Bycatch is the dominant threat affecting Indo-Pacific Bottlenose Dolphins throughout their range. However, there are very few estimates of mortality rates or abundance over time to evaluate the severity of the impact. In Australia, the major threats include bycatch in trawls and gillnets as well as in shark nets set at popular coastal beaches to protect bathers (Hale 1997, Paterson, 1990). A total of 15 Bottlenose Dolphins (species unknown) were confirmed to have died from net entanglement in southern blue-fin tuna feedlots between 1994 and 2000 (Kemper and Gibbs 2001). Research by Mintzer et al. (2018) into the global use of cetaceans as bait in fisheries reported that this practice is much more widespread within Taiwanese fisheries than is presently recorded. From 1974 to 1986, the Taiwanese shark and tuna gillnet fishery exploited the waters off northern Australia, including the Arafua and Timor Seas and was responsible for the incidental catch of a variety of small cetaceans, with Tursiops spp_._ likely aduncus comprising 60% of the total cetacean catch (Harwood & Hembree 1987). The annual mortality of Indo-Pacific Bottlenose Dolphins may have exceeded 2,000 animals—severely impacting local population[s] (Young and Iudicello, 2007). As a result of this bycatch, the fishery was closed in 1986 (Young and Iudicello, 2007).
Off western Kyushu, Japan, mortality of Indo-Pacific Bottlenose Dolphins in bottom-set gillnets was estimated at 12-14 individuals per year, which was considered unsustainable for the local population thought to number only somewhat more than 200 individuals (Shirakihara & Shirakihara 2012). Photo-identification data collected off the coast of Bangladesh indicated that 28.2% of identified individuals had injuries related to entanglement gear (Mansur et al. 2012), and similar methods used in the coastal waters of Mayotte (Comoros archipelago, Mozambique Channel) showed that 19% of identified individuals bore signs of non-lethal interactions with coastal artisanal fisheries (Kiszka _et al._2008). In Bangladesh, between August 2016 and April 2018, 31 small-scale gillnet fishing vessels participating in a Citizen Science Fishermen Safety Network reported fatal entanglements of 14 dolphins and porpoises, of which seven were Indo-Pacific Bottlenose Dolphins, during 177 fishing trips (Wildlife Conservation Society 2018). Off the island of Zanzibar (Tanzania), 43 Indo-Pacific Bottlenose Dolphin captures in artisanal drift gillnets were reported from 1995 to 1999. The magnitude and population-level impact of gillnet bycatch in Zanzibar was assessed in 2003-2004, and it was found that annual bycatch rates were unsustainable as they represented 9.3% of the estimated local population of Indo-Pacific Bottlenose Dolphins (Amir 2010). Bycatch was also evaluated recently in drift gillnets off Zanzibar using landing data (Temple _et al._2019), and there are reports of Indo-Pacific Bottlenose Dolphin mortality in coastal artisanal fisheries off mainland Tanzania, Kenya, and Mozambique (Kiszka 2015, Kiszka _et al._2009). Bycaught dolphins are often used as bait in longline fisheries targeting sharks (Mintzer et al. 2018, Braulik et al. 2017).
In KwaZulu-Natal, South Africa, an average of 34 Indo-Pacific Bottlenose Dolphins per year were accidentally killed by bather protection nets around beaches between 1981 and 1989 (Cockroft 1990). This increased to an average of 45/year from 1990-1999, and from 2000-2009 the estimate was 27/year (Cliff & Dudley 2011). In general, catches have steadily declined since the 1980’s due to better management of nets and the replacement of some shark nets (gillnets) with drumlines (baited hooks) in recent years (Plön, unpublished). However, the majority of bycaught animals are immature individuals and mature females, raising concern about population-level impacts (Plön, unpublished). In most areas limited data on bycatch or a lack of information on population size mean that it is not possible to determine whether annual catch rates are sustainable or not. However, the strong overlap in gillnet fishing effort and the coastal distribution of this species, combined with fairly small population sizes in many areas that have been surveyed, suggest that fishery bycatch is sufficiently high to cause population declines in many places.
