FISHERIES RESEARCH INSTITUTE,MOA,TAIWAN

The volume changes of sea cage nets affect the number of fish to be farmed and farmer's annual income. However, there is a lack of research involving the use of actual cages to investigate how current forces lift the cage nets and thus change their volumes. This study analyzed the influence of current forces on the lifting of a cage net by monitoring four points on the cage bottom, as well as the current velocities and current directions. The results showed that in the afternoon (12:00-18:00) and at night (18:00-24:00) during spring tide and neap tide, southward current flows clearly dominated. In addition, the percentage of occurrence of current velocities exceeded 10 cm/sec during the spring tide was 40%, which was greater than that of neap tide (24%). When the southward current velocity exceeds 10 cm/sec, it easily causes lifting of the cage bottom on the north end of the cage, and with every increase of 10 cm/sec in current velocity, the bottom of the cage is lifted by about 1.3-1.5 m. When the average southward current velocity increases from 10 cm/sec to 30 cm/sec, the net shape coefficient reduces from 0.80 to 0.59 on average, and the stability of the cage net volume is 90%. If the average southward current velocity exceeds 20 cm/sec and lasts for 3-4 hours, it easily causes serious deformation of up to 50-70% of the volume of cage net. These results can serve as a reference for the industry in the development and design of cage nets, as well as in the configuring of nets, to reduce the influence of current forces, which, in turn, would improve productivity by reducing the degree of net deformation to maintain the largest culture spaces possible.