Trophic Structure of the Northwest Hawaiian Islands and Resident Monk Seals (Monachus schaundslandi) During the Twentieth Century

The Hawaiian monk seal ( Monachus schauinslandi ) is an endangered species only found within the Hawaiian Archipelago. The majority of the breeding population for this seal is located around six islands in the Northwest Hawaiian Island chain (NWHI). Overall, both juvenile and adult seals have a wide range in δ ¹³ C and δ ¹⁵ N from 1912-2006 (δ ¹³ C: -12.5‰; δ ¹⁵ N: 12.6‰). Seals in the northern NWHI were enriched in δ ¹³ C by nearly 2‰ and depleted in δ ¹⁵ N by nearly 6‰ during the 96 years. Meanwhile, seals within the middle and southern extent of the NWHI showed little to very slight decreases in δ ¹³ C and δ ¹⁵ N. Preliminary comparison of modern monk seals with selected potential prey in the southern NWHI indicate possible trophic reliance on a mix of reef fish, eels, and lobster. Commercial fishery data post-WWII indicate an overlap between fisheries and monk seal diets in lobster, squirrelfish, and goatfish.