Mouth morphology and behavioural responses of cultured turbot towards food pellets of different sizes and moisture content

The importance of food pellet size and texture in pre-capture selectivity and subsequent ingestion by juvenile turbot, Scophthalmus maximus (Rafinesque), was investigated. Measurements of mouth parts and body lengths were taken to investigate their relation to preferred pellet sizes. Two fish size classes, with mean lengths of 123.9 ± 2.5 mm and 161.3 ± 2.9 mm were tested. Behavioural responses of individual fish offered pellets of five different diameters (2.0, 3.5, 5.0, 8.0 and 13.0 mm) and two moisture levels (10.9% and 28.3%) were investigated. Responses were classed as follows: no reaction; orientation; incomplete approach; complete approach and capture, and scored accordingly. A broad range of acceptable pellet sizes was observed in both fish size groups. In the smaller sized group, acceptable pellet diameters ranged between 4.0% and 6.5% of total fish length, or 40% of mouth gape. This decreased to 20% of mouth gape, or between 2.2% and 5.0% of total fish length in the larger fish group. Juvenile turbot demonstrated an ability to accurately select pellets within the acceptable range and pellets outside this range were largely ignored, eliciting few pre-capture behaviours.