"We found lice levels exceeded what we would find normally, extending for 30 km ?even though the farm is only about 0.2 km long," says John Volpe, the third coauthor from the University of Victoria. "Conservatively this means that the parasite footprint of the farm is 150 times larger than the farm itself."
Sea lice can lower the fitness of salmon - and in some cases be lethal - as they create open lesions on the surface of the fish that compromises a fish's ability to maintain its salt-water balance. When infection rates are high enough, the parasites feed on the fish at rates greater than the fish can feed itself, literally eating the fish alive. Adult salmon can survive sea lice infection, but young salmon are much more vulnerable due to their small size.
Under natural conditions, juvenile pinks and chum do not typically have to contend with sea lice because the adult salmon are far out at sea and are widely dispersed. By the time the migrating pinks and chum normally encounter lice, the juveniles have had the time to put on body mass and build resistance. The location of the farm near the natal rivers undermines this natural coping strategy.
The field study also discovered a new cause for alarm: once the young salmon pass the farm and pick up the sea lice, the migrating school becomes a moving cloud of contagions. Sea lice larvae mature and reproduce on the seaward salmon with each louse producing 300-800 eggs. This second round of lice larvae can re-infect the fish in the school and can spread to other previously unexposed populations coming from geographically disparate regions.
"At about the 30 km mark from the farm, those lice become reproductive," says Volpe. "In effect, the farm has exported its lice generating properties ?a cycli