The habitat preference of Humpback Dolphins for shallow waters places them in some of the world’s most intensively utilized, fished, shipped, modified and polluted waters. The primary threat to the Indian Ocean Humpback Dolphin throughout most, or all, of its range, is incidental mortality in fisheries (‘bycatch’), including in gillnets and in shark control nets in South Africa. The bycatch threat likely emerged in the early part of the 20th century as the use of gillnets in global marine fisheries started to expand dramatically. Global fish production increased at 3.2% per year from 1961–2009, and the increase in global landings began several decades earlier (FAO 2012). Marine mammal bycatch likely escalated with the gradual introduction of monofilament netting in the 1960s and 1970s (Klust 1973, Potter and Pawson 1991, Tarbit 1984). There are no known areas that offer a refuge for this species from the pervasive threat of incidental mortality in fisheries. Although it is impossible to evaluate the magnitude in most areas, in all areas where it has been evaluated, the rate of incidental mortality of this species in fisheries appears to be high, unsustainable and causing rapid local population declines.
With the exception of the KwaZulu-Natal shark control program, there have been no on-board observer studies from which bycatch estimates could be generated. In virtually all countries around the Indian Ocean, fisheries, and particularly those that operate in the near-shore areas where these dolphins occur, are small-scale and artisanal, and involve small boats or canoes, that are frequently oar- or sail-powered. The fishing effort is concentrated within the preferred near-shore habitat of Humpback Dolphins. For example in Pemba in Tanzania approximately 90% of humpback dolphins sighted during recent surveys occurred less than 1 km from shore, and 95% of recorded fishing vessels occurred in exactly the same habitat. Of 27 photo-identified humpback dolphins in the same area, 41% had clear injuries from previous entanglements in fishing nets (Braulik, unpub. data). This demonstrates the degree to which most humpback dolphins must frequently encounter fishing gear and there is every reason to believe that this example from Pemba is the norm throughout the species’ range.
Substantial incidental mortality of humpback dolphins in coastal gillnets is reported from most countries within their range (IWC 2002). The best-studied bycatch is for the KwaZulu-Natal province in South Africa, where 203 Humpback Dolphins (50% in Richard’s Bay) were captured in shark nets in the 30 years between 1980 and 2009 (Atkins et al. 2013). This corresponds to a 4.2% annual mortality rate (Taylor supplemental spreadsheet “Sousa plumbea Natal.xlsx”) assuming the population is potentially able to grow at 3% per year (Moore 2015). A high proportion (8%) of 109 distinctive, catalogued individuals were recognized among the animals bycaught in shark nets between 1998 and 2006 and while individuals naturally emigrate from the Richards Bay area, this points to mortality in shark nets as a driver of the permanent loss of individuals from the area which may also be negatively affecting the wider population (Atkins et al. 2016). The annual rate of decline, assuming that the population is growing at a maximum potential rate, would be 1.3% resulting in a 63% decline in three generations.
The reported mortality rate due to fisheries interactions (both bycatch and directed take) in the southwest region of Madagascar was noted to be almost certainly unsustainable (Cerchio et al. 2015). In Mozambique, intense coastal fishing effort is considered to be the main threat to Humpback Dolphins, and shark fishery gill nets and trawl nets have incidentally killed dolphins throughout the country (Guissamulo 2008).
Incidental catch of humpback dolphins has been documented in gillnets off the south coast of Zanzibar in Tanzania (Amir et al. 2002). There are currently no estimates of the magnitude of the bycatch but given that even two humpback dolphins taken per year would exceed 2% of the population, it is very likely that bycatch represents a threat to the dolphins in the area (Stensland et al. 2006) and is one of the main factors responsible for the large observed decline in abundance (Sharpe et al. 2019).
In Oman, as in India, the high incidence of stranded humpback dolphins was presumed to be the result of interactions with fisheries (Collins et al. 2002, Sutaria unpub. data). In addition, in India, dolphins are known to tear fishing gear leading to economic losses to fishers and risk to the dolphins. In some areas, including in Sindhudurg, Maharashtra, the Gulf of Mannar, and Tamil Nadu in India, retaliatory measures like intentional killing take place. This is a frequent occurrence and a cause of concern (Jog et al. 2017, Muralidharan 2018). The presence of pieces of fishing gear in dolphin gut contents was observed in seven necropsies conducted in India between 2020 and 2021; and at least 20 strandings between 2016–2021 indicated evidence of interactions with fishing gear (Marine Mammal Research and Conservation Network of India October 2022).
