J. theor. Biol. (1995) 173, 41-50 Why Pelagic Planktivores should be Unselective Feeders JARL GISKEt AND ANNE GRO VEA SALVANES University of Bergen, Department of Fisheries and Marine Biology, Hoyteknologisenteret, N-5020 Bergen and tlnstitute of Marine Research, Boks 1870 Nordnes, N-5024 Bergen, Norway (Received on 29 January 1994, Accepted in revised form on 18 July 1994) 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. 1. Introduction Optimal diet width theory was put forward to find the number of prey types an optimal predator should pursue. Under the assumption that the optimal for- agers maximize net energetic gain rate, prey types could be ranked according to their net energetic gain (E/H; energy per handling time) obtained for each prey type. A diet including m - 1 prey types should not be expanded to also include the m-th ranked item if the average gain rate from the m- 1 prey types would be higher than the gain rate from the m prey types: m--I E.,/H., <<. ~ 2j(Ej - E,,,Hj/H.,) (I) j=l (Schoener, 1971; Charnov, 1976; Townsend & Winfield, 1985) where 2/, Ej and Hj are encounter rate, energy content and handling time of prey type j, respectively. (All symbols used in all equations are explained in Table 1.) Handling time is "the time 0022-5193/95/050041 + 10 $08.00/0 41 needed to pursue, handle and swallow a single item" (Schoener, 1987: 12). Time elapsed due to digestion constraints are thus not incorporated in H. There is no emphasis of spatial differences in prey distributions in equation (l)--they are eventually reflected in the 2is. A selective diet implies that the predator encounters potential prey organims which it does not chase. The fundamental parameter is the predator-prey encounter rate 2j. (Often diet selection is claimed on basis of prey abundance indices instead of encounter rates. Only when one can ascertain that the predator and the scientist or the sampling tool has equal prey encounter rates, can abundance indices be used. This is seldom the case in an aquatic environment.) As eqn (I) was developed in the early stages of optimal foraging theory (c.f. Schoener, 1987), the influence of predation risk was not mentioned in these diet selection models. Townsend & Winfield (1985) concluded that doing so "would be a significant step forward". Gilliam (1990) combined the optimal diet (" 1995 AcademicPress Limited