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Motivated by the need for additional tools to disinfect discharge water from well boats, and to prevent distribution of salmon lice, the effect of ultrasonic cavitation on the planktonic stages of the salmon louse, nauplii and copepodids, as well as marine heterotrophic bacteria, and the marine green microalgae Tetraselmis suecica, has been investigated. Survival and morphology were registered after different exposure times. Efficacy of the ultrasonic cavitation treatments varied with exposure time. A reduction in survival was registered even for the shortest exposure time (5 seconds) for both naupliar and copepodid stages of the salmon louse (36.7 AE 11.5 and 67.20 AE 7.2% survival respectively). Survival reached zero after exposure times of 20 and 60 seconds for the nauplii and copepodid stages, respectively. A reduction in 70% was observed for bacteria at all exposure times (5 to 300 s), while a reduction of 95% was observed after 300 s for algal cells. The logged energy transfer to the samples was on average 17.5 J/s. In conclusion, cavitation treatment is destructive for the planktonic stages of salmon lice, and may contribute to reduce discharge of pathogens and parasites from well boats when adapted for this purpose and combined with existing water disinfection methods. K E Y W O R D Scavitation, control strategy, fish transport, salmon aquaculture, sea lice, well boat

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Biofouling is a serious problem in marine finfish aquaculture with a number of negative impacts. Marine growth obstructs net openings, thereby reducing water exchange through the net and affecting fish welfare and health, as well as the spreading of dissolved nutrients, particles and pathogens. Furthermore, additional water blockage leads to increased hydrodynamic forces on fish cages, which potentially threaten the structural integrity of the fish farm. However, detailed knowledge about the effects of biofouling on the flow past, and the resulting forces on fish cages, is limited and systematic investigations of the effects of different types of fouling have been called for. This study investigates the effects of different amounts and sizes of two important fouling organisms in Norwegian aquaculture, blue mussel (Mytilus edulis) and kelp (Saccharina latissima) on the drag on net panels. Drag forces on a number of clean and fouled nets were measured in a flume tank at a flow speed of 0.1 m/s. Net solidity was calculated from images acquired of all nets in the current. The relationship between net solidity and drag was then found for clean nets and for each type of fouling, and biofouling was parameterized by comparing clean and fouled net results: for a given fouled net, a clean net can be found that experiences the same drag. The latter can then be used in numerical models to estimate the effect of fouling on net drag. That means existing models can be used to model the drag effect of fouling. This study found a solidity increase due to mussel and kelp fouling to affect drag roughly at the same rate as an increase in clean net solidity at a flow speed of 0.1 ms−1 and within the tested fouling size range for two net types. Therefore, existing models, describing the relationship between net solidity and drag, can be used directly or with minor alterations (especially at high solidities) to estimate effects of additional mussel and kelp fouling on drag. In contrast, wet weight seems to be unsuitable as a measure to estimate drag on nets fouled with seaweed or mussels. It should be noted that these findings are only valid under similar conditions, and that other fouling types and sizes, as well as test parameters and tank size can affect the relationship between solidity and drag.

This paper presents a study of traditional netting materials subjected to disinfecting chemicals during fish farming and treatment of net cages. A series of tests were performed in order to study the effect of various concentrations of disinfecting chemicals on the tensile strength of Raschel knitted Nylon netting materials. Simulated spill of diluted hydrogen peroxide (HP) to the jump fence during de-lousing did not affect the strength of the applied new and used knotless nylon netting samples. Hydrogen peroxide reacted with biofouling forming gas bubbles, but this did not result in reduced netting strength. The performed tests did not indicate any effect on netting strength from a simulated single, traditional bath disinfection as performed at service stations applying the disinfectant Aqua Des (AD) containing peracetic acid (PAA). However, increasing the AD concentration from 1 to 10% resulted in a strength reduction of 3-6%. Simulated spill of concentrated AD on the jump fence of a net with copper coating residuals resulted in a severe reduction in strength of 45%. This strength loss was probably a consequence of chemical reaction between copper and Aqua Des, and uncoated netting did not experience any loss in strength subjected to the same chemical exposure. These findings from application of AD should also apply to other PAA disinfection chemicals with trade names as, for example, Perfectoxid and Addi Aqua.