The foraging ecology of monk seals in the main Hawaiian islands

Abstract

Diverging trends in population abundance of Hawaiian monk seals (Monachus schauinslandi) are apparent between the Northwestern Hawaiian Islands (NWHI) and the main Hawaiian Islands (MHI). The NWHI population decline, approximately 40/0 per annum, is partially linked to food limitation. The smaller MHI seal population is likely increasing and seals appear to be in better condition overall, suggesting sufficient prey resources. This study combines satellite tracking and fecal analysis for the first comprehensive foraging study of MHI monk seals in an effort to define MHI foraging ecology and investigate what factors related to foraging success or prey explain the different population trajectories. Satellite-linked dive recorders were opportunistically deployed on-twenty seals between January 2004 and June 2008. Average tracking duration was 3.6 months. Most dives occurred between 0 to 20 m. 98% of dives were less than 7 minutes. Foraging trip durations were short, lasting on average 3.8 ± 0.27 d. Foraging behavior was highly variable between age and sex classes. Home-ranges were generally very restricted (310.29 ± 45.69 km") with animals showing high fidelity to particular regions of their island of residence. However, some individuals did travel between islands. Prey remains from feces and regurgitates (n=120) were identified to the lowest possible taxonomic level. The most common prey families, by percent frequency of occurrence, were Balistidae (48.3%), Crustacean (37.5%), Acanthuridae (32.5%), Muraenidae (30.8%), Serranidae (20.80/0), and Cephalopod (18.3%). No striking differences in diet were detected between the MHI and NWHI seals. The incongruity in body conditions of seals between locations likely indicates differences in foraging behavior and habitat use such that MHI seals appear to be investing less time for foraging resulting in a positive energy balance.