Ectoparasitic sea lice (Lepeophtheirus salmonis and Caligus elongatus) infestations of wild, adult, one sea-winter Atlantic salmon Salmo salar returning to Scotland

Abstract

Caligid ectoparasitic copepods are major pathological pests on cultured Atlantic salmon Salmo salar L., and their population biology has been well studied in the farm environment. The ecology of caligid infestations of wild salmon is, by contrast, rather poorly understood. We monitored return migrant one sea-winter wild Atlantic salmon in Scotland annually for infestations of 2 caligids, Lepeophtheirus salmonis (Krøyer) (between 1998 and 2005) and Caligus elongatus Nordmann (between 1999 and2005). Prevalence of L. salmonis was 100% in all years, whereas C. elongatus prevalence ranged from 90 to 100%. Abundances fluctuated markedly between years and L. salmonis mean abundance (min. to max.: 17.4 to 31.0) was significantly greater than for C. elongatus (min. to max.: 2.9 to 23.8) in all except one year. A positive association in abundance of the 2 species, for individual fish within any one year, indicates weak or absent competitive effects on abundances for individual hosts. Individual fish within any one year appeared similarly vulnerable to infestation by either species, although Taylor's power regression showed clear differences in density-related patterns of overdispersion amongst hosts for the 2 species. Host condition factor (expressed either as Fulton's index, K, or the relative mass index, W R ) showed significant variation among years. Parasite species abundances were not, however, determined by host condition factor, and poor condition fish were no more likely to carry high infestations than were high condition fish.

Using sentinel cages to estimate infestation pressure on salmonids from sea lice in Loch Shieldaig, Scotland

“…Burdens of 20+ adult Lepeoph theirus salmonis fish -1 on wild adult Atlantic salmon (Todd et al 2006) and sea trout (Urquhart et al 2008) have been recorded with no obvious detri- lice (all stages) abundance on the sentinel fish and on the fish from 2 farms using both untransformed and de-trended data. The cross-correlation at lag zero is the correlation between counts in the sentinel cages and the farms in the same month; the cross-correlation at a lag of 1 mo is the correlation between counts in the sentinel cages and the farms 1 mo before; and so on.…”

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

“…Plankton sampling discovered higher densities of L. salmonis copepodites in very shallow water along the seashore and in estuarine areas of Ireland and Scotland (Costelloe et al 1995(Costelloe et al , 1998McKibben & Hay 2004). While the same copepodite distribution may be expected in British Columbia, it requires confirmation because the Pacific and Atlantic populations of L. salmonis are genetically distinct (Tjensvoll et al 2006;Todd et al 2006) and may have different evolutionary adaptations. Thus, at least in Europe, empirical data indicated that the infective copepodites of L. salmonis concentrated in the path of migrating salmonids in estuaries.…”

“…However, Todd et al (2006) found that wild Atlantic salmon in poor condition were not more likely to have high lice burdens. At low density (less than three lice per fish or less than 0.65 lice g 21 fish), lice had no effect on the condition of juvenile pink salmon (Butterworth et al 2008).…”

“…Recent reviews have stated that greater than 0.5 to 0.75 mobile L. salmonis per gram host weight, or more than 5 to 10 per fish (greater than 0.1 lice g 21 fish) can be pathogenic to Atlantic salmon, Salmo salar (Costello 2006;Wagner et al 2007). Todd et al (2006) found that 0.2 to 0.7 L. salmonis per gram were pathogenic to the sea trout S. trutta. Although chalimii are not known to be pathogenic, they can become so when they mature into mobile stages if sufficiently abundant.…”

Disease interaction and pathogens exchange between wild and farmed fish populations with special reference to Norway

“…Timeseries data from more than seven years have been collected for L. salmonis and C. elongatus abundances on wild salmon captured in coastal bag nets in marine waters off the north coast of Scotland (Todd et al, 2006). The fish remained live and free-swimming in these traps and thus the data were more reliable.…”