Investigating aetiology and risk factors of ocular lesions and associated mortality in ranched Southern Bluefin Tuna
University of Adelaide
This year (2017), some of the ranching operations reported the increased cumulative mortality. In some severe cases, up to 90% of collected mortalities present some degree of unilateral or bilateral ocular damage ranging from corneal cloudiness, with or without ulcers, up to complete perforation. The anecdotal report of eye lesions has progressively increased since the 2015 season without a definitive cause being identified. Previous reports (Rough et al., 1999; Rough, 2000; Hayward et al., 2007; Hayward et al., 2008a; Hayward et al., 2008b; Hayward et al., 2009; Hayward et al., 2010; Hayward et al., 2011, including FRDC projects No 2003/225 and 2008/228, Nowak et al., 2007; Nowak et al., 2012) identified sea lice of the genus Caligus spp. as a differential cause of eye lesions in SBT. The copepod ectoparasite is thought to damage the eyes by feeding on the cornea epithelium of infested SBT. Lesions worsen when fish flash against the cage’s net to dislodge the itchy copepods. Partial or full vision loss is suspected to impair the capacity of the fish to compete for feed and to result, with time, in the death of affected fish. At this stage, it is unclear: 1 - what is the distribution of the observed increased mortality across the industry; 2 - what is the occurrence and severity of eye lesions across the industry; 3 - if the observed increased mortality is entirely attributable to eye lesions; 4 - if eye lesions are solely caused by C. chiastos or if other causes are involved; 5 - if potential tow-, cage-, and fish-level risk factors are associated with the occurrence of eye lesions and its cause(s).
1. Estimate the frequency and distribution of increased mortality across the industry.
2. Describe the pathology and severity of eye lesions and estimate the frequency and distribution of these lesions across the industry.
3. Investigate potential tow-, farm-, and fish-level risk factors associated with increased mortality and eye lesion occurrence.
4. Investigate the putative role of sealice in causing this episode of eye lesions.