Abstract
Objective Swimming at the sea surface is a behavior exhibited by numerous shark species; however, depth distributions vary considerably by species, individual behavior states, and environmental conditions. Given the growing use of aerial surveillance tools for shark population monitoring and beach safety in New York coastal waters, our objective was to characterize the swimming depth distributions of multiple shark species in the region and assess their availability to aerial detection.Methods Between 2017 and 2024, we tagged 150 sharks of 10 species (Sand Tiger Carcharias taurus, Common Thresher Shark Alopias vulpinus, White Shark Carcharodon carcharias, Tiger Shark Galeocerdo cuvier, Spinner Shark Carcharhinus brevipinna, Blacktip Shark Carcharhinus limbatus, Dusky Shark Carcharhinus obscurus, Sandbar Shark Carcharhinus plumbeus, Scalloped Hammerhead Sphyrna lewini, and Smooth Hammerhead S. zygaena) with depth-sensing pop-up satellite archival tags or acoustic transmitters to characterize swimming depth distributions and availability to daytime aerial monitoring off Long Island, New York. We also compiled data on water clarity and wave height, which further influence aerial detectability.Results Shark daytime swimming depths ranged from the surface to 149 m, but across species, the grand mean swimming depth +/- SD was 7.7 +/- 8.2 m. During daytime, sharks occurred in the top 1 m of the water column, where the probability of detection is greatest, only 15.7% of the time on average. Near-surface occurrence probability varied among species and based on the time of day. Average summer season water clarity was 2.1 m and average wave height was 0.8 m, further exacerbating the limited aerial detectability of sharks and introducing availability bias in assessing their presence.Conclusions Most shark species spent minimal time near the sea surface in New York waters. Even when present in near-surface waters, sharks will frequently be undetectable by aerial observation methods due to the typically poor water clarity and sea surface conditions. Therefore, aerial methods have limited capacity to monitor shark presence, and other methods should be employed for population monitoring and beach safety needs in this region.
Aerial surveillance tools are expanding in use for fishery-independent population monitoring of sharks as well as for supporting beach safety. Using depth-sensing acoustic and satellite tags, we found that sharks in New York coastal waters spend little time near the surface and will typically be visually undetectable even when present. Aerial monitoring methods should not be exclusively relied upon to monitor sharks in this region.