# Automated Monitoring of Bluefin Tuna Growth in Cages Using a Cohort-Based Approach
## 2 Materials and Methods  
### Description of the Subsea Monitoring System  
The monitoring system comprises a subsea sensor platform equipped with a stereoscopic camera, an inclinometer, and a 120 kHz single-beam transducer. This platform was positioned at a depth of 23 m on the bottom of a fattening cage in Grup Balfego, located in the West Mediterranean (40\({}^{\circ}\)51\({}^{\prime}\)33\({}^{\prime\prime}\) N 0\({}^{\circ}\)50\({}^{\prime}\)59\({}^{\prime\prime}\) E). Additionally, a logging subsystem, tethered to the cage structure, was placed at the water's surface. The cage itself is cylindrical, with a diameter of 50 m at the water surface and a height of 35 m at its lowest point, providing an approximate volume of 20,000 m\({}^{3}\) and accommodating 724 BFT. The autonomous monitoring system was installed and operational from 28 July 2020 to 23 May 2021. Recorded data were uploaded to the cloud on a daily basis and processed offline. Figure 1 provides an overview of the system. Figure 1: Overview of the components of the monitoring system installed at the Grup Balfego facilities, consisting of a sensor platform and logging subsystem. The sensor platform was equipped with a stereoscopic camera, among other sensors. Positioned on the bottom of the cage and oriented towards the surface, the camera provided a ventral perspective of the fish. Buoyancy was achieved using buoys, and ropes tied the platform to the cage. This camera arrangement offered two primary advantages: first, the sunlight acts like a backlight system so objects are always darker than water and no additional lighting equipment is required at a depth of 23 m; second, the orientation allowed for clear observation and analysis of fish body bending. Video recordings were captured using a customised stereo camera comprising two Gigabit Ethernet cameras, each with a resolution of 2048 \(\times\) 1536 pixels (3.1 megapixels) and a framerate of 20 fps. The cameras were mounted in an underwater housing, with a baseline of 85 cm and inward convergence of 5\({}^{\circ}\). Camera synchronisation was achieved using the IEEE 1588 Precision Time Protocol (PTP). The system was designed for a depth of 40 m and had an umbilical cable supplying power over Ethernet to the cameras and transferring images to a logging computer, which encoded left and right videos through hardware encoding. Two systems with different focal lengths were employed. Initially, a system equipped with 12.5 mm focal length lenses was deployed. However, during the initial months, it became apparent that water turbidity conditions were considerably worse than those observed in l'Ametlla de Mar. Consequently, on many days, tuna could not be effectively measured across the entire water column. With the 12.5 mm focal length lenses, fish could only be accurately measured at distances starting from 6 m since they could not fit completely into the field of view at shorter distances. As a response to this challenge, in September 2020, we decided to modify the cameras and replace the lenses with 6 mm focal length lenses. This adjustment allowed for effective sizing of the fish at distances as close as 3 m, addressing the limitations encountered with the initial lens configuration. The logging subsystem comprised the following elements: battery, solar panel, logging computer, satellite communication (Iridium), and 4G communications. The battery and solar panel collectively provided the system with an average daily energy autonomy of 5 h from June to October and 3 h from October to January. Iridium communications facilitated remote on/off switching and 4G communications enabled the cloud storage of the recordings.