Tidal influence on spatial dynamics of leopard sharks, Triakis semifasciata, in Tomales Bay, California

Abstract

We used ultrasonic telemetry to determine the movement directions and movement rates of leopard sharks, Triakis semifasciata, in Tomales Bay, California. To analyze tide and time of day effects, we surgically implanted transmitters in the peritoneal cavities of one male and five female leopard sharks, which we located during summer for three to five sampling sessions lasting 12 to 24 h each. All leopard sharks showed strong movement direction patterns with tide. During incoming tides, sharks moved significantly (p < 0.0001) towards the inner bay, apparently to exploit the extensive inner bay muddy littoral zones' food resources. On outgoing tides, sharks showed significant (p < 0.0001) movements towards the outer bay. During high tide, there was no discernible pattern to their movements (p = 0.092). Shark movement rates were significantly (p < 0.0001) greater during dark periods (mean ± SE: 10.5 ± 1.0 m min-1), compared with fully lighted ones (6.7 ± 0.5 m min-1). Movement rates of longer sharks tended to be greater than those of shorter ones (range means ± SE: 5.8 ± 0.6 m min-1 for the 91 cm shark, to 12.8 ± 1.6 m min-1 for the 119 cm shark), but the leopard sharks' overall mean movement rate (8.1 ± 0.5 m min-1) was slower than other (more pelagic) sharks.