Chemical Clues Reveal the Flexible Diet of Korea's Lost Sea Lion

The Japanese sea lion (Zalophus japonicus) once thrived across the waters of Japan, Korea, and Russia, but intense hunting and environmental shifts drove the species to extinction by the mid-20th century. Now, researchers have analysed skeletal remains excavated from Gajae-gull on the island of Ulleungdo to reconstruct the life history of these lost marine mammals.

By examining bone collagen, the team performed stable isotope analysis—a technique that measures specific chemical variants of carbon and nitrogen retained from food. The results revealed a striking dietary shift over time. During the earlier Chulmun period, these sea lions occupied a top position in the food web, feasting on large fish and cephalopods like squid. However, as time moved into the Mumun and historic periods, their diet changed significantly. The chemical markers showed a transition to coastal, mid-trophic prey, suggesting the animals were forced to adapt to changing prey availability.

Radiocarbon dating of a rib bone placed the burial between 1548 and 1952 AD, representing the most recent evidence of the species in Korea. While the study highlights the sea lion’s impressive dietary flexibility, it also underscores how drastically its ecological role shifted before it finally vanished. These findings provide a crucial baseline for reconstructing the historical marine ecosystems of the region.