Effects of larvae ontogeny, turbidity, and turbulence on prey attack rate and swimming activity of Atlantic herring larvae

The effect of turbulence, light level, and ontogeny on herring larva''s attack rate and swimming activity was tested in a previous study. However, during larval seasons (spring and autumn), water clarity is frequently impaired by alga blooms, which also most probably will affect larva feeding rate. Therefore, this study was to investigate the effects of turbidity, turbulence, and ontogeny on the attack rate and swimming activity of herring larvae. By adding diatomaceous earth (DE) to the water, three turbidity levels were established: 0, 35, and 80 Jackson Turbidity Unit [JTU; which coincide with a beam attenuation (c) of 0.1, 2.5, and 4.8 m-1, respectively]. An unfavourable (8×10-6 W/kg) and a favourable turbulence level (1×10-6 W/kg) were chosen based on results from the earlier study. The results show that intermediate turbidity (35 JTU) has a positive effect on the attack rate of smaller larvae (20 mm), while high turbidity (80 JTU) has a negative effect on attack rate of all tested larvae size groups. In general, attack rate was lower at the highest turbulence compared to the low level, independent of turbidity level. However, there was one exception, when turbidity was at the highest, the largest larvae (29 mm) seemed to gain from feeding in the highest turbulence level. The overall activity level was higher in the presented study than in the earlier study without turbidity. The favourable turbidity level (35 JTU) coincides with turbidity levels normally found at the equivalent depth during spring and autumn blooms in the area of where the experimental larvae originate. In addition, turbidity''s effect on light absorbtion and how it influences the maximum feeding depth of the larva are discussed. [Copyright &y& Elsevier]