The occurrence of benthic scavengers and their consumption at tuna farms off Port Lincoln, South Australia

Farming of southern bluefin tuna (SBT) (Thunnus maccoyii) at Port Lincoln, seaward of Boston and Rabbit Islands, involves feeding SBT in 130–150 pontoons, using primarily frozen Australian sardines (Sardinops sagax) amounting to more than 40,000 t per year. During feeding some sardines are not eaten and fall through the pontoon where they are available for scavengers. Benthic baited video camera deployments were conducted during October 2002 and January 2003 at the SBT farms off Boston and Rabbit Islands, respectively. The experiments were carried out during day and night using underwater video surveillance and sardines as bait. The most common scavengers were leatherjackets during the day, notably Degens leatherjacket (Thamnaconus degeni), and sea lice (a group of voracious scavengers on carrion composed of several species of isopods, notably the isopod Natatolana woodjonesi, and amphipods) during the night. Scavengers arrived at the bait within minutes developing a feeding frenzy, but the numbers were variable irrespective of site and day/night. During Oct 2002, consumption was independent of site and day/night. During Jan 2003, a significant effect of site and time was found, showing that consumption was higher at Boston than at Rabbit Island and higher during the day than at night. In all trials, all bait was consumed within 25 minutes of deployment. The average consumption rate of sardines was estimated to be approximately 12 g per minute. The results showed that scavengers play an important role in removing waste feed from beneath SBT pontoons.