An underwater risk scale for monitoring the crowding intensities of Atlantic salmon in commercial net pens using Remotely Operated Vehicles (ROVs), v0.1

Crowding is a common aquacultural procedure and its monitoring and management is key to limiting its potential detrimental effects upon the fish. Crowding intensity in Atlantic salmon aquaculture is typically assessed manually using surface-based observations, such as changes in surface activity, or the numbers of dorsal fins or white sides that are visible during inspections of the crowd. However, surface observations, whilst having utility in many settings, do not always equate with the sub-surface behaviours of fish and the potential welfare risks that occur underwater. Here we suggest an underwater crowding risk scale for Atlantic salmon held in net pens based on underwater observations for crowd monitoring using a Remotely Operated Vehicle (ROV). The scale is based on a survey of fish farm professionals, fish welfare researchers and fish health students. A crowding event will typically fluctuate between different intensity levels over time and intensity and risk level can change in an instant. The scale is therefore based on what an ROV operator can see within a 10 s time frame, so that the ROV operator can help guide the staff managing the crowd in their corrective actions to lower the risk. This is a first attempt to make such an ROV-scale and to introduce stakeholders to the concept. It is therefore labelled v0.1. Given the importance of crowding for fish welfare, this study should be quickly followed up by work to improve and enhance the scale. • An underwater risk scale for crowding Atlantic salmon in net pens is presented. • The scale is based on a survey of fish farm professionals, fish welfare researchers and fish health students. • The scale is based on what an ROV operator can see within a 10 s time frame. • It aims to assist ROV operators in detecting potential risks so that they can guide the staff managing the crowd. • This is a first attempt to make such an ROV-scale and it is therefore labelled v0.1.