| Summary: | 碩士 === 國立臺灣海洋大學 === 環境生物與漁業科學學系 === 101 === As a high-level predator, the carcharhinid shark play an important role in the
marine ecosystem. In recent years, due to increase of fishing effort, sharks, including
carcharhinid sharks, have been subjected to heavy fishing pressure. Therefore, to
understand their potential regulatory effects in the marine ecosystem, it is essential to
quantify and qualify the feeding ecology of these species. In this study, the diet
compositions of three carcharhinid sharks, the spinner, Carcharhinus brevipinna, the
silky, C. falciformis, and the dusky shark, C. obscurus caught by longliners in the
northeastern waters off Taiwan between Jul. 2011 and Feb. 2013 were examined.
According to the preliminary stomach contents analysis, the major food items of
spinner sharks were teleosts. A total of 13 prey taxa, including the Japanese scad
(Trachurus japonicus) and the porcupine fishes (Diodon spp.), were identified. The
ratio of teleosts and cephalopods was similar to that of silky shark. In total, 13 prey
taxa, including the squids (Loliginidae) and the porcupine fishes, were identified for
silky shark. Both teleosts and cephalopods were commonly found in the stomachs of
the dusky sharks with a total of 21 prey taxa, including the snake mackerels
(Gempylidae) and inshore squids (Uroteuthis duvauceli). The differences of spatial
distribution, morphological characteristics and teeth shape lead to the various
interspecific diet composition. Due to its wide-range distribution, dusky shark had a
more diverse diet. The Levin’s standardised measure of niche breadth indicated that
compared to the spinner shark had a high diet generalization, the silky shark had
medium dietary overlap with other sharks, and the dietary overlap between spinner
and dusky sharks was low. In addition the three carcharhinid sharks change their diets
as their size increase. However, the teeth shape of spinner shark restrict its diet to
large preys. On the contrary, the intraspecific diet composition varies many factors,
including by life stage, body size, maturity stage, environment, seasons and
characteristics of prey. All of the small and immature groups had similar prey items,
while the three species shift their diets to different groups as they grow up. As for sex,
the dietary compositions had no significant difference between them. According to
this study, the feeding activity of the three species increase during the night. In terms
of the season, although the dietary compositions were not significantly different
among seasons, prey-specific index of relative importance (%PSIRI) indicated the
variation of prey composition suggesting a seasonal shifting for sharks.
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