The twilight zone, a vast layer of the ocean 200 to 1000 meters below the surface, is home to an enormous but largely unstudied fish population that plays a crucial role in the marine food web.

Research published in Science has found that elephant seals act as ecosystem sentinels, providing data on fish abundance and environmental changes in the oceans over decades.

By tracking how much weight the seals gain during hunting trips, scientists can estimate fish populations in the twilight zone.

Researchers from University of California-Santa Cruz tagged female seals with satellite-linked data loggers to monitor their foraging behavior, with each seal embarking on months-long journeys, diving to twilight zone depths over 140,000 times per year, covering millions of cubic kilometers of ocean.

They can provide real-time deep-ocean data, making them a natural biological survey tool for measuring fish populations in ocean conditions that cannot be studied using satellites or traditional buoys.  

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