Foraging interaction between monk seals and large predatory fish in the Northwestern Hawaiian Islands

Abstract

CRITTERCAMS were deployed on 42 Hawaiian monk seals at French Frigate Shoals, Hawaii, to assess the degree to which interaction with large predatory fish may play a role in their foraging success. Sharks, jacks, and large-bodied snappers were seen near the actively foraging seals approximately 17% of the time. Present at all but the greatest depths (>100 m), the predatory fish were most frequently observed when seals foraged at the summits of neighboring banks. Competing fish were encountered as individuals and in schools (mean 4.5, maximum 46 ind.) and often formed mixed-species assemblages that followed seals around. By traveling with foraging seals, the preda- tory fish could exploit the seals' superior ability to flush cryptic prey from bottom cover. The probing, digging, and rock-flipping activities of seals spook prey into the open, making them available to jacks, sharks, and snappers. The jacks Caranx ignobilis and Seriola dumerili were the most success- ful at obtaining prey items flushed from cover by the seals, but carcharhinid sharks were also seen competing for prey. Some adult seals appeared to feed effectively in prey-rich areas despite the pres- ence of high densities of predatory fish. It is not known if the predatory fish have a larger impact on juvenile seals, the segment of the population with the poorest survivorship.