A Tank Experiment on Selectivity Components of a Trammel-net for Tilapia mossambica

Abstract

In a tank experiment, the selectivity mechanisms of a trammel-net and a gill-net were compared. The selectivity of the tested trammel-net was represented by four components attributable to the wedging and entangling functions of the original gill-net and the additional function induced by the outer nets. The components of wedging and entangling were represented by normal and logarithmic normal curves. The gross selectivity of the trammel-net, the sum of the four components, was found to be consistently high for larger fish. The selectivity for smaller fish was reduced compared to the gill-net. This is attributed to the negative effect of the additional wedging function, i, e, wedging is obstructed in the trammel-net. Consequently, the dominant function of the trammel-net is entangling, therefore, its aggregated function is largely different from that of the gill-net. The trammel-net is presumed to have bimodal selectivity. © 1991, The Japanese Society of Fisheries Science. All rights reserved.