Scott A. Matern | Search USGS

USGS2000

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

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.

Environmental Biology of Fishes↗

USGS1995

Comparative ecology of prickly sculpin, Cottus asper, and coastrange sculpin, C. aleuticus, in the Eel River, California

We documented species' distributions, size structure of populations, abundance in mainstem and tributary streams, habitat use, and diets of prickly sculpin, Cottus asper , and coastrange sculpin, C. aleuticus , in the Eel River drainage of California, to determine the processes allowing coexistence of these very similar fishes. We observed prickly sculpins at 43 sites and coastrange sculpins at 34. The species co-occurred at 26 sites. Young-of-year coastrange sculpins were only observed within 42 km of the ocean, but young-of-year prickly sculpins were present throughout the species range. Mean, maximum, and minimum lengths of coastrange sculpins were positively correlated with distance from the ocean but no significant relationships were found for prickly sculpins. Absolute abundance of both species was highest in mainstem habitat (prickly sculpins = 0.6 sculpins m −2 and coastrange sculpins = 0.4 sculpins m −2 ) . Tributary densities of both species tended to be less than 0.1 sculpins m −2 . The species inhabited very similar habitats and had very similar diets. Coastrange sculpin populations in upstream areas were maintained by immigration from downstream areas in contrast with prickly sculpin populations that produced young-of-year fish throughout their range. Densities were probably not high enough for interspecific interactions to be important. The factors limiting the upstream distribution of the species may include high water temperatures, stability of the stream bed, and behavior of the fish. In the past, the range of sculpins within the Eel River drainage probably fluctuated with changing physical conditions. Recent introductions of exotic species that compete with and prey upon sculpins, and ongoing human activities in the drainage could result in major reductions in the distribution and abundance of one or both species.

California↗