WHY PELAGIC PLANKTIVORES SHOULD BE UNSELECTIVE FEEDERS

Diet width theory is a branch of optimal foraging theory, used to pred ict which fractions of the potential food encountered should be pursue d. For pelagic fish, it is generally assumed that light is the dominan t stimulus for both prey encounter rate and mortality risk. In order t o achieve encounter rates allowing selective feeding, the pelagic pred ator exposes itself to enhanced predation risk for a prolonged time, T he gain in growth obtained by diet selection may seldom outweigh the f itness cost of increased mortality risk. More generally, pelagic feede rs will have a higher reproductive rate by searching the depth where f eeding will be encounter-limited, and hence be opportunistic feeders. Literature reports of pelagic diet selection either fail to distinguis h between the catchability of the prey in a gear and the encounter rat e with its predator or neglects the vertical structure in pelagic prey distribution that may give differences in diets for unselective preda tors 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 ste ep local (i.e. vertical) gradients in mortality risk and (ii) the lack of local shelter where a newly ingested meal may safely be digested.