Preferred temperature of grass carp, Ctenopharyngodon idella (Valenciennes), and brema carp, Megalobrama amblycephala (Yih), (Pisces, Cyprinidae) in horizontal and vertical gradients

The preferred temperatures of grass carp, Ctenopharyngodon idella (Valenciennes), and brema carp, Megalobrama amblycephala (Yih), were determined individually and together in both horizontal and vertical gradients. No significant difference was found by two methods between the preferred temperatures in either species of carp (28.0-28.5 oC) when the fish were placed individually. However, when the two species of carp were placed together, C. idella was displaced thermally, preferring a temperature of 23.5 oC, which was significantly lower than that obtained when it was held separately. This suggests that, if both species of carp are placed in ponds forming part of a Chinese polyculture system, C. idella will seek out a lower temperature as a mechanism to avoid competition, and therefore, the growth of this species will be reduced.