Pathology and mortality associated with the mauve stinger jellyfish Pelagia noctiluca in farmed Atlantic salmon Salmo salar L

Jellyfish can cause significant losses to the aquaculture industry. This report describes the clinical presentation and pathology caused by the mauve stinger jellyfish Pelagia noctiluca (Forsskal, 1775) in cultured Atlantic salmon Salmo salar L. in Ireland and highlights the potential consequences and need for descriptive pathology of such interactions. P. noctiluca is a member of the cnidarian class Scyphozoa (Order Semaeostomeae; Family Pelagiidae) and has worldwide distribution through the warm and temperate waters of the marine environment. This golden brown to purple jellyfish species lacks the benthic polyp stage and has a primarily pelagic habitat, although ocean currents bring it closer to coastal waters. P. noctiluca is frequently observed from the west coast of Ireland especially during the autumn months, but large aggregations are not commonly reported. In a jellyfish survey carried out around Ireland in 2009, P. noctiluca was the most abundant species found, accounting for 93% of the overall catch (total wet weight of gelatinous species) (Bastian et al. 2011). Its distribution and density were found to be heterogeneous, with great variation between regions (apparent North-South gradient in density) and sampling points (Bastian et al. 2011). When in contact with a fish farm, small jellyfish can pass through the nets, while bigger individuals tend to break up into pieces still capable of stinging the fish. In jellyfish blooms, affected fish can suffer from hypoxia, mechanical damage and toxicity via nematocysts discharge (Baxter et al. 2011a; Rodger, Henry & Mitchell 2011). In 2013, large swarms of P. noctiluca were observed intermittently in several Northern and Southern farms in the west coast of Ireland from the beginning of August to mid-November. Jellyfish presence was determined by visual observations from boat and cages and by divers during removal of mortalities. Zooplankton sampling through vertical net tows and posterior species identification was also carried out. The diagnosis of the jellyfish species involved was based on the morphological features described in detail by Russell (1970). Jellyfish appeared in the vicinity and inside cages (Fig. 1a), and associated mortalities ranged from