Canthaxanthin as a potential tracer of salmon feed in mussels (Mytilus spp.) and sea urchins (Strongylocentrotus droebachiensis)

The large amount of fish feed involved in salmon aquaculture results in high nutrient loading to the surrounding environment. The near-field effects of this loading are well documented but the full zone of influence, or far-field effect, remains poorly understood. We investigated the potential of the carotenoid canthaxanthin, a common additive in farmed Atlantic salmon diets, as a biochemical tracer to identify this zone of influence in two benthic invertebrate species that are locally abundant around salmon farms: the blue mussel Mytilus edulis (or Mytilus trossulus), a suspension feeder, and the green sea urchin Strongylocentrotus droebachiensis, an omnivorous grazer. To measure persistence of canthaxanthin in the digestive gland of mussels or the gonad of sea urchins, both species were individually fed three levels of salmon feed (6, 13, 26mgd ^-1 for mussels; 70, 150, 300mgd ^-1 for sea urchins) for 13d to measure pigment uptake rates, followed by a non-pigmented diet for 7d to measure pigment loss rates. Mussels exhibited a relatively rapid uptake of canthaxanthin (1 to 3d), which subsequently declined to zero within 3d following cessation of the pigmented diets. The sea urchins exhibited slower initial uptake (4 to 10d) and the pigment signal lasted up to 46d, suggesting retention of canthaxanthin. To examine the scale of dispersion of feed-derived particulate material in nature, canthaxanthin uptake by sea urchins was measured at fixed intervals along a transect extending 1km from a salmon cage in Passamaquoddy Bay, New Brunswick, Canada. Pigment concentration in the gonad dropped from ~5 to 0.5μgg ^-1 within the first 100m from the cage and remained at this low level along the remainder of the transect, likely reflecting low background levels of salmon farm-derived particles within the bay. Our study demonstrates the potential of canthaxanthin as an organic tracer for salmon aquaculture, and the use of blue mussel digestive gland for short-term detection and sea urchin gonad for long-term detection, in accordance with the rapid ingestion and nutrient storage functions of the respective tissues.