The Use of Food Samples from Sea Birds in the Study of Seasonal Variation 
in the Surface Fauna of Tropical Oceanic Areas 
Myrtle J. Ashmole and N. Philip Ashmole 1 
ABSTRACT: Many parts of the tropical oceans appear to be relatively seasonless, 
but, because of the difficulty of sampling mobile and patchily distributed animals 
and the cost of oceanographic investigations, few data are available on the extent 
of seasonal changes. By regularly collecting regurgitations from sea birds, and 
identifying and measuring the food items, seasonal data could be obtained on the 
availability, size classes, and perhaps reproductive cycles of the fish and squid char- 
acteristic of the surface layer of tropical seas. Flying fish (Exocoetidae), juvenile 
tunas (Scombridae) , and squid of the family Ommastrephidae are especially easily 
obtainable. 
Experience gained during a recent study of the comparative feeding ecology of 
sea birds on Christmas Island (Pacific Ocean) makes it possible to assess the char- 
acteristics of bird species which affect their suitability for such study. Potentially 
useful species include terns (especially Sterna fuscata , Anous stolidus, A. tenuirostris, 
and Gygis alba ) and boobies (especially Sula sula). Samples could be obtained from 
several bird species in the same period, and a program could include sampling of 
inshore waters with the neuston net and making basic oceanographic observations. 
Investigations of this kind could be carried out economically on any of a large num- 
ber of tropical oceanic islands. 
in assessing the results of a study of the feed- 
ing ecology of some sea birds of Christmas 
Island, in the central equatorial Pacific, we 
attempted to make use of published information 
concerning seasonal variations in the fauna of 
the surface layer of the sea in this region. It 
soon became evident, however, that this type 
of information is very sparse, and that those 
workers who have made seasonal comparisons 
have often had to depend on data collected in 
different areas, in different years, or by different 
methods (see, for instance, King and Demond, 
1953; King and Hida, 1957; King and Iver- 
sen, 1962). 
The absence of information on seasonal vari- 
ation in many tropical oceanic areas results 
mainly from the expense of running oceano- 
graphic ships, and the conflicting demands on 
their time, which generally prevent adequate 
sampling in a single area over a period as long 
as a year. Only in a few areas where there are 
1 Peabody Museum of Natural History, and Depart- 
ment of Biology, Yale University, New Haven, Con- 
necticut 06520. Manuscript received January 23, 1967. 
important commercial fisheries are good seasonal 
data available. Even in these areas, however, 
there are generally few data on changes in the 
abundance of the forage animals which form 
the food of most sea birds and of many fish 
used for human food. This is because nets used 
for routine plankton sampling do not catch 
many nekton animals, and even modern nets 
like Isaacs-Kidd trawls catch few fast-swimming 
animals near the surface (King and Iversen, 
1962; Pearcy, 1965). 
It would be of considerable interest to ob- 
tain more information on the extent to which 
seasonal fluctuations in physical and biological 
characteristics actually occur in those parts of 
the tropical oceans which appear to be more or 
less constant throughout the year. For instance, 
ornithologists working on tropical islands in 
several different areas have found that among 
the sea birds breeding on a single island (or 
group) some species have evolved regimes under 
which individuals breed at intervals of less than 
a year, suggesting that seasonal variation in the 
environment is not of great importance to them. 
1 
