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陥穽漁具の機能変化に関する研究


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Title: 陥穽漁具の機能変化に関する研究
Other Titles: Study On Function Changes in Trap Gear during Operation
Authors: 山口, 恭弘
Authors (alternative): Yamaguchi, Yasuhiro
Issue Date: Mar-2000
Publisher: 長崎大学水産学部
Citation: 長崎大学水産学部研究報告, 81, pp.1-41; 2000
Abstract: Trap gear such as set nets and pots are usually operated to fix on the sea bottom for several weeks. In order to be clear function changes in trap gear frameworks of this thesis were consisted of the following three steps by fishing gear constructions. 1) The netting twines consisted of the fundamental part of a fishing gear may be changed the breaking strength and elongation. 2) The nettings consisted of the minimum unit of function of capture. 3) Pot fishing gears are considered to the simplest model of an actual fishing gear. On the other hand, it is important for establishing and carrying out of environment-friendly to improve the fishing technology. In fact, trap gear are well-known as never-overfishing gear. It has never studied to be clear the function changes in trap gear during operations. In the present study, the function changes in trap gear were studied with using material engineering and hydrodynamical methods. The results obtaind can be summarized as follows: 1. The breaking strength and elongation of netting twines after soaked in the sea more than three years were measured. Nylon and polyethylene netting twines having thickness of 220-1980 denier were used. The breaking strength and elongation were sharply decreased for the first 1 year after soaked in the sea, and became a constant during 2-3 years. The retained breaking strength and elongation after 3 years of all test netting twines were about 70% and 40-60%, respectivly. 2. An encrusted condition of marine foulings on nettings for set nets which consider influence to measure a drag on the fouled plane netting by marine foulings were measured. The test netings were soaked around a set net fishing ground during the periods from 15 to 60 days and some water depthes (1.5 to 15m). The test netting consisted of polyester multi-filaments (0.85mm in diameter and 17mm in bar length), which is commonly used for the set net fishing gear. The main marine foulings are fine attaching materials so-called "Nuta", Amphipoda, a kind of hydrozoans Tubulalia mesembryanthemum and a kind of moss animals Bugula neritina. 3. The drag per 1m² for each test net at the right attack angle and 0.5m/s in flow velocity was from 3.7kgw to 19.3kgw. The changing ratio of drag varied from 1.4 times to 8.5 times against the initial (unused) netting. The mean ratio of drag on each soaking period, however, peaked 6.5 times of the 30th day net and then it scarcely changed till 60th day net. 3. The drag per 1m2 for each test net at the right attack angle and 0.5m/s in flow velocity was from 3.7kgw to 19.3kgw. The changing raito of the drag varied from 1.4 times to 8.5 times against the unused net. However, the mean raito og drag on each soaking period peaked 6.5 times of 30 days in soaking periods and then it scarcely changed till 60 days. The porosity (β) of the nettings before and after fouling changed from 76 to 16%. The drag coefficient (CD) of fouled nettings can be expressed by the following equation; CD=35.98(Rh)⁻⁰.⁴³(1.7×10²<Rh<1.8×10³), where Rh is Reynolds number based on hydraulic mean depth of a fouled netting. However, the drag coefficient may be rather influenced the porosity for Rh<5×10². The drag coefficient (CD) of a fouled netting was determined to be 1.3 at 0.6<β<0.8 and 2.2 atβ<0.5, respectively. The results suggest that the drag coefficient of a fouled netting is determined by its porosity. 4. To evaluate the preferable fishing ground for cuttlefish basket traps from bottom condition and CPUE (kg/trap/day) at two different fishing grounds (off Ariake and off Fukae) in Shimabara sound. The Md φ of off Ariake and off Fukae weres -0.3 to 3.0 and 1.0 to 1.5, which were limited range against the whole Shimabara Sound (-5.4 to 5.6). CPUE of off Fukae was 2-fold greater than that of off Ariake. The significance of differences in CPUE between two fishing grounds was verified by taken together Mann-Whitney test (p<0.05). The results suggest that the medium sandy sea bottom is more preferable for capturing of golden cuttlefish by the cuttlefish basket trap. 5. Catches were high at water temperature between 12-14℃. Daily catches varied periodically due to the change in the lunar cycle, and it became maximum at neap tide, or ebb, and minimum at spring tide. In this region, water temperature seems to be an important long-term factor affecting the main fishing period, and tidal current seems to be an important short-term factor controlling daily catch. The amount of catch might be controlled by both water temperature and tidal current, which is therefore the important factors in the cuttlefish basket trap fishery.
Keywords: 陥穽漁具 / trap gear / 機能変化 / function change / 汚損網地 / fouled nettings / 空隙率 / porosity / 流水抵抗 / drag / コウイカ / golden cuttlefish
URI: http://hdl.handle.net/10069/29714
ISSN: 05471427
Type: Departmental Bulletin Paper
Text Version: publisher
Appears in Collections:第81号

Citable URI : http://hdl.handle.net/10069/29714

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