ソコビキアミ ノ ギョカク セイノウ ニ カンスル キソテキ ケンキュウ
Fundamental Studies on the Fishing Efficiencies of the Trawling Nets
肥後, 伸夫 ,
ヒゴ, ノビオHIGO, Nobio
鹿児島大学水産学部紀要=Memoirs of Faculty of Fisheries Kagoshima University
137 , 1972-03-05 , 鹿児島大学 , カゴシマ ダイガク , Kagoshima University
This research was carried out with the use of the three kinds of trawling net using ofby the fishing boats, namely, trawlers, bull trawlers or Danish seiners belonging to thegroups of the fishing boats of the mother ship fishery, fishing on the continental shelf ofthe eastern part of the Bering Sea including near the shelf, during the term from April to October for the three years of 1963, 1965 and 1967.As to the items of the fishing gears used in this research, in table 3, are shown thetonnage of the fishing boat, its number and the scale of the net used in this experiment.Namely, the number of the monther ship is 3 ; the number of the fishing boats belongingto those mother ships, is as in the following : ― the number of the side trawlers with tonnage of 370, is 2 ; that of the stern trawlers with the tonnage of 299, is 3 ; the total ofwhich being 5 ; that of the bull trawler with the tonnage of 121 and with the tonnage of98, is 4 and 2 ; the total number of with being 6 ; the total number of the Danish seiner is, 11 ; with the tonnage range from 65 to 92.In case of those trawling nets, in accordance with varieties in the sort of the net, aconsiderable difference is to be noted both in the weight of the ground rope and in thebuoyancy of the float, but the whole lengths of the nets are almost equal.As may be seen in the introduction of this thesis, the purpose of this research is as inthe following : ― through the three sorts of the fishing net used as the materials of thisexperimental research, namely, the trawl net, the bull trawl net, and the Danish seine,the following 3 items were aimed : (1) the attempted clarifications of the fishing efficiencyof the fishing net by means of actual measurements and the model experiments ; (2) theconfirmation of the fundamental characteristics in the designing and making-up the respectivenet ; (3) accompanied with the analyses of the physical and biological factors in drawing the design of the trawling net in possession of high effectivity.
The experimental methods used in this research are as in the following :1. With the use of Radar equipped on board of the mother ship lying at anchor, sometracings were made of the towing speed and the removing distance concerning the netstowed by trawlers, bull trawlers and Danish seiners and as the result of the tracings thesweeping area of the actual net towing was counted, and after combining this counted value with the value of the filtering volume, which will by mentioned in item 3, some numericalvalue, which is assumed to be indispensable for the determination of the fishing efficiencyof the fishing net, was obtained.2. Concerning Danish seine, in the three stages of the commencement of net to wing, its finishing and the end of net-lifting, the gradual changes in the status of boat and nethappening in the lapse of time, were ascertained with the use of experimental models in50m.-pool, with the counting clarification of the towing speed, interval between the wingends, and the sweeping area of the towing rope.3. On the three sorts of trawling net (trawl net, bull trawl net, Danish seine), modelexperiments were carried out ; and in those experiments, first the area of the net mouth under towing was counted, and with the use of the counted value, the filtering volume wasfixed out, and after combining this with the sweeping area mentioned in item 1, somenumerical value was fixed, and by which the fishing efficiency of the various kinds of fishingnet was compared.
4. For the purpose of using as the fundamental criterion in drawing the basic designsof the respective trawling net, the following experiments were carried out : ―a) Three sorts of trawling net having the bag nets made to be equal typed, in spite oftheir being made of 2 sheets, 4 sheets and 6 sheets of net, each of which being typicalof the trawling net, were designed, and with the use of these, some model experimentswere carried out, and the respective characteristics and efficiency were brought undercomparative examinations.b) Concerning the shape of the net under towing, its resistance against the flowingwater and the behaviours of the fish schools entering into the net ; the causes of suchphenomena were put under considerate examinations, and the distribution of the currentvelocities in and out of the net were measured by using model nets.c) The border line of the limited possibilities of making the turning movements orthe respective fish body within the bag net, in other words, the dimension of the swimming capabilities was fixed experimentally.d) The problem of what behaviour is to be taken by the fish schools against the variations of the flowing velocity within the bag net,was put under consideration to bringit to a reasonable solution, in connection with the size of the fish body and in that withthe size and the scale of the fish schools, respectively ; and at the same, the net formation and the net resistance at that time, were put under investigations.e) In the 50m.-pool, using gold fish, the behaviours of the fish schools to the rope were brought under grasping, with the investigations of the driving-in effects of the netperformed.