Scientists have long been puzzled by the fluctuating numbers of fish in the oceans. An international team of researchers, led by Martin Krkošek from the University of Otago, New Zealand, compared the survival of wild salmon that received parasite medication with those that did not. The authors conclude that parasites can have a significant impact on fisheries and conservation.
The scientists analysed data from 24 trials, which tagged 283,347 young Atlantic salmon between 1996 and 2008. Paired groups of control and anti-parasite treated salmon were released into ten areas of Ireland and Norway. All experimental fish were infection free when released and a proportion of each group were recovered as adults returning to coastal waters one or more years later.
Treatment had a significant positive effect on survival. The untreated salmon were 1.29 times more likely to die. The parasites were probably acquired during migration in areas that host large populations of domesticated salmon, which elevate local abundances of parasites. The concern is not only for a loss in salmon abundance, but also the loss of genetic variability and its associated potential for adaptation to other environmental changes.