Central Mozambique Channel and Western Madagascar IMMA

Criterion A – Species or Population Vulnerability

This region is a breeding and feeding ground for several threatened marine mammal species, particularly vulnerable sperm whales and the endangered Indian Ocean humpback dolphin (Cerchio et al. 2012b, 2015), as well as a migratory corridor for endangered blue whales (Best 2007, Branch et al. 2007, Cerchio et al, 2018). Passive acoustic monitoring suggests that pygmy blue and Antarctic blue whales occur in the central Mozambique Channel from April to July (Siret 2016).  Aerial survey data collected during summer 2009-2010 estimated a density of 0.03 sperm whales per 100 km2 (Laran et al. 2017).   Dugongs listed as Vulnerable on the red list and Critically Endangered in East Africa occurred here historically and may also occur occasionally in coastal areas (Cerchio et al. 2012a, Davis et al. 2022).  False killer whales, listed as Near Threatened on the Red List also occur regularly.

Criterion B: Distribution and Abundance

Sub-criterion B2: Aggregations

Aerial survey data collected at numerous locations across the south west Indian Ocean during summer 2009-2010 recorded the highest densities of cetaceans in the entire region in this IMMA (Laran et al., 2017).  Including a correction for availability bias, densities of all cetaceans combined peaked at 1 individual per km² in the central part of the IMMA (Laran et al., 2017).  Because much of the data from aerial surveys does not allow for individual species identification, although this area clearly is important habitat that supports exceptionally high cetacean densities the data is lacking sufficient detail for this criteria to be satisfied for most individual species.  However, the most abundant species recorded during aerial surveys were delphinids (mostly T. truncatus and, to a lesser extent, T. aduncus, lumped as Tursiops sp.) with a relative density of 0.36 individuals km-² (CV: 23%, availability bias corrected), primarily in deep oceanic waters and over the shelf (Laran et al. 2017a). Dorémus et al. (2009) also noted large groups of T. truncatus during vessel-based surveys offshore, near to and south of Juan de Nova, and this there is evidence that this IMMA contains habitat important for supporting concentrations of Tursiops truncatus and Tursiops aduncus. Small delphinids (likely Stenella sp.) were estimated at a density of 0.26 (CV: 27%, availability bias corrected), while all large delphinids (subfamily Globicephalinae) represent about 0.35 individuals.km-², availability bias corrected).

Criterion C: Key Life Cycle Activities

Sub-criterion C2: Feeding Areas

The high densities of cetaceans, particularly both species of Tursiops that occur in this IMMA are likely to be present because the habitat, including the eddies, slope, and oceanic islands create productive habitat supporting abundant prey for these species.

Sub-criterion C3: Migration Routes

The region is a known migratory corridor for humpback whales belonging to the Breeding stock C3 and C2 (Rosenbaum & Choux, 2022) during the austral winter. Among eight tagged whales in the Comoros, five travelled through or stopped along the west coast of Madagascar, within the IMMA (Fossette et al. 2014). A boat-based line-transect survey between Toliara and Mahajanga during Sept-Oct 2010 recorded 40 humpback whale sightings for an estimated density of 0.017 individuals km-2 (95% CI: 0.009 – 0.032) and estimated abundance of 554 individuals (CV: 31.8%); these sightings were primarily south of the IMMA, but it was assumed that these were late-season southbound migrants (Cerchio et al. 2012b).  It is also a migratory corridor for blue whales and possibly other baleen whales (Branch et al. 2007). Passive acoustic monitoring suggest that pygmy blue and Antarctic blue whales occur in the central Mozambique Channel from April to July (Siret 2016).

Criterion D: Special Attributes

Sub-criterion D2: Diversity

A high diversity of cetaceans is found in the Malagasy waters of the central Mozambique Channel with at least 20 species documented for north and south Mozambique waters, including six Mysticetes (Cerchio et al., 2022).  A total of 13 different species were encountered during about 10 000km of aerial transects during the REMMOA survey conducted between January to the beginning of February 2010 (Figure 3, Laran et al. 2017). The cetacean species richness predicted from occupancy analysis of marine mammals shows the highest value in the entire southwest Indian Ocean was for the waters inside this IMMA. In addition to small delphinids (Stenella sp. and Tursiops sp.), larger delphinids such as melon-headed whales, beaked whales (Cuvier’s and Mesoplodon beaked whales), short-finned pilot whales, false killer whales, sperm whales and Risso’s are also abundant in the area (Van Canneyt et al. 2010, Laran et al. 2017).

Dugongs were common in the Barren islands in the past, but based upon socio-ecological interviews with local fishers in the region its thought this species has most likely been extirpated due to hunting and bycatch (Cerchio et al. 2012a). At the time of writing no dugong sightings were recently reported in the area (Davis et al, 2022), dugongs were reported by aerial survey REMMOA (Van Canneyt et al., 2010; Laran et al., 2017) north of the IMMA, and there are sightings reported by fishers less than 300 km to south of the IMMA (Davis et al., 2022) so it is possible dugongs occasionally occur.

There are a few sightings of Indian Ocean humpback dolphins that have been documented along this coast, along with a high incidence of reports in interviews with fishers in the Barren Islands and on the coast at its south border (Cerchio et al. 2012b, 2015, Laran et al. 2017), and the IMMA includes significant areas of potential humpback dolphin habitat; it is likely that this species occurs regularly in the IMMA.