Analysis On Elver Net Resulting In Damage Of Early Fish Resource

To clarify the variation of by-catch fish numbers and species composition in elvernet in Yangtze River estuary. The sampling which focused on spatial-temporal variationof by-catch fish CPUE and numbers of species change by tide, and different types ofwater areas and nets had bearing on damage ratio of commercial fish. This work canhelp fishery administration administrate elver net in Yangtze River estuary reasonably.From January-May in2011-2012, Three types of water were selected and10stations were set. These stations included set net and stow net. Samples were collectedfrom two nets when neap tide and spring tide in each station. Samples fixed in theformalin of5%concentration.A total of54species,25families,15180individuals of by-catch fish were collectedin500nets, included25marine species,16estuarine species,10fresh water species and3diadromous species. Developmental stage of by-catch fish include from postflexion toyoung stage. The most abundant species Coilia nasus was26.08%of total individuals,followed with Taenioides rubicundus(9.73%), Coilia mystus(7.46%), Cynoglossusjoyneri(6.01%), Pseudolaubuca engraulis(5.99%), Liza haematocheila(5.30%). These6species got70.57%in total abundance. The by-catch fish species in Sts.4-5were morethan other stations. The average density of Sts.1-5and Sts.9-10were more than20pernet. Peak value of average density were appear in March to April in each station exceptSt.1and St.9.In2011, the species and CPUE of commercial were great different in each station.The CPUE of stow net were more than set net except St.2. The fish species was themost in St.5and the least in St.6. In2012, the fish species was the most in St.5and theleast in St.2and Sts.7-8. The numbers of elver collected in2012was less than2011except St.2and St.9. Elvers collected from fresh water area were more than other waterareas.The damage ratio made by elver net in Sts.1-3and Sts.6-8in2012was higher than2011and other stations was lower than2011. Compare with damage of two years ineach station, the most difference was appeared in St.9. The damage of Sts.6-8were lowthan other station. Compare Shannon-Winner diversity index in spring tide and neap tide, except St.1and Sts.8-9, other stations’ diversity index in spring tide is higher thanneap tide. And diversity index of fish in salt water is higher than other water areas.By-catch fish number of species in Sts.3-4, Sts.6-7and St.10in spring tide is more thanneap tide.Coilia nasus is most dominant species in this study. A total of5477individualswere collected in elver net. The average density is10.95per net. In2011, averagedensity is more than18.45per net in Sts.1-4and Sts.9-10, and less than6.83per net inSts.6-8. The average density in2012is less than2011in total. The CPUE in fresh wateris more than other water areas. The average density in February and April is more thanMarch in2011-2012. The average density in May is more than other4months in2012.Average body length of Coilia nasus in2012is longer than2011in total. Average bodylength of Coilia nasus in mixed salt and fresh water and fresh water is longer by monthfrom February to April in2011. Average body length of Coilia nasus was fluctuated bymonth in each water area in2012.Our results indicate that early life stages of economic fishes were damagedseriously by elver nets, especially for Coilia nasus. It is one of the important reasons ledthe Coilia nasus resource declining in the past. In order to protect fish recruitment stockand recover fish resources in Yangtze River estuary, the management should not onlylimit the quantity and operational periods of elver nets, but also regulate the mesh size.