|Other Abstract||Anadromous Coilia ectenes (Jordand et Seale) is a commercially important fishery resource in the middle and lower reaches of the Yangtze River, the resource of which has declined dramatically since late 1970s. Fish Sampling were conducted in the Yangtze Estuary at Chongming (31.58oN, 121.73oE), Jingjiang (31.95oN, 120.27oE) and Anqing (30.5oN, 117.05oE) in the lower reach of the Yangtze River as: for females and males at Chongming in each month from April 2006 to August 2007; females and males at Jingjiang and Anqing in each month from April to August in 2006 and 2007; mature females( Stage Ⅳ or stage Ⅴ) at Chongming in May and June 2008 and mature females at Jingjiang and Anqing in July 2008. A total of 1115 individuals were collected. The otolith annuli were validated with the individuals collected at Chongming from April 2006 to August 2007, the age structure, growth and gonad development of the individuals collected at the three locations in 2006, 2007 and 2008 were investigated, the differences in age structure, standard length and the first annuli width among reproductive population with different migration distance were analyzed with the mature males and females collected at the three locations from April to August in 2006, 2007 and 2008, reproductive traits of the mature females collected at the three locations from April to August in 2006, 2007 and 2008 were analyzed and the changes in proximate composition and estimates of energetic costs during the spawning migration were analysed with the individuals collected at the three locations from April 2006 to August 2007.
1. Examination of transverse plane of the sectioned sagittal otoliths revealed a periodic straight/curved growth pattern. The straight zone occurred during April and December, and curved zone during October and June. 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 annuli mark.
2. The collected specimens were consisted of 5 age classes ranged from 0 to 4 years, dominated by age-2 individuals, but a harvest of C. ectenes between 1973 and 1975 was consisted of 7 age classes ranged from 0 to 6 years, dominated by age-3 and age-4 individuals (Yuan et al.，1978). Again, the maximum lengths of age 3 and age 4 females and age 2 to age 4 males were at least 10% smaller than the harvest with the same age and gender between 1973 and 1975 (Yuan et al.，1978), revealing that the harvest during the present 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.
3. The standard length of females and males at age 0 has no difference; but that of age 1, age 2 and age 3 males were smaller than that of the same age females. The standard length (SL) and age (t) data were fitted to the von Bertalanffy growth function, represented as SL=362.55(1-e-0.42(t+0.30)) (n = 392, r2 = 0.65, p < 0.05) for males and SL=348.25(1-e-0.53(t+0.23)) (n = 723, r2 = 0.73, p < 0.05) for females.
4. Mature males were consisted of individuals aged 1 to 3 years, including 3 age classes, which were of age 1 (3.2%), age 2 (93.6%), and age 3 (3.2%) at chongming, age 1 (7.1%), age 2 (35.7%) and age 3 (57.2%) at Jingjiang, and age 1 (2.6%), age 2 (92.3%) and age 3 (5.1%) at Anqing. Mature females were consisted of individuals aged 1 to 4 years, including 4 age classes, which were of age 1 (13.2%), age 2 (84.6%), and age 3 (2.2%) at chongming, age 2 (64.9%), age 3 (31.2%) and age 4 (3.9%) at Jingjiang, and age 2 (87.3%) and age 3 (12.7%) at Anqing. While Standard lengths of age 2 mature females and males were significantly larger at Anqing than at Jingjiang and at chongming, that of age 3 mature females were significantly larger at Anqing than at Jingjiang. The first annular radius in sectioned sagittal otoliths of females and males were significantly smaller at Anqing than at Jingjiang and at chongming. As the spawning season at Anqing was delayed than at Chongming and at Jingjiang (Li et al., 2007), growth season of the offspring from birth to the first annulus formation was shorter for offspring at Anqing, which might have resulted in a smaller first annular radius, which were corresponding to the smaller first annulus radius of the spawning females at Anqing indicated that they may be offspring born there, giving evidence for a homing migration.
5. The gonado somatic index (GSI) at Chongming peaked in May and June, that at Jingjiang in June and July, and at Anqing in July and August. The gonad development revealed that a major part of the individuals at Chongming matured in May and June, that at Jingjiang in June and July, and at Anqing in July and August, indicated that the population at Chongming reproduced in May and June, that at Jingjiang reproduced in June and July, and at Anqing reproduced in July and August. For age 2 mature females, condition factor was smaller at anqing than at Chongming and Jingjiang, GSI was highest at Anqing, egg diameter was highest at Jingjiang, while fecundity was higher at Anqing than at Chongming and Jingjiang.
6. The proximate composition and total energy of immature females (stageⅡand Ⅲ ) among the three locations had some significant differences: the protein and lipid contents of somatic tissue and liver-pancreas were lower at Anqing than at Chongming and Jingjiang and the lipid content of mesenteric lipid tissue was lower at Anqing than at Chongming and Jingjiang, indicated that the lipid stored in the somatic, liver-pancreas and mesenteric lipid tissue were an important energy source during the course of migration. Both the absolute and relative energy loss (absolute energy loss×100)/ initiative energy at the beginning of migration%) increased with standard length. Both these values were higher at Anqing (16%-50% for relative energy loss)than at Jingjiang (<12% for relative energy loss). The proximate composition and total energy content of mature and immature females (stageⅡand Ⅲ ) in each location also had significant differences: the lipid contents of somatic and mesenteric lipid tissues of mature females at Chongming and Jingjiang were lower than that of immature females and the lipid contents of liver-pancreas of mature females at Jingjiang and Anqing were lower than that of immature females, indicated that the lipid stored in the somatic, liver-pancreas and mesenteric lipid tissue were also an important energy source during the course of gonad development. Both the absolute and relative energy loss increased with standard length. Both these values were higher at Jingjiang (25%~53% for relative energy loss)than at Chongming (＜35% for relative energy loss). The ovarian energy of mature females at Anqing were similar to that at Jingjiang which indicated that the energy loss during the course of gonad development at Anqing should be similar to at Jingjiang, while the total energy, energy density and lipid contents of mature females were similar to that of immature females at Anqing. Furthermore, the stomachs of 33% females were full of food, indicated that the females at Anqing should supply additional energy to fulfill the needs of gonad development by feeding, rejecting the viewpoint that anadromous C. ectenes do not feed after entering the estuary (The research labs of resource and catch in Yangtze River Fisheries Research Institute, 1977).
In conclusion, anadromous C. ectenes has been overexploited presently. This fish has some special reproductive strategies: spawning occurs in the estuary in May, which will be delayed with increasing migratory distance from estuary to freshwaters. The females with increasing migration distance would spend more energy, so will have lower GSI, smaller egg diameter but higher fecundity, which all indicated that they would contribute more to the population recruitment. Considering this, we should strengthen measures to conserve the spawners with longer migration distance. The offsprings born at Anqing might be homing to Anqing itself for reproduction when they grow up (homing migration). The individuals with longer migration distance may suffer higher catching pressure, and the overfishing of these populations may induce shorter migration distance, in fact the migration distance had shortened from Dongting Lake to the area near to Poyang Lake(Shi and Gong，2003), which would be a challenge to fisheries management and conservation for anadromous C. ectenes. Reducing fishing intensity is the fundamental fisheries management and resource conservation option for this fish in the Yangtze River. The annual closed season for fishing in the middle and lower reaches of the Yangtze River (April 1 to June 30) covers only part of the spawning season of anadromous C. ectenes. The ban on fishing this species should be expanded typically to the upper spawning grounds.|