Larvae and Juveniles in the Surf Zone of Yangtze River Estuary

The Coilia nasus (Clupeiformes, Engraulidae) is a diadromous species widely distributed in China river estuary as a main commercial fish and high in production. To clarify daily-age structure, hatch period, growth characteristics and stay period of Coilia nasus larvae and juveniles in the surf zone of Yangtze River estuary, A total of 9358 individuals of Coilia nasus larvae and juveniles were collected monthly by a seine net (1×4m, mesh aperture 1mm) at 13 stations in the surf zone of Yangtze River estuary from May to October 2007, respectively. 594 individuals (body length 3.0-30.5 mm) were measured randomly by month on samples, and sagittal otolith were removed from specimens and fixed on a microscope side face up with epoxy resin. Rings outside the nucleus of the sagitta were counted with a light microscope at ×400, and the mean of two replicate count was used as the estimated ring number. Daily ages were estimated from the ring numbers add 1. Hatch dates were estimated from the daily ages and collection dates. The daily-age of Coilia nasus ranged from 7 to 34 days, mainly including 13 to 18 days, which accounting for 50.1% of the total abundance. Body length range was widened from June to August, and smaller individuals were little appeared in September and October. Body length (L, mm) and daily-age (D, days) were represented by a linear model whose fitted expression was: L = 0.73D + 5.09,R²=0.74. Back-calculated hatching dates and the spawning season of Coilia nasus in the Yangtze River estuary was spread over a long period. Hatching began on 23 may 2007 and continued until 4 October with a prominent peak from late May to early August, but little was observed after mid-August, indicating that after mid-August only a few eggs were hatched. The Coilia nasus larvae and juveniles began to appear in the surf zone at about 7 days after hatching, and then remained continually in the surf zone for about 23 days.