The gill maggot Salmincola salmoneus as an indicator of repeat spawning in Atlantic salmon Salmo salar

The potential of the gill maggot Salmincola salmoneus for use as an indicator of repeat spawning in Atlantic salmon Salmo salar was studied in adult fish captured during their return migration to the River Alta (n = 659) and River Namsen (n = 540) in Norway. Eighty‐eight and 49% of previous spawners identified by scale readings were infected with S. salmoneus in the two rivers, respectively. Salmincola salmoneus can be used as a reliable, rapid and objective field indicator of repeat spawning in S. salar as nearly all infected fish (99·4%) were identified as repeat spawners, although it is important to have appropriate background information on S. salmoneus prevalence on the postspawning individuals within the same population.