| The silver gudgeon, Squalidus argentatus (Sauvage and Dabry de Thiersant,1874) belongs to Genus Squalidus, subfamily Gobioninae, family Cyprinidae, order Cypriniformes. The silver gudgeon is a small-sized freshwater fish broadly distributed in the major river system, China. Age, growth, mortality and reproduction of silver gudgeon from the Tian-e-zhou Oxbow in the middle reach of Yangtze River were studied. A total of873specimens were collected from March2011to January2012. The objective of this paper was to describe the biological characteristics of silver gudgeon and provide the basic information for the conservation and management. The main results are as follows:1. The scales were used to determine the age of silver gudgeon, the typical pattern of annuli was clearly thicken ridges at the annuli-formed area, and few scales formed cutting-over type in the parazona. It was demonstrated that one annulus was formed per year from April to July, mostly in April and May, based on marginal increment analysis. The estimate precision was measured by percent agreement and coefficient of variation (CV). The percent agreement and CV for two scale readings were86.79%and6.94%, respectively. It was demonstrated that the scale was a fit material for age determination.2. Standard length of all specimens ranged from2.3to12.1cm, while weight varied between0.16-31.59g. The results showed that the ages for both sexes ranged from1to6years old, and most individuals were1to3years old for males and females accounting for87.01%and87.35%of total, respectively, but the proportion of3years old individuals for females was much more than males. The standard length-weight relationship between males and females showed an allometric growth, and could be expressed as W=0.009SL3.2254andW=0.009SL3.2285, respectively. The von Bertalanffy growth equation was fitted on the back-calculated standard length data resulting in parameter values of L∞=11.5069cm,K=0.4074,to=-0.5764for male and L∞=17.1797, K=0.1820, to=-1.2673for female. For males and females, the natural mortality rate was M=1.1459per year and M=0.6048per year, indicating that the mortality of silver gudgeon was much higher than modal mortality (M=0.2-0.3per year); and the exploitation rate E=0.4183and E=0.5711. The exploitation of female silver gudgeon in Tian-e-zhou Oxbow was overexploited, so we should strengthen management on them to protect better in the future.3. Standard length, weight and age for males and females at first sexual maturity (SL50%) were5.4cm,2.08g,0.8years and5.1cm,1.72g,0.9years, respectively. The observed minimum standard length, weight and age at maturity for males and females were4.7cm,1.43g,1years and4.9cm,1.47g,1years, respectively. The overall sex ratio for females:males was1.94:1, which was found to be very significantly different from the hypothetical distribution of1:1. The sex ratio was variable with the size classes and different month. Small size groups (<6.8cm) of silver gudgeon had the same number of male and female individuals, but with the standard length growth, females dominated. Females were predominant in the non-spawning season, but the male-female ratio was not significantly different in the spawning period. The phase Ⅳovaries for the population maintained for May and September based on monthly variations of gonadosomatic index (GSI) and gonadal stages analysis for females. The oocyte diameter in mature ovaries showed only one distributional mode on may, July and September. According to the size frequency distributions of oocyte and histological stage of ovary all indicated that silver gudgeon was a single batch spawning fish. Batch fecundity of females ranged from850to20268eggs (mean5760±3656eggs) in fish ranging5.2-11.9cm SL,1.81-35.59g W. Relative fecundity ranged from153to2134eggs/g (mean961.06±415.79eggs/g). In the Tian-e-zhou Oxbow, the ovary never reached phase V, past-spawner and regressing subphase, and the stage of oocyte development by the histological staging methods only entered migratory nucleus, but never observed hydrated oocytes and post-ovulatory follicles (POF). The sectioned lapillus were used to determine the daily rings of ten underyearlings which were collected between July and September, which indicating the initial spawning event from the end of April to start of June, and the end of may to start of June was dominated, but the fish did not spawned in Tian-e-zhou Oxbow. The ovary developmental phase, oocyte stages and back-calculated by the daily rings spawning date all showed that silver gudgeon can not spawn in Tian-e-zhou Oxbow, the whole fish migrated from Yangtze River. In a word, we should take action to let more fry produced in the Yangtze River into the oxbow and strictly ban catch of this species in the future, in order to maintain the population quantity. |
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