The effects of mixed-species assemblage on the behaviour and welfare of fish held in home aquaria

While there is growing scientific research into the welfare of fish held under aquaculture conditions, there has been little research into the welfare of ornamental fish kept in home aquaria. With the ever increasing popularity of home aquaria, there is now an urgent need to address our current lack of knowledge regarding welfare of mixed-species assemblages. Here we investigated the welfare of four commonly kept species of ornamental fish held in different species combinations and group sizes. Specifically we asked two questions (1) whether the presence of angelfish (Pterophylum scalare) altered the behaviour of a mixed group of white cloud mountain minnows (Tanichthys albonubes), neon tetras (Paracheirodon innesi) and tiger barbs (Barbus tetrazona) and (2) whether group size affected behaviour and welfare in a mixed group assemblage of angelfish, white cloud mountain minnows and neon tetras. Behaviours including intra and interspecific interactions (darting and aggressive attacks), shoaling, and use of environmental enrichment were measured. Angelfish appeared to have a beneficial effect on the welfare of small shoaling species by reducing aggression but had little effect on other behaviours. For white cloud mountain minnows and neon tetras, larger group sizes resulted in increased natural behaviours (i.e. a tendency to shoal). Interestingly, the present study highlighted the problems in predicting the effects of environmental enrichment with both species and group size altering the way enrichment was used (e.g. as shelter from competitors or as a resource worth aggressively defending). Setting scientifically determined guidelines on appropriate species assemblages or stocking densities for ornamental fish is challenged by the plethora of group sizes and combinations found within home aquaria; research as in the present study serves to provide insights into welfare issues which may not become apparent within unrepresentative, single-species studies.