Assessment of the production and dispersal of faecal waste from the sea-cage aquaculture of spiny lobsters

Abstract

Despite the rapid expansion of spiny lobster aquaculture, very little is known about its potential environmental impacts. In an effort to understand the potential impacts of sea-cage culture of spiny lobsters, the faecal output and settling velocity of faeces of spiny lobster were quantified. These data were then used in hydrodynamic modelling to assess the dispersal pattern of faecal material from two sizes of hypothetical spiny lobster farms, so that inferences about likely benthic impacts could be made by modelling different scenarios of farm stocking density and food conversion efficiency. The overall mean settling velocity of lobster faecal strands was 0.9 ± 0.05 cm s−1. There was a positive correlation between both the weight and density of faecal strands and their settling velocity. Conversely, neither the length nor the surface area of faecal strands was correlated with the settling velocity. Hydrodynamic modelling showed that faecal material deposition mainly occurred directly beneath the sea-cages. The density of faecal material deposition increased proportionately with higher stocking density and FCR values. Overall, the results indicate that the deposition of faecal waste from sea-cage lobster aquaculture is unlikely to cause widespread benthic impacts when operating to the current highest commercial stocking and feeding levels.