Gathering populations of northern elephant seals could hold the secret to measuring fish abundance across the vast Pacific Ocean and deep within its ‘twilight zones’ – a feat that was previously impossible by marine researchers looking to better understand fish stocks.

This is the latest development from a 60-year study in which marine biologists from UC Santa Cruz have been monitoring the behaviour of northern elephant seals as they journey to the nearby Año Nuevo Natural Reserve.

Over the course of six decades, generations of researchers have been able to amass some 350,000 observations on more than 50,000 seals who – over the years – have gathered by their thousands to breed and molt.

Now – and thanks to the development of new technologies – researchers have been able to extend those observations into the waters of the Pacific Ocean, monitoring the behaviour of tagged individual seals to gather real-time data on their fitness, foraging success, at-sea behaviour, and population dynamics. 

And it’s all in the hope of delivering a clearer picture on what the health of ocean’s fish populations currently looks like.

Having inherited the monitoring programme from those before her, Roxanne Beltran has led a new study – published last month in Science in which she suggests northern elephant seals can “act as smart sensors for monitoring fish populations” crucially so within the vast regions too far out of the reach of human-built measurement tools – the ‘twilight zone’.

This is the layer of water between 200 and 1,000 metres below sea level, where sunlight penetration all but stops, and today’s ocean monitoring tools often struggle to reach. Ships and floating buoys, for example, can only measure fractions of the ocean at a time, while satellites can’t reach below the surface, where fish occur.

But not only is this where they occur, this is where the majority of the planet’s fish biomass is held. And, because this is also where seals feed, researchers tracking their foraging success can provide what Beltran describes as a “previously impossible way to measure the availability of fish population across the vast ocean.”

“Given the importance of the ocean for carbon sequestration, climate regulation, oxygen production, and food for billions of people, there is an urgent need to measure changes in marine ecosystems,” said Beltran, an assistant professor of ecology and evolutionary biology at UC Santa Cruz. 

“Our research shows that the vast foraging extent and millions of feeding attempts by elephant seals make them a fantastic ecosystem sentinel, both for fish populations and top predators in the open ocean.”

One seal, on average, makes around 750,000 foraging attempts during their seven-month, 6,000 mile journey across the Pacific Ocean. By tracking just 14 seals each year, researchers could harness data and provide estimates for fish availability across 4.4 million cubic kilometres of ocean volume.

Beyond that, weighing individual seals could also allow researchers to measure long-term fluctuations in prey abundance. This could prove vital information for the establishment of sustainable fisheries management , particularly as commercial fleets increase the depth and breadth of their harvesting in response to environmental change.

Fish in the ‘twilight zone’ are ecologically important prey for economically important species. While little is actually known about fish in this region of the ocean, scientists agree that reduction in their numbers could have huge impacts on many other species.

The new study is the latest in a long line of discoveries born of this ongoing research project. Previous discoveries have included the long-distance foraging migrations of elephant seals, the high frequency of elephant seals foraging attempts for small fish, and the likely possibility that seal pup quality is, too, linked to ocean health through maternal foraging success.

Beltran’s study goes one step further, showing that foraging success of elephant seals was tightly linked to a broad-scale oceanographic index that can be measured by orbiting satellites.

‘This linkage allowed us to measure the ocean’s pulse and estimate fluctuations in fish availability five decades into the past and a few years into the future,” said Beltran. 

Insights and tools from this study, therefore, provide a critical ecological baseline for sustainable fishing while allowing for assessment of the impacts of anthropogenic environmental changes on fish populations at the scale of entire ocean basins.

Click here for more from the Oceanographic Newsroom.