Effect of particle size and concentration on the feeding behaviour 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: filterfeeding and particulate-feeding.Particles of less than 1230 pm 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 pm, with a mean value of 11.78 2 4.91 1 fish-' min-l.This value is 93 % of the calculated maximum clearance rate for flltenng f ~s h , Implying that pilchard are highly effic~ent at feedlng over this size range.Particles of less than 206 pm 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 slze to a predicted saturation value of 46.53 1 fish-' min" Pilchard display size-selectivity during parbculate-feed~ng, preferentially removing larger prey Items.Results indcate that pilchard is primarily a filter-feeder, obtaining the majonty of ~t s food from the smaller end of the size spectrum of available prey Mlcrozooplankton, and to a lesser extent phytoplankton, are major dietary components.T h ~s IS in contrast to Cape anchovy Engraulis capenas, which is primarily a particulate-feeder and derives most of ~t s food from the larger end of the size spectrum of available prey.The different feeding responses of these 2 CO-occurring species are discussed w ~t h reference to resource partitioning.