Improving fish welfare through the application of CFD

Safetec has applied CFD (Computational Fluid Dynamics) to support the design of one semi-closed fish tank in 2019-2020. This article describes the challenges of the design of semi-closed fish tanks with regard to and solutions to an optimized design by application of CFD.

Why Semi-closed fish farming unit

The development of semi-closed fish farming units (also called semi-closed fish tanks) has gained substantial interest in recent years due to the following advantages:

  • The sea water is taken from deep water, and hence sea-lice problems are avoided.
  • The solid walls separate the fish from the outside environment, preventing direct interaction with the sea.
  • The facilitation of larger fish density (leading to higher productivity).

Challenges on design of semi-closed fish farming units

The technology is at an early stage of development, with some challenges needing to be solved. Some challenges on the welfare environment for the fish are:

  • Water speed distribution: In an open net fish farming cage, the seawater flows through the open net cage mainly horizontally, and hence the water speed distribution is directly correlated with the current and wave conditions. However, in a semi-closed fish tank, the walls block the horizontal seawater flow, and the seawater enters and flows out of the fish tank through the bottom (see Figure 1). Hence the water speed distribution is decided by the sea current, the waves, the tank geometry, and extra water pumping. Inside the fish tank, the flow is three-dimensional (3D) and complex.
  • O2 supply: Sufficient O2 supply is a basic requirement for a fish tank. In a semi-closed fish tank, the fresh seawater carrying dissolved O2 enters and leaves the fish tank only through the bottom. Hence the water flow rate through the fish tank is much smaller than for an open net cage. This leads to less exchange of water, and consequently less O2 supply.
  • Discharge of ammonia, solid particles, and CO2The main source of ammonia and solid particles in a fish tank is feces, while fish breathing generates CO2. The concentration of ammonia, solid particles, and CO2 shall not exceed the corresponding threshold limits in order to avoid a negative impact on fish health. Turbidity resulting from suspended solid particles is an important indicator of water quality. However, the limited water flow rate in semi-closed fish tanks might not remove these pollutants efficiently. Hence, extra measures might be needed.

Figure 1 Flow streams of sea water inside and around a semi-closed fish tank

Solutions and Application of CFD

Solutions involve freshwater pumping, openings in the walls and extra oxygen injection by an oxygen generator. But for all these measures, the basic design parameters need to be optimised. Safetec has developed a methodology based on CFD (Computational Fluid Dynamics) simulation for such optimisation. Complicated distribution of water speeds (see Figure 2 and Figure 3), O2, ammonia, solid particles, and CO2 could be extracted from the CFD simulation results. In addition, water exchange time could also be calculated based on the simulations. For a given design, design parameters (i.e., geometry, hatches and pumping systems) for different current conditions could be optimised by running multiple groups of CFD simulations.

Figure 2 Distribution of Flow vectors

                     Figure 3 Statistical data of flow speeds in fish tank

The Way Forward

Next step, Safetec will include more scopes in the CFD simulations in order to assist optimised design and operation of fish tanks. These include modelling of fish feeds and behaviour of dead fish.