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Abstract
Predatory interaction of marine mammals with longline fisheries is observed globally, leading to partial or complete loss of the catch and in some parts of the world to considerable financial loss. Depredation can also create additional unrecorded fishing mortality of a stock and has the potential to introduce bias to stock assessments. Here we aim to characterise depredation in the Patagonian toothfish (Dissostichus eleginoides) fishery around South Georgia focusing on the spatio-temporal component of these interactions. Antarctic fur seals (Arctocephalus gazella), sperm whales (Physeter macrocephalus), and orcas (Orcinus orca) frequently feed on fish hooked on longlines around South Georgia. A third of longlines encounter sperm whales, but loss of catch due to sperm whales is insignificant when compared to that due to orcas, which interact with only 5% of longlines but can take more than half of the catch in some cases. Orca depredation around South Georgia is spatially limited and focused in areas of putative migration routes, and the impact is compounded as a result of the fishery also concentrating in those areas at those times. Understanding the seasonal behaviour of orcas and the spatial and temporal distribution of “depredation hot spots” can reduce marine mammal interactions, will improve assessment and management of the stock and contribute to increased operational efficiency of the fishery. Such information is valuable in the effort to resolve the human-mammal conflict for resources.
Highlights
IntroductionSince the modernisation of longline fishery technology in the 1950s, incidents of depredation (fish removed from the gear by predators during hauling) of longline fisheries by toothed whales have been reported with increasing frequency and are a global occurrence
Since the modernisation of longline fishery technology in the 1950s, incidents of depredation of longline fisheries by toothed whales have been reported with increasing frequency and are a global occurrence
Whale sightings reported by the observers versus those reported by skippers were largely consistent (F 1, 1748 = 1.941 e04, adj R2 = 0.92, p < 2.2 e-16 for orcas, F 1, 2354 = 2.411 e04, adj R2 = 0.91, p < 2.2 e-16 for sperm whales); on average skippers saw 0.3 more orcas and 0.6 more sperm whales per longline set in 2011 and 2012
Summary
Since the modernisation of longline fishery technology in the 1950s, incidents of depredation (fish removed from the gear by predators during hauling) of longline fisheries by toothed whales have been reported with increasing frequency and are a global occurrence. Depredation by toothed whales can lead to considerable loss of the catch in some parts of the world [1] and can have implications for the economic viability of longline fisheries, the target stock. Marine Mammals and Longline Fisheries publish, but contributed to the preparation of the final manuscript
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