Gut contents (and other data) show that tuna may frequent areas where specific prey
are aggregating for feeding or mating. Around the mid-Atlantic São Pedro and São
Paulo Islands just north of the equator there is an aggregation of flying fish
(Cypselurus cyanopterus) from November to January. Yellowfin move inshore to feed
on them, and flying fish become the dominant meal in tuna guts (Vaske et al. 2003).
The observations are certainly biased toward flying fish as prey, because the tuna are
captured by hand-line fishing under night lights deployed to attract the flying fish.
However, Vaske et al. found they were only 42% of observed meals in 210 fish with
gut contents (of 395 total), and an ommastrephid squid, Stenoteuthis pteropus, was
also common (27%).
(^) Another example occurs in the Gulf of Guinea several degrees north of the equator.
Zooplankton are relatively rare along that latitude, and Vinciguerria nimbaria, usually
diel vertical migrators, apparently cannot gain enough food at night to support their
metabolism. So, they remain at the surface, feeding throughout the day. From sonar
and trawl results, Ménard and Marchal (2003) showed that they form schools about 15
m thick and 30 m diameter (some schools larger) at between 60 and 75 m depth, and
the schools cluster in non-random patterns. The fish, mostly adults 38–48 mm long
and ∼0.6 g, are fed upon by skipjack and juveniles of both bigeye and yellowfin tuna,
all ∼46 cm long and weighing 1.9 kg, that are in turn preyed upon by a purse-seine
fishery. Indeed, the gut contents of tuna from the fishery are V. nimbaria; neglecting a
few empty stomachs there were 1 to 150 (mean 45) of these small fish recently eaten
by each tuna. Using approximate digestion times, Ménard and Marchal estimated the
daily ration at 66 to 133 g d−1, or 3.5 to 7% of tuna body mass.
(^) Similar examples and similar variations can be found for a wide variety of pelagic