The species’ near-shore distribution makes it vulnerable to environmental degradation, direct exploitation, and fishery conflicts (Curry and Smith 1997, Wells and Scott 1999; Reeves et al. 2003). Until hunting was outlawed in 1990, Tursiops aduncus was the target of a large-scale drive fishery in Taiwan’s Penghu Islands. Indo-Pacific Bottlenose Dolphins occur in southern Japan and due to confusion with the commonly hunted Common Bottlenose Dolphin it is not clear whether they were ever specifically targeted by Japanese coastal cetacean fisheries in the past (Kasuya 2017). Some Indo-Pacific Bottlenose Dolphins are taken in the small cetacean fisheries in Sri Lanka (Ilangakoon 1997) and Indonesia (Mustika 2006). Socio-ecological interview surveys in southwest Madagascar in 1999 indicated extensive hunting of coastal dolphins by Anakao fishermen of the Vezo community (Andrianarivelo, 2001). Dolphin species most often taken were the Spinner Dolphin, the Indo-Pacific Bottlenose Dolphin and Indian Ocean Humpback Dolphin. Vezo fishers hunted coastal dolphins for local consumption and sale of meat (Cerchio et al. 2015). Andrianarivelo (2001) estimated that over 6,000 individual dolphins, predominantly Stenella spp. and Tursiops spp. were taken in drive hunts between 1985 and 1999 from a single village, Anakao, and there was a substantial increase in catches evident during 1995–1999 accounting for 57% of all catches (Cerchio et al. 2009). The trend suggested an increase in intensity of hunting and impact on populations, likely associated with a change in hunting technique (from harpoons to nets) in the mid to late 1980s, and depletion of other food resources (e.g. decrease in fish populations), and subsequent increased adoption of cetacean hunting and consumption throughout the 1990s (Cerchio et al. 2015). Subsequent interview surveys conducted between 2010-2013 demonstrated continuing high levels of both hunting and by-catch reported for Tursiops spp. along the entire west coast of Madagascar (Cerchio et al. 2015). Indo-Pacific Bottlenose Dolphins were one of the most common species targeted by a drive-hunt in Zanzibar that stopped in 1996 (Berggren et al. 2007, Stensland and Berggren 2007). Anecdotal evidence suggests that several species of delphinids are hunted in the Seychelles (despite national prohibition), including Indo-Pacific Bottlenose Dolphins (Kiszka 2015). Although illegal, Indo-Pacific Bottlenose Dolphins were recorded as hunted in a harpoon fishery in southern Myanmar with six carcasses documented as being sold in March 2006 at the Maungmagan market (Tint Tun 2006). This species is one of the more common small cetaceans that are used as bait to catch sharks. There is no current evidence of traditional use/trade for consumption or medicinal use across the species range in Australia although it may have happened in the past.
Indo-Pacific Bottlenose Dolphins are among the cetacean species most commonly kept in captivity in Asia (Wang et al. 1999, Reeves et al. 2003). Since the early 1960s, live-captures of T. aduncus for oceanarium display have occurred in many areas, including South Africa, Australia, Taiwan, Japan and Java (Best and Ross 1984, Tas’an and Leatherwood 1984, Reeves et al. 1994, Cawthorn and Gaskin 1984, Kasuya et al. 1984). In 1974, 58 Indo-Pacific bottlenose dolphins were captured from Amami Oshima, Japan, for the Okinawa Expo and the aquarium which held them became the Churaumi Aquarium in Okinawa (Kasuya et al. 1984). In 2003, a live-capture export trade was initiated in the Solomon Islands and a minimum of 108 Indo-Pacific Bottlenose Dolphins were exported between 2003 and 2011 to Mexico, UAE, China, the Philippines and Singapore (Oremus et al. 2013; Parsons et al. 2010). This level of removals from the local population(s) is probably unsustainable (Reeves & Brownell 2009). In the mid 2000’s in Australia, 32 Bottlenose Dolphins, including both T. aduncus and T. truncatus, were held at Sea World, Surfers Paradise, Queensland, and four Tursiops aduncus were kept at Pet Porpoise Pool, Coffs Harbour, New South Wales (International Species Information System 2009). In China in 2014, 39 captive facilities housed 491 cetaceans, most of them Tursiops spp. most from Japan and not T. aduncus (Chinese Cetacean Alliance 2015). Twenty-three Indo-Pacific Bottlenose Dolphins are held at Ocean Park in Hong Kong, and one in Sea World, Durban (www.Species360.org, reported March-2019).
