Research Technical Report

Diet, Daily Ration And Gastric Evacuation Of The Leopard Shark (Triakis semifasciata)

Kao, J.S. (June 2000)

Masters Thesis, California State University Hayward and Moss Landing Marine Laboratories

ABSTRACT

The trophic interactions of an apex predator such as the leopard shark (Triakis semifasciata) are potentially important factors in structuring the estuarine benthic ecosystem. To measure a predator's impact, its diet and consumption rate must be determined. The food habits of the leopard shark were reexamined after 25 years of large-scale physical and biotic changes in its prey community, which were reflected in the diet of this opportunistic generalist feeder. Feeding chronologies and forage areas differed for small and large sharks in Elkhorn Slough. Small sharks (<120 cm TL) were found to feed on fishes and fat innkeeper worms, Urechis caupo within the shallow coves of the Elkhorn Slough Estuarine Research Reserve. Large sharks (>120 cm TL) were found to feed on fishes and fat innkeeper worms nocturnally in the deeper main channel portions of Elkhorn Slough. Leopard sharks displayed a gorge and digest rather than a continuous feeding regime. The gastric evacuation rates of small and large leopard sharks were examined for their primary prey types; small sharks evacuated U. caupo in a linear fashion and large sharks evacuated the staghorn sculpin, Leptocottus armatus in a linear fashion. Digestion was faster in large leopard sharks than small sharks for their primary prey under controlled conditions. Daily ration estimates of small and large sharks differed for bioenergetic and conventional ration models on a % body weight consumed basis. Small shark ration estimates appeared lower than large shark estimates using non-bioenergetic models, however no significant differences existed between ration estimates using resampling-generated confidence estimates. Bioenergetic estimates of small shark daily ration were much higher than non-bioenergetic estimates of small sharks, and much higher than all ration estimates of large sharks. Differences in ration estimates may be due to diel differences in thermal regime for small and large sharks, which may affect in situ digestion rates; conversely, large-scale diel migrations may increase the metabolic needs of large leopard sharks. Small and large leopard shark diets have become increasingly similar since the 1970s, most likely as a response to increased erosion and reestablishment of the California sea otter, Enhydra lutris in Elkhorn Slough and their combined effect upon the benthic community. Small and large shark diet has come to include more U. caupo and fish prey than in the early 1970s.