| dc.description.abstract | Field studies have shown that young gizzard shad (Dorosoma eepedianum) are carnivores, visually feeding on individual zooplankton. Shad larger than 30 .mm are omnivores, feeding predominantly as filter-feeders on both
phytoplankton and zooplankton. This study experimentally identified and quantified the causal mechanisms determining the feeding selectivity and feeding rate of filter-feeding gizzard shad. Laboratory observations found shad to filter-feed by inhaling water and food through expansion of the buccal and
opercular cavities. Shad did not visually select and attack individual zooplankter prey items, but swam through the water inhaling water containing prey with a rapid series of undirected suctions. Shad filtering rate, the volume of water inhaled per minute, was equal to the multiple of the volume of the expanded buccal cavity and the pumping rate. I determined buccal volume by making plaster of Paris molds of the expanded buccal cavity. The volume of the expanded buccal cavity increased as a power function of shad length. Pumping rate, measured by high speed movie films and visual observation, decreased with shad length. Filtering rate increased as a power function of shad length with shad 17 cm in standard length filtering over one liter of
water per minute. The actual rate that particles were inhaled and ingested was determined.by the shad's capture efficiency and filtering efficiency. Capture efficiency is a function of both the shad's capture and the prey's escape mechanisms. A shad's suction pump mechanism creates a flow into the mouth similar to flow into a pipe. I simulated a fishlike suction intake using a siphon system which afforded control over the three variables of fish suction intakes; mouth opening size, buccal volume, and buccal expansion rate. The simulated suction inhaled 10 ml into a tube 1.0 cm in diameter in 0.4 sec. The capture probability of the. simulated suction for zooplankter prey was highest for the cladocerans Ceriodaphnia reticulata (P = •96), Daphnia galeata mendotae (P = .92), and Daphnia pulex (P = .76); intermediate for cyclopoid copepods (mostly Cyclops sp. and Mesocyclops sp.) (P = .28) and Cyclops scutifer (P = .24); and lowest for the calanoid copepod Diaptomus pallidus (P = .07). To test the results of the capture experiments, the relative feeding rates of gizzard shad on a mixture of different zooplankton were determined in laboratory experiments and compared to predictions based on the capture probabilities. Shad feeding rate constants, k (liters/hr), were lowest on Diaptomus spp. (xk = .67), intermediate on cyclopoid copepods (xk = 1.37) and highest on D. galeata
mendotae (xk = 3.60), C. reticulata (xk = 3.00) and copepod nauplii (xk = 4.01). These experiments show that differential capture probabilities of nonvisual-feeding planktivores result in an apparent feeding selectivity for zooplankton which have poor escape ability. Particles inhaled into the mouth are filtered from the water by the gill rakers. The shad's filtering efficiency was determined by measuring the interraker spaces. Cumulative frequencies of interraker distances weighted for raker length show that filtering efficiency for particles 1 to 70 microns was a hyperbolic function of particle size with particles 7 0 microns or larger filtered with 100% efficiency. This filtering efficiency would result in an apparent feeding selectivity for large algae versus small algae.
The feeding rate of filter-feeding gizzard shad on a particular prey type was equal to the multiple of 4 factors: (1) prey density, (2) shad filtering rate, (3) shad capture efficiency, and (4) shad filtering efficiency. This feeding rate model was confirmed in an experiment which compared computer simulated to observed shad feeding rates. | |
