Modelling carbon deposition and dissolved nitrogen discharge from sea cage aquaculture of tropical spiny lobster

Abstract

Abstract The tropical spiny lobster, Panulirus ornatus, is farmed in floating sea cages situated in shallow coastal waters in many parts of the Asia-Pacific region. Despite the rapid expansion of this aquaculture activity, very little is known about its environmental impacts. This study combines computer modelling with previous laboratory measures to provide information on benthic carbon deposition and the production of dissolved inorganic nitrogen (DIN) from hypothetical sea cage aquaculture of spiny lobsters. Modelling scenarios were run with two different lobster aquaculture stocking densities (3 and 5 kg m−3) and various feed conversion ratios (FCRs) using natural seafood or artificial lobster diet (FCR 1.28–28). Simulations from the model showed that cumulative benthic carbon deposition varied from 0.1 to over 0.8 kg C m−2 year−1, while the mean DIN levels around sea cages ranged from 5.6 up to 25 µg N l−1 and the maximum DIN levels ranged from 10.8 to 165 µg N l−1. The results showed that feeding lobsters with seafood resulted in a markedly higher benthic carbon loading and release of DIN when compared with artificial lobster feed. Therefore, the elimination of the use of trash fish would greatly reduce the environmental impacts of spiny lobster aquaculture. Overall, the effects from spiny lobster aquaculture were spatially localized with the highest concentrations of carbon deposition and DIN directly beneath the sea cages. Therefore, it seems unlikely that spiny lobster aquaculture in sea cages will cause adverse environmental effects unless the lobsters are heavily stocked and supplied with poor quality feed.