fish. Hardy’s (1924) herring study was based on gut-content analysis. Tuna are a
convenient subject because they eat relatively big prey (but also small zooplankton
like hyperiid amphipods, those perhaps as riders on swallowed jellyfish) that remain
identifiable in the stomach. Different species differ somewhat in diet, but all are visual
predators feeding primarily in daylight and crepuscular periods. They swim at high
speeds (bursts to five body lengths per second), usually attacking relatively large prey
and sometimes working in groups to corral them. Olson and Boggs (1986) provide a
gut-content analysis (Table 9.1) for yellowfin tuna (Thunnus albacares) caught in
purse seines in the eastern tropical Pacific (ETP). At least in this region and on across
to Samoa (Buckley & Miller 1994), fish are the dominant prey, particularly mackerel
and smaller tuna. Squid are important, especially for younger tuna. Galatheid crabs
eaten in the ETP are the small epibenthic Pleuroncodes planipes that occasionally
swarm at the surface. Over island banks near Samoa, crustacean prey are stomatopods
(mantis shrimp). Vertically migrating mid-water fishes like Vinciguerria are probably
eaten during dusk as they arrive near the surface. Recent work with pressure-
recording internal tags shows that tuna also make occasional dives into mesopelagic
layers, where they likely feed on bioluminescent prey (squid, gonostomatids,
myctophids). It appears that some prey, squid for example, decrease in the diet as
yellowfin tuna grow larger, while other prey like frigate tuna increase. Data in Table
9.1 are percentages of the overall diet, and the shifts with age are relative. In fact, at
13.4% of 10 g of food in its stomach, an “average” age-1 tuna has eaten 1.3 grams of
squid, while at 1.5% of 310 g of food, an age-4 tuna has eaten 4.7 g of squid. Some
foods, like pompano, a fish, do drop from the diet as the tuna grows, but most are still
on the menu, and the population impacts of growing tuna increase for most of their
prey.
Table 9.1 Food of yellowfin tuna, Thunnus albacares, in the eastern tropical Pacific
(5° to 15°N from Central America out to 140°W). Data are gut-content analyses by
Olson and Boggs (1986). Data are percentage of the total mass of prey in the guts.