Boundaries

A diversity of freshwater habitats occur in northwestern Madagascar, ranging from large rivers running through deciduous forest, to steep and swift-flowing rivers draining the mountains, and to floodplain lakes. This ecoregion includes all westward flowing drainages from the northern tip of Madagascar (near Antsiranana) to and including the Mahavavy du sud drainage basin located to the southwest of Mahajunga. The satellite island of Nosy Be is also part of this ecoregion. 

Freshwater habitats

The Northwestern basins of Madagascar, especially those draining the Tsaratanana Massif, receive more annual rainfall [>2,500 mm annually (Aldegheri 1972)] and are more diverse geomorphologically and biologically than the drier drainages of Western Madagascar [579] and Southern Madagascar [582] ecoregions. Compared to western rivers, those in the northwest are also less subject to large seasonal fluctuations in flow (Aldegheri 1972; Donque 1972). Northwestern and western basins have, on average, a higher pH (nearly neutral vs. mildly acidic), lower conductivity, higher temperatures, and significantly higher levels of dissolved carbonates (i.e., alkalinity) than eastern rivers (Riseng 1997). The oligotrophic floodplain lakes in this ecoregion exhibit higher conductivity than lakes in eastern Madagascar (Riseng 1997). 

Description of endemic fishes

Freshwater habitats include tsingy (karst) formations of the Ankarana reserve in the far north; steep and rocky, clearwater rivers and streams draining the Tsaratanana Massif and Montagne díAmbre; large, shallow oligotrophic floodplain lakes; large perennial rivers and their tributaries; and numerous crater lakes on the island of Nosy Be. A number of rivers and their tributaries within this ecoregion, most notably the Mahavavy du Nord, Sambirano, Ankofia-Anjingo, and Mangarahara-Amboaboa (Sofia tributary), have high habitat diversity for Madagascar, and as a result contain rich and highly endemic freshwater fish faunas. For example, all but one species of Paretroplus, Madagascar\'s most diverse cichlid assemblage, have localized distributions restricted to northwestern riverine and lacustrine habitats (Sparks 2002). Frequently these distributions comprise only a single basin, few of which are currently protected. Thus, even localized disturbances could potentially encompass the entire range of a species. In addition, northwestern Madagascar is home to a small species assemblage of killifish of the genus Pachypanchax; members of the endemic cichlid genus Ptychochromis, including at least two undescribed species; and distinct inland populations of an undescribed ariid catfish. The endemic atherinid genus Teramulus is currently known only from two extant populations, both located in isolated and relatively undisturbed forested sections of this ecoregion.

Justification for delineation

This ecoregion is defined by those basins in northwestern Madagascar with an annual average of over 1400 mm of rain (Food and Agriculture Organization & International Institute for Applied Systems Analysis 2000). Compared to the dry western and southern basins, the wetter northwestern basins are substantially more diverse in habitat, and are not as influenced by vast seasonal fluctuations in flow rate that could easily wipe out existing fish populations during periods of complete desiccation.

Level of taxonomic exploration

Fair. Following a number of recent ichthyofaunal surveys throughout northwestern Madagascar, focused primarily on endemic cichlids, the northwestern basins are now recognized as supporting diverse communities of native freshwater fishes. Within this ecoregion, our knowledge base is adequate for isolated basins (e.g., many of the larger lakes including Kinkony, Andrapongy, and those of the Sarodrano region, and portions of some rivers, including the Amboaboa, Ankofia, and Betsiboka) and extremely poor for others, especially the headwaters and upper to middle reaches of most rivers, including all of those draining the Tsaratanana Massif. The middle to lower reaches of many larger rivers within the ecoregion are somewhat better studied. Overall, survey coverage in northwestern Madagascar is patchy, and with each new expedition a number of species new to science are encountered. Poorly surveyed and relatively undisturbed habitats within this ecoregion should be a focus of future research efforts.