Elephant Seals Unveil Deep-Ocean Patterns

Northern elephant seals may hold the key to unlocking the secrets of the open ocean's twilight zone (~200 – 1,000 meters deep). According to a new study, these deep-diving creatures can help estimate fish abundance by providing a rare window into the elusive prey dynamics in one of the planet's most mysterious and remote ecosystems. Ecosystems are dynamic, with resource fluctuations – natural or human-induced – shaping species interactions and food webs. These processes are well studied in terrestrial ecosystems but not in deep, open ocean ecosystems, which hold the majority of global fish biomass. It is unclear whether deep-sea populations undergo cascading fluctuations like those seen on land. Monitoring this region is hindered by technological constraints, as data collection is costly, sporadic, and surface-focused. Wildlife have long been recognized as vital sentinels of ecosystem health, however, offering insights into human impacts and environmental changes that are otherwise challenging to quantify. However, no dedicated deep-ocean sentinel species exists. Here, Roxanne Beltran and colleagues evaluated whether northern elephant seals (Mirounga angustirostris) could be used as ecosystem sentinels to monitor the health of the northeast Pacific Ocean twilight zone. Elephant seals primarily feed on twilight zone fish across large areas of the open ocean. Using data from satellite-tagged seals, Beltran et al. discovered a strong correlation between seal foraging success and long-term oceanographic conditions measured at the ocean's surface. Notably, fluctuations in seal mass gain corresponded with changes in these conditions observed two years earlier, suggesting they serve as valuable proxies for prey availability in the twilight zone. This connection enabled the hindcasting of prey abundance over the past 45 years and forecasting two years into the future, highlighting distinct periods of high and low fish populations. The findings suggest that prey populations in the twilight zone experience rapid fluctuations, with cycles of abundance and scarcity occurring every three to five years – patterns otherwise impossible to measure through direct observation.