Introduction

Abstract

The productivity of the oceans ultimately determines the numbers of seabirds, and regional variations in the abundance of food have a profound effect on their distribution. It has been argued that food is the resource that is most commonly limited in supply, as far as seabirds are concerned (Evans, 1980), and restricted food niches may be partly responsible for the relatively small number of species of seabirds. Thus, out of a total of 8,600 species of birds, approximately 285 may be classified as seabirds. They belong to fifteen families representing four orders (Fig. 1). The exact number of species depends on the definition of a seabird on the one hand, and a species or race on the other. It is apparent from Fig. 2 that approximately 97% of all species of birds are confined to one third of the world’s surface, while only 3% utilize the remaining surface, the oceans. This distribution suggests a general scarcity of available food for seabirds, and indeed vast central areas of the oceans are relatively unproductive, particularly in the tropics and subtropics. In general, the areas of highest productivity are the seas overlying the continental shelves, and in the best areas, the sea can grow as much food as the most fertile land.