Predation by toothed whales on fish hooked by longline and dropline gear is becoming more prevalent throughout Australian waters. The duration of the predation period appeared to be seasonal in the north-western Coral Sea fishery during the 1980's. However over recent seasons the duration of the mammals’ presence in the Coral Sea appeared to increase and observations of adults teaching young animals to predate on hooked fish became common. Predation by toothed whales in the Falkland Island Patagonian toothfish fishery appears to be a learned response with the problem dramatically increasing from a short seasonal problem prior to 2001, to a full season problem in 2002.
The development of a combination of predation mitigation methods is urgently required. Losses to the fisheries due to predation, and the increasing likelihood that mortality to mammals may occur with resultant poor publicity for the fishery and increased scrutiny of biodiversity and sustainability issues, necessitate immediate action. Predation by toothed whales in the eastern, southern and western Australian longline fisheries is expected to increase. Apart from an informal review of SENTMAC operations in the blue-eye trevalla fishery by Tasmanian Parks and Wildlife (Rosemary Gales, pers. comm.), no fisheries data are available for the incidence of predation issues by toothed whales in Australian waters. The AFMA Observer Program has commenced monitoring of all forms of predation damage in Australian fisheries, including seals, sharks and toothed whales.
Terminal stage predation events in the north-western Coral Sea fishery have only recently been observed as mammals appear to have become less apprehensive about approaching fishing vessels. The nature of the predation evidence on the fish is now more varied in the Coral Sea than in the mid-1980’s when it was first reported. This suggests that more than one species may be involved, each with its own characteristic bite pattern (McPherson et al. 2003). Whether the terminal stage of each event is acoustic, visual, or a combination of both is not known. Observations of the method of approach of the mammals during predation events, would assist in the development of appropriate mitigation and gear modification methods.
Predation has been increasing on marlin hooked by the Cairns-based game fishery. Predation is now occasionally reported by smaller toothed whales such as dolphins (family Delphinidae) from coastal fisheries (including Queensland waters) and is likely to increase in commercial and recreational fisheries all State waters as it has in coastal Hawaiian waters.
At the Queensland Fisheries Service (QFS) "Invitational workshop on acoustics to reduce marine mammal interactions" Brisbane April 2002, the keynote speaker Prof Jon Lien (Chair of Ministers Advisory Committee on Fisheries and Oceans, Canadian Minister of Fisheries and Oceans) brought to Australia by NHT funding indicated that predation issues are now reaching extreme levels in at least a dozen countries. No research was being conducted on predation issues, and there was no clear certainty within fisheries circles about what research to conduct.
While the mammal predation issue does not pose a major threat to the continuation of the longline fishery, there is real concern that mammal interaction issues may not be considered on a scientific basis. Bache and Evans (1999) pointed out that while Australian fisheries albatross bycatch policy was influenced by scientific data, environmental non-government organisations (NGO’s) were instrumental in determining Government dolphin bycatch policy. Policy development catalysed by NGO action was considered by Bache and Evans (1999) as least likely to achieve consensus resolution. Commonwealth and State fisheries policy would best be served by achieving consensus resolution through scientific monitoring of the interactions.
Interaction problems are not restricted to economic returns to the fishery and direct threats to the well being of toothed whales. Increasing predation rates reflect a probable change of diet for some species. Based on interactions between dolphins and some trawl fisheries, there could also be a disproportionate population increase of some species that would put many animals under threat if fishing effort, hence opportunities for predation, were to reduce.
