Madagascar Ridge IMMA

Criterion A – Species or Population Vulnerability

The Madagascar Ridge area is an area used by pygmy blue whales (B. m. brevicauda) for feeding during the austral summer (Kasuya and Wada 1991, Best et al. 2003, Branch et al. 2007), sperm whales (Physeter macrocephalus) and a migratory corridor for southern right whales and humpback whales (Best 2007, Fossette et al. 2014). Surveys conducted in the region by Gambell et al. (1975) suggest that sperm whale is the most abundant large cetacean in the Madagascar Ridge area.

Criterion B: Distribution and Abundance

Sub-criterion B2: Aggregations

This region is an important habitat for blue whales, based on catch and sighting data (Branch et al. 2007), primarily pygmy blue whales. The sperm whale is the most abundant large cetacean species in the Madagascar Ridge area (Gambell et al. 1975). This region may have the highest densities of sperm whales in the southwestern Indian Ocean (Gambell et al. 1975). A compilation of available sighting records of blue and sperm whales throughout the western Indian Ocean (1972-2017) suggest that the Madagascar Ridge is one of the most critical habitats for these two species in the region (Tetley, Kiszka and Hoyt, unpublished data, Figure 3).

Criterion C: Key Life Cycle Activities

Sub-criterion C2: Feeding Areas

The Madagascar Ridge area is important for feeding pygmy blue whales (B. m. brevicauda) during the austral summer (Kasuya and Wada 1991, Best et al. 2003, Branch et al. 2007). Best et al. 2003 estimated blue whale abundance as 424-472 in December 1996 from the southwest tip of Madagascar and along the Madagascar Ridge.  Under a scientific research permit, Japanese whalers took large numbers of Bryde’s whales (>100) in the vicinity of the Madagascar ridge during 1976-1979, with detailed description of stomach contents indicating feeding (Kawamura 1980, Ohsumi 1980). These whales were described as Balaenoptera edeni brydei, which is distinct from coastal South African and southeastern Atlantic populations (Best 2001).  It could be used as a foraging ground for a large number of other species, but we currently do not have supporting evidence.

Sub-criterion C3: Migration Routes

The region is a migratory corridor for humpback whales (Best et al. 1998, Fossette et al. 2014, Cerchio et al. 2016) and southern right whales (Best 2007). The southern Madagascar shelf is important breeding habitat for Megaptera novaeangliae, as satellite telemetry data indicated extensive use of this habitat during the height of the breeding season (Cerchio et al. 2016; Trudelle et al. 2016; Dulau et al. 2017). Blue whales, male sperm whales and other species of baleen whales such as fin, Bryde’s and Antarctic minke whales also use this region as a migratory corridor (Gambell et al. 1975, Kasuya and Wada 1991, Best 2007).  Kasuya and Wada (1996) report B. physalus sightings immediately south of Madagascar (25°-30°S, 45°-55°E) during December, likely on southern migration, but well south of 30°S from January to April in presumed feeding habitat.

Criterion D: Special Attributes

Sub-criterion D2: Diversity

A total of 23 species of cetaceans have been recorded in the Madagascar Ridge area, including 9 species of baleen whales and 14 species of toothed cetaceans (Cerchio et al 2022).  These include Humpback whale (Megaptera novaengliae), Southern right whale (Eubalaena australis), Antarctic minke whale (Balaenoptera bonaerensis), Pygmy right whale (Caperea marginata), Cuvier’s beaked whale (Ziphius cavirostris), Arnoux’s beaked whale (Berardius arnouxii), Gray’s beaked whale (Mesoplodon grayi), Strap-toothed whale (Mesoplodon layardii), Killer whale (Orcinus orca), False killer whale (Pseudorca crassidens), Common dolphin (Delphinus delphis), Pantropical spotted dolphin (Stenella attenuata), Rough-toothed dolphin (Steno bredanensis), Striped dolphin (Stenella coeruleoalba), Risso’s dolphin (Grampus griseus), Common bottlenose dolphin (Tursiops truncatus), Fraser’s dolphin (Lagenodelphis hosei), Pygmy killer whale (Feresa attenuata) (Best et al. 1998, Fossette et al. 2014, Cerchio et al. 2016, Gambell et al. 1975, Kasuya and Wada 1991).