Research on methodology to develop bottom seine nets for exclusion of bycatch

通过水箱实验和作业调查相结合的方式对丹麦围网渔具进行了研究；(1)对对虾在小水箱中穿过网眼的行为进行了观察，阐明了对虾被排除在外的概率取决于一次相遇穿过一个网目的概率和一只对虾遭遇的概率数量。围网的逻辑选择性是由于生物反复与网目相遇而产生的。基于该模型的模拟较好地反映了丹麦围网的选择性曲线。(2)在上述模拟的基础上，设计了丹麦围网排除副渔获鱼的实验模型，并对其选择性进行了检验。网格的放大和打开效果与预期一致。提出了设计兼捕专用渔具的网目尺寸和鱼类尺寸管理图。(3)潜水观察了鱼体对丹麦围网的捕食行为。在主网前观察到的鳍鱼很少，推测它们被排除在更靠前的位置。丹麦围网的选择性被假设为受到拖绳的影响，因此，它们在被拖曳时的运动在空气中进行了实验。一种结构变化的绳索在地面固定条件变化下的运动差异在绳索的前部位置附近出现。这表明对围网选择性的研究必须考虑到绳索对生物的刺激作用。(4)虽然上述实验研究分析了先前调查的丹麦沿海小型围网中的副渔获物丢弃记录，但澄清了基于新引入的概念，即单位努力丢弃量(DPUE)对丢弃物的评估和估计是最准确和可靠的。

Danish seine gear was studied on a combination of a tank experiment and operational survey with a gear newly designed for exclusion of bycatch as;(1) Observation of behavior of prawns to pass through meshes in a small-tank clarified that probability for prawns to be excluded is ruled by probability to pass through one mesh at once encountering and the probabilistic number of encountering for one prawn. The logistic selectivity of seine nets is induced by repetition of encountering to meshes by organisms. Simulation based on this model well represented the selectivity curve of Danish seine nets obtained from previous trials.(2) An experimental Danish seine net for exclusion of bycatch finfishes was designed on the basis of the above simulation and its selectivity was examined. Effects of enlargement and opening of meshes was as expected. A mesh size and fish size management diagram to design bycatch-exclusive fishing gears was proposed.(3) Behavior of finfish to Danish seine was observed underwater by diving. Few finfish were observed in front of the main net and they were conjectured to be excluded more anterierly. Selectivity of Danish seine was hypothesised to be affected by towing ropes, therefore, their motion while being towed was experimented in air. Difference in motion of ropes of a variation in structure under a variation of set condition on ground appeared around anterior positions of the ropes. This indicates that studies on selectivity of seine nets must take stimulation by ropes to organisms into account.(4) While the above experimental studies, record of discards of bycatch in small coastal Danish seine previously surveyed was analysed and it was clarified that evaluation and estimation of discards based on a newly-introduced concept, i.e. discard per unit effort (DPUE) is most accurate and reliable.