Marine mammals – Dugong, Whales and Dolphins

There are approximately 36 species of whales and dolphins in the WIO region, of which 8 are baleen whales, 2 to 3 sperm whales, 13 toothed whales and 13 dolphins. Dugong are reduced to scattered remnant populations, probably totaling no more than 500 animals in the WIO, of which over 300 are known to exist in the Bazaruto Archipelago.
Information on cetaceans in the WIO is limited, and among the main sources has been reviews conducted to determine the effectiveness of the Indian Ocean Whale Sanctuary (IOS) reviews consider the status of populations, and trends in threats to cetacean stocks. Dugong information is patchy throughout the region, with the most recent research and monitoring of Dugong in the Bazaruto Archipelago strongly showing signs of over 300 individuals. 
Species known to have reduced in numbers in the WIO include the dugong (dugong dugon) blue whales (Balaenoptera musculus), fin whales (B. physalus), sei whales (B. borealis) and humpback whales (Megaptera novaeangliae). The severe depletion of almost all stocks of ‘great whales’ in the Southern Hemisphere is well documented.

Dugong (Dugong dugon) - Dugongs range in near shore tropical and subtropical coastal and island waters of the Indo-Pacific between southern Mozambique in the west and Vanuatu to Japan in the east, between latitudes approximately 27o north and south, in waters warmer than about 18°C. Population estimates have been made in three areas, Australia; the eastern Red Sea; and the Arabian Gulf. While the status of the dugong over a large proportion of its range remains poorly known, their populations are significantly depleted in most locations, and believed to be declining in over two-thirds of their range, and locally extinct in some locations.
Historically, WIO dugong distribution extended from Somalia to Mozambique and northern South Africa, and on the islands of the Comoros, Seychelles, Madagascar and Mauritius. Throughout this range, dugong populations have suffered a steep decline since the 1960s. Although dugongs are protected across all the WIO states, enforcement and consequent protection is strongly limited by capacity and resources.
Historically, WIO dugong distribution extended from Somalia to Mozambique, and on the islands of the Comoros, Seychelles, Madagascar and Mauritius. Throughout this range, dugong populations have suffered a steep decline since the 1960s. Although dugongs are protected across the range of all the WIO states, enforcement and consequent protection is currently limited by capacity and resources.
Dugong in Mozambique: - Surveys in the late 1960s suggested that dugongs were common along the Mozambique coastline from Maputo Bay, Chidenguele, Inhambane Bay, Bazaruto Bay, Mozambique Island and Pemba Bay and the Quirimbas Archipelago. Dugong herd sizes of 8 - 10 individuals were reported for Inhaca Island in 1992, although this area is now thought to support only 2 or 3 individuals. While dugong were observed throughout Inhambane Bay in October 1994, no animals were recorded during a survey of the bay in 2007. The Bazaruto Archipelago supports the largest dugong population in the WIO, and although there were suggestions that the population was declining, recent surveys suggest about 300 individuals. Only one dugong was recorded in aerial surveys of the Quirimbas archipelago of northern Mozambique in 2007, and an incidental sighting of a lone individual was made in 2009.

Humpback whales (Megaptera novaeangliae) - Humpback whales are widely distributed throughout WIO with distinct population structure that is still under study. Four subpopulations are suggested by the IWC: 1) on the East coast of Africa from South Africa to Kenya. This is a large substock, likely >6000 animals; 2) in the Comoro Islands, a smaller aggregation, with Mayotte having a high sighting rate of mothers with calves and potential high residency, and a late season peak; 3) around Madagascar, likely the largest substock with 7000-8000 animals; and 4) the Mascarene and Seychelles Islands, a smaller aggregation, possibly a recent range expansion following recovery of numbers since protection from whaling. Current genetic evidence suggests the above may form 2 genetic substocks, with the islands (Comoros, Madagascar, Mascarenes, Seychelles) experiencing significant mixing. It may be that the Comorian stock represents a connection to the East African one.
Migration of humpbacks is primarily along north-south routes, with populations from the southern Ocean migrating along the East African coast, through the Mozambique channel and up the Madagascar Plateau. A major wintering concentration has been identified in Antongil Bay in NE Madagascar. Key areas for humpback observations, sightings above water and for hearing whalesong underwater include, among other locations, Antongil Bay (NE Madagascar), Tofo (S Mozambique), Nacala/Quirimbas (N Mozambique), Reunion and Malindi (Kenya). Recent work suggests a major wintering ground for humpbacks off Bazaruto, Mozambique, and significant concentrations off Zanzibar.
Boat-based surveys of the waters surrounding Mayotte have been conducted in the austral winters of 1995-2002. A total of 102 individuals have been identified. Mother and calf pairs make up a large number of the total observed, and have been seen in the region as late as November. They make extended stays in the region and have been re-sighted over a two-week period. Very little behaviour associated with mating activity (competitive groups and singing) has been observed in this area. Given the high percentage of mother/calf pairs and low frequency of mating activity, the waters of Mayotte may serve as a critical wintering habitat or resting point along the migration route for mothers and calves. Some photographic matches have been made with other humpback whale wintering areas in Madagascar, providing some evidence of migratory links.

