Settling velocity of faecal pellets of gilthead sea bream ( Sparus aurata L.) and sea bass ( Dicentrarchus labrax L.) and sensitivity analysis using measured data in a deposition model

Abstract

Settling rates of faecal material from three size categories of cultured gilthead sea bream, Sparus aurata, and sea bass, Dicentrarchus labrax, were determined. Faecal material was collected from underneath commercial cages and subsequently transferred to a settling column. Particle settling velocity was determined using particle tracking software. Image analysis software was used to determine dimensions of selected faecal particles from which particle volume was then estimated. Faecal particles (all fish sizes) had a mean settling velocity of 0.48 cm/s (range 0.05–3.94 cm/s, n = 1021) for sea bream and 0.70 cm/s (range 0.10–6.27 cm/s, n = 1042) for sea bass. The faecal material largely consisted of very small particles and mean particle size was 0.71 mm ( n = 151) and 1.12 mm ( n = 150) for S. aurata and D. labrax, respectively. More than 50% of the total particle volume had a settling velocity less than 2.0 cm/s in all S. aurata size categories. In D. labrax, more than 75% of the total particle volume had settling velocities greater than 2.0 cm/s. The particle tracking DEPOMOD model was highly sensitive to different representations of these data. The predicted seabed flux of waste faecal material (g/m 2/year) for D. labrax using the mean settling velocity at 0 m and 50 m from the cage was 3196 g/m 2/year and 248 g/m 2/year, respectively. However, when using particle settling velocity and volume distribution data, the predicted flux was over three times greater at 0 m but five times less at 50 m. For S. aurata, the predicted flux using the mean settling velocity was 3018 g/m 2/year and 464 g/m 2/year at 0 and 50 m, respectively. However, where distribution data were used, the values were over twice as high at 0 m and but were halved at 50 m. These results indicate that use of a single mean settling velocity value in this type of modelling does not accurately predict the extent of benthic flux. In addition, species-specific faecal settling rates should be used when modelling polyculture operations.