Growth and cataract development in two groups of Atlantic salmon (Salmo salar L) post smolts transferred to sea with a four-week interval

The aim of this trial was to study the impact of sea transfer Lime on growt h and cataract development in Atlantic salmon post smelt. In addition, loca lisation of changes within the lens, severity and progression of cataracts were monitored on individual salmon over time. One-year-old pre smelt were individually tagged, randomly divided into two sub groups and transferred t o sea with a four-week interval. Based on results of seawater challenge tes t performed shortly prior to transfer and the external appearance of the sa lmon, both groups were considered well smoltified. Growth and cataract were monitored through sampling in freshwater (FW) and seawater (SW) over a tot al period of 7 months and possible correlations between cataract and growth on individual and group level were studied. The two groups showed differen t growth performance, measured as specific growth rate (SGR), with the late -transferred group growing slower in the period it was retained in fresh wa ter and in the first 80 days following transfer, therefore reaching a lower end weight. At the first sampling point in sea a high incidence of osmotic (reversible) lens opacities was noted. Both groups also developed irrevers ible cataracts in SW and the severity of these was highest in the late-tran sferred group. In both groups there was a positive correlation between spec ific growth rate and cataract severity in the first period after transfer. However, the correlation at a later stage turned to be negative, indicating that cataracts impair growth performance. Irreversible cataracts were prim arily diagnosed in the posterior and/or anterior cortex. Subsequently, opac ities were also found in perinuclear regions, while, towards the end of the trial, a layer of new, unaffected tissue was found in the periphery of the lens.