Preference of cultured sole (Solea senegalensis) for different substrates differing in material, texture and colour

Sole (Solea solea and Solea senegalensis) culture has been intensely investigated but their special benthonic and nocturnal habits still pose specific challenges to develop appropriate husbandry methods. In nature sole remain buried in the sand, but in culture the use of sand as a substrate has been considered a serious obstacle for hygienic conditions. In the present work different materials, plastic and concrete, are evaluated compared with sand, using different textures and colours. The methodology used relies on the fish itself for the assessment of preference. Images of tagged fish that could freely choose between different substrates are taken continuously during 72 h and further analyzed to assess frequency of preference and immobility time in each substrate. Promisingly sole showed a similar frequency of preference for sand and plastic, when plastic is dark and rough, but there seems to be still a lack of comfort in the plastic substrate since the fish do not remain immobile in it. There is not a clear hierarchy between colour and texture as far as preference is concerned. Sole seem to prefer light sand whatever the texture, and rough plastic whatever the colour, so the preference should be assessed for each specific combination. Results obtained with concrete do not permit to conclude a special preference. Further experiments should be developed to improve the texture of plastic and thus increase immobility and to assess the long-term consequences on growth, stress and health.