Why pelagic planktivores should be unselective feeders

Diet width theory is a branch of optimal foraging theory, used to predict which fractions of the potential food encountered should be pursued. For pelagic fish, it is generally assumed that light is the dominant stimulus for both prey encounter rate and mortality risk. In order to achieve encounter rates allowing selective feeding, the pelagic predator exposes itself to enhanced predation risk for a prolonged time. The gain in growth obtained by diet selection may seldom outweigh the fitness cost of increased mortality risk. More generally, pelagic feeders will have a higher reproductive rate by searching the depth where feeding will be encounter-limited, and hence be opportunistic feeders. Literature reports of pelagic diet selection either fail to distinguish between the catchability of the prey in a gear and the encounter rate with its predator or neglects the vertical structure in pelagic prey distribution that may give differences in diets for unselective predators operating at different depths. The principal differences between the pelagic habitat and habitats where diet selection will be expected will include one or both of the following: (i) the continuous and steep local (i.e. vertical) gradients in mortality risk and (ii) the lack of local shelter where a newly ingested meal may safely be digested.