Sea lice – Research
BY DR ARMIN STURM
Cryptic CLUE Does camouflage in sea lice affect cleaner fish efficacy?
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EA lice are parasitic copepods that live on the body surface of marine fish and graze on the mucus and skin of their hosts. As sea lice infections can cause serious health problems in fish, their control is essential in salmon farming. Traditionally, sea life infections have been managed using a limited range of licensed veterinary medicines. While in the North Atlantic, sea lice have developed resistance against most available pharmaceutical products, a number of innovative non-medicinal control methodologies are now widely implemented at industrial scale, including delousing by water jets (Hydrolicers), warm water (Thermolicers), and modified cage designs reducing infection pressure (snorkel cages and lice skirts, for example). In addition, co-culture of salmon with cleaner fish, originally proposed in the 1990s, has recently experienced a remarkable renaissance. A number of wrasse species, as well as lumpfish, have proven to be efficient in removing sea lice from infected salmon. The application of cleaner fish in salmon farming has increased dramatically over the last decade, with a total of 37.4 million cleaner fish having been deployed in Norway and two million in Scotland in 2016, compared to less than two million in Norway and none in Scotland in 2008. The wide range of non-medicinal sea louse control strategies used commercially today is a positive development, as this vastly diversifies the arsenal of tools available for the management of these parasitic copepods. At the same time, the speed at which resistance has developed in the past underlines the considerable ability of these parasites to adapt, and provides a reminder that the development of novel control strategies needs due consideration of the need to prevent resistance formation.
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A range of factors may have contributed to the rapidity of past resistance formation in sea lice. First, populations of the main sea louse species affecting salmon production in the North Atlantic, the salmon louse Lepeophtheirus salmonis, show a high degree of genetic exchange, meaning that resistance can spread rapidly once it develops locally. Second, sea lice have a short generation time and high reproductive output, which again favours the fast spread of resistance. Finally, similar drug selection pressures have been brought to bear at many salmon production sites spanning the North Atlantic. In consequence, genetic variants allowing lice to survive treatments in one region may have provided similar benefits elsewhere. The increasing use of cleaner fish in the major salmon producing countries of the North Atlantic could potentially constitute a selection pressure favouring genetic variants that make lice less prone to be removed by cleaner fish. These fish are visual feeders and show a preference for larger as com-
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05/02/2019 13:56:57