Patterns of Insular Shark Dynamics Based on Fishery Bycatch and Lifeguard Surveillance at Oahu, Hawaii 1983–1992

Incidental shark sightings recorded by beach lifeguards and records of shark bycatch from fishery catch reports were evaluated as possible long-term indices (1983-1992) of insular shark dynamics. The daily lifeguarding of the 17 Oahu beaches provided a documentation of effort not available from fishery data. Identified seasonal and spatial trends in shark sightings were found to persist through successive years of surveillance and were roughly consistent with distributions of fishery bycatch. However, it was possible to attribute shark bycatch to changes in accessibility of fishing grounds and size of the fishing fleet. An evaluation of the potential biases in lifeguard data indicated that variables such as wind/surf conditions and beach attendance did not govern the frequency with which sharks were reported. Summer increases in sightings coincided with shark pupping activities, and winter pulses were roughly associated with periods of increased rainfall. Relationships with coast and season were evident, with significantly more sightings on the island's leeward coast. A survey of shark-related news stories compared with reports of shark sightings suggested that shark sightings increased on a limited scale with high media exposure. No consistent trend in abundance of sharks was detected over the full 10-yr period. The interannual pattern of shark sightings (scaled for effort) remains unexplained, despite comparison with fishery data, island runoff, sea-surface temperature, and known El Niño events.