"Part of the contentious debate about sea lice is that there hasn't been this level of precision in the sampling and modeling," says coauthor Mark Lewis, a mathematical ecologist from the University of Alberta known for his work on West Nile virus and invasive species. "We have worked out the spatial footprint ?the data are strong and show that the farms are having a significant impact over a very large area."
The researchers isolated the effect of the salmon farm by studying the infection levels of migrating juvenile pink and chum salmon as they approached and passed a salmon farm. Within the study area, the salmon farms are anchored in a long thin fjord. Here the wild salmon have no choice but to pass by the farm on their seaward migration.
The scientists developed new, non-lethal sampling techniques that allowed them to examine thousands of the fish for parasites, taking measurements every 2 to 4 km. The fish start in the rivers and head to sea, traveling en masse in large mixed schools. Thus the scientists could see the effect on the salmon as they moved toward the farm. They found that the juvenile salmon carried almost no sea lice prior to the farm but became heavily infected as they approached it.
Unlike other species, pink and chum salmon leave their natal rivers while still much smaller than your baby finger ?3 cm long ?and weighing only half a gram. "The youngest fish can be only days old when they encounter a farm," says Krkosek. "Some of these fish are so young their egg sacks haven't been fully absorbed ?even fish this small are infected."
The anchored farms, open cages of closely packed salmon, are aquatic feedlots pro