Effects of cnidarian biofouling on salmon gill health and development of amoebic gill disease

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.

Biofouling in marine aquaculture: a review of recent research and developments

“…Direct contact with cnidarian biofouling can be harmful to the fish, as organisms bearing nematocysts (stinging cells), such as the hydroid Ectopleura larynx and the anemone Anthothoe albocincta, have the potential to cause gill and skin damage (Baxter et al 2012;Wybourne 2013;Bloecher, Powell, et al 2018). In addition, biofouling poses a health risk to cultured fish as it can facilitate and amplify the presence of pathogens by harbouring viral, bacterial, and parasitic organisms that cause various diseases (reviewed in Fitridge et al 2012).…”

“…Upon contact, cnidarian biofoulers expel nematocysts that can penetrate fish skin and deliver poison (Helmholz et al 2010;Cegolon et al 2013), even after fragmentation following pressure-washing . For example, the hydroid Ectopleura larynx causes gill injuries in Atlantic salmon (Baxter et al 2012;Bloecher, Powell, et al 2018), and white-striped anemones Anthothoe albocincta are suspected to cause skin damage in Chinook salmon (Wybourne 2013). Management methods for other issues (eg parasitic sea lice) can further exacerbate these impacts.…”

Epidemiology of marine gill diseases in Atlantic salmon (Salmo salar) aquaculture: a review

“…In addition to the presence of putative pathogens, a number of other potential risk factors for CGD have been proposed. One major type of risk factor may be environmental insult to the gills, such as exposure to harmful phytoplankton, gelatinous zooplankton species in the water column or biofouling organisms dislodged into pens during in situ net washing (Rodger et al 2011a; Bloecher et al 2018; Kintner & Brierley 2019). Bath treatments involving the use of chemotherapeutants such as formalin (Speare et al 1997) or hydrogen peroxide (Kiemer & Black 1997; Rodger et al 2011a) can be directly damaging to gills or may exacerbate existing gill conditions and may represent a risk factor for the development of CGD.…”

“…To counter this, nets can be cleaned using pressure washers, but fish in cages have been observed to exhibit avoidance behaviour from the dense clouds of debris that come off the nets during the cleaning process. Experimental challenges showed that this debris can cause pathological changes in the gills, such as epithelial sloughing, necrosis and haemorrhaging (Baxter et al 2012; Bloecher et al 2018).…”

Gill diseases in marine salmon aquaculture with an emphasis on amoebic gill disease.

“…Finally, Bloecher et al [56] recently confirmed the negative impact that the hydroid nematocyst Ectopleura larynx can have on salmon gill health and highlighted the potential risks that net cleaning poses to fish welfare. However, in situ measurements of gill health before and after net cleaning conducted in the field are necessary to validate these findings.…”