Selectivity of Wire Basket Traps for Tilapia (T. Nilotica, T. Galilaea and T. Zillii)

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Abstract
Tilapia represents more than 80 per cent of the commercial catch from the north delta lakes. Trammel nets and wire basket traps (chicken-wire traps) are the main gear used. The selective action of the wire basket traps are studied to determine the average selection length for the different tilapia species. Three mesh sizes of traps were tried (mesh bars 15.5, 17.4 and 22.3 mm) and the mean selection length was calculated according to Holt's equation (Lm = Kθ). Holt's constants for the three species were estimated to be 6.507 for T. nilotica, 6.276 for T. zillii and 5.940 for T. galilaea. The mean selection lengths for the three species were likewise calculated. From the normal curve distribution for the three species, the 50 per cent and more retention lengths for T. nilotica lie within a range of 2.7, 2.4 and 3.7 cm for the three meshes respectively. For T. zillii the range was 2.2 and 2.3 cm for meshes 15.5 and 17.4 mm. For T. galilaea the range was 3.3 and 3.6 cm for meshes 17.4 and 22.3 mm. For the proper management of the tilapia fisheries, it was recommended that the wirebasket traps mesh size should not be less than 25 mm (mesh bar) to give mean selection length of 16 cm for T. nilotica, 15.5 cm for T. zillii, and 15 cm for T. galilaea.

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