This mysterious layer of water, between 200 and 1,000 meters below the surface, lies in  almost complete darkness, for the sun's rays can scarcely penetrate that far. Thus it is difficult or impossible to monitor using traditional tools. But while ships, buoys, and satellites can only capture a fraction of what’s happening below the waves, the seals’ foraging patterns can offer a far more expansive view of ocean life. 

The study draws on a vast archive of historical and real-time data, gathered from over 350,000 observations of more than 50,000 seals. This extensive data set covers a wide range of factors, including the seals’ fitness, foraging success, behaviour at sea, and population trends. It has revealed much about how the seals hunt, survive in the ocean’s depths, and contribute to the broader marine ecosystem. Thus the scientists have seen how elephant seals can serve as valuable ecosystem sentinels, providing critical insights into fish populations in the open ocean.

Why is this so important? In spite of its darkness, the twilight zone holds the majority of the world’s fish biomass, and it is also where these seals hunt for food. By tracking their foraging success, scientists can gain insight into the fish populations that are using this unknown region. The information is crucial, especially with the pressure from industrial fisheries to catch these fish to serve the global demand for protein-rich foods—traditional fish stocks are severely depleted. 

A single seal will make an average of 75,000 foraging attempts during its seven-month, 6,000-mile journey across the Pacific Ocean. If scientists track just 14 seals each year, they can estimate fish availability across an area of the ocean measuring 4.4 million cubic kilometres.

In addition to tracking foraging success, weighing the seals provides valuable information about the abundance of prey over time. As commercial fishing fleets expand their reach to meet growing food demands, this type of data is essential for sustainable fisheries management. Understanding fish population trends and how they respond to environmental changes will be crucial in maintaining the balance of ocean ecosystems for future generations.

Therefore, the research offers an innovative way to monitor this largely unknown region of the ocean, which is critical for ensuring the health of the oceans' ecosystems in general and its fish populations in particular.