Abstract
Jansen et al. (2016; Aquacult Environ Interact 8:349�350) question the regression analysis presented in Serra-Llinares et al. (2014; Aquacult Environ Interact 5:1�16), which corre- lates lice abundances on farmed and wild fish. Jansen et al. (2016) argue that the correlation might not reflect a cause-effect relationship but be instead a mere artifact of the spatio-temporal covari- ance in lice abundance on farmed and wild fish driven by temperature. In this Reply Comment we revisit the analysis presented in Serra-Llinares et al. (2014) and further re-analyze our data follow- ing the statistical approach used by Helland et al. (2015; Aquacult Environ Interact 7:267�280), to rule out the potential confounding effect of temperature. We conclude that Jansen et al. (2016) were correct in conveying part of the observed correlation to the effect of temperature; however, there is solid evidence of a significant influence of lice originating from nearby farms on the observed lice abundances on wild fish, even after the effect of temperature is accounted for.
Highlights
Serra-Llinares et al (2014) presented a correlation between the abundance of salmon lice Lepeoptheirus salmonis Kröyer on wild salmonids and the infestation pressure imposed by nearby farms, expressed as the daily release of copepodites within a 30 km distance from farmed sites
Partial correlation analysis indicated that the production of copepodites in nearby farms by itself explained 21.8% of the variation in mean lice abundance on wild fish while controlling for the effect of temperature
No temporal auto-correlation pressure imposed by nearby farms and lice abun- was included in the analyses, because attached lice dances on wild fish appears to be weaker than counted in Period 1 would have developed into preclaimed by Serra-Llinares et al (2014)
Summary
Serra-Llinares et al (2014) presented a correlation between the abundance of salmon lice Lepeoptheirus salmonis Kröyer on wild salmonids (sea trout Salmo trutta L. and Arctic charr Salvelinus alpinus L.) and the infestation pressure imposed by nearby farms, expressed as the daily release of copepodites (the infective stage of L. salmonis) within a 30 km distance from farmed sites. To make our results more comparable to those from using a summary statistic to describe the lice infesta- Helland et al (2015), we changed our infestation tion levels observed on wild fish (such as mean lice pressure variable from daily production of copeabundance, as used by Serra-Llinares et al 2014), podites to total numbers of female lice in nearby. As in Serra-Llinares et al (2014), only farms (all stages) observed on each individual fish as the located less than 30 km from each wild fish sampling response variable in a generalized linear mixed location were included in the calculation, and a linmodel (GLMM), in which fish length, temperature, ear correction was applied to account for the effect of salinity and infestation pressure Temperature, infestation pressure from nearby fish farms and year (categorical variable with the levels 2010 and 2011) were used as Intercept
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