Measurement of organic loading under an open-ocean aquaculture cage, using sediment traps on the bottom

Organic loading under a submerged fish cage in commercial operation has been quantified for the first time in the open ocean. Sediment traps out to 100 m sampled the loading continuously over the 15 months of a complete grow-out cycle for cobia (Rachycentron canadum). Typically 4% or 5% of the feed arrived directly to the sediment, although this benthic percentage became much higher in the last two months of this study. Almost all the loading (90%) lands within 30 m of the cage mooring block. The loading consists of fragments of feed pellets that wash out from the mouths and gills of the fish. The fragments sink rapidly and almost vertically; they are not carried horizontally into large dilution volumes. Dispersal on the sediment surface is much more extensive than dispersal in the water. This study developed expeditious and cost-effective techniques for sampling and analyzing organic loading, using a minimum of technological resources.