Effects of starvation on the breathing and feeding of filter-feeding silver carp (Hypophthalmichthys molitrix Val.)

The respiratory process of silver carp, a typical filter-feeding fish, works in conjunction with its feeding mechanism when it filters plankton from water. In the present study, a variety of parameters associated with breathing and feeding of silver carp were measured in response to decreases in dissolved oxygen (DO) levels when the fish were fed microalgae Chlorella ellipsoidea. This approach enabled exploration of the relationship between breathing and feeding in starved or non-starved carp while filtering microalgae particles at different DO levels. The results showed that (1) in the no-feeding group (starved group), respiratory frequency (fR) and oxygen consumption rate (VO2) of fish did not show significant differences among DO levels of 0.89 to 7.43 mg·L−1 (P > 0.05). Normal DO levels (5.28-7.43 mg L−1) did not significantly affect respiratory stroke volume (VS.R), gill ventilation (VG), VG/VO2, filtration rate (FR) or filtering efficiency (E) of fish (P > .05).When DO levels declined to 4.14 mg L−1, the VS.R, VG and VG/VO2 of fish began to increase significantly (P < .05), while the FR and E of fish began to decrease significantly (P < .05). There are no significant differences in the clearance rate (CR) of fish among DO levels of 3.17 to 7.43 mg L−1 (P > .05). The oxygen extraction efficiency (EO2) of fish increased significantly with decline of DO levels from 7.43 mg L−1 to 0.89 mg·L−1 (P < .05). (2) In the feeding group (non-starved group), normal DO levels (5.28-7.43 mg L−1) did not affect significantly the fR, VO2, VS.R, VG and VG/VO2 of fish (P > .05), but these parameters began to increase significantly, while DO levels declined to 3.17 mg L−1 (P < .05). With the decline of DO levels, EO2, FR and CR of fish increased and reached the peak at a DO level of 3.17 mg L−1, and these parameters later began to decrease, while DO levels further declined to 1.90 mg L−1. The E of fish decreased significantly when DO levels declined from 7.43 mg L−1 to 0.89 mg L−1 (P < .05). (3) The fR, VO2, VG, VG/VO2, FR and CR of fish in the no-feeding group were significantly higher than those in the feeding group at DO levels ranging from 4.14 to 7.43 mg L−1 (P < .05), while the EO2 and E of fish in the no-feeding group were significantly lower than those in the feeding group at DO levels of 3.17 to 7.43 mg L−1 and 5.28 to 7.43 mg L−1, respectively (P < .05). In addition, an anti-filtering response appeared at DO levels of 1.90 mg L−1 and 0.89 mg L−1 in both groups.