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“Perfect storm” brought sea louse epidemic to BC salmon: University of Toronto study

July 20, 2016

Likely due to high temperatures and uncoordinated treatment

Toron­to, ON – High ocean tem­per­a­tures and poor tim­ing of par­a­site man­age­ment like­ly led to an epi­dem­ic of sea lice in 2015 through­out salmon farms in British Columbi­a’s Queen Char­lotte Strait, a Uni­ver­si­ty of Toron­to-led study has found.

The sea lice spread to migrat­ing juve­nile wild salmon, result­ing in the high­est num­bers of sea lice observed on wild salmon in a decade.

In spring of 2015, a team of U of T ecol­o­gists led by post­doc­tor­al researchers Andrew Bate­man and Stephanie Pea­cock found that more than 70 per cent of fish the team sam­pled in the Strait’s Broughton Arch­i­pel­ago had at least one sea louse: the high­est preva­lence of such par­a­sites since 2005.

“It was sort of a per­fect storm of envi­ron­men­tal con­di­tions and mis­man­age­ment of treat­ment,” says Pea­cock, a post­doc­tor­al fel­low in the U of T’s Depart­ment of Ecol­o­gy & Evo­lu­tion­ary Biol­o­gy when the research was con­duct­ed. “A lot of peo­ple talk about how sea lice are nat­ur­al, but in farms, you have these par­a­sites in larg­er num­bers. Juve­nile wild salmon are then exposed as they migrate past these areas.”

Because farmed salmon are in open net pens and share water with near­by wild salmon, the par­a­sites can trans­mit to young wild salmon who would­n’t nor­mal­ly encounter sea lice until lat­er in life. These young fish are some­times as small as three cen­time­tres in length, while sea lice them­selves can be close to one cen­time­tre in diam­e­ter.

“Get­ting sea lice at such an ear­ly age affects young salmons’ health and their abil­i­ty to fend off preda­tors,” says Pea­cock.

Based on the num­bers of lice on juve­nile salmon in 2015, researchers pre­dict­ed an addi­tion­al 9 — 39 per cent decline in return­ing pink salmon due to the out­break.

In order to deter­mine the cause of this out­break, the team assessed the evi­dence for four con­tribut­ing fac­tors:

  • influx of lice on return­ing adult pink salmon
  • sea sur­face tem­per­a­ture
  • the tim­ing of chem­i­cal treat­ments to con­trol sea lice on salmon farms
  • resis­tance of sea lice to chem­i­cal treat­ment on farms

“Even though par­a­site treat­ments on farms were effec­tive, we saw that treat­ments failed to pro­tect wild salmon, and this hap­pened at a time of unex­pect­ed warm­ing in ocean water in these regions,” says Mar­tin Krkosek, assis­tant pro­fes­sor in U of T’s Depart­ment of Ecol­o­gy & Evo­lu­tion­ary Biol­o­gy. Krkosek was Bateman’s and Pea­cock­’s super­vi­sor and study co-author.

The fall of 2014 did have a healthy return of adult pink salmon, bring­ing sea lice into near-shore waters where they could infect farmed salmon. High ocean tem­per­a­tures dur­ing win­ter months then like­ly accel­er­at­ed sea-louse devel­op­ment, enabling pop­u­la­tions to grow quick­ly and reach high­er num­bers than they would under nor­mal ocean tem­per­a­tures.

Also in 2015, indi­vid­ual salmon farms did not coor­di­nate anti-louse treat­ments, with some farms delay­ing treat­ment until just pri­or to the time when juve­nile salmon migrate past farms. As a result, sea lice from those farms could have spread to adja­cent farms, ham­per­ing area-wide con­trol of the out­break.

“Fur­ther­more, dur­ing the juve­nile wild salmon migra­tion, farms are sup­posed to treat for sea lice with­in 15 days of when a thresh­old num­ber of lice are found on adult farmed salmon,” says Pea­cock. “But in 2015, some farms wait­ed sev­er­al months before they treat­ed.”

“The strat­e­gy might have been ‘wait until the migra­tion to treat’ but it kind of back­fired because the louse pop­u­la­tions were allowed to grow in the mean­time, and the lice spread to oth­er near­by farms. It like­ly cre­at­ed a big­ger prob­lem down the road.”

“We were sur­prised to see that some farms don’t seem to be fol­low­ing their license con­di­tions,” says Pea­cock. “We thought maybe there were more infrac­tions in 2015 that may have led to high­er num­bers of sea lice, but when we looked back over 10 ‑15 years, the rate of such infrac­tions was about the same. The big dif­fer­ence in 2015 real­ly seems to be the com­bi­na­tion of a lack of proac­tive par­a­site treat­ment and high­er water tem­per­a­tures.”

The team sug­gest­ed the solu­tion to such an out­break in the future would like­ly be an ear­li­er, coor­di­nat­ed par­a­site treat­ment effort between salmon farms – some­thing the researchers say is lack­ing in this area and across the coun­try.

“Sea lice used to be a prob­lem a decade or so ago, then par­a­site man­age­ment changed and it seemed like the prob­lem had been effec­tive­ly man­aged for sev­er­al years. Then it was­n’t man­aged well all of a sud­den,” says Krkosek. “This is a les­son about the impor­tance of bet­ter coor­di­nat­ing and tim­ing region­al treat­ment and being aware of warm­ing ocean tem­per­a­tures.”

The results of the team’s study appear in a paper pub­lished in the July edi­tion of the Cana­di­an Jour­nal of Fish­eries and Aquat­ic Sci­ences.

This work was sup­port­ed by Nat­ur­al Sci­ences and Engi­neer­ing Research Coun­cil (NSERC) and Kil­lam post­doc­tor­al fel­low­ships to Bate­man, an NSERC Dis­cov­ery Grant to Mark Lewis at the Uni­ver­si­ty of Alber­ta that sup­port­ed Pea­cock, an NSERC Dis­cov­ery Grant and Sloan Fel­low­ship to Krkosek, a Glob­al Green­grant from the Maris­la Foun­da­tion, and an infra­struc­ture grant from the Pacif­ic Salmon Foun­da­tion.

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Andrew Bate­man
Post­doc­tor­al Researcher
Depart­ment of Ecol­o­gy & Evo­lu­tion­ary Biol­o­gy
Uni­ver­si­ty of Toron­to

Stephanie Pea­cock
Post­doc­tor­al Researcher
Depart­ment of Ecol­o­gy & Evo­lu­tion­ary Biol­o­gy
Uni­ver­si­ty of Toron­to

Mar­tin Krkosek
Assis­tant Pro­fes­sor
Depart­ment of Ecol­o­gy & Evo­lu­tion­ary Biol­o­gy
Uni­ver­si­ty of Toron­to

Larysa Woloszan­sky
Media Rela­tions Offi­cer
Uni­ver­si­ty of Toron­to