The patterns identified in this study indicate a link between local salmon farm production cycles and infestations of wild trout smolts, consistent with high levels of infestation during years when lice levels on farms are high (Revie et al. 2002
; Lees et al. 2008
) and with previous small-scale studies (Butler 2002
; Hatton-Ellis et al. 2006
). The significant relationship across years at the intensively studied Shieldaig site was mirrored through 2002–2003 across sites throughout the west coast. Because the year of production cycle coincided in 9 of the 10 sites in the spatial analysis, the possibility that, in these data, infestation was associated with variation in calendar year rather than farm cycle cannot be ruled out. During 2002–2003, salmon farms in their second year of production were known to have higher levels of lice than those in their first year, irrespective of calendar year (Lees et al. 2008
). Insufficient data were available to factor in possible effects of inter-annual variation in other environmental parameters, such as salinity. In the current analysis, significant relationships were evident between year of production and incidence of fish with parasite burdens exceeding a threshold level considered by Wells et al. (2006)
to ‘provide a clear indication of the proportion of sea trout within a population that are subject to physiological stress and potential death from sea lice infestations’.
A strength of the study is that by comparing years of production within sites it is possible to control for spatial variation in a wide variety of environmental factors that are known to affect sea lice levels. Sea lice tend to be lost from hosts in brackish water, thus introducing variation among lice levels on fish caught under different salinity conditions, irrespective of initial rates of infestation (Revie et al. 2009
). Furthermore, because sea lice dispersal is thought to be mainly driven by wind, the physical geography and juxtaposition of the farm and sampling sites are likely to influence lice levels and distribution in the environment, and in turn on trout (Amundrud & Murray 2009
). These among-sites variables are controlled for in studies comparing within-site and between-year of farm production cycle but not in direct comparisons among sites (MacKenzie et al. 1998
), where they would introduce noise and reduce power to detect an effect.
Although the results suggest a general relationship between lice burdens and fish farm production cycles, there were exceptions to this pattern (; ). Such exceptions may reflect local between-year variations inconsistent with the typical incidence of lice during the farm cycle. For example, lice on farms may be lowest in the second year of the cycle because of treatment of fish (Lees et al. 2008
). Indeed, the apparently anomalous 2004–2005 Shieldaig trout data coincided with unusual patterns of lice numbers on local farms and in the water column (Penston & Davies 2009
). Furthermore, spatial concurrence of fish and parasite may vary among years, for example, owing to wind direction affecting dispersal of lice (Amundrud & Murray 2009
The data presented in this study add to the evidence from a number of countries that, in general, the sea lice burdens of wild sea trout are related to the occurrence of salmon farming (Gargan et al. 2003
; Heuch et al. 2005
). However, although lice burdens can be used to estimate likelihood of individuals surviving (Bjørn & Finstad 1997
; Wells et al. 2006
), it is not currently possible to use such data to predict the effect of sea lice infestations on the source populations (Revie et al. 2009
). This is because sampling may be biased with respect to lice infestation level and also would not provide information regarding those fish that had already died. However, the data presented here suggest that there is a link between Atlantic salmon farms and lice burdens of wild trout in the west of Scotland.