Food Choices of Fish

But optimal foraging strategies may not remain identical throughout an organism’s lifespan and many species exhibit a trophic ontogeny or shift  in food types and sizes, or feeding styles at different stages in their life cycle. These are necessitated by the fishes own increase in size, by morphological developments or by habitat shifts that accompany growth.

Adult fishes consume a great variety of foods and exhibit a great variety of feeding styles, although basically all feeding mechanisms may be categorize as either biting, ram feeding, or suction feeding based on how food gets into the mouth. Different types of food are then concentrated and processed for “swallowing” by several methods. Foods range in size from microscopic phytoplankton, bacteria, and detritus, through a progressively  larger  series of zooplanktons, and culminating in larger invertebrates and other vertebrates. Some fish also consume multicellular algae and vascular plants.

Although in general there is a positive correlation between the size of a fish’s mouth and the size of the prey it consumes, there are some notable exceptions. Some of the largest fishes, for example whale sharks (Rhincodon typus) and basking sharks (Cetorhinus maximus), use their huge mouths to engulf and filter large quantities of small organisms. But what is “small” can be relative a 20 meter whale shark may filter feed on juvenile tunas that are themselves 25–30 cm in length. At the other end of the size range of sharks, the diminutive 15–50 cm cookie-cutter sharks Isistius brasiliensis and I. plutodus feed on whales and other large oceanic creatures, although they do so one small bite at a time.