No. 12. GILPAT.pdf - Marine Institute Open Access Repository

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NDP <strong>Marine</strong> Research Sub-Programme 2007-2013corona). The gill damage observed would have had a significant clinical impact on fish health andits ability to survive. Importantly, a highly significant, positive correlation between theabundance of harmful jellyfish (M. atlantica, P. quadrata and S. corona) and the average daily fishmortality was identified, although pancreas disease also affected fish at this site (Baxter et al.2011a). These results suggested that M. atlantica and S. corona were the causative agents of thegill disorders identified in Bantry Bay and may also have had an impact on the observedmortalities.3.1.3. Conclusions from the monitoring programmeZooplankton Monitoring:A higher density of jellyfish was found in the south west compared to the westcoastGill disease in the south west was correlated with abundance of M. atlantica, P.quadrata, and S. coronaGill disease was also linked to the occurrence of Trichodina sp. and TenacibaculummaritimumSeasonal abundance of jellyfish can vary from year to yearFurther investigations into the role of hydroids in the pathogenesis of gill diseaseare requiredThe role of jellyfish as potential vectors of bacteria should be investigated further.3.2. Challenge Trials3.2.1. BackgroundCommon jellyfish (Aurelia spp.) are among the most cosmopolitan jellyfish species in theoceans, with populations increasing in many coastal areas. The interaction between jellyfish andfish in aquaculture remains a poorly studied area of science. Furthermore, the potential directhealth problems posed to marine-farmed salmonids by the biofouling hydroids, and thepotentially harmful effects of in situ net cleaning to remove fouling organisms, has received littleconsideration. A recent fish mortality event in Ireland, involving A. aurita spurred theinvestigation into the detrimental effects of the common jellyfish on salmon gill tissues (Mitchellet al. 2011).For the challenge trials, Atlantic salmon smolts were exposed to macerated A. aurita orbiofouling hydroids (attached to the nets and loose in the water) for 10-11 hours (before32
NDP <strong>Marine</strong> Research Sub-Programme 2007-2013removal) under experimental conditions. Gill tissues of control and experimental treatmentgroups were scored with a semi-quantitative gill scoring system according to Mitchell et al,(2012).3.2.2. Findings from the common jellyfish challenge trialThe results revealed that A. aurita rapidly and extensively damaged the gills of salmon, with thepathogenesis of the disorder progressing even after the jellyfish were removed (Fig. 3.4). At 2hours after exposure, significant multi-focal damage to gill tissues was apparent. The natureand extent of the damage increased up to 48 hours from the start of the challenge. Althoughthe gills remained extensively damaged 3 weeks from the start of the challenge trial, shorteningof the gill lamellae and organisation of the cells indicated an attempt to repair the damagesuffered. These findings, reported by Baxter et al. (2011b), clearly demonstrate that A. auritacan cause severe gill problems in marine-farmed fish.3.2.3. Findings from the hydroid challenge trialOn gross examination, fragments of the hydroids were observed in the gills of the salmon fromthe experimental treatment groups throughout the first hours of the challenge trials (Fig. 3.5).Focal areas of epithelial sloughing, necrosis, and haemorrhage were seen on the gills of theAtlantic salmon smolts exposed to E. larynx. A similar degree of damage was also seen in thecontrol groups after several hours of the experiment. However, no gill damage was observedin the smolts sampled prior to the start of the experiment (Fig. 3.6). Therefore, it is likely thatthe results observed may be due to a contamination of the control groups through the recirculationsystem used to conduct the challenge trails.33
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