How pebble size affects logperch (Percina caprodes) foraging behavior

Background/Question/Methods

The logperch (Percina caprodes) is a benthic darter species native to central North America that forages by using its unique conical snout to push away or flip over pebble or gravel particles to uncover the invertebrate prey. Our objectives were to (i) better describe “stone flipping” behavior, and (ii) experimentally test the degree to which stone size (dimensions and weight) influences logperch foraging success (i.e., the fish’s ability to flip stones and consume prey items). We hypothesized smaller fish would only be able to shift small stones, whereas larger fish would be able to shift stones across an increased range of substrate sizes. Experiments were conducted in aquaria using wild-caught logperch acclimated to the laboratory. The experimental tank contained a layer of uniform rounded bean-sized aquarium gravel. Prey “cups” were uniformly spaced in a 3 x 3 matrix and embedded in gravel. Each cup contained two frozen chironomids. Alike substrates were placed atop each cup. Our four treatments used stones of dimensions and weight: 1.3 x 1.3 cm (4.1 g), 2.7 x 2.7 cm (18.3 g), 3.5 x 3.5 cm (30.9 g), and 4.8 x 4.8 cm 30.4 g). Seven fish were individually tested in each treatment. Individuals were deprived of food for 24 h, placed in the test aquarium, and videotaped for 8 h to record its behavior and the time and in which order the fish dislodged available stones.

Results/Conclusions

Video playback analyses showed logperch dislodge stones three distinct ways: (i) they push them aside using their snouts and vigorous swimming motions (a “push”); (ii) they nudge them slightly and use their snouts to roll them aside (a “nudge-roll”); (iii) they wedge their snouts beneath the stone margin and flip the stone over and away (a “flip”). After moving the stone, uncovered prey were then consumed. Preliminary analyses support our hypothesis that small fish tended to “push” or “nudge-roll” the smaller stones. Large fish generally “flipped” the stones and were the only fish able to dislodge larger stones. Preliminary analyses appear to confirm our hypotheses that foraging success is negatively related to fish size and substrate size. In the wild, invertebrate numbers, substrate embededdness, and substrate sizes are likely influenced by changes in basin land use patterns and the resulting hydrograph patterning. Changes in logperch foraging success are likely to reflect such changes, and may be one factor controlling their declines in various locations.