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Research Papers: Ocean Space Utilization

Flow Fields Inside Stocked Fish Cages and the Near Environment

[+] Author and Article Information
Lars C. Gansel

Norwegian University of Science
and Technology,
Trondheim 7010, Norway
e-mail: Lars.Gansel@sintef.no

Siri Rackebrandt

Carl v. Ossietzky University Oldenburg,
Oldenburg 26129, Germany
e-mail: Siri.Rackebrandt@uni-oldenburg.de

Frode Oppedal

Institute of Marine Research,
Matredal 5984, Norway
e-mail: FrodeO@imr.no

Thomas A. McClimans

SINTEF Fisheries and Aquaculture,
Trondheim 7010, Norway
e-mail: Thomas.A.McClimans@sintef.no

Contributed by the Ocean, Offshore, and Arctic Engineering Division of ASME for publication in the JOURNAL OF OFFSHORE MECHANICS AND ARCTIC ENGINEERING. Manuscript received July 7, 2011; final manuscript received May 20, 2014; published online June 12, 2014. Assoc. Editor: Hideyuki Suzuki.

J. Offshore Mech. Arct. Eng 136(3), 031201 (Jun 12, 2014) (8 pages) Paper No: OMAE-11-1059; doi: 10.1115/1.4027746 History: Received July 07, 2011; Revised May 20, 2014

This study explores the average flow field inside and around stocked Atlantic salmon (Salmo salar L.) fish cages. Laboratory tests and field measurements were conducted to study flow patterns around and through fish cages and the effect of fish on the water flow. Currents were measured around an empty and a stocked fish cage in a fjord to verify the results obtained from laboratory tests without fish and to study the effects of fish swimming in the cage. Fluorescein, a nontoxic, fluorescent dye, was released inside a stocked fish cage for visualization of three-dimensional flow patterns inside the cage. Atlantic salmon tend to form a torus shaped school and swim in a circular path, following the net during the daytime. Current measurements around an empty and a stocked fish cage show a strong influence of fish swimming in this circular pattern: while most of the oncoming water mass passes through the empty cage, significantly more water is pushed around the stocked fish cage. Dye experiments show that surface water inside stocked fish cages converges toward the center, where it sinks and spreads out of the cage at the depth of maximum biomass. In order to achieve a circular motion, fish must accelerate toward the center of the cage. This inward-directed force must be balanced by an outward force that pushes the water out of the cage, resulting in a low pressure area in the center of the rotational motion of the fish. Thus, water is pulled from above and below the fish swimming depth. Laboratory tests with empty cages agree well with field measurements around empty fish cages, and give a good starting point for further laboratory tests including the effect of fish-induced currents inside the cage to document the details of the flow patterns inside and adjacent to stocked fish cages. The results of such experiments can be used as benchmarks for numerical models to simulate the water flow in and around net pens, and model the oxygen supply and the spreading of wastes in the near wake of stocked fish farms.

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Figures

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Fig. 1

Location of field experiments

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Fig. 2

Setup of the experimental fish farm of the Institute of Marine Research in Matredal, north of Bergen, Norway. Dye experiments were conducted in cage 3.

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Fig. 3

Dye spreading in cage 3 within the first 170 s at 14:57 (Oct. 31, 2009). The arrows indicate the flow direction and are obtained by tracking the movement of dye blobs between two pictures. The lowest plot is a combination of the arrow plots in the middle. The ambient flow speed was approximately 0.06 m/s.

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Fig. 12

Same as Fig. 8, but for Sept. 2, 2008 at 23:00

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Fig. 11

Same as Fig. 10, but for Sept. 4, 2008 at 12:00

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Fig. 10

Same as Fig. 8, but d is for 10 m depth and e is for 12 m depth and for Sept. 2, 2008 at 17:00

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Fig. 9

Same as Fig. 8, but for Aug. 23, 2008 at 07:00

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Fig. 8

Current direction and speed at five locations around cage 5 (see Fig. 3) at four different depths on Aug. 22, 2008 at 17:00. Figures (a)–(d) show the currents at 2 m, 4 m, 8 m, and 15 m, respectively. The length of the arrows indicates the current speed and the orientation of the arrows indicates the current direction. The black arrows are the results from the 5 current meters. The bold arrow in the lower left corner shows the ambient current.

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Fig. 7

Vertical distribution of fish biomass from Sept. 2, 2008 to Sept. 5, 2008. Full day numbers mark midnight.

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Fig. 6

Variation of density with depth at Solheim on Aug. 22, 2008 and on Sept. 2, 2008. (Sigma-T is density minus 1000 kg/m3.)

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Fig. 5

Setup of the current measurements at the Solheim site. 3A shows the location of cage 5 in the fish farm setup and 3B shows cage 5 only. The dots mark the positions of 5 Nortek vector current meters.

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Fig. 4

Dye spreading in cage 3 within 120 s at 15:10 (Oct. 31, 2009). See Fig. 4 for details. The ambient flow speed was approximately 0.09 m/s.

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