Operational interactions between marine mammals and the Patagonian toothfish (Dissostichus eleginoides) fishery were assessed in southern Chile through 7 surveys with observers on board industrial vessels between April 2002 and March 2003. Sperm whales (Physeter macrocephalus) occurred in higher frequency than any other species during fishing operations. In 60% of all monitored sets sperm whales were present, while orcas (Orcinus orca) were found in only 10% of these. When cetaceans were present, evidence of damaged catch included lips, heads and trunks and a total of 121 lips, 16 heads and 3 trunks were recovered. Mean rate of predation was 3% (± 2% CI 95%; n=180 sets) and ranged between 0% and 100%. Considering that in 153 (84%) of the effectively monitored sets no interaction whatsoever was recorded and that the mode and median of the predation rate was 0, the global impact of cetaceans over the fishing yield is considered to be low. When mixed sightings of orcas and sperm whales were encountered (n=12) the rate of predation decreased to 0%. Upon detecting orca presence, sperm whales modified their surface behaviour, grouping into tight parallel formations. This suggests that orcas prefer attacking aggregated sperm whales instead of the line. High sperm whale densities were found associated to different hotspots which had high fishing yields. This relationship tends to support the hypothesis that the richest fishing grounds are also traditional feeding grounds for sperm whales. Monetary loss associated with operational interactions associated with predation reach USD$92,684 (C.I. 95% USD$47.302 - $153.745) for the whole fleet, with a mean loss per set of USD$138 (C.I. 95% USD$74,76 - 249,3).
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