Does exposure to sea lice from aquaculture have a population-reducing effect on wild Atlantic salmon? A systematic review protocol

Wild Atlantic salmon are increasingly threatened throughout their distribution in north Atlantic rivers. Ectoparasitic sea lice have the potential to cause severe damage to salmon, especially the smolts early in the marine migration; these lice proliferate in areas where fish farming is prevalent due to the high density of hosts. Many studies have attempted to investigate how lice infestation pressure affects salmon population recruitment, however, there has not been a compelling systematic review of this topic. Here, we present a protocol for a systematic review that will collate and synthesize the evidence base related to population-level effects of sea lice (both Caligus and Lepeoptherius) from aquaculture on wild Atlantic salmon (Salmo salar). This review will include a comprehensive assessment of the comparative validity of the selected studies (i.e., critical appraisal), a step that is currently lacking from previous synthesis efforts, but one that is essential to improve our understanding of the existing evidence base and ensure review results are reliable. This systematic review will use published and grey literature without date restriction. Searches will be performed using three bibliographic databases, two search engines, and networking tools to identify relevant literature. Eligibility screening will be conducted at two stages: 1) title and abstract, and 2) full text. Studies written in English or Norwegian that evaluate the impact of sea lice exposure from copepod salmon louse (Lepeophtheirus salmonis) and Caligus elongatus on the abundance, marine survival, or adult returns of wild and/or hatchery produced Atlantic salmon will be included. Included studies will undergo critical appraisal of internal and external study validity. We will extract information on study characteristics, study design, exposure and comparator details, measured outcomes (abundance, survival, adult returns), and potential sources of heterogeneity (e.g., release location, time period). A narrative synthesis will be used to describe the quantity and characteristics of the evidence base, while quantitative synthesis (i.e., meta-analysis) will be conducted to estimate an overall population-level effect of sea lice from aquaculture on wild Atlantic salmon when sufficient numbers of similar studies are available.