Introduction
Dissolved oxygen (DO) is a key variable in Atlantic salmon sea cages, as sufficient oxygen is needed to ensure good fish welfare and high growth. The effects of insufficient oxygen leading to hypoxia range from reduced appetite, stress responses, reduced feed conversion and growth to acute mortality, depending on how low DO levels are encountered (Oppedal et al., 2011, Remen et al., 2016). Alver et al. (2022) presented a mathematical model of 3D distributed DO levels in salmon cages, seeking to combine our knowledge of the advective and diffusive transport of oxygen, the oxygen consumption by individual fish, and the spatial distribution of the fish , to estimate DO levels as a function of environmental conditions . In this study , the model is used along with an extensive measurement program to investigate oxygen conditions in one of six commercial scale cages containing biomass at a location in Mid-Norway.
Materials and methods
The mathematical model is based on an advection-diffusion equation operating on a regular grid with horizontal and vertical resolution of 2 m , with a simplified representation of the fish to compute feed ingestion and oxygen consumption rates. Feeding dynamics are computed based on Alver et al. (2016), and oxygen dynamics based on Alver et al. (2022). M odel inputs for temperature, ambient oxygen level, current speed and direction are based on measurements over a 9 day period in June 2022, and model output is compared to measured oxygen levels at 12 points for one of the cages (centre and three points along the edge of the cage, each at 5, 10 and 15 m).
The model is tested when simulating the observed cage only, and when simulating the full farm (Fig. 1) . The e ffect of neighbouring cages partly depleting the water of oxygen needs to be taken into account when simulating a single cage only, and this was addressed with an ad hoc model modification that reduces boundary levels of oxygen dependent on current direction and speed. An additional modification was tested to account for increased biofouling over the study period leading to gradually more damping of the current speed within the cage. The different model varieties were evaluated based on their agreement with measured oxygen.
Results and discussion
Comparisons between simulated and observed DO levels at the sensor positions are summarized in Fig. 2. The model variants are denoted basic (single cage model, no modifications), VAB (single cage model with adjustment to boundary conditions and current damping rate) and VFB (full farm model with adjustment to current damping rate). The basic model has a clear overall positive bias, with a positive trend over the study period. The VAB model has near zero bias and no similar trend, showing that the ad hoc modifications lead to better model fit. The VBF model also significantly reduces the bias and the positive trend, showing that taking the full farm into account clearly improves the overall model fit. All model variants have highest RMSE at 5 m and lowest at 15 m, indicating that dynamics in the upper part of the water column in the cage is the most difficult to capture in the model.
References
Alver, Morten Omholt, Martin Føre, and Jo Arve Alfredsen, 2022 . "Predicting oxygen levels in Atlantic salmon (Salmo salar ) sea cages." Aquaculture 548: 737720.
Alver, M.O., Føre, M. and Alfredsen, J.A., 2023. Effect of cage size on oxygen levels in Atlantic salmon sea cages: A model study. Aquaculture , 562, p.738831.
Alver, M.O., Skøien, K.R., Føre, M., Aas, T.S., Oehme, M., Alfredsen, J.A., 2016. Modelling of surface and 3D pellet distribution in Atlantic salmon (Salmo salar L.) cages. Aquacultural Engineering 72, 20–29.535.
F. Oppedal, T. Dempster, and L. H. Stien, 2011. Environmental drivers of Atlantic salmon behaviour in sea-cages: A review. Aquaculture, 311(1):1–18.
M. Remen, M. Sievers, T. Torgersen, and F. Oppedal, 2016. The oxygen threshold for maximal feed intake of A tlantic salmon post-smolts is highly temperature-dependent. Aquaculture, 464:582–592.