Blue whales (Balaenoptera musculus) - Blue whales are poorly documented throughout the WIO, but have a high conservation importance globally, as their numbers are estimated to be at 1-2% pre-whaling abundance in the Southern Hemisphere. Two sub-species are recognized, and the one in the WIO is the pygmy blue whale B. m. brevicauda.  These are likely restricted to waters north of about 55°S, and likely breed in tropical waters, though very little is known about distribution. The other subspecies is the Antarctic blue whale B. m. intermedia, whichsummers in the Antarctic but has unknown wintering/breeding areas. Based upon vocalization types (song “dialects”), there appear to be at least 3 populations of pygmy blue whales in the Indian Ocean – a Madagascar call type, a Sri Lanka call type, and an Eastern IO/Western Australia call type.  The Antarctic blue has a single distinctive call type.
Blue whale sightings have primarily been in the Mozambique Channel, and off the SW and SE coasts of Madagascar, and on the Madagascar Plateau. Approx 450 pygmy blue whales are estimated for the south of Madagascar on the Madagascar Plateau. Based on acoustic observations, there is some evidence for range overlap of the different call type populations, with the Madagascar Plateau having only the Madagascar call type, the northern tip of Madagascar having both Antarctic and Sri Lanka call types, and the Crozet Islands having Madagascar and Antarctic call types. Historic data from whaling suggests there is a pygmy population NW of the Seychelles (offshore Kenya/Somalia), but its current status is unknown and possibility for migratory connections with southern populations is similarly unknown.

Southern Right Whale (Eubalaena australis) - The primary distribution for this species is outside of the WIO, on the southwest coast of South Africa, but recent sightings have included southern Mozambique, and around Madagascar on both the east and west coasts, in Antongil Bay, off Fort Dauphin, Toliara/Anakao, and Andavadoaka. The current population is estaimate at 1,500 – 4,000 individuals. The population is undergoing recovery, and the sporadic WIO sightings may indicate expansion back into a prior range, and/or may represent remnants of a more widely distributed population that was extirpated.

Beaked whales - Recent aerial surveys are providing new information on this previously little known group. The Mozambique Channel emerges as a very important high diversity hotspot, particularly off the mid-west coast of Madagascar. High diversity is also found in SW Madagascar and around the Comoro Islands extending east to NW Madagascar in off-shelf waters.  These surveys did not include the mainland coast of Africa, but many species are also reported along the mainland coast.

Coastal Dolphins - The 13 dolphin species known from the WIO are widespread, though four coastal specIes are of primary interest for conservation:
Indo-Pacific Humpback Dolphin (Sousa chinensis) is widely distributed along the west coast Madagascar, and eastern African Mainland from South Africa to Tanzania, and in northern Kenya. It is typically in small group sizes, and may form mixed species groups with bottlenose and other dolphins. The species prefers sheltered shallow (< 30 m) coastal waters, and is not found on Madagascar east coast sites such as Antongil Bay and Fort Dauphin. In Madagascar, it is well documented only in Toliara/Anakao, Mahajunga, Loza Bay, Nosy Be region, Mahjunga and Nosy Mitsio. It is extremely vulnerable to human activities.
Indo-Pacific Bottlenose Dolphin (Tursiops aduncus) is widely distributed and common throughout the region. The species is well documented in E and W Madagascar, as well as in Tanzania, Kenya, South Africa, Mozambique, Mayotte, Reunion and Mauritius. It is typically in near-shore, shallow waters, but can also be found in deeper coastal waters. In most locations it is not hunted, except on the west coast of Madagascar, but is vulnerable to human activities as there is a significant bycatch in coastal aritsanal fisheries.
Spinner dolphin (Stenella longirostris) is widely distributed and common throughout WIO. While it is also typically found in near-shore, shallow waters, it can also frequent deeper coastal waters and may exhibit a diel pattern of close near-shore, shallow water distribution during daylight (resting phase), and offshore, deep water distribution during nighttime (feeding phase). Unlike the other two species, group sizes can be large, in the 100s.
Spotted dolphin (Stenella attenuata) are most abundant than than other species of dolphin, in surveys conducted off western Madagascar and Mozambique, and also abundant in aerials surveys conducted in Kenya.
All four dolphin species have high conservation value as their coastal ranges, life history and habits make them vulnerable to human activities. This also gives them good valuable as an indicator species of general human impacts in the coastal marine environment. While they are typically not hunted, dolphin hunting is a threat on the southwest and west coast of Madagascar.
An interesting aspect regarding cetaceans in the WIO is that there are a number of similarities between the cetaceans in the WIO, the eastern tropical Pacific and the Gulf of Mexico. Firstly, the same species were common or rare, regardless of ocean. Second, these differences in abundance were due primarily to differences in encounter rate, and less to school size. Third, regardless of ocean, three species comprised the majority of the cetaceans encountered, Stenella attenuata, S. longirostris, and S. coeruleoalba.