In South Africa, of the six dolphin species (n = 90) incidentally captured in shark net installations or stranded off the east and south coasts between 2005 and 2009, Indo-Pacific Bottlenose Dolphins had the second highest levels of persistent organochlorines and approximately half of adult T. aduncus had PCB concentrations above the previously determined threshold (17,000 ng g−1 lw) for impairment of immune functions in aquatic mammals (Kannan et al. 2000; Gui et al. 2016); however it is important to note that other small cetaceans often report levels much higher than this. Despite this, systematic health assessments conducted on animals incidentally caught in shark nets between 2010 and 2012 indicated that Indo-Pacific Bottlenose Dolphins were generally in good health (Lane et al., 2014). In Zanzibar, levels of organochlorine pesticides in Indo-Pacific Bottlenose Dolphins were lower than those reported from other regions, but methoxylated polybrominated diphenyl ether levels were higher (Mwevura et al. 2010). Along the coast of La Réunion, DDTs, PCBs, and methoxylated-PBDEs were commonly found in blubber tissues of Indo-Pacific Bottlenose Dolphins (Dirtu _et al._2016). However, concentrations were low compared to other locations, including Zanzibar (Mwevura et al. 2010).
Several diseases appear to be increasing in occurrence in coastal small cetaceans, raising concern for the health of these animals. Since the early 2000s, there have been several dolphin mortality events in Australia that have included T. aduncus (Kemper et al. 2016). In these events all the dolphins tested positive for the cetacean morbillivirus. There are reports of a lobomycosis-like disease and other poor skin conditions in Indo-Pacific Bottlenose Dolphins from Mayotte and Japan which is a concern for these locally restricted populations (Kiszka et al. 2009, Van Bressem et al. 2013, Tajima et al. 2015). The etiology of the disease was not established, but it may be related to the degradation of their coastal habitat (Kiszka et al. 2009). The bacterial pathogen of the genus Brucella has also been reported from Indo-Pacific Bottlenose Dolphins. Forty of 58 serum samples from T. aduncus from the Solomon Islands tested positive for antibodies to Brucella (Tachibana et al. 2006). This pathogen in dolphins can be a severe disease and cause abortions, male infertility, neurobrucellosis, cardiopathies, bone and skin lesions and death (Moreno et al. 2012).
Climate change, especially increases in sea surface temperature that are prevalent throughout coastal areas of the Indian Ocean, are negatively affecting coastal habitats such as coral reefs and seagrass beds used by Indo-Pacific Bottlenose Dolphins. For example, in Shark Bay, Western Australia, an unprecedented marine heatwave in 2011 caused catastrophic losses of seagrass meadows along with mass mortality in invertebrate and fish communities. Long-term demographic data on Shark Bay’s resident Indo-Pacific Bottlenose Dolphin population revealed a significant decline in female reproductive rates following the heatwave (Wild et al. 2019).
Indo-Pacific Bottlenose Dolphins are among the species most commonly targeted by dolphin-watching operators in the Indo-Pacific region and in some places where regulation is poor, the animals are harassed. Although dolphin-watching tourism is certainly not a threat to the species as a whole, changes in dolphin behavior have been observed in areas with intense dolphin-watching activity off Amakusa-Shimoshima Island, Japan (Matsuda et al. 2011) and in Menai Bay, Zanzibar (Christiansen et al. 2010) and around Mauritius (Webster et al. 2014). In Shark Bay, Western Australia, an increase in dolphin tourism coincided with a 14.9% decline in local dolphin abundance (95% CI=−20.8 to −8.23) (Bejder et al. 2006).
The disturbance caused by marine construction and demolition, port development, reclamation, dredging, and other forms of habitat destruction and degradation, together with associated anthropogenic noise, is hard to quantify, but such activities and processes are widespread throughout the coastal range of this species. Individually and cumulatively, they may be causing population declines through increased mortality, impaired health, and/or reduced reproductive fitness as well as by habitat displacement.
