Site management factors influencing mortality rates in Atlantic salmon ( Salmo salar L.) during marine production

Abstract

Mortality rates of up to 48% have been experienced in Irish farmed Atlantic salmon during their first year of production at sea with disease outbreaks making a significant contribution to these mortalities. In this study, epidemiological techniques were used to elucidate any associations between site management practices and mortality rates. Mortality, management and disease data were collated from 11 marine Atlantic salmon farms in Ireland over the period 1988–1992 and include 34 years of production at sea. To enable routine analysis of this large data set, the data were configured on the relational database system ORACLE. Analyses of the database indicated that total mortality was significantly higher in years where sites were not fallowed ( P < 0.05), where more than one generation of fish were reared on site ( P < 0.05), where slaughtering of fish occurred on site ( P < 0.05) or where farm staff moved between farm sites ( P < 0.05). The individual effect of each of these practices on mortality rates could not be determined as farmers generally practice all four methods in conjunction. There was no association between mortality rates and the number of years of production on site, site depth or net clearance of the sea bed. The results suggest that the interruption of parasitic life cycles and/or the reduction in pathogenic organism load on site by fallowing, single generation rearing, and the practice of basic hygiene methods could be used as significant control measures for reducing the severity of disease outbreaks in populations of Irish farmed Atlantic salmon.