Wild Adult Salmon Aren't Exacerbating Lice Epidemic

Marine biologists were surprised to find that the numbers of sea lice on juvenile salmon didn't climb when louse-infested wild adults swam by on their way to spawning.

Mature pink salmon passing through Fife Sound in the Broughton Archipelago of British Columbia each carried nine lice that were bearing eggs.

But the intensity of new louse infestations on juvenile pink and chum salmon in the Sound was actually lower during the run than in the months before.

During June and July, 43% of the juveniles had lice attached of the species Lepeophtheirus salmonis, and 7.2% were newly infested with copepods, the stage that first attaches to a fish. Infested fish altogether averaged 1.2 lice.

When 200,000 adult pinks passed through in August and September, 40% of juveniles carried lice, and 4.8% with copepods, averaging one louse per infested fish. The prevalence of lice on young salmon was lowest in the weeks when wild adult salmon carrying lice were around.

Throughout the research, salinity and temperature conditions were optimal for reproducing sea lice. The study site was along a major salmon migration route with no active fish farms in the immediate vicinity. Young salmon would have acquired lice prior to the adult run from one of the 18 operating farms scattered among the islands and inlets in the area. These house millions of Atlantic salmon in open net pens.

Farmed salmon are a known source of sea lice on wild pink and chum salmon that have recently entered the ocean. Once the juveniles are infected, they can keep getting reinfected as the lice reproduce.

Geographic separation of wild adult Pacific salmon at sea and juvenile salmon entering the ocean naturally prevents young fish from becoming infested when they're too small to tolerate the parasites. By the time adult pinks showed up in August, the younger salmon had been growing in marine waters for five months.

Authors of the Fife Sound study call their results "perplexing". They don't know why more sea lice didn't, as expected, attach to juvenile salmon when infested adults returned. Wild pink salmon have likely adapted to avoid passing sea lice from adults to the less resilient juveniles.

There are several reasons for the findings, all of which could be involved. For instance, even though young and mature salmon occur together in the same passage between islands, they don't intermingle. Juveniles linger close to shore, while adults swim in mid-channel. Also, the two generations of salmon encounter each other on the outer edge of the archipelago where good tidal flushing of sea water may inhibit louse transfer.