Introduction
Mortality in Norwegian salmon aquaculture averages at around 15% during time at sea, with gill disease being a major cause of mortality [1]. Gill health is impacted by many factors including pathogens, fish handling, and sea lice treatments, as well as waterborne particles such as harmful algae, jellyfish, and biofouling fragments [1, 2]. Biofouling assemblages occlude pen nets and are commonly removed via regular (up to weekly) in-situ net cleaning operations [3, 4]. During this process, cleaning waste is released into the water column and able to disperse within the pen and farm site. Contact with hydroid particles can cause gill injuries, lasting for up to 7 days [5]. In addition, hydroids and other biofouling organisms can harbour pathogens [6, 7], and dispersal of their fragments via currents may contribute to the spread of disease between pens or farm sites [8].
The study used a hydrodynamic particle tracking model to examine the spread of biofouling particles arising from in-situ net cleaning operations at a case-study farm located in mid-Norway. We determined in particular the potential transport of these particles to neighbouring farms.
Material and methods
The particle distribution model was set up for two similar sized farms in close vicinity (4 km apart) located off the coast of mid-Norway [9]. Input variables to the model were (i) hydroid particle abundance, (ii) hydroid particle size distribution and (iii) hydroid particle sinking rates. Abundance and particle size distribution were estimated based on an assessment of biofouling present before and after cleaning, and analysis of cleaning waste concentration and composition measured in situ. Particle sinking rates were measured in a laboratory study. Results are presented as particle concentrations per square meter accumulated for the upper 35 m of the water column (i.e. average depth of a sea cage) for individual pens, as well as for surrounding waters.
To put the dispersal of hydroid particles from our case-study model farms into a broader spatial context, we quantified the distance of 936 salmon farms along the Norwegian coast to the salmon farm nearest to it. We then determined the proportion of nearest-neighbour farm distances around Norway’s entire salmon aquaculture industry that was equal to or smaller than the dispersal distance of hydroid material quantified by our model.
Results and discussion
Hydroid particles released during simulated net cleaning at the two model farms resulted in a maximum concentration of 958,135 particles m-2 (integrated across the upper 35m of the water column) within individual pens. Particles released from one pen were transported into adjacent pens, leading to small ‘peaks’ in particle concentration even in pens that were not being cleaned. As a consequence, fish in individual pens were exposed to hydroid particles multiple times while they, or surrounding pens were being cleaned, increasing the risk for encountering harmful particles and gill injury. With recovery times after one-time exposure taking up to 1 week [5], repeated exposure may contribute to worsening gill conditions in salmon during time at sea.
Maximum dispersal of biofouling material released via cleaning occurred 48 hrs following onset of net cleaning operations (Figure 1). The largest unidirectional dispersal distance for hydroid particles was > 4.8 km. Of the 936 salmon farms along the Norwegian coast, 63% have at least one nearest neighbour farm within this distance. While our study was restricted to a case study location, these results suggest that inter-farm dispersal of biofouling material may also take place elsewhere.
Net cleaning may facilitate the dispersal of gill damage inducing particles between farm sites, including particles that are known to harbour pathogens. Novel cleaning tools that avoid the release of biofouling particles via containment or more regular cleaning (grooming) of nets [4] have the potential to reduce gill damage arising from biofouling maintenance operations.
References
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