The Galápagos Fur Seal (Arctocephalus galapagoensis) is the smallest of all eared seal species. Endemic to the Galápagos Archipelago, this species survives in a tropical environment, which is highly unusual for a fur seal. Its existence in this equatorial region is made possible by cold, nutrient-rich ocean currents, such as the Cromwell and Humboldt currents, that flow through the islands. These currents create pockets of productive, cool water that sustain the seals’ specialized diet. The fur seal population is non-migratory and depends entirely on the food resources found surrounding the islands.
Primary Prey Species
The diet of the Galápagos Fur Seal is highly focused, consisting overwhelmingly of small, schooling organisms that inhabit the deeper ocean layers. Stomach content and scat analyses have consistently shown that the vast majority of their food comes from the deep scattering layer, a dense aggregation of marine life that migrates vertically in the water column. This layer is rich in small fish and cephalopods, which the seals efficiently target as their primary energy source.
Among the fish, lantern fish (myctophids) and bathylagids are numerically dominant in the fur seal’s diet. Studies analyzing fish ear stones found in fur seal scats report that these two groups of small, deep-sea fish account for 84% to 99% of all fish consumed. Lantern fish are small, bioluminescent fish that form massive schools, making them a dense and reliable food source. Fur seals consume these fish whole, targeting prey small enough for rapid consumption while underwater.
The cephalopod component of the diet is primarily made up of small, pelagic squid, including Onychoteuthis banksi and various species of omastrephids. These invertebrates are typically inhabitants of the deep scattering layer, reinforcing the seal’s reliance on this specific ecological zone. While the fur seal’s diet can technically include up to 26 different prey species, only a few contribute significantly to their energy intake, demonstrating a specialized feeding strategy.
Nocturnal Foraging and Hunting Behavior
The fur seal’s feeding strategy is intricately linked to the behavior of its prey, utilizing a phenomenon known as Diel Vertical Migration (DVM). This specialized hunting routine involves foraging almost exclusively at night. The seals capitalize on the fact that deep-sea organisms, such as lantern fish and squid, migrate from the dark depths to the surface waters during the night to feed.
By hunting under the cover of darkness, the seals can access their prey when it is concentrated and within a manageable diving range. Foraging trips by adult females often last around 16 to 18.8 hours, covering distances that can exceed 40 kilometers from the colony. The average foraging dive depth is relatively shallow, around 26 meters, with most dives occurring in the 0-40 meter range where their prey is most abundant at night.
However, adult females are capable of much deeper dives, reaching maximum depths of over 100 meters, with some recorded dives reaching 106.5 meters and lasting up to 4.5 minutes. This range of diving ability allows them to hunt efficiently in the upper water column while also occasionally pursuing prey that has not fully migrated upwards. This nocturnal focus minimizes competition with other surface-feeding predators.
Impact of Environmental Shifts on Diet
The food web that supports the Galápagos Fur Seal is highly susceptible to major oceanic events, primarily the El Niño-Southern Oscillation (ENSO). The cold, nutrient-rich water that sustains their prey is easily disrupted by these events, which cause a significant warming of the surface waters. As sea surface temperatures rise, the thermocline—the boundary between warm surface water and cold deep water—deepens considerably.
This warming and deepening thermocline severely impacts the primary food sources, as the cold-water species, like myctophids, either die or move to deeper, cooler waters that are beyond the seals’ sustainable diving range. The resulting scarcity of prey places the fur seals under intense dietary stress. During the severe 1982-1983 El Niño event, for example, the population suffered drastically, with approximately 30% of the adult population dying and nearly all pups being lost due to starvation.
Such environmental shifts force the seals to cope with periods of extreme food shortage, as they are largely unable to switch to other, more suitable prey species. The increased frequency and intensity of El Niño events, which are linked to global climate change, represent a persistent threat to the fur seal population. Their specialized diet and dependence on the cold-water upwelling make them highly vulnerable to these fluctuations in the productivity of the marine ecosystem.