Use of laser scanning to evaluate turbot (Scophthalmus maximus) distribution in raceways with different water velocities

Fish distribution can be a reliable indicator of husbandry conditions but, as happens with other behavioral parameters, it is not easy to assess with objectivity and repeatability. Here, we used a laser scanning method to monitor the distribution variations of a turbot (Scophthalmus maximus) population in a tank under different water velocity treatments, using (1) two uniform water velocities throughout the rearing unit, by changing the water depth but keeping the water flow; (2) different water velocities inside the rearing unit, which allowed the fish to choose a preferred zone; and (3) baffles to create a higher velocity and turbulence above and below them. Preliminary results on water velocity preferences or tolerance for 22-cm turbot determined that velocities between 0.33 and 0.46 BL s−1 promoted a homogenous turbot distribution, but that when the fish could choose, they avoided swimming against water flows over 0.58 BL s−1. With a velocity over 0.98 BL s−1, turbot no longer appeared to be able to maintain their position. We also tested the viability of the method to estimate the total biomass of turbot in a high-density tank (from 280 to 320 percentage of coverage area [PCA]). The resulting coefficient of variation between samplings was lower than 10%. The laser scanning has proven to be a useful tool to monitor flatfish distribution and to estimate the total biomass in flatfish culture, and it offers several advantages: (1) it is objective and non-intrusive, thus minimizing stress; (2) it can be used with a high stocking density; (3) it analyzes biomass population; and (4) it can be adapted to existing commercial facilities.

► A laser scanning method is used to monitor turbot distribution in a raceway tank.
► Tank hydrodynamic conditions can modify turbot distribution inside the raceway.
► Laser scanning allows to estimate flatfish biomass, even with high PCA.