Prey Detectability for Fish-Eating Birds in Relation to Fish Density and Water Transparency

Two models are developed that demonstrate how changes in density of freshwater fish and ensuing changes in water transparency affect prey encounter rate in fish-eating birds. "Pursuit divers" (e.g. divers and mergansers), which search for food while swimming with their eyes below the water surface, may be compensated for reduced densities by increases in transparency. "Surface plungers" (e.g. terns, fish-eating raptors) can reach prey only in the uppermost volume of water, so increases in transparency cannot compensate for declines in fish density. Only by an increase in search height can the extent of the surveyed volume of water where fish are available be increased. A test of the model for two "pursuit divers", Gavia arctica and Mergus merganser, using field data concerning the selection of 16 lakes in relation to fish density, water transparency, and a "fish detectability index" (combining data on fish density and water transparency), suggests that this model applies for these species in oligotrophic lakes in south-west Sweden.