Effect of algae density on breathing and feeding of filter-feeding silver carp (Hypophthalmichthys molitrix Val.)

• The fish can reduce its clearance rate by reducing its gill ventilation, etc.
• Anti-filtering response of the fish is another adaptation to high plankton biomass.
• It can improve its oxygen extraction efficiency in high algae biomass environment.

The respiratory process of silver carp, a typical filter-feeding fish, works in conjunction with its feeding mechanism when it filters plankton in water. In the present study breathing and feeding of silver carp were measured in response to increases of algae density from 0 to 242 mg/L in order to explore the relationship between breathing and feeding in higher algae biomass environments. The results showed that (1) the oxygen consumption rate (VO2) of the fish increased significantly with increases in algae density (P < 0.05), but there were no significant differences in VO2 among fish at algae densities of 63.3–242 mg/L (P > 0.05). The respiratory frequency (fR), gill ventilation (VG) and VG/VO2 of the fish did not show significant differences among algae densities of 0–23.8 mg/L (P > 0.05). However, when algae density increased to 63.3 mg/L, the fR, VG and VG/VO2 increased significantly and reached a peak, but then declined significantly with further increases in algae density (P < 0.05). With increases of algae density, oxygen extraction efficiency (EO2) first declined and then increased, with the lowest value occurring at an algae density of 63.9 mg/L. The EO2 was negatively correlated to VG in the present study (P < 0.05). There were no significant differences in respiratory stroke volume (VS.R) among fish at algae densities of 0–242 mg/L (P > 0.05). (2) The filtration rate (FR) of silver carp increased significantly with increases of algae density, but did not show significant differences at levels of 63.3–242 mg/L (P > 0.05). The changes of the clearance rate (CR) and filtering efficiency (E) of the fish showed the same trend, in which the highest values occurred at algae densities of 63.3 mg/L and 23.8 mg/L, respectively (P < 0.05). In addition, an anti-filtering response occurred at algae densities over 138 mg/L. The present studies indicate that in order to acclimate to environments with higher algae biomass, silver carp can actively reduce clearance rate through a decline of filtering efficiency and/or gill ventilation.