The prerequisite for anadromous juvenile salmonids to enter sea water (SW) is the development of SW tolerance during a process called smolting or parr-smolt transformation. This complex and crucial life stage is accompanied by several behavioural, physiological and morphological changes allowing successful transition from freshwater to marine environment (Hoar 1976). In sh hatcheries or research facilities, the readiness of smolts for SW transfer is typically estimated using a salinity tolerance test or SW challenge hypoosmoregulatory test (Clarke, Saunders & McCormick 1996). The former test measures mortality/ survival of smolts in hypersaline water and the latter measures plasma ions at a determined time (e.g. 24 h) after an abrupt transfer to SW. However, given the increasing regard for animal welfare (e.g. Huntingford, Adams, Braithwaite, Kadri, Pottinger, Sandoe & Turnbull 2006), it would be desirable to develop a reliable, non-invasive and non-lethal method for assessing smolt readiness..
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