Diet Composition and Ontogenetic Shifts of Skipjack Tuna (KATSUWONUS PELAMIS) in Hawai'i Inferred From Stomach Contents and Stable Isotope Analyses
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Vazquez, Alondra Islas
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Marine Science
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Skipjack tuna (Katsuwonus Pelamis) contribute high socio-economic value to tuna fisheries in the Pacific Ocean. Understanding their feeding ecology and the food webs that support them is critical to predict how environmental variability will affect their stocks. Skipjack tuna are opportunistic predators and consume a wide range of prey, including fish, squid, and crustaceans. Previous studies have found dietary differences across oceanographic domains, with varying regional importance of fish and crustaceans. Additionally, studies have documented ontogenetic diet shifts, where different sizes of tuna consume different prey. This is the first study describing the diet composition of skipjack tuna, ranging from 40 to 84 cm fork length (FL), in Hawaii using stomach-contents and stable isotope analyses. Tuna stomach contents were classified into three broad groups: fish, squid, and crustaceans. DNA barcoding was used to identify prey to the lowest taxonomic level possible (family, genus or species). From the stomach-contents, a species richness of 24 prey taxa (16 fishes, 5 crustaceans, 3 mollusks) was described, with the majority of the species diversity consisting of reef-associated fishes. However, the most important prey group were crustaceans with an index of relative importance (IRI) of 78.4%. Using time-integrated diet estimates from Bayesian isotope mixing models, two crustacean functional groups together contributed the highest proportion (61.1%) to the diet. The complementary approaches of stomach-content and stable isotope analyses demonstrated crustaceans dominate the diet of skipjack tuna around Hawaii. This study did not detect an ontogenetic diet shift for skipjack tuna based on their δ15N values. However, skipjack tuna showed a significant change in their carbon source at a size of 68.8 cm fork length. Based on this δ13C difference, this result indicates that small (< 68.8 cm FL) and large (> 68.8 cm FL) skipjack are supported by different food webs and likely foraging at different locations
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Hawaii Pacific University