Predation by the gull Larus crassirostris on benthic invertebrates: First report of avian predation on the abalone Haliotis discus hannai

摘要

We conducted field surveys to study avian predation on invertebrates inhabiting the intertidal and shallow subtidal zones in Otsuchi Bay on the Sanriku Coast. The black-tailed gull Larus crassirostris was the most frequently observed avian predator, and we observed the gull actively foraging on benthic invertebrates at the study site. In particular, we found frequent predation activities by the black-tailed gull on the abalone Haliotis discus hannnai. The abalone accounted for 56.3% of the total amount of the prey remains, and the sea urchin Strongylocentrotus intermedius and mussel Mytilus galloprovincialis were also major components of the prey remains and accounted for 14.6% and 11.8% of the total number, respectively. While the gull preys on the abalone over a wide size range (27.0-105.7 mm shell length), smaller size class abalone individuals (40 mm shell length) were dominant in prey remains compared to the live abalone surveyed in the surrounding habitats. Among four time periods in a day, predation rate on the abalone was highest in the late afternoon. Predation rate on the abalone was higher on rising tides than falling tides, although no clear relationship was confirmed between the predation rate and the tidal height. The predation by the black-tailed gull might have large impacts on the shallow rocky reef ecosystem.