8 
PACIFIC SCIENCE, VoL XXII, January 1968 
haps Gygts alba feeding at dawn on animals 
which visit the surface during the night), all 
the terns studied are dependent on schools of 
tuna and other predatory fish to drive fish and 
squid to the surface: many of the prey animals 
are caught by the birds while they are actually 
jumping out of the water in efforts to escape the 
predators below. The larger birds like Phaethon 
rubricauda and S. sula, which can penetrate the 
water to a depth of at least 1 meter, may be able 
to catch prey which are not disturbed by other 
predators, and so may be partially independent 
of the presence of predatory fish. 
Tuna schools are by no means randomly dis- 
tributed over the tropical oceans, but occur 
largely in those areas where the animals on 
which they feed are most concentrated. In an 
earlier paper (Ashmole and Ashmole, 1967) 
we have discussed the evidence that in the open 
ocean convergence at "fronts” produces local 
concentrations of plankton and nekton, even in 
areas where the overall density of organisms is 
very low, and that these concentrations attract 
tuna schools. The existence of large popula- 
tions of sea birds — like Sterna fuscata — which 
can range great distances from their islands 
even when breeding, probably depends on the 
presence of rich feeding areas of this kind. 
Clearly a knowledge of the areas where the wide- 
ranging species are feeding will help in inter- 
preting the data obtained by the analysis of 
their food. The inshore-feeding sea birds, on 
the other hand, make use of the concentrations 
of prey animals, and of surface-feeding tunas, 
which are often present in the lee of islands. 
Thinking that there might be a close corre- 
spondence between the food of surface-caught 
tunas and the food of the birds, we studied 
records of stomach contents of Yellowfin Tuna, 
Neothunnus macropterus (Temminck and 
Schlegel) , kindly made available by the Hono- 
lulu Biological Laboratory of the U.S. Fish and 
Wildlife Service. The data used were for Yel- 
lowfin caught at the surface within 10 miles of 
Christmas Island and neighboring islands, but in 
different years from our sampling of the food 
of the birds. We found that there were strik- 
ing differences between the diets of the tunas 
and of the birds, even if the comparison was 
restricted to the birds which fed close inshore. 
The tunas took a far higher proportion of reef- 
originating fish than did any of the birds, 
and presumably this reflects the fact that tunas 
can quickly and easily change from feeding 
right at the surface to feeding at considerable 
depths or around reefs, while the birds must 
wait for their prey to come to the surface. 
After they have completed larval life, typical 
reef-inhabiting fish such as Acanthuridae evi- 
dently do not often come to the surface even 
when pursued, and so are rarely eaten by birds ; 
however, they were eaten by the tunas in con- 
siderable numbers. (The pelagic larvae were 
also eaten by tunas; they may also sometimes 
be taken by birds, and it must be remembered 
that we were able to identify only a small 
proportion of the partly digested larval fish 
found in samples from A. tenuirostris at certain 
times of year.) In contrast, Exocoetidae were 
unimportant in the diet of the tunas, but are of 
great importance in the diets of all the sea 
birds studied; probably the ability of these fish 
to escape submarine predators by flying renders 
them especially vulnerable to aerial predation by 
birds. 
Attempts to sample forage animals near the 
surface of the central Pacific during the daytime, 
by means of oblique hauls with Isaacs-Kidd 
and other trawls, yielded so little material that 
they were abandoned in favor of night hauls, 
which produced catches containing far more 
organisms of generally larger size (King and 
Iversen, 1962). Although the greater number 
of organisms in night hauls partly reflects the 
movement to the surface layers at night of 
animals which are mesopelagic during the day, 
it is of interest that very few Exocoetidae, 
Scombridae, or Gempylidae, or squid of the 
family Ommastrephidae, were identified in 
either the day or night samples studied by 
King and Iversen. Since members of all these 
groups are commonly caught by birds feeding 
at the surface, it may be that they are such fast 
swimmers that they can generally dodge the 
nets (cf. Pearcy, 1965:266). Alternatively, they 
may be extremely patchily distributed, or so 
strictly confined to the surface that they are 
only rarely encountered by nets used in oblique 
hauls. In any case, it is clear that members of 
these groups are much more efficiently sampled 
by the birds. 
In view of the commercial importance of tuna 
