Article

Environmental Biology of Fishes

, Volume 96, Issue 7, pp 865-878

Demography and movement patterns of leopard sharks (Triakis semifasciata) aggregating near the head of a submarine canyon along the open coast of southern California, USA

  • A. P. NosalAffiliated withCenter for Marine Biodiversity and Conservation, Scripps Institution of Oceanography, University of California – San Diego Email author 
  • , D. C. CartamilAffiliated withCenter for Marine Biotechnology and Biomedicine, Scripps Institution of Oceanography, University of California – San Diego
  • , J. W. LongAffiliated withSt. Petersburg Coastal and Marine Science Center, United States Geological Survey
  • , M. LührmannAffiliated withInstitute of Biological Sciences, University of Rostock
  • , N. C. WegnerAffiliated withCenter for Marine Biotechnology and Biomedicine, Scripps Institution of Oceanography, University of California – San DiegoFisheries Resource Division, Southwest Fisheries Science Center, National Marine Fisheries Service, National Oceanic and Atmospheric Administration
  • , J. B. GrahamAffiliated withCenter for Marine Biotechnology and Biomedicine, Scripps Institution of Oceanography, University of California – San Diego

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Abstract

The demography, spatial distribution, and movement patterns of leopard sharks (Triakis semifasciata) aggregating near the head of a submarine canyon in La Jolla, California, USA, were investigated to resolve the causal explanations for this and similar shark aggregations. All sharks sampled from the aggregation site (n = 140) were sexually mature and 97.1 % were female. Aerial photographs taken during tethered balloon surveys revealed high densities of milling sharks of up to 5470 sharks ha−1. Eight sharks were each tagged with a continuous acoustic transmitter and manually tracked without interruption for up to 48 h. Sharks exhibited strong site-fidelity and were generally confined to a divergence (shadow) zone of low wave energy, which results from wave refraction over the steep bathymetric contours of the submarine canyon. Within this divergence zone, the movements of sharks were strongly localized over the seismically active Rose Canyon Fault. Tracked sharks spent most of their time in shallow water (≤2 m for 71.0 % and ≤10 m for 95.9 % of time), with some dispersing to deeper (max: 53.9 m) and cooler (min: 12.7 °C) water after sunset, subsequently returning by sunrise. These findings suggest multiple functions of this aggregation and that the mechanism controlling its formation, maintenance, and dissolution is complex and rooted in the sharks’ variable response to numerous confounding environmental factors.

Keywords

Shark aggregation Acoustic telemetry Sexual segregation Site fidelity Marine reserve Wave height