A number of mortality events involving different jellyfish species have been reported in the literature over the years. Small jellyfish can pass through the nets, while bigger individuals tend to break up into pieces still capable of stinging the fish.
Affected fish can suffer from hypoxia, mechanical damage and toxicity via nematocysts discharge (Baxter et al. 2011). During the summer and autumn months of 2013, large aggregations of the mauve stinger jellyfish Pelagia noctiluca were present along the west coast of Ireland causing significant losses to the affected Atlantic salmon farms.
Associated mortalities ranged from <1 per cent to 70 per cent depending on farm and pen location, and the sea temperature ranged from up to 21ºC in August down to 10ºC in November.
In recent years, marine gill disorders have become one of the most important health issues for the salmon industry, and these comprise infectious and non-infectious conditions including jellyfish.
However, jellyfish impact is usually underestimated and only severe events are reported. In the absence of complementary water samples or observations, low levels of morbidity/mortality and unspecific gill pathology are generally attributed to water borne irritant damage.
This is why from our industry and veterinary perspective we considered important to highlight the potential consequences and to describe the pathology of such interactions.
The observations and findings from last year’s cases were put together in a short communication published in the Journal of Fish Diseases (Marcos-López M, Mitchell SO, Rodger HD (2014) Pathology and mortality associated with the mauve stinger jellyfish Pelagia noctiluca in farmed Atlantic salmon Salmo salar L. DOI: 10.1111/jfd.12267).
This note is a short summary of what reported there.
Clinical presentation and pathology
Affected fish showed respiratory distress, loss of appetite, lethargy and/or increased jumping behavior. On the most severe cases, up to 80 per cent of the fish examined presented signs of jellyfish damage in the gills and skin. Gross skin lesions greatly varied in size and included erosion, scale loss, swollen and/or congested/haemorrhagic lesions and ulcers (Fig. 1B). Gill damage comprised necrosis, haemorrhage and/or loss of tissue. Some lesions showed a yellow-brownish colour suggesting bacterial infection.
Histology samples were taken to characterize the type of pathology. Overall, the histopathology assessment of the skin lesions revealed a significant acute dermatitis with associated dermal necrosis and focal ulceration (Fig. 1C). The affected gill filaments showed acute haemorrhage, congestion, infiltration, oedema, necrosis, lamellar epithelium sloughing and/or tissue loss (Fig. 1E).
Chronic lesions also showed lamellar epithelium hyperplasia and fusion, and occasional presence of giant cells within the affected lamellar epithelium. Bullae-like formations at the edges of the filaments were also observed in some samples (Fig. 1F).
In some of the samples, large aggregations of filamentous bacteria (Tenacibaculum sp.) were seen colonizing the necrotic filaments (Fig. 1D). Open lesions are prone to secondary bacterial infections, however some zooplankton species including P. noctiluca have been shown to carry out and host Tenacibaculum maritimum (Delannoy et al. 2010). The most affected pens were treated with oxytetracycline (orally 8 to 10 days at 100mg/kg body weight), resulting in prevention of secondary bacterial infections due to the skin damage and decreased mortalities.
Further insights
P. noctiluca has already been associated with mortalities in salmon aquaculture. The most well-known episode occurred in Northern Ireland in 2007, when a large swarm of this species killed an entire Atlantic salmon farm (~250,000 fish) (Doyle et al. 2008).
Despite previous reports, the skin and gill pathologies induced by P. noctiluca have not been previously described. The lesions described are believed to be important and caused by a combination of mechanical and toxic damage. We believe that the characterization of the pathology caused by different environmental agents (i.e. phytoplankton and zooplankton species) will improve the differential diagnosis of gill disorders for which histopathology is a key diagnostic tool.
Unlike most terrestrial livestock farming, marine aquaculture is highly affected by the environmental conditions. In recent years, there exists a worldwide concern that jellyfish blooms are increasing.
However, their cyclic nature and the lack of long-term data make it difficult to draw definitive conclusions. Increased eutrophication due to anthropogenic activities and other human activities (e.g. over-fishing) may favour jellyfish multiplication (Purcell et al. 2007).
References
Baxter E.J., Sturt M.M., Ruane N.M., Doyle T.K., McAllen R., Harman L. & Rodger H.D. (2011a) Gill damage to Atlantic salmon, Salmo salar, caused by the common jellyfish, Aurelia aurita, under experimental challenge. PLoS ONE, 6(4), e18529.
Delannoy C.M.J., Houghton J.D.R., Fleming N.E.C. & Ferguson H.W. (2010) Mauve stingers (Pelagia noctiluca) as carriers of the bacterial fish pathogen Tenacibaculum maritimum. Aquaculture 311(1-4):255-257.
Doyle T.K., De Haas H., Cotton D., Dorschel B., Cummins V., Houghton J.D.R., Davenport J. & Hays G.C. (2008) Widespread occurrence of the jellyfish Pelagia noctiluca in Irish coastal and shelf waters. Journal of Plankton Research 30:963-968.
Marcos-López M., Mitchell S.O., Rodger H.D. (2014) Pathology and mortality associated with the mauve stinger jellyfish Pelagia noctiluca in farmed Atlantic salmon Salmo salar L. DOI: 10.1111/jfd.12267.
Purcell J.E., Uye S. & Lo W. (2007) Anthropogenic causes of jellyfish blooms and their direct consequences for humans: a review. Marine Ecology Progress Series 350:153–174.
August 2014