There are very few areas within the known range of Indian Ocean Humpback Dolphins where anthropogenic alteration of habitat has not occurred. The destruction of inshore habitats is likely to be one of the greatest threats to humpback dolphins, particularly in the southern Africa region (Atkins et al. 2015, Plön et al. 2016) and in the Arabian/Persian Gulf and many other rapidly developing urban coastal areas (Karczmarski 2000, Baldwin et al. 2004). Dredging, land reclamation, construction blasting, port and harbour construction, pollution, boat traffic, oil and gas exploration and development (including inshore seismic surveys), and other coastal development activities occur, or are concentrated within, humpback dolphin habitat and threaten their survival in ways that are challenging to quantify but likely to be impactful (IWC 2002). Development of both large and small ports and the associated manufacturing and processing is expanding incredibly rapidly around the Indian Ocean rim, and the areas often selected for ports are estuaries and protected bays that are the preferred habitat of humpback dolphins (Sutaria et al. 2015). The continued presence of Humpback Dolphins in degraded habitats does not rule out that habitat degradation has had adverse behavioural or health effects (IWC 2002, Piwetz et al. 2015). This is a pervasive threat that is increasing throughout the species’ range and there is no reason to expect this trend to change in the foreseeable future.
In comparison to other marine mammals with wider and more oceanic ranges, the exposure of Indian Ocean Humpback Dolphins to environmental contaminants and their bioaccumulation is likely to be very high (Jefferson and Karczmarski 2001). Many of the large urban centres and ports around the Indian Ocean, including Mumbai, Karachi, Dubai, Aden, Mombasa, Mogadishu, Dar es Salaam, Maputo and Durban, release a toxic cocktail of untreated human and industrial waste into coastal waters inhabited by Humpback Dolphins. Of six dolphin species in South Africa, Sousa plumbea was found to be the most contaminated by total Persistent Organic Pollutants (POPs) and concentrations of DDTs in this species were among the highest levels reported in delphinids globally, above the likely effect threshold for impairment of immune function (Gui et al. 2016). Flame retardants, PBDEs and dechloranes were also found in levels as high as other industrialized countries (Aznar-Alemany et al. 2019). Elsewhere the levels and impacts of such pollution on Humpback Dolphins have yet to be studied but may be severe (Gore et al. 2012). For example, in Pakistan very high levels of chemical pollution in creeks of the Indus delta are toxic enough to cause fish kills and are likely to have adverse effects on cetaceans (Kiani and Van Waerebeek 2015). In Gadani, Pakistan up to 100 ships per year are scrapped and dismantled leading to the release of large amounts of heavy metals, asbestos, dioxins and other persistent organic pollutants in coastal water. These problems are widespread and increasing in many countries; their impact on coastal dolphins has not been evaluated but pollution is likely contributing to local declines in range and abundance and may have caused extirpation adjacent to major industrial centres.
Other threats that appear somewhat less serious (possibly because of lack of information) or only affect a certain portion of the species’ range include direct killing, boat traffic/harassment, underwater noise, and oil spills and exploration (IWC 2002, Piwetz et al. 2015). There is little evidence for intense hunting of humpback dolphins except for those in Madagascar (IWC 2002); in the southwest of that country, dolphins were historically taken with harpoons but are now targeted with gillnets or in a drive hunt. Andrianarivelo (2001) estimated a minimum of 61 deaths of S. plumbea between 1985 and 1999 in Anakao related to directed takes including drive hunts (Cerchio et al. 2015). Given the relatively small population sizes reported throughout the region, the reported mortality rate due to hunting is likely unsustainable and contributing to local population declines (Cerchio et al. 2015). Dolphin hunting used to occur in Menai Bay in Zanzibar; the last hunt there occurred in 1996, taking 23 individuals, assumed to be S. plumbea and Tursiops aduncus. This would represent annual mortality of close to 12% for a combined population estimate of around 200 animals for the two dolphin species in the area. The removal rate was certainly unsustainable, resulting in a negative impact on the dolphin populations off the south coast of Zanzibar (Stensland et al. 2006). Humpback dolphins were hunted in former years in the Arabian Gulf, and there was some evidence of continued dolphin hunting from small, motorized boats using harpoons until the early 2000s in Oman (Baldwin et al. 2004), but this is now thought to have ceased. In Maputo Bay, Mozambique dolphins were also reported to be hunted for meat in intertidal shallows and estuaries (Guissamulo 2008).
Blast fishing using dynamite or other explosives is a considerable threat to humpback dolphins in the few countries in which it occurs frequently, including Tanzania (Cagua et al. 2014, Braulik et al. 2017b) and Sri Lanka (Cornelis et al. 2008). This illegal activity is concentrated in near-shore areas that are also specifically humpback dolphin habitat.
In South Africa, where the Indian Ocean Humpback Dolphin has been most extensively studied, it was concluded that there no single cause for the rapid decline could be identified, and that the cumulative effects of multiple stressors, which are difficult to pinpoint and mitigate, are responsible for the decline in numbers (Plön et al. 2021).
