SARF098C: PAMP Refreshment Study -: The association between emamectin benzoate use and crustacean assemblages around Scottish fish-farms

The louse Lepeophtheirus salmonis is a crustacean ectoparasite that infests salmon. The control of lice is the major issue currently facing the Scottish salmon farming industry. The infeed anti-louse chemical emamectin benzoate (EMB) is consented for use in Scotland. Following ingestion, EMB enter the fishes’ tissue which becomes toxic to feeding lice. EMB is excreted by the fish over an extended period (~200 days) and enter the environment via faeces. Previous research (SARF098A and SARF098B) identified an association between a reduction in benthic Crustacea and increasing EMB use in sediments around fish farms. This negative association was apparent even at Reference stations. The objectives of the current research was to evaluate the evidence as to whether the negative association applied to particular functional groups or crustacean families.
The functional group-categorisation split Crustacea based on diet (particulate organic matter v. others) and motility (mobile v. sedentary). The analysis focussed on data from stations located around the allowable zone of effects (AZE) and Reference stations. These stations are located, typically, 50 – 100 m and 250 – 1000 m from the farm respectively. Current speed, particle size and sampling depth were used as covariables the statistical models linking EMB use and Crustacea.

The analysis split the data into Local Authority (LA) areas and, from each LA, the use of EMB was assessed as a function of farm-distance (AZE and Ref) and sampling methodology. The data were highly unbalanced and, as a consequence, only data from Shetland, Reference stations supported robust analysis. The Shetland, Reference samples were dominated by the crustacean families (in numerical order), Urothoidae, Ampeliscidae, Corophiidae, and Lysianassidae. Multivariate analysis indicated that patterns in crustacean family abundance were linked to all the measured environmental covariables including EMB and provided evidence of a negative association between the most common family (Urothoidae) and EMB use. Of the four functional groups, only those classified as ‘Mobile’ showed an interpretable association with EMB and the model predicted that, under average environmental conditions, a mobile-crustacean reduction of between 17 and 83% would occur following treatment with 3.6 kg EMB. However, this negative relationship was only predicted following removal of statistical outliers. The outliers were dominated by the high abundance of otherwise unusual opportunistic scavengers.

These results are based on the analysis of observational data - further research is required to establish any causal relationship between EMB sedimentary concentration and crustacean assemblage change. This research should be based on manipulative studies where the actual concentration of EMB is controlled, and measured, alongside measurements on the crustacean community.