Aquacultural waste dispersion modeling

The number of marine fish farms is rapidly increasing due to both the increment in global fish consumption and the decrease in wild fish stocks. The potential detrimental effects of fish farms on coastal ecosystems raise also concerns. Mathematical models can be employed to assess aquaculture impacts for both existing farms and for establishing new ones.

Our research is focused on the development of a coupled, three-dimensional modeling framework which takes into account the hydrodynamics of the study area and the environmental response to the organic load from the cages.

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We have coupled together three different models as shown in the figure above. In the figure, solid (dashed) lines indicate online (offline) coupling. Hydrodynamical calculations are performed thanks to our customized version of the Princeton Ocean Model (POM) which we mainly use to simulate the Ligurian Sea currents. Vertically-integrated currents calculated in POM are then passed to our dispersion particle model, the Lagrangian Assessment for Marine Pollution 3D (LAMP3D) model. LAMP3D uses the theory for the Ekman spiral to obtain a three-dimensional velocity field and calculate the fate and the dispersion of a large number of particles. To consider the natural capability of the seafloor in absorbing part of the organic load, we have also developed our degradative benthic model, the Finite Organic Accumulation Module (FOAM). To take into account the sediment stress levels in FOAM, we have first used different remineralization rates observed for the Atlantic salmon production and then new data from in situ measurements carried out in Mediterranean fish farms. The final benthic impact index (or degradative parameter) provided by FOAM is related to the ratio between the oxygen supply calculated using the model velocity at the bottom and the oxygen demand due to the model carbon flux to the sediment.

The FORTRAN source codes for LAMP3D and FOAM are available after registration and under the terms of the GNU General Public License at Andrea Doglioli's webpage.

During this work we have realized the importance of hydrodynamics for a correct assessment of the particles' fate. Our plan is to directly use in the future the three-dimensional velocity field calculated by different models as the Regional Ocean Model System (ROMS) or the Massachusetts Institute of Technology general circulation model (MITgcm).

To validate our models, we study the offshore fish farm (black square in the figure above) located in the Ligurian Sea (Northwestern Mediterranean) in front of Lavagna. Solid lines in the figure indicate bathymetry and S1-S4 are the control stations (see below). The farm is composed of 8 fish cages and the bottom depth ranges between 38 and 41 meters. Hydrodynamics in the area are strongly affected by the presence of a blunt headland, the Portofino Promontory which we have extensively studied in our project on turbulent structures around capes. On average, currents are mainly along-shore accounting for a northwestward transport. They are also strongly variable in time however, with important periods of inversion of the current direction. Hydrodynamical simulations are validated against observations from currentmeters in the area.

Shown below is an example for the time evolution of the sediment status (shaded colors) calculated by the models under the cages. The 5-, 10- and 50-millimole-per-square-meter organic carbon concentration isolines are shown in black. White arrows indicate bottom velocity. The 30-, 40-, 45- and 50-meters isobaths are shown with white dashed lines. Current direction and intensity strongly influence the position of the impacted area and the degradation of the settled matter thanks to two different processes. On one hand, a strong current brings more oxygen to the sediment and makes degradation more efficient. On the other hand, the same strong current increases waste dispersion resulting in a wider impacted area and in lower waste concentrations on the bottom. The animation clearly shows that current reversals are fundamental for a correct estimate of the stress level at the bottom.

The average carbon flux on the seabed at the four S1-S4 control stations around the farm are reported in the figure below. Results are presented for different simulated scenarios, namely for Atlantic (old runs, asterisks) and Mediterranean (new runs, filled and empty circles) conditions. Model trends compare well with the in situ measurements (grey bars). The measured carbon flux toward the sediment is highest in station S2 and lowest in S4, where a strong removal of carbon and a subsequent negative flux are observed. This is an agreement with the average northwestward transport explained above. Higher degradation rates for July conditions result in the lowest values of organic carbon flux (empty circles). When October degradation rates are used, the highest carbon fluxes are registered only in the stations with the greatest organic loads (S1 and S2). As expected, a worse agreement is noticed when Atlantic rates are adopted (asterisks).