ANNULUS VALIDATION AND AGE AND GROWTH ESTIMATION OF ANADROMOUS COILIA ECTENES IN THE YANGTZE ESTUARY

Anadromous Coilia ectenes (Jordand et Seale) is a commercially important fishery resource in the middle and lower reaches of the Yangtze River.It usually feeds and overwinters in the Yangtze Estuary and the coastal waters Adults migrate up into the Yangtze River and its affiliated lakes in the spawning season.After spawning,they migrate down into the sea.Offsprings feed in freshwater and migrate down into the sea before overwintering.Resource of this fish has declined dramatically since late 1970s.The present research validated annuli in sagittal otolith and analyzed the age and growth of C.ectenes from the Yangtze Estuary to evaluate the resource condition of the population.A total of 576 individuals of C.ectenes were collected in the Yangtze Estuary monthly during April 2006 to August 2007.No pe-riodic opaque/translucent zones were visible in the transverse plane of the sectioned sagittal otolith when examined with either transmitted or reflected light.Instead,periodic straight-curved zones,similar to those in sagittal otoliths of Coilia mystus10,were observed along the ventral growth axes in the transverse plane under transmitted light.The growth axes from the core were generally straight initially,and then gradually curved to the distal side until a sudden break formed.After the break,the growth axes were straight,and the straight-curved zones were typically repeated,particularly in large individuals.In April,the margin of the growth axis was straight in some individuals,and the proportion increased in May and June.The margin of the growth axis was straight for all the individuals during July through September.In October,a small proportion of the sampled individuals displayed a curved growth axis,and this proportion increased in December.Marginal increment ratio analysis also showed an annual periodicity of the straight-curved growth increment.The shift from curved growth to the next straight growth occurred during April and June.We defined this shift as an annulus mark.The collected specimens were consisted of 5 age classes ranged from 0 to 4 years,dominated by age-1 and age-2 individuals.Both the males and females showed a similar age structure and growth pattern.The standard length (SL) and age (t) data were fitted to the von Bertalanffy growth function SL=327.691?e-0.51(t+0.28) (n = 576,r2 =0.71,P 3.36 (n = 576,r2 = 0.95,P< 0.05).Compared to the age and size structures of the harvest in 1970s,the harvest during our research was younger in age and smaller in size of the same age class fish.This change in age composition and size structure indicated that the anadromous C.ectenes has been overexploited.Reducing fishing intensity is fundamental for fisheries management and resource conservation of this fish in the Yangtze River.