EXPERIMENTS ON THE CURRENT OVERCOMING ABILITY OF SOME FRESHWATER FISHES

In desiging the magnitude of current velocity at the inlet of a fishway, the currentovercoming ability of fishes should be considered at first. A series of experiments wereconducted in the flume of a hydraulic laboratory. Various kinds of fishes were selectedfor experiments, such as carps (Cyprinus cxrpio); Crucian carps (Carassius auratus);silver carps (Hypophthalmichys molitrix); grass carps (Ctenopharyngodon idellus);mullets (Liza soiuy); whitefish (Erythroculter erythropterus); snake head (Ophiocephalusargus); catfishes(Parasilurusasotus); blunt--headed bream(Megalobrama amblycephala)etc. At the water temperature 10--27℃, and water depth 25--50 cm., the starting velocityfor various kinds of fishes is about 0.2m/sec. The optimum velocity ranges from thelower limit of 0. 3--0. 4 m/sec. to the upper limit of 0. 5--0. 8 m/sec., But the limitingvelocity for different kinds of fishes varies comparatively large. The upper limit of optimum velocity is generally considered as the preferable veloci-ty for fishway inlet. The value is about 15--30% less than the limiting velocity. The experiments denots denote that the current overcoming ability of fishes concernsmainly with their body shapes, body length and the variation of the water temperature. By analyzing the resllts of the experimentsand the observed date from some existingfishways, it indirates that the current overcoming ability of a fish is closely related to itsbody length. so that an empirical formula is proposed for estimation.