Ecological Insights Into the Extinct Korean Sea Lion (Zalophus japonicus) in Korea Based on Stable Isotope Analysis of Bone Collagen

Abstract

Rationale: The Japanese sea lion (Zalophus japonicus), once abundant in Japan, Korea, and Russia, went extinct by the mid-20th century due to hunting and environmental change. This study examines the diet and ecological role of Korean Z. japonicus using stable carbon and nitrogen isotope analysis of bone collagen from archaeological and historical contexts.

Methods: Skeletal remains of the extinct Korean sea lion (Z. japonicus) were excavated from Gajae-gull, Ulleungdo, in 2021. A rib bone was analyzed for radiocarbon dating using AMS and calibrated with the Marine20 curve, applying a ΔR correction. Stable isotope analysis of extracted bone collagen (δ¹³C, δ¹⁵N) was performed via EA-IRMS. Isotopic niches of Z. japonicus and modern marine mammals were compared using SIBER in R, with ANOVA and post hoc tests.

Results: Radiocarbon dating of a rib from Gajae-gull, Ulleungdo, indicated burial between 1548 and 1952 cal ad, representing the most recent evidence of Z. japonicus in Korea. Stable isotope analysis revealed dietary shifts from high δ¹⁵N values in the Chulmun periods, indicating consumption of large fish and cephalopods, to lower δ¹⁵N and higher δ¹³C values in the Mumun and historic periods, indicating reliance on coastal mid-trophic prey. Comparisons with modern marine mammals showed clear isotopic niche partitioning.

Conclusions: These findings underscore the dietary flexibility of Korean Z. japonicus and its shifting ecological role in response to changing prey availability and environmental conditions. Future research incorporating additional samples and diverse isotopic markers will enhance paleoecological reconstructions of this extinct species and its historical marine ecosystems.

Keywords: Zalophus japonicus; bone collagen; dietary patterns; stable isotope analysis.

FIGURE 1

Archeological sites in Korea where Zalophus japonicus remains have been discovered (yellow dots), with the shaded grey area representing its estimated historical range (Choy et al., 2012; Choy & Richards, 2009, 2010; this study).

FIGURE 2

Excavated bone samples of Zalophus japonicus from Gajae‐gull on Ulleungdo, Korea.

FIGURE 3

Stable carbon (δ¹³C)(a) and nitrogen (δ¹⁵N)(b) isotope values of Zalophus japonicus from Gajae‐gull on Ulleungdo, compared with previously published data from different archaeological periods (Choy et al., 2012; Choy & Richards, 2009, 2010).

FIGURE 4

Biplot showing trophic dynamics between Zalophus japonicus and two modern marine mammal species in the East/Japan Sea, based on stable carbon (δ¹³C) and nitrogen (δ¹⁵N) isotope values. Shaded ellipses represent the sample size‐corrected standard ellipse area (SEAc), indicating core isotopic niche space, while dashed lines represent the Convex Hull outlining the total isotopic area. The inset (bottom left) shows Bayesian posterior distributions of the standard ellipse area (SEAb): shaded boxes represent 50%, 75%, and 95% credible intervals; black dots represent the SEAb mode; and red crosses denote maximum likelihood SEAc estimates. Data sources: Choy et al., 2012; Choy & Richards, 2009, 2010; Ko et al., 2016; MOF, 2020, and this study.