The effect of environmental variables, gear design and operational parameters on sinking performance of tuna purse seine setting on free-swimming schools

There are three main types of tuna purse seine operations: setting on free-swimming schools, setting on fish aggregation devices, or setting on marine mammal associated schools. In this research, we measured sinking performance of 88 purse seine sets on free-swimming skipjack (Katsuwonus pelamis) schools in the western and central Pacific Ocean from October 2006 to May 2013. The sinking behavior of a purse seine is influenced by many factors (e.g. netting materials, sinker weight, length-height ratio, mesh size, purse line length, setting position, setting speed, current, wind, etc.), it may be difficult to quantify the effect these factors have on the sinking performance. A purse seine set is considered to have a better sinking performance if its purse line reaches a greater depth in a shorter time period, i.e., higher sinking speed and greater sinking depth. A generalized additive model (GAM) was used to analyze sinking depth and sinking speed under different sea conditions (current speed and direction at different layers), gear design (length-height ratio, towing line length and purse line length), and operational methods (shooting duration and setting speed). The results in this study showed that the sinking performance of a purse seine with a lower length-height ratio was better than a seine with a higher ratio, current speed at 120 m depth was the most important environmental factor affecting sinking depth of the purse seine. Sinking depth was strongly associated with length-height ratio, shooting duration and purse line length. Sinking speed was related to length-height ratio, shooting duration, current speed at 60 m, purse line length and towing line length. The sinking performance models obtained from this study can be used to predict sinking behavior in relation to operational and environmental conditions, which is essential for the success of tuna purse seine operation on free-swimming tuna schools.