12
Apr
Ocean Health: First Reports of Salmon Lice Resistance in the Pacific Ocean Threatens Local Ecosystems
(Beyond Pesticides, April 12, 2022) A recent study published in Scientific Reports warns that parasitic salmon lice (Lepeophtheirus salmonis) in Pacific Ocean open-net fish farming operations are becoming resistant to emamectin benzoate (EMB), an active ingredient used to control salmon lice population in North America, both in the U.S. and British Columbia, Canada. Previously, researchers believed parasitic salmon lice only had high rates of chemical resistance in the Atlantic region due to the mixing of farmed and wild salmon. However, Pacific salmon lice are exhibiting similar rates of decreased sensitivity to EMB from various sources, including a decrease in the wild Pacific salmon population, overuse of chemical treatments, and reliance on single chemical treatments.
The aquaculture industry (e.g., farmed seafood/fish) repeatedly faces sustainability issues, failing to adhere to environmental regulations and threatening marine health. 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. These parasites endanger both farmed salmon and wild salmon, in addition to other local species of fish. In this context, pesticide treatments contributes to resistance among lethal pest populations, especially in ecologically vulnerable and interconnected ecosystems like ocean basins. The researchers caution, “Salmon lice in the Pacific Ocean appear to have evolved EMB resistance based on two lines of evidence. First, lice from BA salmon farms experienced decreased sensitivity to EMB in bioassays conducted between 2010 and 2021. Second, the field efficacy of EMB treatments on these farms declined over the same period. […] Whatever the cause, the emergence of resistant salmon lice in the Pacific poses serious challenges for controlling outbreaks to protect wild salmon in the coming years, further exacerbating the negative consequences of lice on salmon predicted in a warming climate.”
The U.S. Department of Agriculture (USDA) defines aquaculture as any “farming of aquatic organisms, including baitfish, crustaceans, food fish, mollusks, ornamental fish, sport or game fish, and other aquaculture products.” Farmed fish, like salmon, use one of the most high-risk aquaculture practices, open-net pens in coastal and offshore regions. These pens allow easy exchange of waste (i.e., feces), chemicals (i.e., pesticides and pharmaceuticals), and parasites/diseases (i.e., sea lice) between the farm and the surrounding ocean environment. The discharge of waste, chemicals, and parasites/pathogens can have a disastrous impact on marine organisms and plants, disrupting ecosystem services. Generally, these pens are in relatively remote areas, somewhat “hidden” from public view. However, these fish live in very crowded conditions, unlike wild-caught fish. The fish consume food that may contain various pharmaceuticals (e.g., antibiotics) or insecticides to control diseases and pest infestations that frequently occur in these conditions. Furthermore, the farm pens can attract predators, such as marine mammals, that can tangle and drown in fish farm nets.
Emamectin benzoate (EMB), also known by the trade name SLICE® and others, is the most common treatment applied to salmon farms to control fish lice. However, pesticide treatments to control fish lice are causing higher rates of chemical resistance among the species in the Pacific Ocean and around the world.
Researchers used an EMB bioassay (to measure the potency of a chemical) and quantitative measurements of sea louse populations on a farm in Broughton Archipelago (BA), Canada. This region of Canada encompasses many salmon farms in the Pacific, and the results demonstrate a dramatic decrease in pacific louse sensitivity and exponential growth in resistance to EMB treatments between 2010 and 2021. Moreover, the real-world effects of EMB treatments on fish farms declined over the same period. The researchers suggest that substantial EMB resistance among pacific salmon lice evolved recently. Therefore, controlling salmon-louse outbreaks may be difficult in the future.
Aquaculture farming industries routinely use pesticide treatments, such as emamectin benzoate, in fish feed to minimize the impacts on farmed fish living in an enclosed marine environment. Yet, salmon lice are the greatest challenge to aquaculture production and environmental sustainability. These parasites attach to the fish’s skin and feed on their blood and mucus, creating sores that lead to infection or death. Under normal conditions, lice populations decline in the winter with a shift in salmonid population dispersal. However, the crammed, over-treated nature of farm fishing 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 current through the barrier due to their small size. For instance, in 2017, over a quarter-million salmon died from lice infestations at two Gray Group salmon farms in the Bay of Fundy in New Brunswick, Canada.
