| Tuna purse seine was a fishing gear which operation by surrounding up the fishschool and herding into bunt, depending on the aggregation characteristic of tuna, andthen fishing them. During the process of rounding up, fish school would escape fromthe beneath of foot line if it does not sinking as predicted after the net closedsuccessfully. So, sinking performance of gear was one of the key factors indetermining whether the purse seine fishing was success or fail, it was also the criticalproblem which must be solved by fishing scientists. However, the study on sinkingperformance of purse seine found the typical issues as follows:(1) To what extentdoes the sinking performance affect the results of cast net when the tuna purse seineworks.(2) Most studies concerned only on the relationship between sinkingperformance and impact factors, and the mainly methods were linear. It imposesrestriction on inner link between sinking performance and impact factors and the bestmodel of analyze the sinking performance of tuna purse seine influenced by impactfactors was not mention.(3) The net model was not designed as the scale of thefull-scale purse seine fully and the condition of model test was different from actualoperation because of the complex structure of full-scale purse seine, which leads todeviation between the results and net model. Thus, whether the model test can reflectthe sinking performance of full-scale net or not needs to be solved. According to theproblems above, this paper will analyze in four aspects:(1) The impacts of tuna purseseine gear’s sinking performance on the fishing success;(2) Analysis on impactfactors of sinking performance of tuna purse seine by sea trials;(3) Analysis onsinking performance of tuna purse seine by model test;(4) Evaluation of tuna purseseine performance between from model test and sea trials. The result showed that:(1) the data obtained from on sea observations indicated that a significantdifference can be found in the fishing success, with which different sinking speeds corresponded, where the fishing success increased sharply from20.0%to66.7%asthe sinking speed rose, but no regular variations on the fishing success appearedwithin the different sinking depth layers; the screening result by the full Logisticregression model showed that the sinking depth was not a significant factor for fishingsuccess (p>0.01), i.e., the probability of fishing success depended only on the sinkingspeed. Moreover, capture rates at different sinking speeds predicted by the modelwere in agreement with the observed results. This study supported that improving theperformance of the sinking capacity of the fishing gear will increase the probability offishing success, future study, however, should be devoted to enhancing the sinkingspeed but not deepening the sinking depth of the fishing gear. In parallel theperformance of the fishing gear was advocated as an independent variable to fullyreflect combined influences on the fishing success by a variety of factors as comparedto previous studies.(2) By using the generalized additive model (GAM) to analyze the observeddata,the results showed that: the relationship between the sinking depth (H) and time(t) for the middle section of leadline can be described by the formula: D=-0.0002t2+0.4086t+1.8099(R2=0.9993); the sinking speed decreased with the increasing depth,the relationship between the sinking speed (V)of the middle part of the leadline andtime (t) can be described using the formula: S=2.5×10-7t2-6×10-4t+0.4412(R2=0.9852); the analysis of stepwise GAM showed that four factors had significantimpacts on the sinking depth (P<0.05), in which the significance faded V120, L,T andV60; two factors had a significant impact on the sinking speed(P<0.05) and thesignificance can be ordered by L1,V60; L1and V60can be controlled to improve thesinking speed, thus improving the fishing success of free swimming school. Both thesinking depth and the sinking speed were non-linearly correlated with other factorsi.e., the estimated degree of freedom value was not1(EDF1), however, The GAMcan provide more information about the relationship between the sinking performanceand influencing factors, also can help to screen the factors of the model, and fit thebest model. Therefore, in the present study, the GAM model is more suitable for studyof impact factors of sinking performance for tuna purse seine.(3) The model purse seine was made following the Tauti’s modeling law with=20and=1.The main dimensions of the model purse seine were: float line length80.9m, the maximum height of the net15.6m, lead line length98m, basic weight of the lead line0.663kg/m, buoyancy of the float line25N/m, the same hanging ratio betweenthe model net and prototype is kept. The results showed that: the relationship betweenthe sinking depth (D) and time (t) for the middle section of lead line can be described bythe formula D=-0.0014t2+0.276t–0.6476（R2=0.9953）; The lead line weight hadsignificant effect on sinking performance, while the setting speed had insignificanteffect on the sinking performance, but the interaction effect caused by the above twofactors were significantly affecting the sinking depth; Within the range from0.531to0.663kg/m, with the lead line weight increased the sinking depth showed a significantincreasing trend, which was not the case for the sinking speed; From0.663to0.759kg/m, the sinking depth slightly decreased, while the sinking speed greatlyelevated. When lead line weight is0.663kg/m, the sinking speed showed an increasingtrend with the setting speed increased; With the sinking time increased, tension of leadline in the time range0-20s larger fluctuated, maintain a steady state, thereafter.(4) The result of Bootstrap showed that: the average sinking depth ofstandardized full-scale purse seine (203.07～226.49m) is75～78%of net model’saverage sinking depth（275.94～286.51m）; the average sinking speed（0.095～0.107m/s）of standardized net model is52%of that of full-scale purse seine（0.185～0.203m/s）. The main reason is that the complex environmental factors, no strictcontrol of hauling time in model test, the choice of standard Statistical methods andthe difference of influential factors are all exerting impacts on the results ofcomparative analyses. |
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