The spring of 2015 saw a resurgence of sea louse parasites on juvenile pink and chum salmon in the Broughton Archipelago of BC. Weekly monitoring conducted at Salmon Coast Field Station picked up the high infestation rates early in the season, and through back channels we learned that salmon farms had also been suffering from more lice than usual.
This was alarming, because the salmon farming industry has been pretty well on top of sea-louse management for the last decade. Based on past research, we would expect the lice we saw on juvenile pinks in 2015 to be associated with 9-39% additional mortality for pink salmon that would otherwise return to spawn in 2016.
Along with a number of coauthors, and using long-term monitoring data collected by Alex Morton, Steph Peacock and I set out to try and understand what was going on.
Using data from a number of different sources (government, the salmon farming industry, nongovernmental organisations), we evaluated several hypotheses. The leading contenders were: uncharacteristically warm ocean temperatures, healthy returns of pink salmon to the area in the fall of 2014, mistiming of anti-louse treatment on farms, and lice having evolved resistance to treatment.
The pink salmon run in 2014 was healthy, but not exceptional, and – luckily – lice in the Broughton don’t appear to have evolved resistance to chemical treatment.
The main things that seem to have caused the sea louse outbreak in 2015 were warm water – winter temperatures were a degree warmer than the long-term average – and mis-timed treatment on farms. While the approach taken by farms doesn’t seem to have been different to that employed over the last decade, things seem to have gone wrong in the face of the unusually warm water. It looks like farms were forced to apply more “reactionary” treatments during the spring, when juvenile salmon are migrating, than in other years. In some cases, louse counts on farms were allowed to get out of control during the winter, which might have led to cross-infection among farms.
Our perspective is that we need better communication and coordination across multiple parties (government, industry, independent science) to help manage sea lice on farms in the future. We already knew that warm water makes sea lice develop faster and churn out more infective larvae, and multiple groups independently noticed high louse counts around the time the juvenile salmon were swimming out to sea. If we can work together in the future, and manage regional farms as a system, rather than in isolation, we might be able to avoid similar problems in the future.
Check out the associated paper in the Canadian Journal of Fisheries and Aquatic Sciences.
Some good news: although ocean temperatures were again high in 2016, we were happy not to see as many sea lice on wild salmon.