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Flow Fields Inside Stocked Fish Cages and the Near Environment

[+] Author Affiliations
Lars C. Gansel

Norwegian University of Science and Technology, Trondheim, Norway

Siri Rackebrandt

Carl v. Ossietzky University Oldenburg, Oldenburg, Germany

Frode Oppedal

Institute of Marine Research, Matredal, Norway

Thomas A. McClimans

SINTEF Fisheries and Aquaculture, Trondheim, Norway

Paper No. OMAE2011-50205, pp. 201-209; 9 pages
doi:10.1115/OMAE2011-50205
From:
  • ASME 2011 30th International Conference on Ocean, Offshore and Arctic Engineering
  • Volume 5: Ocean Space Utilization; Ocean Renewable Energy
  • Rotterdam, The Netherlands, June 19–24, 2011
  • ISBN: 978-0-7918-4437-3
  • Copyright © 2011 by ASME

abstract

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 the effects of biofouling and especially fish behaviour on the flow patterns around and through fish cages. 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 non-toxic, fluorescent dye, was released inside a stocked fish cage for visualization of 3-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 towards the center, where it sinks and spreads out of the cage at the depth of maximum biomass. Furthermore, the converging surface water swirled in the direction of the swimming fish. In order to achieve a circular motion, fish must accelerate towards 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. The 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.

Copyright © 2011 by ASME
Topics: Flow (Dynamics)

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