Study On Gear Performance And Change Of Floatline Shape Of Tuna Purse Seine Based On Field Measurement

Tuna purse seine fishing is the most efficient fishing methods in modern tuna fishing. The principle of purse seine is, when finding a fish school, the fishing vessel releases a long strip shape net around the fishing school, netting in the water formed wall and pursing the purse line close the bottom of the net, to make catches concentrating to the bunt and captured. Therefore, the sinking and rising performance are the two important characteristics of purse seine. According to the operating mode,the tuna purse seining can be separated as log school and free swimming school, which log school is mainly to FADs. The log school are general relatively stable, pursing speed is very slow; free swimming school are active, when its frightened and easy to change swimming direction or sinking, which require fast pusing in capturing them. So understanding the difference in sinking and rising performance of two catching method is very important, and it has an important guiding role to improve the catching fish success. At the same time, the change of floatline is also important, the net shape by the influence of different factors, the change of net shape determines the surrounding area.This paper based on the data collected by large tuna purse seine operating in the Western and Central Pacific Ocean from February to May 2014, March to June 2015 and September to November 2015, and the writer carried out the ministry of science and Technology 863 “Deep-sea tuna purse seine fishing technology and ultra low temperature storage key technology research”. A total five trips investigation at sea, two trips in Jinhui No.7 and three trips in Jinhui No.9, collected 128 setting of net data（FAD setting 63 groups and free school setting 65 groups）. Analyzing the sinking and rising performance of different parts of leadline of tuna purse seine, and through videoanalysis software processing the change of floatline and surrounding area in pursing.Summarizing the general rules about gear performance and change of floatline of tuna purse seine, in order to provide a reference for researchers to improve design of tuna purse seine and fishing men improving fishing operation.Through processing the field data of tuna purse seine, the main research results can be summarized as the following points:（1） In sinking stage of tuna purse seine, the natural sinking depth presents successively increasing trend from the bunt to the back wing end, the change trend of the maximum sinking depth is higher at the central part and lower at two wing ends the pursing time has a significant influence on the sinking performance of the tuna purse seine. The natural sinking presents a decreasing trend from the bunt to the back wing end, the average sinking speed shows no difference maintained at about 0.16 m/s. Stop pursing of purse line has a significant influence on depth and speed of leadline of the net.（2） The relationships between the rising depth of different parts of leadline and time can be described by different formulas:the bunt: h₁=-1.3×10-5t₁²- 0.0474t₁ + 173.37（R2=0.9879）;the middle part: h₂=-6.2×10-5t22- 0.0784t₂ + 234.33（R2=0.9879）;the back wing end: h₃=-8.3×10-6t32- 0.0438t₃ + 177.67（R2=0.9732）.The relationships between the rising speed of different parts of leadline and time can be described by different formulas:the bunt: v₁=1.2×10t₁² + 7.6×10-5t₁ + 0.0252（R2=0.9469）;the middle part: v₂=-1.8×10-7t22 + 0.0005t₂- 0.0246（R2=0.9814）;the back wing end: v₃= 4.2×10t32 + 6.8×10-5t₃ + 0.0337（R2=0.9120）.The current speed of 70 m, 120 m have a significant impact on the rising speed（P<0.01）.（3） During pursing process, the speed of leadline of two kinds of fishing method shows a increasing trend again after first decreasing, then decreasing. The relationship of depth of leadline and time of catching free school can be described by formulas:h₁=-1.25t₁²+25.49t₁+37.59（R2=0.9805）;The relationship of depth of leadline and time of catching FAD school can be described by formulas:h₂=-0.67t₂²+21.03t₂+76.52（R2=0.9918）.The pursing sinking depth and rising depth of catching free school are lower than that of catching FAD school, while the pursing sinking speed and rising speed higher than that of catching FAD school.（4） The pursing speed of purse line and bridle, change of rudder angle of the ship,the direction and speed of the wind have a significant influence on the change of floatline. When the average pursing speed of bridle is 0.4 m/s and the average pursing speed of purse line is 0.6 m/s,the surrounding area in pursing bridle change more than stop pursing bridle, and both show liner decline; the relationships between surrounding area and pursing time before the back wing end hauling to the boat can be described by formulas: S1=-4.11t1×104+7.7×105, the surrounding area from 7.70×105 m2 to 3.57×105m2; the relationships between surrounding area and pursing time after the back wing end hualing to the boat can be described by formulas: S2=-2.24t2×104+3.7×105 m2,the surrounding area from 3.57×105 m2 to 1.38×10⁵ m²;...