DEPREDATION AROUND SOUTH GEORGIA AND THE IMPLICATIONS ON STOCK ASSESSMENT OF D. ELEGINOIDES

Observer data collected on longliners between 2003 and 2009 was analysed to look at the levels of depredation caused by killer whales (Orcinus orca) and sperm whales (Physeter macrocaphalus) around South Georgia. Since 2003 cetaceans have been observed on 3000 out of 14644 observed lines (20.5%) with killer whales present on 562 lines (3.8%) and sperm whales on 2588 lines (17.7%). By comparing the catch per unit efforts (CPUEs) with and without the presence if cetaceans, the amounts lost were also examined and varied from 0.9% to 5.8% of the TAC per year over the time period. Additional work was also conducted which looked at the behaviour of cetaceans around fishing vessels in the area. To aid identification and give an idea of population size, a photographic catalogue of individual animals was developed. To date, a total of 35 killer whales and 65 sperm whales have been seen, with re-sightings occurring both in season and between years. Hydrophones deployed on longlines recorded both vocalisations of cetaceans and noises made by the vessels, generated by propeller cavitations, gearing and drive shafts, the hydraulics of line machinery and the fishing gear itself. Distinctive acoustic patterns were seen both when winches were engaged and when the drive on the vessel was engaged during hauling. Recordings made from a distance of 3.4nm away were picked up by the hydrophones and it is thought that these acoustic cues could be responsible for attracting the cetaceans.