Introduction
Ballan wrasse (Labrus bergylta) is a significant cleaner fish species used as a biological control in Atlantic salmon (Salmo salar) aquaculture against ectoparasitic sea lice (Lepeophtheirus salmonis) (Overton et al., 2019; Torrissen et al., 2013). The relationship between rearing conditions, ballan wrasse-specific welfare, and delousing efficiency are poorly understood, thus presenting the opportunity to identify optimal husbandry practices to promote the development of effective cleaner fish.
Materials and Methods
This study included the allocation of three treatments to 600 fish across nine replicate tanks. A pellet-only diet was used as the control, with treatments consisting of feeding blocks only or a combination of blocks and pellets. The rationale behind using feeding blocks is to imitate and promote the "nipping" action required for their role in sea lice management. The wrasse were exposed to their treatments across a 16-week period with data collection occurring at the baseline and three-week intervals. At every interval, neophobia tests were conducted with a LEGO novel object to analyse boldness as a behavioural trait associated with delousing. Growth and welfare were assessed via morphometric measurements and adapting pre-existing operational welfare indicators (OWIs) to be applied in a species-specific context (Bosakowski & Wagner, 1994; Hoyle et al., 2007; Geitung et al., 2020; Treasurer & Feledi, 2014). Gut microbiome samples were taken to provide insights into the effect of feed method on physiological condition. Welfare and morphometrics were sampled at the baseline, week 10, and week 16, with microbiome data representing the baseline and week 16.
Analysis
Data analysis is currently ongoing via mixed effects modelling, including feeding treatment as a fixed effect whilst accounting for tank-level variation. Analysis incorporates the differences between treatments as well as across the 16-week exposure period. Behavioural factors assessed include number of contacts, proximity to, and latency to make contact with the novel object. Morphometrics, Fulton's condition factor, and OWI scores provide the insights into implications to growth and physical welfare. 16S sequencing will examine differences in gut microbiome.
Discussion
By integrating behavioural, morphometric, welfare, and microbiome measures, this study identifies and enhances ballan wrasse-specific OWIs, addressing a current gap in cleaner fish welfare assessments. By identifying whether the feeding method provided during rearing encourages behaviours that enhance delousing efficiency, optimal husbandry methods can be implemented prior to sea cage deployment.
References
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