Changes in Northwest Hawaiian Island Monk Seal (Monachus schauinslandi) Populations as Evidenced by Stable Isotope Ratios

The endangered Hawaiian monk seal has been undergoing dramatic population declines for several decades. These declines may be linked to food resources or environmental changes and this is reflected in the stable isotope analysis of the monk seals. The use of stable carbon (δ13C) and nitrogen (δ15N) isotope ratios on Hawaiian monk seal bone collagen samples collected from 1912 through 2006 determined that changes within the environment and food web of the Hawaiian monk seal may be factors contributing to the decrease in the population. Over the ninety-four year period the overall δ15N of the monk seals was depleted by approximately 3‰ and the δ13C had minimal changes. Monk seals located within the northern extent of the NWHI exhibited the most dramatic changes in stable isotopes. From 1923 through 2006 the δ15N of the Hawaiian monk seals within this area was depleted nearly 6‰, while δ13C was enriched by 2‰. This significant depletion in δ15N, along with the enrichment within the δ13C for the northern NWHI could be caused by an increase in the primary productivity within the area, leading to a shorter food web. This inverse relationship within the northern NWHI could also be a representation of the monk seals foraging more on benthic rather than pelagic prey, or foraging inshore rather than offshore in this region. This northern region was in contrast to the Central and southern islands within the NWHI. The central NWHI seals had nominal change in their δ15N and δ13C from 1912 to 2006; whereas, the southern NWHI seals exhibited a 3.5‰ depletion in δ15N and a nearly 1‰ depletion in δ13C over the span from 1951 through 2006. Within the central NWHI the juvenile monk seals were more depleted in δ15N as compared to the adult monk seals, which could indicate a prey base change for these seals. Within the southern NWHI there was a significant difference within δ15N and δ13C over the decades of the study which could indicate a decrease in the regional productivity. The adult monk seals within the southern region also had an increase in δ15N which could be a indicator of starvation for the seals within this region. Stable carbon and nitrogen isotopes indicate modern monk seals (2000-2006) foraged on a number of different teleost, crustacean, eel, and cephalopod species, correlating to earlier and current studies being conducted on the food sources of the Hawaiian monk seal