Threats - The near-shore zone throughout the WIO is highly productive and subject to high levels of artisanal exploitation and development, especially in nearshore habitats such as seagrass and coral reefs. Dugongs are entirely dependent on such seagrass and reef habitats. An increase in large mesh gill netting from the 1970s onwards, along with a lack of law enforcement, seine netting, commercial trawl operations, palisade fish traps, habitat destruction of seagrass beds and increased anthropogenic disturbance all contribute as threats to dugong populations in Mozambique, and in the WIO.
The greatest threats known to whales and dolphins (similar to the Dugong) include fishing net entanglement either directly (whaling) or indirectly causing death through drowning. Chemical pollution (heavy metals, pesticides and other toxins) can cause direct harm to the animals by accumulation in their tissue, via ingestion of contaminated prey. Deep water beaked whales and delphinids are known to be sensitive to acoustic disturbance in areas with rapidly expanding exploration for offshore petroleum (including seismic surveys and bathymetric mapping, implicated in disturbance and strandings in other regions). A key area is NW Madagascar where there are active offshore petroleum concessions, high species diversity including deep-water species, and a recent mass stranding of offshore dolphin (melon headed whale, Peponocephala electra) that was coincident with petroleum exploration activities. There was also a mass stranding of common bottlenose dolphins off Bazaruto  in October 2007, suspiciously temporally close to some offshore seismic surveys off Mozambique!

Conservation of marine mammals - The Bazaruto Archipelago is an existing MPA, and has the largest and possibly last viable dugong population in the Eastern African region. The survival of the dugong ultimately depends on the maintenance of adequate habitat, notably the sea grass beds.
Amongst the efforts of the International Whaling Commission (IWC) to facilitate the recovery of the great whales was the establishment of the Indian Ocean Sanctuary (IOS) in 1979. This Sanctuary consists of those waters of the Northern Hemisphere from the coast of Africa to 100°E (including the Red and Arabian Seas and the Gulf of Oman) and those waters of the Southern Hemisphere between 20°E and 130°E from the equator to 55°S. The Sanctuary offers protection from commercial whaling to the great whales. The IOS is generally established for limited tim periods, and has been extended continuously to date, in 1989, 1992 and most reently in 2002.
There is critical need to investigate the status of dolphin populations and threats to them in the countries bordering the Indian Ocean. More research emphasis should in future be placed on investigating by-catch and the possible overfishing of delphinid prey stocks, and on abundance and distribution particularly for offshore cetacean species. Within South Africa and Mozambique, there is limited evidence that the IOS has played a role in stimulating research on cetaceans to date. In Zanzibar, research on dolphins based on mortality as fisheries bycatch has revealed information on diet and population structure. Whale or dolphin watching, if developed carefully, could bring much needed income to developing countries. This has recently been developed in a number of countries including South Africa, Kenya, Seychelles and Mauritius.

Key References - Best et al. (1996); Branch et al. (2007); Cerchio et al. (2008); Cerchio et al. (2009); Cherfas 1989; Clark & Lamberson 1982; Cockcroft (1995); Cockcroft & Krohn (1994); Cockcroft & Young (1998); Cockcroft et al. (1994); Dulau-Drouot et al. (2011); Findlay et al. (1998); Kiszka (2010); Kiszka et al. (2007); Kiszka et al. (2008); Kiszka et al. (2010); Laws (1985); Leatherwood & Donovan 1991; Leatherwood et al. (1984); Marsh & Lefebvre (1994); Marsh et al. (2001); Peddemors (1999); REMMOA (unpublished); Rosenbaum et al. (1997); Sirenews (2001). --> References