POPULATION ECOLOGY OF LEIOCASSIS LONGIROSTRIS (GÜNTHER) (PISCES,BAGRIDAE) WITH REFERENCE TO THE PROBLEM OF MAXIMUM SUSTAINED YIELD

The long-snout catfish—Leiocassis longirostris (Günther) has long been a valuablecommercial fish in the Yangtze valley,yet practically no information on its biologicalfeatures can be obtained from the literature.Age-composition,growth,spawning,feeding and the problem of maximum sustained yield of this fish have recently beeninvestigated for the first time.As material for age-determination,the pectoral spine,urohyal and vertebra havebeen used for comparison,and the first-named structure is found to be more suitablefor this purpose.Formation of annulus in the pectoral spine occurs in the middleof June to July.The relationship between the age and the body length can be expressed by theequation L_t=9801-e-0.24(t-0.22).Owing to the fact that individuals of L.longirostris over 78.5 cm in length havea body form much more elongated than those under this length,the weight-lengthrelationship is better denoted by separate equations:For individuals under 78.5 cm in length:Wt=107301-e-0.24(t-0.22)2.9914;For those over 78.5 cm in length:Wt=86701-e-0.24(t-0.22)2.085.The relationship between the radius in the section of the pectoral spine and thelength of body is in linear regression.The length back-computed from the radius ofthe annulus generally coincides with the actual measurement of the body length,although there is indication of the so-called“anti-Lee phenomenon”in such back-computations.First maturity of L.longirostris occurs in individuals varied from 45.5 to 55.5cm in length and from 3 to 5 years of age.After the first maturity,reproductionrecurs in the successive years.It is found that the region from Chiengli to Hoshuiin the Yangtze River is the regular spawning ground.The individual absolute fecundity varies from 1,184 to 145,410 (69,264 on theaverage).Its relation to body length is expressed as R=0.0003311 L2.3613.The comparative spawning power is highest at 4 and 5 years old.The reproductivestock is composed of many age-classes,but the re-spawners are always dominant over the new spawners in number.L.longirostris is a predacious fish with a broad recipe,over 40 kinds of animalsbeing recovered from stomach contents.Small fishes of various kinds and shrimpsform the chief components.The total mortality of L.longirostris is calculated to be 0.425,with a naturalmortality of 0.235 and a catching mortality of 0.19.The actual harvestable age atpresent is from 4-years old onwards.The problem of maximum sustained yield isdiscussed.If the minimum harvestable age is to be raised to 5 years old and at thesame time fishing effort be intensified so as to elevate the fishing mortality to 0.30,the total yield of this fish might be expected to increase by 14 per cent,while thesize of the reproductive stock might still maintain a state of stability.