Complexities in commercial scale use of non-invasive controls against parasites in aquaculture

Abstract

Salmon lice (Lepeophtheirus salmonis) are one of the major challenges faced by the Atlantic salmon (Salmo salar) aquaculture industry. Due to the risk of poor welfare outcomes and high mortality during treatments against salmon lice, as well as increasing resistance towards many of the available chemical therapeutants, prophylactic measures that mismatch host and parasite environments are emerging. For salmon lice, these depth-based strategies exploit the positioning of free-living lice larvae in the upper part of the water column before they attach to salmon skin. They work by uncoupling salmon from mostly surface-dwelling lice larvae while still providing surface air access required for salmon swim bladder reinflation, buoyancy control and optimal welfare.

One of the most extensively studied depth-prevention technologies is the snorkel sea cage. It consists of a standard cage fitted with a roof net to keep fish deeper and an enclosed tarpaulin tube (a snorkel), where salmon have access to the surface air used for filling their swimbladder while still avoiding surface waters where lice larvae are most abundant. Previous work show they can reduce salmon lice infestation levels in sea cages without major impacts on salmon welfare. However, long full-scale studies, which are crucial to understand the real-world consequences of these technologies on salmon lice infestation, are lacking. Knowledge is also needed on i) how additional lice removal strategies might work in combination with lice prevention technologies and ii) the effects of these controls on other co-occurring salmon parasites.

The purpose of this thesis was to examine the impact of commercial-scale snorkel sea cages on external (L. salmonis and Paramoeba perurans) and internal parasites (Eubothrium sp.) of Atlantic salmon and investigate possible in situ control methods (cleaner fish and optical laser) for reducing remaining salmon lice infestations that develop. This knowledge will help reveal the successes, challenges, and solutions in managing parasites with snorkel sea cages in salmon farming and will provide insights on the ramifications of other lice barrier technologies combating the salmon lice problem.

In a study observing the long-term effects of depth-based technology at commercial scale, salmon lice infestations on Atlantic salmon were examined in triplicate snorkel and standard sea cages over a 12–month production cycle. Snorkel sea cages reduced newly settling lice on Atlantic salmon by 75 % and salmon lice treatments by nearly half throughout the study, confirming that snorkel sea cages can effectively control lice over commercial production cycles. Lice reductions depended on an environment free of layering with surface brackish water (salinities < 28 ppt) and warm water (temperatures > 16 °C), highlighting the importance of considering local environment conditions when applying depth-based prevention technologies.

With the potential for depth-based technologies to influence salmon parasites other than lice, we document that snorkel sea cages reduced both prevalence and abundance of marine tapeworms (Eubothrium sp.) in salmon guts. In a study comparing commercial snorkel and standard sea cages, tapeworm prevalence was 3–5 times lower and tapeworm abundance 10–20 times lower in snorkel sea cages. In separate studies, there are indications that the presence of snorkels might increase the risk and intensity of infestation by the marine amoeba P. perurans, the causative agent of amoebic gill disease (AGD). This problem seems to increase with shielding depth. However, creating a low salinity surface layer inside the snorkel may limit these infestations if salmon enter for sufficient time to reduce AGD levels.

Continuous deployment of lice-eating cleaner fish and lice-shooting optical lasers are increasingly used to remove lice from farmed salmon. However, information about their effects are lacking. This is especially important in depth-based prevention cages, in which adding efficient lice reducing controls could prevent the need for removing the prevention technology to perform other de-licing procedures, saving time and effort for salmon farmers. In this thesis we document that using optical lasers in combination with 16 m deep snorkel sea cages during winter did not lower the infestation density of mobile salmon lice compared to cages without laser nodes installed. Additionally, based on high mortalities and minimal feeding by ballan wrasse and a possible mismatch between lumpfish and salmon swimming depths in standard salmon cages, which may be even more pronounced in depth-based prevention cages, it was suggested that cleaner fish may have low effectiveness against salmon lice over autumn-winter.

The main conclusions from this thesis is that snorkel sea cages have the potential to reduce both salmon lice and marine tapeworm infestations in commercial scale sea cages, while the risk and intensity of AGD seem to be increased compared to standard cages. However, freshwater filling inside the snorkel show promise as an in situ control method for the amoeba. On the other hand, even with several options available (e.g. cleaner fish and optical delousing), none of the in situ lice control methods stood out as a clear lice removal method to be used in combination with preventive technology. Optical lasers did not reduce lice compared to cages without lasers and cleaner fish experience high mortality, poor welfare and possible opposing depth distribution to salmon. More work focussing on depth distribution for both cleaner fish and salmon is needed to improve the efficiency of the lice removal options for depth-based prevention technologies.