日本沿海捕鳗笼的网目选择性

1995~2000年在东京湾进行的套网法实验,结果表明:网目尺寸为21·0、18·1、15·5、13·6、11·6mm的笼网目选择率在星康吉鳗全长035·50cm、l>32·50cm、l>27·50cm、l>26·50cm、l>24·00cm时,全长选择率几乎接近于1·00,50%选择全长分别为29·80、26·80、23·00、22·00、20·00cm;选择域为3·80、4·30、3·50、3·00、3·00cm。依据AIC理论,套网和实验网的网目接近时,套网的影响显著;Single curve model所得到的主选择性曲线可更好地反映网目选择性。50%选择相对体周长MGc1·11,选择域为0·52。01·60,选择率趋向于1·00。     

Size selectivity of a trammel net for oval squid Sepioteuthis lessoniana

ABSTRACT:   The size selectivity of a trammel net for herded oval squid Sepioteuthis lessoniana in Tateyama Bay, Chiba Prefecture, was estimated by comparison between the mantle length frequency distributions of oval squid caught by a trammel net and by a set net. The measured mesh sizes of the inner net of the trammel net and of the final section of the set net were 85.3 and 11.3 mm, respectively. In the trammel net fishery where oval squid are herded into the net, most of the oval squid are caught in the bag-shaped inner net. Hence, the logistic function was employed for the size selectivity curve of the trammel net. The 'share each length's catch total' (SELECT) model was implemented for the estimation of the selectivity curve. The size selectivity r ( l ) of the trammel net for the oval squid was expressed as a logistic function of the mantle length l : r ( l ) = exp(−18.57 + 0.88  l )/[1 + exp(−18.57 + 0.88  l )]. From these logistic parameter estimates, the 50% selection mantle length and selection range ( L ₇₅– L ₂₅) were calculated as 21.07 and 2.49 cm, respectively. The selection probability of oval squid whose mantle girth was equivalent to the mesh perimeter of the inner net was 0.09. Accordingly, oval squid of a girth smaller than the mesh perimeter were likely to pass through the mesh to escape from the net.     

A static analysis of the tension and configuration of submerged plane nets

A non-linear finite element method is employed to determine the equilibrium configuration and tension distribution of a net set in a uniform current. The method models each mesh bar in the net as an infinitely flexible rope element connected at each end to other elements by a frictionless hinge. The Newton-Raphson method is used to solve the basic non-linear simultaneous equations with respect to tension in mesh bar and knot displacements. Global convergence to the correct solution from an initially estimated state is ensured using a loading iteration in addition to the common configuration iterative procedure in order to accurately model the shape-dependent characteristic of hydrodynamic forces. The method is shown to predict the shape and tension distribution of fishing nets with reasonable accuracy in the case examined in the present study.