The average annual catch of tuna and related species in the Indian Ocean was just over 1.5 million tonnes during 2008-12. Of this, almost 1.1 million tonnes (71%) came from the western and central Indian Ocean. The main fisheries for tuna and tuna-like species in the region are gillnet (40% of reported catch during 2008-12), purse seine (26%), longline (12%), handline and troll (11%) and pole-and-line (9%).
Major gillnet fishing nations include Iran, India, Sri Lanka, Pakistan, Oman and Yemen. Cetacean bycatch must be large, but is poorly documented. A rough estimation, based on the limited ublished information available, suggests that something in excess of 60,000 small cetaceans might be taken as bycatch each year.
There is an urgent need for monitoring and management of these fisheries including the development of mitigation methods to reduce cetacean bycatch. Large-scale gill netting on the high seas (using nets in excess of 2.5km length) is banned by both UN convention and IOTC resolution, but is being carried out by Iran, Pakistan and possibly also other countries; compliance is required. More generally, the large and still expanding gillnet capacity within the region needs to be assessed, and if appropriate either capped or reduced.
Purse seining in the western and central Indian Ocean is dominated by French and Spanish fleets. An increasing proportion of sets is made on drifting fish aggregating devices (FADs) but there has been, and continues to be, a considerable number of sets made on free schools (i.e. non-FAD-associated tuna schools).
Most cetaceans do not regularly associate with FADs and the major potential cetacean interactions are with free school sets. During 1981-1999, 9.6% of all sets were reported to have been made in association with baleen whales, probably Bryde’s whales (Balaenoptera brydei).
When encircled, most whales are reported to escape by breaking through the net. Mortality is unknown, but may have been of the order of 10s annually. The association of free schools of large yellowfin tuna with dolphins (mostly spotted dolphins Stenella attenuata and spinner dolphins Stenella longrostris) is more contentious.
This association (which is common in the Eastern Tropical Pacific and is exploited by the purse seine fishery there) has always been reported to be rare in the western Indian Ocean. However, the tuna-dolphin association is common in many coastal areas of the region and widespread in the high seas of the western Indian Ocean north of 10°S.
Setting on dolphin schools has been also reported to be rare, but its true scale is questioned. Setting on cetaceans has recently been banned by EU regulation (2007) and IOTC resolution (2013), so cetacean bycatch and mortality should be much reduced in the future. 100% coverage by international observers would be ideal.
Longline fisheries were dominated for several decades by East Asian nations, but now increasing catches are made by coastal countries, notably India, Sri Lanka and Seychelles. A major issue for longliners is depredation – removal of bait and dam age of hooked fish by sharks and cetaceans. Several species of cetacean have been implicated, but the main one appears to be the false killer whale (Pseudorca crassidens). There is also some entanglement of cetaceans in longlines (likely follow ing attempts at depredation). Development of mitigation measures is on-going and needs to be continued. It is possible that some longline fishermen are deliberately killing cetaceans.
Several coastal countries have handline fisheries for large yellowfin tuna, which fish ermen locate by their association with dolphins (mainly spotted and spinner dolphins). There is anecdotal evidence that some dolphins are hooked. Although they invariably break free or are released, the scale of any post-release mortality or of sub-lethal impacts is unknown. From the Maldivian pole-and-line fishery, there are reports of dolphins (probably Indo-pacific bottlenose dolphins, Tursiops aduncus) taking fish attracted by the lights used during night bait fishing. The scale and potential impacts of these interactions require assessment.
There has been a widespread failure to monitor and manage cetacean bycatch in Indian Ocean tuna fisheries, and to develop and implement mitigation measures.
The enormous, and still growing, gillnet capacity in the region should be of particular concern. There is a need for increased observer coverage of all fisheries, supplemented by electronic monitoring. Fishery-independent surveys of cetacean distribution and abundance in the western Indian Ocean are also required to inform management.
