A model of salmon louse production in Norway: effects of increasing salmon production and public management measures

Abstract

Salmon lice Lepeophtheirus salmonis Krøyer have caused disease problems in farmed Atlantic salmon Salmo salar L. since the mid-1970s in Norway. High infection intensities and premature return of wild sea trout Salmo trutta L. were first reported in 1992. Later emaciated wild Atlantic salmon smolts carrying large amounts of lice have been observed both in fjords and offshore. The Norwegian Animal Health Authority regulations to control the problem, which came into operation in 1998, included compulsory louse level monitoring in farms and maximum legal numbers of lice per fish. Here, we present a model of salmon louse egg production in Norway and show that the effect of the current public management strategy is critically dependent on the yearly increase in salmon production. This is because the infection pressure is the product of the number of fish in the system, and the number of lice per fish. Due to the much larger number of farmed than wild salmonids, it is highly likely that lice originating from farmed salmon infect wild stock. Estimated tolerance limits for wild salmonids vary widely, and the level of louse egg production in farms which would be neaded to decimate wild populations is not known. Two possible thresholds for total lice egg production are investigated: (1) 1986 to 1987 level (i.e. before adverse effects on sea trout were recorded), and (2) a level corresponding to a doubling of the estimated natural infection pressure. The farm lice per fish limits that would have to be observed to keep louse production within the 2 thresholds are calculated for the period 1986 to 2005. A steady decrease in the permitted number of lice per fish may keep the total louse production stable, but the number of salmon required for verification of lice numbers will increase as the prevalence to be verified is decreased. At threshold (2), the model estimated that lice limits should have been 0.05 louse per fish in 1999. This would require 60 fish from each pen to be collected, anaesthetised and examined for a good estimate at a confidence level of 95%. Such sample numbers are likely to be opposed by farmers. The use of national delousing programs to solve the problem is discussed.

How sea lice from salmon farms may cause wild salmonid declines in Europe and North America and be a threat to fishes elsewhere

“…The source of the sea lice causing the epizootics on juvenile salmonids in spring must be either wild salmonids that did not migrate offshore, escaped or feral farm fishes, salmon farms or non-salmonid wild hosts. Escaped farm fishes are a significant reservoir of lice in Norway in comparison with wild salmonids (Heuch & Mo 2001), but whether they may be a reservoir elsewhere is not clear.…”

Host density thresholds and disease control for fisheries and aquaculture

“…For example, populations of farmed salmon (usually Atlantic salmon, Salmo salar) are orders of magnitude larger than sympatric wild salmonid populations in salmon farming regions of Norway, Scotland, Ireland, New Brunswick, and British Columbia (Heuch & Mo 2001, Butler 2002, Krko$ek 2009. In western North America, populations of the non-indigenous Pacific oyster Crassostrea gigas in aquaculture facilities dwarf the remnant and depleted populations of native Olympia oysters Ostrea lurida, the only native oyster in the region (McGraw 2009).…”

Can the river location within a fjord explain the density of Atlantic salmon and sea trout?

“…Heuch et al (2009) demonstrated that higher salinity in the outer fjord areas correlates with a higher sea louse infection at salmon farms in the Hardangerfjord. However, as noted by Heuch & Mo (2001) and further corroborated by Brooks (2005), local prevailing oceanographic conditions are important to the area-specific interaction between sea lice and salmon smolt, and it is not certain that similar spatial patterns will emerge in the other fjord systems. We strongly recommend that regional analysis of spawner density should be conducted in other similar areas, and especially in areas with lower concentrations of fish farms.…”

“…Consequently, the species are vulnerable to a series of anthropogenic stressors and many populations have declined or have become extinct in the last century (Parrish et al 1998). Aas et al (2011) considered river regulation and pollution as the most relevant threats to salmon and sea trout in freshwater, while overfishing and aquaculture related effects are considered to be highly relevant for sea survival (Heuch et al 2001;Costello 2006;Skaala et al 2006). Vøllestad et al (2009) examined two data sets on catches of salmon from Scottish and Norwegian rivers and concluded that in addition to temporal trends in catches in the different regions, there was also an indication of a strong local-scale effect (i.e.…”