Sea Technology

AUG 2017

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www.sea-technology.com August 2017 / st 25 tidal currents. This current meter uses a biaxial electromagnetic induction system and has a current velocity mea- surement range of 0 to ± 500 cm/s, a resolution of 0.02 cm/s, an accuracy of ±1 cm/s (± 2 percent), a bearing reso- lution of 0.01°, and a precision of ± 2°. The dynamic behavior of the cage was measured by observing the hori- zontal and vertical shapes. First, for the horizontal shape, a horizontal cross section was observed while changing the depth of the sonar system in 1-m units during slack water when the cur- rent was very weak. For the vertical shape, the vertical cross section was measured by vertically changing the detection plane of the image sonar, us- ing a suspension frame to fix the sonar system to the top frame of the cage. The frame was installed so that the detection plane of the sonar system could observe the cut surface at the center of the cage net while suspend- ed about 1 m below the surface. The vertical cross section was measured at high tide while a strong current was flowing, which was expected to cause the shape of the cage to change. The obtained images were compared with the current direction and current speed obtained with a moored current meter. In addition, to investigate the ac- tion of sessile organisms on the cage net, the 4-mm copper-alloy net, con- ventional non-antifoulant fiber netting and anti-foulant netting were attached to square plastic frames (50 cm long by 50 cm wide) and immersed to a depth of 2 m in the installation area for 3 months. Then, the growth of sessile organisms on the nets was measured and analyzed. Test Results As a result of analyzing the hori- zontal cross section of the copper-al- loy net, which had been continuously measured using the sonar system, the upper part of the copper-alloy net con- nected to the buoyancy pipe exhibited a true square shape, which was well maintained down to a depth of 6 m. Beyond 7 m, however, the sides of the copper-alloy net were found to deform inward, and this tendency worsened as the depth increased. Furthermore, the bottom net of the cage was detected from a depth of 11 m, although part of the bottom net was also detected at a depth of 12 m. From a depth of 7 m, the side nets of the cage were inclined inward and the bottom net sagged downward due to its own weight. It ap- peared that, because the bottom net of the copper-alloy cage is heavy, the cen- ter of the bottom net fell, causing the side nets to fold inward. These shape characteristics were also observed in the measurement of the vertical sec- tion. The measurement of the vertical cross section of the cage revealed that the bottom net sagged downward due to the weight of the bottom net, form- ing a convex shape, which was identi- cal to the shape of the horizontal cross section. When using the sonar system to measure the cage shape, the net shape near the system was well detected, but the reflected signal was not well detected in the case of net shapes that were far from the system. This is prob- ably because of acoustic scattering caused by the fish inside the net. To measure the changes in the un- derwater shape of the copper-alloy net cage with the current, the copper-alloy net cage and a neighboring fiber-net cage were measured during a high tide with fast currents. The copper-alloy net cage showed no significant change in shape, but the shape of the fiber cage changed rapidly as the current increased. This is caused by the mesh of the fiber net being half the size of that of the copper-alloy net such that the fluid resistance acting on the net is higher than that of the copper-alloy net. Furthermore, its shape changed greatly with the current because the material is very flexible. The fish environment in the cage net is greatly affected by the internal vol- ume of the cage. In this study, the cop- per-alloy net cage exhibited almost no change in its internal volume despite changes in the current, and therefore provided sufficient space for the fish in the cage, relieving them from stresses such as density. The shape of the net mesh did not change with changes in the current, resulting in the natural flow of sufficient seawater into and out of the cage, as well as sufficient oxy- gen supply to the inside of the cage. In addition, food waste or carcasses that could remain inside the net flowed out with the seawater and this would be very helpful for promoting the growth of the fish while preventing disease. The amount of sessile organisms that grew on the sample squares was 1.95 kg, 2.55 kg and 1.7 kg in the copper-alloy net, the non-antifoulant fiber net and the anti-foulant fiber net, UNDERWATER LISTENING SYSTEMS Versatile Acoustic Recorders Real-Time Listening Remote Buoys New Noise Processing Features www.rtsys.eu 25 rue Michel Marion 56850 Caudan - France +33 (0)297 898 580 - info@rtsys.eu International Standard and Guideline Compliant (ANSI, MSFD...)

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