A fitness-based indicator for the effect of aquaculture-produced salmon lice on wild sea trout

Abstract

Sea trout, the anadromous phenotype of brown trout (Salmo trutta), is severely affected by high salmon lice infestation pressure that occurs in areas of Atlantic salmon (S. salar) aquaculture. Yet, and in contrast to Atlantic salmon, there is currently no effect indicator implemented for sea trout for regulating the Norwegian aquaculture industry. Here, we present such an indicator centred on the concept of fitness. Our numerical example, based on previous published estimates of reduced survival and reduced marine feeding time of sea trout due to spillback of salmon lice from aquaculture, demonstrates that the indicator is readily implementable. There are several benefits of our indicator: (i) multiple effects of salmon lice on sea trout can be implemented in a single biologically meaningful indicator; (ii) the indicator can be customized and simplified depending on data availability and knowledge; (iii) the simplifications provide a conservative yet effective indicator; (iv) the indicator is on the same scale as the already implemented Atlantic salmon indicator, making it possible to use the same management threshold values; and (v) the changes in fitness measured by the indicator can provide scenarios for the evolution of anadromy.