Apex Predation by Yellowfïn Tuna (Thunnus albacares): Independent Estimates from Gastric Evacuation and Stomach Contents, Bioenergetics, and Cesium Concentrations

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Abstract
Three approaches for estimating predation by yellowin tuna (Thunnus albacares) were compared: (1) stomach analysis adjusted for gastric evacuation; (2) food energy required as a function of swimming speed in yellowfin tracked at sea; and (3) food intake needed to maintain observed cesium concentrations. Gastric evacuation data from captive yellowfin were best fit by linear functions of time for four foods. Fish with high lipid content (mackerel, Scomber japonicus) were evacuated at a slower rate (proportion per hour) than smaller fish (smelt, Hypomesus pretiosus), squid (Loligo opalescens), and small fragile fish (nehu, Stolephorus purpureus), all of which had lower lipid contents. Tuna captured in the eastern Pacific had daily rations averaging 3.9% of body mass based on stomach contents and gastric evacuation rates, 5.2% based on bioenergetics estimates, and 6.7% based on the cesium estimate. Swimming costs accounted for one-third to one-half of the energy budget. Annual predation by the eastern Pacific yellowfin population averaged 4.3-6.4 million metric tons during 1970-72, depending on the method used for estimating ration; 34% was frigate tunas (Auxis spp.). High growth and turnover rates (P/B ratios) of tropical tunas in contrast with low conversion and trophic transer efficiencies suggest a trophic structure that differs from more productive ecosystems.

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