Filter-Feeding Behavior and Particle Retention Efficiency of Sacramento Blackfish

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Abstract
We analyzed the feeding behavior and particle retention efficiency of pump filter‐feeding Sacramento blackfish Orthodon microlepidotus presented with plastic microspheres and zooplankton. The rate of water processing by Sacramento blackfish was the product of buccal volume and pumping rate. Buccal volume increased exponentially with fish size whereas pumping rate decreased with fish size. Pumping rate, determined from slow‐motion videotape playback, was positively correlated with temperature and unaffected by prey density. The amount of time spent feeding was correlated with temperature and prey density, but uncorrelated with fish size. Feeding experiments with plastic microspheres indicated that Sacramento blackfish from 5.1 to 14.8 cm standard length retained particles larger than 35 μm, while larger fish (28.4–30.8 cm) retained particles larger than 25 μm. Although fish ranging from 5 to 31 cm standard length all filter‐fed, their behavior changed as they grew. Small fish tended to feed visually more than did larger fish. This resulted in higher selectivity for large zooplankton by small fish. Filtering rates were very similar to those observed for other pump filter‐feeding fishes.