Abstract
The high-frequency net cleaning regime in Norwegian salmon aquaculture, which is highly driven by ensuring cleaner fish delousing efficacy, has detrimental effects on the cage environment, fish health & welfare, and farm economy. There is currently no standard on maximum recommended biofouling conditions on cage nets, and there is conflicting evidence on the actual impact of biofouling on cleaner fish behaviour in terms of habitat usage. The aims of this study were to characterise biofouling conditions at the time of net cleaning, the effect of spatial factors on biofouling, and investigate current management practices. Also, the potential impacts of biofouling abundance on cleaner fish behaviour in terms of proximity to net walls was investigated to subsequently develop a threshold of recommended biofouling cover on net walls. Five sampling events were conducted at four unique salmon farms from September through November 2019. This study quantified % biofouling abundance at the time of net cleaning at 1, 5 and 10m depth on four transects (North, East, South, West) within one or two cages at each site. Data was collected through vertical filming of net walls and subsequent image analysis. The presence/absence of lice skirt was also linked to each image. To determine the effect of net fouling on cleaner fish habitat usage, net walls were filmed in horizontal transects and resulted in cleaner fish counts per sampled minute. 'Maximum' biofouling conditions (i.e., biofouling abundance on cage nets at the day of net cleaning) differed significantly between sampling events and varied between 10.4% and 58%. Algae and hydroids were identified on image samples, and the dominance of the species groups varied between sampling events. The variability between cages, depth, and cardinal direction all influenced total biofouling abundance and algae abundance. Hydroids were not affected by cardinal direction. There was significantly more biofouling in samples of the first meter below the lice skirt edge than in samples from the last meter still protected by lice skirts. The abundance of the two species groups responded inversely to lice skirt cover, where hydroid abundance was significantly higher in the selected samples from below skirt edge than above, and algae abundance was higher above than below. Across all sampled cages, cleaner fish numbers observed before net cleaning were significantly higher than after net cleaning. Although cleaner fish numbers showed a strong positive relationship with increasing biofouling abundance on cage nets, the relationship was only true for cleaner fish observed before net cleaning took place. When data of cleaner fish observed before and after net cleaning were analysed together, said relationship was much weaker but still significant. This study demonstrated that there is no standard in maximum biofouling conditions on net walls before net cleaning is conducted. Spatial factors influenced biofouling abundance and community composition, but not in a consistent pattern, indicating that within-farm knowledge is vital. Although cleaner fish abundance along net walls was greater before net cleaning than after, and the relationship between cleaner fish and increasing biofouling was significant, the results were weak, indicating that the effect of biofouling may not be as great as fish farmers have formerly believed. Regardless, based on the current findings, a biofouling threshold of 40% may be reasonable with regards to cleaner fish numbers observed in close proximity to net walls. However, if a standard is to be developed, regardless of the effect on cleaner fish or cage environment and fish welfare, a simple and accurate way to monitor biofouling abundance in situ must be developed for farmers.