Effect of waterborne exposure to 4-tert-octylphenol and 17β-estradiol on smoltification and downstream migration in Atlantic salmon, Salmo salar

Groups of Atlantic salmon parr (November, Exp. 1) or pre-smolts (March, Exp. 2) were exposed to estradiol-17β (E2 conc.: nominal 500 ng l−1/actual 8-16 ng l−1) and two doses of tert-octylphenol (OP: nominal 25 μg l−1/actual 4.5-6.5 μg l−1 and OP: nominal 100 μg l−1/actual 10-30 μg l−1) for 26 days in fresh water, and the effects on physiological and behavioural aspects of parr-smolt transformation were investigated. Vitellogenesis was induced by all treatments, as indicated by elevated levels of plasma vitellogenin (Vtg) and hepatosomatic index. Elevated Vtg levels were still found in OP-100 and E2-treated fish 4-5 months after cessation of treatment, indicating a slow clearance of Vtg from circulation. Smolting was compromised by E2 and OP-100 treatment as judged by reduced gill Na+, K+-ATPase activity and impaired ability to regulate plasma osmolality and muscle water content in 24-h sea water (SW) challenge tests during the period of smolting. Downstream migratory behaviour was monitored from late April to July (Exp. 2) by implanting passive integrated transponder tags into subgroups of treated and control smolts and placing them in a stream raceway. Irrespective of treatment, nocturnal downstream movement was initiated in all groups on April 23, switching to diurnal movement in late May. Average swimming speed was estimated to be higher than current speed, indicating active migration. E2 and OP-100 fish migrated at lower frequency than control fish, suggesting a reduced migratory drive. The data suggests that waterborne exposure of salmon to xenoestrogens reduce both physiological and behavioural components of smoltification, even when exposure occurs several months prior to smolting.