BIOLOGICAL STUDIES OF BRACHYMYSTAX LENOK(PALLAS)IN THE YALU RIVER

Brachymystax lenok(Pallas), a landlocked salmonid fish, is one of the most important commercial fishes in the Yalu River. Morphologically, its characteristics generally agree with the description of the same species reported from other river-systems; the numbers of pyloric caeca, gillrakers, and scales along the lateral line among different populations appear to have certain overlapping.Reproduction of this fish was studied, at one of its spawning grounds located about 5 kilometers above the mouth of Shi-san-dao-gou stream. Spawning takes place from early April to late May, but mostly in late April. Spawners are mainly 4 years of age and the smallest mature female collected was 309 mm in length. The absolute fecundity of individual fish ranging 285—438 mm in length and 315—1098 g in weight varies from 1629 to 3119(2194 in average). The relation ship between fecundity and body length can be described as N = 9.54924L—1297.3246, and that between fecundity and body weight as N=2.2207W+641.634. Eggs, 3.5—4.0 mm in diameter, are deposited in gravels. Under indoor conditions and at a temperature of 5.3—9.0℃, the hatching of the artificially fertilized eggs takes about 625 hours.Brachymystax lenok is a voracious fish, feeding mainly on benthic insects and small fishes, such as fry and juveniles of Leuciscus waleckii, Lota lota, etc.Scales have been used for the determination of age and growth of the fish. It shows that a three-year-old individual attains 356 mm in length and a five-year-old individual reaches 389 mm in length. The length-weight relationship for females can be described by the equation W = 0.004509L2.8978 and that for males by W = 0.002486L2.9146.Suggestions concerning fishery management of this fish are proposed.