Investigating ill thrift in yellowtail kingfish Seriola lalandi

The midwest area of Western Australia, location indicated by the square in Fig. 58.1, benefits from the warm Leeuwin current (ranging from approximately 16e28C) and was chosen as a suitable location for commercial sea cage aquaculture of the endemic, white fleshed yellowtail kingfish Seriola lalandi. The industry began in Jurien Bay in 2008, but in recent years has focused on the waters off Geraldton, including the development of a dedicated marine finfish zone in the waters adjacent to the Abrolhos Islands. The fish were cultured in cages with a circumference of 65e80 m in 12 m of water and fed sinking extruded pellets with a fish meal content of at least 40% and fish oil to supply the necessary essential fatty acids plus supplements including vitamin E, taurine, vitamin C, and trace elements and minerals. Nutritional deficiencies including vitamin E and taurine initially resulted in health problems and suboptimal growth in the early cohorts between 2010 and 2012, but the majority of these issues have been subsequently resolved through improved understanding of management techniques and nutritional requirements. Early industry development was slow and spasmodic due to a number of constraints including poorly understood health issues when the fish were introduced to the sea cages (Stephens and Savage 2010). In contrast, when the same cohorts of fish were held in land-based tanks with flow through sea water, they either remained healthy or had similar but less severe health issues. This chapter focuses on a cohort of fish which were stocked into a sea cage in late September 2011. The fish had been growing well with no health issues until a mortality event in early November. The event was preceded by severely low dissolved oxygen overnight and several days of suboptimal dissolved oxygen concentrations due to a number of management and environmental factors. There had also been a change of diet 10 days beforehand to one which was subsequently revealed to contain suboptimal levels of taurine and vitamin E. The peak of mortality occurred on day 2, but fish continued to die at a decreasing rate for the following four days. Approximately 70% of the cohort died, with the larger fish comprising the majority of mortalities. Despite extensive acteriological, histological, and water quality analysis, including examination for phytoplankton, no significant cause for the mortalities was identified at the time. Regular histological sampling of a random sample of five fish had been undertaken every two weeks prior to the event and continued each month until all of the remaining fish were harvested in June 2012. Every batch of feed was evaluated by an independent laboratory, and feed intake, changes in feed batch size and specification were recorded at the aquaculture facility. Fish that survived the mortality event failed to thrive and were considered to be suffering from “ill thrift.”
From January 2012, the regular random sampling found significant lesions in tissues in the coelomic cavity including cystic and fibrotic changes in the exocrine pancreas, cholangitis, and hepatitis. At harvest in June 2012, a random sample of 87 fish from the cage was examined. Body weight and condition index were recorded, and blood samples were collected for basic hematology and biochemical examination. Three fish with a low body weight and condition index were selected for detailed histological examination in an attempt to better understand the pathogenesis of the ill thrift. Hepatic, biliary, and exocrine pancreas lesions were often seen in the fish during the grow-out period, and similar lesions had been seen on occasion in the previous cohort of fish at the site and on rare occasions to the present day.