Dynamics of prey moving through a predator field: a model of migrating juvenile salmon

The migration of a patch of pl-ey through a field of relatively stationary predators is a situation that occurs frequently in nature. Making quantitat ive predictions concerning such phenomena may be difficult, however, becaus e factors such as the number of the prey in the patch, the spatial length a nd velocity of the patch, and the feeding rate and satiation of the predato rs all interact in a complex way. However, such problems are of great pract ical importance in many management situations; e.g., calculating the mortal ity of juvenile salmon (smolts) swimming down a river or reservoir containi ng many predators. Salmon smolts often move downstream in patches short com pared with the length of the reservoir. To take into account the spatial de pendence of the interaction, we used a spatially-explicit, individual-based modeling approach. We found that the mortality of prey depends strongly on the number of prey in the patch, the downstream velocity of prey ill the p atch, and the dispersion or spread of the patch in size through time. Some counterintuitive phenomena are predicted, such as predators downstream capt uring more prey per predator than those upstream, even though the number of pl-ey may be greatly depleted by the time the prey patch reaches the downs tream predators. Individual-based models may be necessary for complex spati al situations, such as salmonid migration, where processes such as schoolin g occur at fine scales and affect system predictions. We compare some resul ts to predictions from other salmonid models. (C) 2000 Elsevier Science Inc . All rights reserved.