Effect of particle size and concentration on the feeding behaviour of adult pilchard Sardinops sagax

Abstract

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.

Respiration rate of adult pilchard Sardinops sagax in relation to temperature, voluntary swimming speed and feeding behaviour

“…Although there is no statistical difference between the mean RQ values of these 2 clupeoids ( t = -0.616, p = 0.55), the higher value for pilchard suggests a greater carbohydrate utilization by this species. This lends support to the hypothesis (Davies 1957, van der Lingen 1994) that phytoplankton is a more important dietary component for pilchard than for anchovy, although both species consume phytoplankton (James 1988).…”

“…The slopes of the predicted respiration rate/swimming speed relationships derived in this study demonstrate that at any given swimming speed, filter-feeding by adult Sardinops sagax is energetically cheaper than particulate-feeding. This result would tend to support the hypothesis that filter-feeding is the princ~pal food acquisition mode of pilchard (Davies 1957, King & Macleod 1976, van der Lingen 1994. However, the feeding mode that realises the greatest return, in terms of energy expenditure versus energy gain, is dependent upon the size and concentration of food particles present.…”

“…However, the feeding mode that realises the greatest return, in terms of energy expenditure versus energy gain, is dependent upon the size and concentration of food particles present. Pilchard have been shown to exclusively filter-feed on particles less than 1230 pm total length, and to filter-feed or particulate-feed on particles greater than this size, depending upon concentration (van der Lingen 1994).…”

“…Pilchard used for oxygen consumption rate experiments were collected and maintained as described by van der Lingen (1994). Nineteen experiments (Table 1) were conducted using a school of 10 adult fish (256.0 ?…”

“…Although all 3 species are planktivorous, each consumes different plankton fractions. Pilchard feed primarily upon phytoplankton (Davies 1957, King & Macleod 1976) and microzooplankton (van der Lingen 1994) and are essentially nonselective filter-feeders (van der Lingen 1994). Anchovy are predominantly particulate-feeders (James & Findlay 1989), with mesozooplankton such as copepods and euphausiids comprising their major dietary component (James 1987).…”