An open water existence limits the foraging options available to pelagic fishes. As a result, the fishes feed on phytoplankton, zooplankton, or each other. Many clupeoids utilize phytoplankton directly by swimming through plankton concentrations with an open mouth, thereby filtering the particles out of the water in a pharyngeal basket that has densely packed gill rakers (100–300/cm) and includes an epibranchial organ that releases digestive enzymes while the food is still in the oral region. The digestive tract is long and has numerous pyloric caeca. Food passes very rapidly through this system, often taking less than an hour, but these fish can utilize a broad array of food types and are very efficient at converting food into protein.
The foraging and migratory patterns of such pelagics as tunas and billfishes become clearer when the nature of food availability in open tropical seas is considered. Estimates of zooplankton resources in the central Pacific indicate average densities on the order of 25 parts per billion. Large pelagic predators are feeding at even higher trophic levels, so their food is scarcer by one or two orders of magnitude. Since no animal is going to survive on food distributed evenly at such low densities, the success and rapid growth rates of many tunas attest to the extreme patchiness of food on the high seas. A nomadic life style, driven by high metabolism and rapid swimming, makes sense when vast expanses must be covered in search of such patchily distributed resources (Kitchell et al. 1978).
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