Farmed salmonids drive the abundance, ecology and evolution of parasitic salmon lice in Norway

Abstract

Sea cage fish farming is typically open to the environment, with disease transmission possible between farmed and wild hosts. In salmonid aquaculture, salmon louse Lepeophtheirus salmonis infestations cause production losses, reduce welfare for farmed fish and increase infestation rates for wild fish populations. The high density of hosts in farms likely also shifts the coevolutionary arms race between host and parasite, with ecological and evolutionary consequences for the salmon louse. Using farm-reported salmon and louse abundances and publicly reported estimates of wild salmonid host abundances and the salmon lice they carry, we estimated (1) the relative abundance of farmed and wild salmonid hosts and (2) the relative importance of each for the abundance of salmon lice for the coastal zone of Norway from 1998 to 2017. Farmed hosts increased in importance over time with the expansion of the industry. From 2013 to 2017, farmed salmonids outnumbered wild salmonids by 267-281:1. By 2017, farmed salmonids accounted for 99.6% of available hosts and produced 99.1% of adult female salmon lice and 97.6% of mated (ovigerous) adult female salmon lice in Norwegian coastal waters. The persistent dominance of farmed hosts has clear implications: (1) management decisions that aim to limit lice abundance can be guided by lice data from farms alone, as lice on wild salmonids make a trivial contribution to the national lice population; and (2) strategies to prevent or treat lice infestations are vulnerable to the evolution of resistance, as the pool of wild hosts is inconsequential and will not act as a refuge large enough to stem the evolution of resistance. As the Norwegian salmon industry expands and salmon lice infestations continue, farmed salmon will drive the ecology and evolution of salmon lice.

Applying genetic technologies to combat infectious diseases in aquaculture

“…Wild populations can act as refugia, provided they are large enough 665,668 . In the Atlantic, however, farmed salmon are significantly more abundant than wild hosts 34 . It might be more effective, then, if some farms were to act as refugia instead, by being stocked with susceptible host strains, but it seems unlikely that such deliberate degradation of fish welfare on a whole‐farm basis would be acceptable to the authorities and public.…”

“…Strict delousing regimes are used to keep lice infestation intensities low, but some delousing regimes (involving crowding, pumping and lice removal effects) can pose severe welfare risks 27–29 . The lice larvae produced on‐farm can also infest wild salmonid populations 30–35 . Sea lice cost the Norwegian salmon sector at least 7.3 billion kroner (~US$ 810 million) per annum 36 (up from US$ 435 million estimated for 2011 37 ) and are of major concern for other salmon industries around the world.…”

Innovation in the Norwegian aquaculture industry

“…50 As is too often the case when treatment is the attempted solution to a regularly occurring health issue, resistance start to develop. 34,51,52 As such, the industry is currently using a host of approaches including chemical treatments, feed additives and cleaner fish. While feed additives and cleaner fish may have some prevention effect, effective prevention remains elusive.…”

“…A number of approaches have also been developed to treat against salmon lice 50 . As is too often the case when treatment is the attempted solution to a regularly occurring health issue, resistance start to develop 34,51,52 . As such, the industry is currently using a host of approaches including chemical treatments, feed additives and cleaner fish.…”

A novel method for the rapid enumeration of planktonic salmon lice in a mixed zooplankton assemblage using fluorescence

“…Due to the patchy distribution of their hosts and the scale of the marine environment, marine parasites typically have a low probability of transmission to their next hosts (Marcogliese, 2005). In the case of salmon lice, however, farmed salmonid hosts are readily available throughout the year, and account for more than 99% of the available hosts (Dempster et al, 2021). Thus, epidemic outbreaks of lice can originate on farms and subsequently spread to wild salmonid populations (Heuch & Mo, 2001; Pike & Wadsworth, 1999).…”