Threat to Ocean Health: Pesticide Resistant Fish Lice Plague the North Atlantic Ocean

June 3, 2021 | (A report published in Royal Society Open Science finds pesticide-resistant parasitic lice (Lepeophtheirus salmonis) are endangering wild and farmed fish populations in the North Atlantic. Extensive use of pesticides to rid the parasite has led to widespread resistance to multiple pesticides, prompting increasing infection rates among North Atlantic salmon populations. Some fisheries market aquaculture practices, like fish/seafood farming, as a solution to overfishing. However, the aquaculture industry repeatedly faces sustainability issues and fails to adhere to environmental standards that threaten marine health. Over the past two decades, organophosphate and pyrethroid insecticides have been the two main chemical classes used to control parasitic salmon lice. However, laboratory studies find increasing resistance among salmon lice to these chemicals, in addition to multi-resistance after in vitro crossbreeding. Since laboratory studies identify that multi-resistance to both chemical classes can occur via crossbreeding, researchers suggest this same resistance transpires in the field. From 2000 to 2017, researchers sampled 1,988 lice from Northeastern wild Atlantic salmonid (salmon, sea trout, and farmed salmon) populations. Researchers analyzed parasites for genetic markers for both pyrethroid and organophosphate resistance. The study results find genetic resistance among salmon lice has a spatiotemporal (location and time) evolutionary pattern. This pattern means that lice demonstrate simultaneous resistance to organophosphate and pyrethroid insecticides across the entire North Atlantic, except Canada. The crammed, overtreated nature of farmed fish creates an environment for these parasites to persist through regular winter die-offs. Resistant lice appear in farm pens a few years post-treatment and leak via currents through the barrier, due to their small size. All oceans connect to one another, cycling nutrients, chemicals, and organisms throughout the world. [Borretzen, Helene et al. Losing the ‘arms race’: multiresistant salmon lice are dispersed throughout the North Atlantic Ocean. Royal Society Open Science. May 2021.]