Acute viral and bacterial infections elevate water cortisol concentrations in fish tanks

Freshwater rainbow trout and Atlantic salmon were experimentally challenged with Viral Haemorrhagic Septicaemia virus and Aeromonas salmonicida respectively, and daily measurements made of cortisol release into the water for 5-7 days post-challenge. There was a surge in cortisol release into the water one to two days before mortality became apparent in both challenges. These experiments and a review of other literature demonstrate that cortisol elevation is a common feature of fish experiencing acute infections. The cortisol response of fish to pathogen infections, and the possible use of water cortisol surges for early warning of acute disease outbreaks on fish farms, are discussed.