Evaluating a crowding intensity scale and welfare indicators for Atlantic salmon in sea cages

Abstract

A 5-level crowding intensity scale for directing and auditing the crowding of Atlantic salmon in sea cages based on surface observations is currently included in standards, manuals, and guidelines for fish farmers. Here we test the feasibility of using this scale to create distinct crowding levels, the effects of these different levels upon fish welfare, and the suitability of a set of possible operational welfare indicators (OWIs) and laboratory-based welfare indicators (LABWIs) to be included in toolboxes for monitoring and assessing fish welfare in relation to the crowding of salmon in sea cages. Crowding level 1 was not included in this study since this is a very light level of crowding, and also not level 5 as this level clearly would harm the fish and lead to mortalities. We were able to use the scale to create three distinct crowding levels in two of three separate crowding events in 12×12m2 sea cages. Although the farm personnel were experienced, it soon became evident that underwater monitoring of fish behaviour and how the net was tightened around the fish was important to make sure that no pockets or irregularities that could harm the fish were formed during the crowding. Despite evidence of increased stress and epidermal damage with increased crowding intensity, there were no clear indications that this led to any long-term detrimental effects on fish welfare. In conclusion an OWI-toolbox for crowding should include both surface and underwater observations, monitoring of oxygen conditions, and morphological injury data to steer decisions to prevent welfare problems and mortalities. In addition, qualitative assessment of fish behaviour, plasma cortisol, and skin histology can be included in a LABWI-toolbox if more in-depth information on the effects from the crowding is wanted.

Keywords

• Salmon aquaculture
• Crowding
• Fish welfare
• Welfare indicators
• Fish behaviour
• Cortisol