EFFECT OF PARTICLE-SIZE AND CONCENTRATION ON THE FEEDING-BEHAVIOR OF ADULT PILCHARD SARDINOPS-SAGAX

Laboratory experiments investigated the effect of food particle size and concentration on the feeding behaviour of adult pilchard Sardinops sagax. This species employs 2 methods of feeding: filter-feeding and particulate-feeding. Particles of less than 1230 mum total length elicit a filtering response while larger particles elicit particulate-feeding at low concentrations and filter-feeding at high concentrations. Clearance rate during filtering is independent of particle size and concentration over the size range 393 to 1227 mum, with a mean value of 11.78 +/- 4.91 1 fish-1 min-1. This value is 93 % of the calculated maximum clearance rate for filtering fish, implying that pilchard are highly efficient at feeding over this size range. Particles of less than 206 mum are cleared at reduced rates, with retention efficiency inversely proportional to particle size. Clearance rates during particulate-feeding are greater than those for filtering, and increase with increasing particle size to a predicted saturation value of 46.53 1 fish-1 min-1. Pilchard display size-selectivity during particulate-feeding, preferentially removing larger prey items. Results indicate that pilchard is primarily a filter-feeder, obtaining the majority of its food from the smaller end of the size spectrum of available prey. Microzooplankton, and to, a lesser extent phytoplankton, are major dietary components. This is in contrast to Cape anchovy Engraulis capensis, which is primarily a particulate-feeder and derives most of its food from the larger end of the size spectrum of available prey. The different feeding responses of these 2 co-occurring species are discussed with reference to resource partitioning.