Influence of tank geometry and flow pattern in fish distribution

The effect of hydrodynamics on fish distribution (Danio rerio) was studied in two tank configurations: rectangular, with three waterfalls, and circular. The flow pattern of each tank was characterised in a previous paper by using particle tracking velocimetry (PTV) techniques. Image analysis was used as a non-intrusive method to determine spatial fish distribution in the tank by collecting fish position coordinates from the recorded sequences. The tank was divided into cells in order to establish the fish distribution pattern. A ratio of fish occupation in each cell was determined and statistically analysed for three replicates of the experiment. The fish occupation ratio was also correlated with the average velocity in the corresponding cell. Furthermore, a fish distribution coefficient of uniformity (FCU) was defined to evaluate the homogeneity of fish distribution in the tank volume. The rectangular tank showed a strong gradient in the fish distribution pattern from water inlet to outlet, where the average velocity was very low, and a low uniformity in fish distribution was obtained in the whole tank (FCU = 0.26). The circular tank showed a gradient in the fish distribution pattern and in velocities from the centre to the outer perimeter, providing velocities from 0.54 to 1.76 BL s−1, respectively. A more uniform fish distribution in the tank was obtained (FCU = 0.70), indicating a better use of the entire rearing volume. Velocity in this tank strongly influenced fish occupation, with a positive correlation (r = 0.85) and a significant linear regression (P < 0.05).

Research highlights▶ Fish distribution (Danio rerio) in rectangular and circular tanks is compared. ▶ Fish distribution is more heterogeneous in rectangular tank with multiple waterfalls. ▶ Fish tend to occupy the first part of rectangular tank, where turbulence is higher. ▶ In circular tank (tangential inlet) fish occupation increases from centre to wall.