Although EMB is a growing concern as louse resistance increases, pesticide treatments are just as toxic. For instance, organophosphate and pyrethroid insecticides are pesticide classes commonly used to control parasitic salmon lice. However, laboratory studies find increasing chemical resistance among lice, sometimes resulting in resistance to multiple chemical treatments. Since laboratory studies identify that multi-resistance to both chemical classes can occur via crossbreeding, researchers suggest this same resistance transpires among parasitic salmon lice resulting from reduced sensitivity to chemical compounds in the North Atlantic region. All oceans connect, cycling nutrients, chemicals, and organisms throughout the world. Hence, pesticide-resistant lice can potentially spread their resistance gene across the ocean basin. These mutant parasites have already made their way from Atlantic to Pacific waters, even in areas where farmers never used chemical pesticides.
While this is the first study to identify the evolution of EMB resistance in the Pacific Ocean, the authors suggest that industry and the federal regulator have already known about the emerging issue for some time. There are similar reports about the adverse effects of farmed fish on Scotland’s west coast and Northern Isles. The use of antibiotics and pesticides in local marine ecosystems results in coastal habitat loss and genetic and health risks to wild marine populations. Marine species biodiversity is already rapidly declining due to overfishing, global warming, pathogens, and pollution. Thus, further biodiversity loss can change aquatic and terrestrial ecosystem functions and reduce ecosystem services.
The Pacific Ocean was the last sanctuary for salmon as louse were susceptible to chemical treatments. At first, scientists determined the emerging resistance to be isolated incidents of ephemeral (short-lived) reduction in EMB sensitivity among lice. However, 2021 bioassay data demonstrates that salmon-louse population control requires a higher concentration of EMB to kill at least 50 percent (EC50) of the lice population (EMB treatment amount for; males 907 parts per billion (ppb), females 340 ppb. These results demonstrate a fivefold increase in EMB treatment concentrations for males and a 16-fold increase for females between 2010 and 2021. Moreover, researchers did not obtain this bioassay data through industry or federal regulators’ reports, but rather through an Indigenous group, First Nation, who obtained data from salmon-farming companies under a legal obligation.
Similar to this study, other research supports that resistance among parasitic lice is genetic and depends on spatiotemporal (location and time) evolutionary patterns. This pattern means that lice demonstrate simultaneous resistance to parasiticides across North America. Although this study could not perform a genetic diagnostic of salmon lice resistance mechanisms, evidence of emerging resistance from a decade ago points to a change in genes rather than phenotype plasticity (changes in an organism’s behavior, morphology, and physiology in response to a unique environment). The authors conclude, “Our results highlight the need for assessments of the frequency of this rare genotype, ideally with full public reporting and independent verification, as an integral part of EMB bioassays until a full diagnostic test is developed.”
The oceans are essential to human health and well-being, feeding billions, supporting millions of jobs, and supplying medicinal materials. However, environmental contaminants like pesticides and the subsequent effects of exposure, such as pest resistance, have profound impacts on the ecosystem and all its inhabitants. Pesticides are pervasive in all water ecosystems—from rivers, lakes, and oceans to glaciers in the Arctic, exacerbating the ubiquity and distribution of pesticide resistance among sea lice populations across the globe. Therefore, it is essential to understand how parasites may develop resistance to pesticides used to control populations in order to safeguard human, animal, and environmental health. Toxic pesticide use must end to protect the nation’s and world’s waterways and reduce the number of pesticides and resistant parasites found in our food, water, and wildlife resources. Learn more about how pesticides are hazardous to wildlife and what you can do through Beyond Pesticides’ wildlife program page.
There are many resources individuals can use to help gain knowledge and apply practices to avoid pesticide use and its adverse effects. These include news stories, local organizations, school pesticide policies, regulatory contacts, and least-toxic pest control operators. Organic practices can successfully eliminate toxic pesticide use. Replacing pesticides with organic, non-toxic alternatives is crucial for safeguarding public health and ecosystems from pesticide toxicity. Buying, growing, and supporting organic helps to eliminate the extensive use of pesticides in the environment and from your diet. For more information on why organic is the right choice for consumers and the farmworkers who grow our food, see the Beyond Pesticides webpage, Health Benefits of Organic Agriculture.
Source: Narwhal, Scientific Reports
