Assessment of shark sighting rates by aerial beach patrols
Published by the NSW Department of Primary Industries
Authors: William D. Robbins, Victor M. Peddemors and Steven J. Kennelly
Aerial surveys are a recognised technique to identify the presence and abundance of marine animals. However, the capability of aerial observers to reliably sight coastal sharks has not been assessed, nor have differences in sighting rates between aircraft types been examined. In this study we assessed the ability of fixed-wing and helicopter observers to sight 2.5 m artificial shark analogues constructed of marine ply. The use of artificial analogues allowed us to control the depth and spatial distribution of potential sightings while providing a realistic visual image for aircrew observers. Analogues were traced from the silhouette of a white shark, with some of the heads modified to represent tiger sharks and hammerhead sharks. The depth at which the shark analogues could be seen was very shallow, averaging only 2.5 m and 2.7 m below the water surface for fixed-wing and helicopter observers, respectively. Substratum depth and analogue head type did not affect sighting rates. A succession of shark analogues was then deployed at ~2 m depth in a 5 km grid, and their sightability to observers in both aircraft assessed through a series of transects flown up to 500 m distant. Low overall sighting rates of only 12.5% and 17.1% were recorded for fixed-wing and helicopter observers, respectively. Helicopter observers had consistently higher success rates at sighting analogues up to a distance of 250 m, however neither aircraft observers sighted more than nine percent of analogues deployed greater than 300 m from their flight paths. Environmental observations showed that the helicopter observations were more affected by air and sea conditions, while the range of water turbidities recorded during the study had no effect on sighting rates of either aircraft observers. We conclude that aerial observers have limited ability to detect the presence of submerged animals such as sharks, especially when the sharks are deeper than 2.5 m, or more than 300 m distant from the aircraft’s flight path. This raises serious concerns about the utility of programs such as aerial beach patrols as a warning system for sharks. Examination of sighting records from 20 actual coastal aerial patrols confirmed this concern, with very low shark sighting rates (0.69 – 1.18 sharks 100 km-1), likely underestimating the presence of many shark species known to occur in the area.
Fisheries Final Report Series No. 132. Cronulla, NSW, Australia. 38pp. ISSN 1837-2112.