抄録
Introduction: A general practice with the operation of otoshi ami, the most prevalent type of trap nets in this country, is to provide two units of the net, each one serving under water from 10 to 60 days after the other (Fig. 2). The procedure will be repeated all through the season so that the net fabrics may be protected from deterioration. However, a close observation would reveal that the strength of thread woven into the fishing net gradually declines with the lapse of time until reaching beyond a minimum of utility when the life of the net comes to an end. In this regard it should be reminded here that in case of synthetic fiber nets having gained prevalence, the threads presumably have not to be so strong as natural fiber threads, as the former is more durable than the latter. For that reason it is important for designing and operating a net to know the minimum strength of various net materials including both natural and synthetic fibers.
The present attempt is to study the process through which net fabrics wear out in the course of use and to determine the minimum strength of them workable for the trapping operation. The samples adopted in the experiment were naturaa fiber nets which had been used for the walls of the bag of traps at various localities.
Method: Some of preliminary information are the number of days for which the net had been submerged in the sea and the estimated remaining life of the net from which the samples were collected. After unknotting the nets, breaking strength of the threads was examined for each sample. The results are indicated in Table 2 together with the number of served days of the sample nets. Additional data such as the localities, the water temperatures, velocities of tidal current, and the maximum catch at a haul in each fishing are summarized in Table 1.
Discussion: Relationship between the number of serving days n of the net strength T (kg) of the thread is given in Fig. 1. From the diagram it is obvious that the decline of the thread strength is almost linear with an increase of the serving days. Therefore, if the thread strength is expressed as T0 in case n=0, Tu=T0-Kn ………………………………………………………………(1) Where K is the coefficient of decreasing breaking strength.
Values of K of the sample nets each from different fishing grounds are calculated as shown in Table 3. In case the Manila twine net treated with coal tar is continuously used for more than 40 days, the deterioration proceeds rapidly, and the value of K becomes very large.
In regard to the sample fabrics of the bags in various fishing grounds, the minimum strengths for utility have been calculated, as shown in Fig. 1, from three factors: the strength of threads obtained from the fabrics of unserviceable nets: the estimated resmaining life of the materials of serviceable nets: the lines of decreasing strength presumed to follow as in Fig. 1.
