To understand the catch effect of different scales or nets in Taiwan’s coastal fishing grounds, five different gill net mesh sizes (9.4, 9.8, 10.7, 11.3, and 11.8 cm) and trammel nets (inner panel mesh size: 10.3 cm) were used on black porgy (Acanthopagrus schlegelii) as the target species in the coastal water of Budai, Chiayi in February to March 2004. Results indicated that the caught effects on the species and its mantissa were similar between the small mesh gill net and the trammel net. Small size or immature fish were caught with greater ease using the 9.4 cm mesh size gill net and the trammel net, while the black porgy of higher IPUE was caught with greater ease using the 10.7 cm mesh size net. The average weight of the fish captured with the trammel net was lower than those caught with the other gill nets; however, due to the higher total weight, the trammel net had the highest overall economic value. The proportion of economy catches and total catch value (C/T) of all survey nets were higher than 0.82, indicating that the captures of the different mesh sizes of gill nets and trammel nets are subject to intensive utilization in the coastal zone.
In terms of the relationship between the fork length of black porgy and mesh selectivity, if the mesh was selected properly (i.e., the relationship of Fork Length (FL) and Mesh size (M) is FL / M = 2.63), the catch effect and incomes of the gill net were greater than those of the trammel net during the fishing season for the black porgy. If only the total catch was considered, however, the catch effect and incomes of the trammel net were greater than those of the gill nets. Therefore, in order to reduce the impact on the environment and increase the conservation of fishery resources, while maintaining catches income, we recommend that the most appropriate mesh size gill net (10.7 cm) should be chosen for the black porgy fishing season.