Salmon lice-induced mortality of Atlantic salmon postsmolts experiencing episodic acidification and recovery in freshwater

Acid rain has reduced several salmonid stocks in Norway and salmon lice have been identified as a major population regulation factor. The combination of these two factors has also been seen to reduce postsmolt survival. In the present study, we have addressed the effects of an episodic exposure to acidic water and later recovery in good freshwater quality (Acid/Recovery groups) followed by salmon lice infestation in seawater in more detail. The ecological perspective of this can be directly addressed to changes over the last decades from chronic acidification over Norway and Europe, to more episodic spring acidification of rivers prior to or during downstream migration of smolts. The results showed that salmon lice-induced mortality increased in all episodic Acid/Rec groups. However, the group given the longest recovery period experienced the lowest mortality compared to the other treatment groups. A period of recovery after acid exposure may eventually restore the fish back to normal physiological level, but in the present experiment a period of 14 days of recovery after a 2 day exposure to acid water was too short to fully restore the fish back to normal levels. Even short-time episodic acidification followed by recovery during springtime and the vulnerable smoltification process, may therefore have negative and often unnoticed effects in wild salmonids until the postsmolts meet other stressors in the marine phase such as salmon lice and other fish diseases.