Abstract
This study examines the potential implications of biofouling management on the development of an infectious disease in Norwegian farmed salmon. The hydroid Ectopleura larynx frequently colonises cage nets at high densities (thousands of colonies per m2) and is released into the water during regular in-situ net cleaning. Contact with the hydroids’ nematocysts has the potential to cause irritation and pathological damage to salmon gills. Amoebic gill disease (AGD), caused by the amoeba Paramoeba perurans, is an increasingly international health challenge in Atlantic salmon farming. AGD often occurs concomitantly with other agents of gill disease. This study used laboratory challenge trials to: (1) characterise the gill pathology resulting from the exposure of salmon to hydroids, and (2) investigate if such exposure can predispose the fish to secondary infections–using P. perurans as an example. Salmon in tanks were exposed either to freshly ‘shredded’ hydroids resembling waste material from net cleaning, or to authentic concentrations of free-living P. perurans, or first to ‘shredded’ hydroids and then to P. perurans. Gill health (AGD gill scores, non-specific gill scores, lamellar thrombi, epithelial hyperplasia) was monitored over 5 weeks and compared to an untreated control group.Nematocysts of E. larynx contained in cleaning waste remained active following high-pressure cleaning, resulting in higher non-specific gill scores in salmon up to 1 day after exposure to hydroids. Higher average numbers of gill lamellar thrombi occurred in fish up to 7 days after exposure to hydroids. However, gill lesions caused by hydroids did not affect the infection rates of P. perurans or the disease progression of AGD. This study discusses the negative impacts hydroids and current net cleaning practices can have on gill health and welfare of farmed salmon, highlights existing knowledge gaps and reiterates the need for alternative approaches to net cleaning.
Highlights
Gill diseases are one of the major health challenges in Atlantic salmon (Salmo salar) farming worldwide and can cause significant production losses [1,2,3,4]
At 1 dphe, the prevalence of fish with positive non-specific gill scores was significantly related to previous exposure to hydroid material, and considerably greater than in the control fish (C-group: 38%, H-group: 50%; p = 0.023)
The study provides the first experimental evidence that nematocysts of the cnidarian E. larynx contained in cleaning waste and fragmented hydroid tissues can remain active after high-pressure treatment, and that exposure to such net cleaning waste can damage the gills of Atlantic salmon
Summary
Gill diseases are one of the major health challenges in Atlantic salmon (Salmo salar) farming worldwide and can cause significant production losses [1,2,3,4]. Amoebic Gill Disease (AGD), caused by the amoeba Paramoeba perurans Neoparamoeba perurans), is a prevalent disease in farmed salmon in Australia. Following the first discovery of AGD and P. perurans in Norway in 2006 [8] neither the disease nor the amoeba were diagnosed during the following 5 years. Since 2012, AGD has been a persistent issue on salmon farms located on the west coast of Norway. AGD can be fatal to farmed salmon [7]
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