| The Japanese eel, Anguilla japonica, is a catadromous fish. It distributes in thewaters of the western Pacific between Hokkaido and Philippines. It spawns nearSuluga seamount which is located east to Mariana Trench, east of Philippines. Due tothe difficulty of Japanese eel artificial breeding, the breeding eels have been relyingon the harvesting of natural fry. Unique ecological conditions and effects of Kuroshioand Taiwan Warm Current make the Yangtze River estuary to be important area ofJapanese glass eel in China. Every year large of glass eels swim into Yangtze River togrow and develop. Surveys on catch of glass eels in the Yangtze River estuary werecarried out in2012. Daily catch and some environment factors such as watertemperature, salinity, tide and lunar phase et al. were recorded to analyze changes ofperiod and relationships between eel yield and environment factors. Analyses on otherfishes in the elver net were conducted to study damages of elver net on other fishes.1. To study temporal and spatial distributional features of elvers of Japanese eelAnguilla japonica in the Yangtze River estuary, surveys were carried out in waters ofDongwangsha, Sheshan Island, Qingcaosha,DongfengXisha, Jiuduansha and Nanhuiin the Yangtze River estuary from Jan. to Apr.. The results showed that the main floodperiod of glass eel was from Feb. to Mar. and that apex time of flood period graduallydelayed from south to north and from estuary external to internal which wasconsistent with the migrating route of eels. The main distribution regions of glass eelswere waters of Nanhui, Sheshan Island and Dongwangsha located in external of theYangtze River Estuary. The yield of Nanhui waters was the maximum, andQingcaosha and DongfengXisha which were located in internal got less yield. Themaximum yield was14.39ind/d·100m2in Nanhui monitoring site and Qingcaosha gotminimum yield0.04ind/d·100m2. The total yield of glass eel in the Yangtze RiverEstuary in2012was about16.8millionind and worth of50million RMB estimated ondata of fixed-point monitoring and questionnaire surveys. To achieve the sustainableutilization of resource of glass eels of Japanese eel, this paper proposed adjustedallowable fishing period to two months (Feb. to Mar.) and limited the fishing areas tothe waters from Nanhui to Sheshan Island where were in external of the YangtzeRiver Estuary.2. Studies were conducted to analyze relationships between eel anadromousphenomenon and important environment factors based on the environmental dataobtained in eel catch period in eight monitoring sites from Jan. to Apr. in2012.Positive correlations between eel yield and water temperature were shown in eight monitoring sites except late catch period. The yield increased with water temperaturerised and decreased with water temperature declined. So declining yield in Feb. canbe due to the water temperature dropped in Feb.. The peak periods of eel anadromousphenomenon in the Yangtze River Estuary in2012were found within the watertemperature6-8℃range, which was not consistent with other researchers. Thechanges of environment in recent years in the Yangtze River estuary could explain thisphenomenon. Eel yield in Dongwangsha site were positive correlated with salinityand showed changes with rise and decline in salinity. Sheshan, Jigujiao and Nanhuimouth got a negative correlation between eel yield and salinity. Positive and negativecorrelations were both existed in Jiuduansha and Nanzhi Lightship West sites.Qingcaosha and DongfengXisha located in internal estuary had little fluctuation insalinity. Studies on relationships between datum tide height and eel daily catchshowed that Sheshan, Jigujiao and Nanhui mouth in external and Jiuduansha andDongfengXisha in internal got a daily yield fluctuation consistent with fluctuation ofdatum tide height. Daily yield increased with tidal amplitude addition and got peakyield in the rising flood season. DongfengXisha sites some periods showed hysteresisand obtained high yield after flood season. Daily yield in Nanzhi Lightship West sitesshowed both positive and negative correlations with tidal amplitude and peak yieldappeared in little tidal season. Each site had higher average yield near new moon andfull moon than that near first quarter moon and last quarter moon, and got maximumyield near new moon and full moon. But some monitoring sites appeared high yieldnear first quarter moon and last quarter moon. So relationships between eel daily yieldand lunar phase in the Yangtze River Estuary were not determined and deserved morefurther researches.3. The mesh of elver net was generally1mm which led to low selectivity to otherresource and produced damages on early resource of economic fish and shrimp andcrab bait biological resources. This paper studied other resources in elver net in theYangtze River Estuary in2012. A total of43species of fish which belong to14families and10orders were collected during monitoring period. The species numberof Perciformes had the maximum value, followed by Cypriniformes. The number ofeconomic species was14,32.56%of the total. Species had more frequency ofoccurrence were Coiliamystus, Collichthyslucidus, Repomucenusolidus,Tridentigerbarbatus, Chaeturichthysstigmatias and Cynoglossusgracilis. The value ofrelative importance of index greater than10was identified as dominant species. Therewere4dominant species, such as Coiliamystus, Cynoglossusgracilis,Chaeturichthysstigmatias and Collichthyslucidus All dominant species wereconstitute with smaller body length and dominant length groups were7-11cm,4-8cm,13-17cm and8-12cm respectively. Analysis of shrimps and crabscatch in elver net showed11species. Palaemon gravieri, Exopalaemon annandaleiand Pseudograpsus albus were the first three in frequency of occurrence, abundanceand biomass. The abundance and biomass of fishes, shrimps and crabs were far morethan those of glass eel in same net, which led to certain degree of damages on otherfishery resources. |
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