Recorded declines of Hawaiian monk seals (Monachus schauinslandi) have occurred over the past half century in the Leeward Chain of the Hawaiian Islands. Preliminary evidence from stable carbon (13C/12C) and nitrogen (15N/14N) isotope ratios in bone collagen from the seals (n=20) shows a shift in both isotopes between the 1910s and 1970, indicative of a diet switch, possibly from a benthic diet (crustaceans, mollusks) to a more pelagic diet (fish), as well as a potential environmental change. We surmise the advent of commercial fishing of potential prey items post-World War II led to the decline of preferential prey items and forced the seals to utilize prey species potentially less nutritious and/or which require more energy to capture. Less nutritious prey could be at least partially responsible for the reproductive failure and overall decline of Monachus throughout its range. These historic isotopic values are compared to modern seals and potential prey from the main Hawaiian Islands with the current assumption that the isotope signatures are similar with the Leeward Islands of the chain.
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