Nippon Suisan Gakkaishi 69 (4), 596-601 (2003)

Necessary and sufficient conditions of mesh size and hang-in ratio and hang-out ratio for fish to pass through the mesh

MASASHI NATSUME1 AND TAKASHI MATSUISHI2

1Hokkaido Central Fisheries Experimental Station, Yoichi, Hokkaido 046-8555, 2Graduate School of Fisheries Sciences, Hokkaido University, Hakodate, Hokkaido 041-8611, Japan

For a fish to pass through the mesh of a net, the mesh perimeter must be at least equal to the fish's maximum girth. Assuming the maximum girth as the circumference of an ellipse calculated from the maximum body depth (Dm) and maximum body width (Wm), We assessed the necessary conditions of the mesh size (Ln) for a fish passing through the mesh without entanglement and the sufficient conditions of the mesh size (Ls) for the cross-sectional shape of the fish body encountering the four sides of the mesh at four tangential points. For mesh size in a sufficient condition, the hang-in ratio (Si) and hang-out ratio (So) were assessed. The necessary conditions of the mesh size can be expressed as, Ln=(π/4)(1.5(Dm+Wm)−expression), that is as half the circumference of the ellipse. Sufficient condition of the mesh size can be expressed as Ls=Dm+Wm, that is as the total of the maximum body depth and maximum body width. The hang-in ratio and hang-out ratio were obtained as Si=1−expression, So=expression−1.

Based on measurements of the maximum body depth and maximum body width of Arabesque greenling, Pleurogrammus azonus, of about 30cm standard length, the average values of the approximation were: Ln=94.7mm, Ls=119.0mm, Si=0.376, So=0.604.


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