Mesh Selectivity of Unmarketable Trash Fish by a Small Trawl Fishery in the Seto Sea. Tadashi Tokai,* Shigeyuki Omoto,* and Ko Matuda*

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1 Nippon Suisan Gakkaishi 60(3), (1994) Mesh Selectivity of Unmarketable Trash Fish by a Small Trawl Fishery in the Seto Sea Tadashi Tokai,* Shigeyuki Omoto,* and Ko Matuda* This paper describes mesh selectivity curves of unmarketable fish as a means of decreasing by-catch. For six main unmarketable fish species, Engraulis japonicus, Apogon lineatus, Leiognathus rivulatus, Acentrogobius pflaumii, Cryptocentrus filifer, and Repomucenus valenciennei, mesh selectivities are obtained from a covered-net fishing experiment using a shrimp beam trawl with codends of six mesh sizes. This paper uses a modified version of Tokai and Kitahara's method of determining the mesh selectivity curve. In this modified method, mesh selectivity is approximated by a function of G/P, where G and P are body girth and mesh perimeter, respectively. The master curves of mesh selectivity are determined as a logistic function by the maximum likelihood method for the six unmarketable fish species. If the mesh size of the codend is increased from 20.6mm commonly used in this area to 24.9mm, the catch numbers of these unmarketable fish are reduced from 53.4% to 16.7% without reducing the catch of the main target species, small shrimps, e.g., southern rough shrimp Trachypenaeus curvirostris. This also saves the labor required to sort usable fish. Œ ž ŽY åšw (Department of Marine Science and Technology, Tokyo University of Fisheries, Konan, Minato, Tokyo 108, Japan).

2 D=aDL+ƒÀD (1) W=awL+ƒÀW (2) Fig. 1. Method of estimating maximum body girth from body depth and body width. G, Body girth; D, Body depth; W, Body G=W2+D2/Wtan-12WD/D2-W2 (3) width. G=W2+D2/Dtan-12WD/W2-D2 (4)

3 Table 1. Regression equations of body depth and body width against body length for the respective species Fig. 2. Master curves of mesh selectivity against relative body girth to mesh perimeter for six species, Engraulis japonicus, Apogon lineatus, Leiognathus rivulatus, Acentrogobius pflaumii, Cryptocentrus filifer, Repomucenus valenciennei.

4 Table 2. The master curve equations of mesh selectivity are obtained as logistic equation for the respective species Fig. 3. Comparison of body length distributions caught by 20.6mm mesh codend and by 24.9 mm mesh codend for the six species, Engraulis japonicus, Apogon lineatuș Leiognathus rivulatus, Acentrogobius pflaumii, Cryptocentrus filifer, Repomucenus valenciennei.

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