Transmission dynamics of parasitic sea lice from farm to wild salmon

Abstract

Marine salmon farming has been correlated with parasitic sea lice infestations and concurrent declines of wild salmonids. Here, we report a quantitative analysis of how a single salmon farm altered the natural transmission dynamics of sea lice to juvenile Pacific salmon. We studied infections of sea lice (Lepeophtheirus salmonis and Caligus clemensi) on juvenile pink salmon (Oncorhynchus gorbuscha) and chum salmon (Oncorhynchus keta) as they passed an isolated salmon farm during their seaward migration down two long and narrow corridors. Our calculations suggest the infection pressure imposed by the farm was four orders of magnitude greater than ambient levels, resulting in a maximum infection pressure near the farm that was 73 times greater than ambient levels and exceeded ambient levels for 30 km along the two wild salmon migration corridors. The farm-produced cohort of lice parasitizing the wild juvenile hosts reached reproductive maturity and produced a second generation of lice that re-infected the juvenile salmon. This raises the infection pressure from the farm by an additional order of magnitude, with a composite infection pressure that exceeds ambient levels for 75 km of the two migration routes. Amplified sea lice infestations due to salmon farms are a potential limiting factor to wild salmonid conservation.

Figure 1

Log variance versus log mean for all 41 samples of copepodids (black circles) chalimi (grey squares) and motiles (clear triangles). There are 79–237 fish per sample. The solid line is the variance=mean line which accounts for 96% of the variation. The dashed line is the best-fit linear model, which has slope 1.08 and accounts for 97% of the variation. Compare with fig. 5 in Shaw & Dobson (1995).

Figure 2

Abundance of parasitic louse stages on juvenile pink and chum salmon at points along their migration routes relative to a salmon farm located at x=0 (farm A). Salmon migrate in the rightward (seaward) direction. Columns correspond to datasets and rows correspond to louse stages. Error bars are bootstrapped 95% confidence intervals. Solid lines are the maximum likelihood best fits of model Ψ₂ to each dataset. Sample sizes are in the range 79–237 fish per sample.

Figure 3

The spatial distributions of planktonic copepodids inferred by model Ψ₂ on a relative scale. Juvenile salmon migrate in the rightward (seaward) direction. The thick grey line is the total abundance of copepodids produced by all sources. The horizontal lines near zero are the ambient infection pressures. The thin dark curves oriented about x=0 are the distributions of copepodids produced directly by farm A and the second curves to the right are the distributions of copepodids produced by the farm-origin cohort of lice on the juvenile salmon. The latter distribution was found by solving the model with κ=0 to eliminate any contribution of lice from natural sources. Corresponding datasets are I-April (a), I-May (b), II-April (c), and II-May (d).

Figure 4

The spatial distributions of sea lice larvae around a point source. (a) The expected differences in the spatial distributions between nauplii (dashed line) and copepodids (solid line). (b) Differences in the spatial distributions of nauplii produced by a pulse release (dashed lines at successive time intervals) and by a continuous constant source (solid line) under strong advection. The dynamics of an oscillating source of amplitude half the mean (c). The frequency of oscillations is 14.8 per year, calculated from Revie et al. (2002). Three solutions for the spatial distributions of nauplii about the oscillating source are plotted in (d) for the time points indicated by the vertical dashed lines in (c). The expected distribution for the constant mean source is shown by the filled circles.