twilight zone contains the greatest fish biomass and covers roughly 60 per cent of the Earth’s surface and 20 per cent of the ocean’s volume. The biologists therefore decided to track a predator with an enormous hunting range in the twilight zone of the North Pacific: the northern elephant seal (Mirounga angustirostris). They follow the animals annually using data loggers, recording a total of 143,201 dives across an area of 4.4 million cubic kilometres, and measure the annual variation in body mass and prey consumption. They then quantify the relationship between prey consumption and various oceanographic parameters.
Mass gain
A particularly strong correlation emerges between the mass gain of northern elephant seals and the Southern Oscillation Index – a measure of air pressure differences along the equator – two years earlier. Using this index, the biologists then estimate prey numbers for elephant seals in the past and up to two years into the future.
The Año Nuevo Reserve near Pescadero on the California coast, where the elephant seals return each year after a migration of thousands of kilometres across the North Pacific, where they forage on fish and squid. Photo: Daniel Costa, UC Santa Cruz.
The biologists conclude that the amount of prey in the mesopelagic zone where the elephant seals hunt was very low between 1984–1985, 1994–1996, and 2021, but high between 1975–1978, 2019–2020, and 2022–2024. They observe a pattern of rapid peaks and troughs that repeat roughly every three to five years. Deep, open-ocean ecosystems thus appear to exhibit large and frequent fluctuations in population dynamics that propagate across trophic levels, much as terrestrial systems do. In other words, populations in these relatively isolated ecosystems do not appear to remain more stable over time than populations on land.
The researchers suspect that southern elephant seals in the Southern Hemisphere could, by the same means, shed light on patterns in fish numbers.
See also:
– Elephant seals in the Pacific serve as deep-ocean sentinels, revealing patterns otherwise hard to measure – EurekAlert, 13 February 2025.