Study blames salmon farms for sea lice
Delay in treating infestation might have allowed deadly parasites to infect wild fish
Randy Shore
Delays in treating sea lice infestations on salmon farms in the Broughton Archipelago in 2015 may have contributed to the transfer of the parasites to wild fish, according to a new study.
While some farms applied parasiticides during the winter, others didn’t treat for sea lice until just before or during the wild fish out-migration months later.
Last year’s sea lice outbreak was a perfect storm of events, starting with the transfer of sea lice from abundant wild pink salmon to oceanbased salmon farms in 2014, according to the study’s lead author Stephanie Peacock.
Unusually warm seas in early 2015 created ideal growing conditions for sea lice and a management challenge for fish farms.
“The number of sea lice we observed on wild juvenile salmon and the proportion of juvenile salmon that were infected was significantly higher in 2015 than the period from 2006-14,” she said.
“There were high numbers of sea lice all along the B.C. coast last year.”
Wild fluctuations in sea lice infestations on wild pinks and chum salmon between 2001-05 gave way to a decade of low sea lice levels, until last year.
“We have seen very low numbers since 2006 and we were thinking maybe we’ve figured (sea lice management) out,” she said.
Peacock and fellow researcher Andrew Bateman analyzed data collected since 2001 under the direction of independent scientist Alexandra Morton, a co-author of the study.
Juvenile wild salmon are naturally protected from the transfer of sea lice from returning pinks, she said.
“Sea lice get transferred to farmed salmon in the fall, they overwinter on the salmon and get transferred to wild salmon in the spring,” she said. “Ordinarily the returning pinks and out-migrating wild salmon would never come in contact.”
Sea lice numbers on salmon farms were higher in 2015 compared with the recent past, according to data from Fisheries and Oceans Canada.
The researchers also found that the proportion of precautionary winter treatments on farms had dropped and the number of farms treating during the wild salmon migration had increased.
“There seemed to be a period during which farms appeared to be hosting large numbers of sea lice,” she said.
The authors say that, in addition to sea lice management thresholds, the industry should consider areawide management to prevent farms from reinfecting each other when treatments are months apart.