Catching efficiency of trammel net with different slacknesses and mesh sizes of inner net.

Abstract

The experimental operations concerning the catching efficiency and the fish selectivity for bottom gillnet and trammel net were examined with 3 different mesh sizes and 3 different slacknesses (the ratio of the height of inner net to that of outer net) in the Tokyo Port during 1981 to 1982.The results obtained are summarized as follows:1) The catch pattern of gizzard shad Clupanodon punctatus, sardine Sardinops melanostictus, and big eyed herring Herklotsichthys zunasi changes due to difference of mesh size of inner net.2) The slackness of trammel net seems to affect to some degree the catches, while in the case where the mesh size of inner net is suitable for the size of fish, the slackness of inner net does not affect the catches of trammel net so much.3) In the case where both the mesh size of inner net of trammel net and the mesh size of bottom gillnet are suitable for the size of fish, it seems that the catching efficiency of gizzard shad by bottom gillnet exceeds that by trammel net.4) It follows from the selection curves of trammel net and bottom gillnet obtained by this experiment that when the slackness is 1.1, the sharpness of selection curve for the trammel net has the same tendency as that of the bottom gillnet. In the case where the slackness is more than 1.5, it is expected that the larger size of fish is caught more easily by trammel net than by bottom gillnet.5) The selection curve of the trammel net has the same character as that of bottom gillnet. This means that the smaller size of fish is selected severely by the mesh sizes.(Tokyo University of Fisheries, Konan, Minato, Tokyo 108, Japan).

Size selectivity of trammel nets in southern European small-scale fisheries

“…Trammel nets are generally considered to be less size selective than gill nets, with size frequency distributions frequently skewed to the right (Millner, 1985;Dickson, 1989;Fabi et al, 2002;Fitzhugh et al, 2002). The selectivity curve of trammel nets is domed but has a flatter shape than that for fin-fish caught with gill nets, as reported by authors who used the same methodology to fit the selectivity curve (Koike and Matuda, 1988;Losanes et al, 1992a). If a significant proportion of individuals, especially the larger ones, are pocketed or trammelled, then the selectivity curve may not fall to zero or even have a descending limb, implying that very few fish escape after coming into contact with the trammel net (Salvanes, 1991;Losanes et al, 1992b).…”

“…The catches of trammel nets depend primarily on the mesh size and vertical slack of the inner net (Purbayanto et al, 2000) with several studies reporting wider selection ranges with increasing inner net slackness (Kitahara, 1968;Koike and Takeuchi, 1985;Koike and Matuda, 1988;Salvanes, 1991;Losanes et al, 1992a,b). Trammel nets with a slackness of more than 1.5 are expected to be more effective in catching larger sized fish than gillnets of the same mesh size (Koike and Matuda, 1988).…”

“…In contrast, size selectivity was clearly a function of the smaller mesh of the inner panels, with modal length generally increasing with inner panel mesh size for many species. Other authors also report that the catches and the size selectivity of trammel nets depends primarily on the mesh size of the inner net (e.g., Kitahara, 1968;Koike and Takeuchi, 1985;Koike and Matuda, 1988;Salvanes, 1991;Purbayanto et al, 2000;Losanes et al, 1992a).…”

“…Vertical slack is reported to have no effect on the modes of the selectivity curves of trammel nets of the same mesh size (Koike and Matuda, 1988;Losanes et al, 1992a). Koike and Matuda (1988) carry out experiments with 5.1 cm inner mesh trammel nets with vertical slackness of 1.1, 1.5 and 2.0 and find that the selectivity curves for Clupanodon punctatus differ only in the steepness of the descending limb, reflecting the increase in selection range with greater slackness.…”

“…Koike and Matuda (1988) carry out experiments with 5.1 cm inner mesh trammel nets with vertical slackness of 1.1, 1.5 and 2.0 and find that the selectivity curves for Clupanodon punctatus differ only in the steepness of the descending limb, reflecting the increase in selection range with greater slackness. Losanes et al (1992a) fish gizzard shad with trammel nets with vertical slacks of 1.1, 1.5 and 2.0 and report that the master curves were all skewed to the right and had the same modal length/mesh size value of approximately 3.8.…”