Kosi Bay system

Site description (2001 baseline):

Site location and context
The Kosi Bay system is situated in a warm, humid, subtropical climate 470 km north-east of Durban; Mozambique borders it in the north and the Indian Ocean in the east. The system is composed of four interconnected, roughly circular, fresh to brackish lakes (Makhawulani, Mpungwini, Nhlange and aManzimnyama), a broad channel leading to an estuary that opens to the Indian Ocean, and three extensive areas of swamp.

The lakes are separated from the ocean by a strip of forested sand-dunes. Two principal rivers feed the system. The Sidhadla river is c.30 km long, receives water from 12 tributaries, and enters Lake aManzimnyama. The Nswamanzi river runs for c.15 km, receiving water from nine tributaries before feeding into the western shore of Lake Nhlange. Numerous sandy mudbanks, emergent at low tide, occur in the lower part of the system. The wetland shows complex patterns and interactions in thermal properties, salinity and nutrient levels through the various lakes.

The main vegetation-types include marshes, sedge-beds, submerged plants (in lakes, pans and streams), swamps and other aquatic communities dominated by reed Phragmites, sedge Cladium and the fern Achrostichum. Various semi-emergent plants such as water-lilies Nymphaea are common along the edges of the system. The swamp-forest contains Barringtonia trees and conspicuous climbers such as Stenoclaena, and in places it is dominated by raffia palm Raphia. The swampy vegetation is surrounded by undulating grassland, among which palms Phoenix are interspersed. The surrounding woodland includes trees of Syzygium, Acacia, Trichilia, Albizia and Dialium.

Key biodiversity
See Box and Tables 2 and 3 for key species. The avifauna is prolific and diverse, largely as a result of the undisturbed condition of the marginal vegetation along the water’s edge. Only 85 of the 296 bird species that have been recorded at Kosi Bay are estuary-associated. The system is important for various species of migratory and nomadic birds. Phoenicopterus ruber and P. minor are almost always present, and the open water occasionally supports Sterna caspia, Pelecanus rufescens and P. onocrotalus. It is assumed that Kosi Bay acts as a staging post for migrating waders as part of the east coast flyway.

The swamp-forest and associated overhanging vegetation supports several rare, localized and specialized bird species, including Gorsachius leuconotus, Podica senegalensis and Scotopelia peli. The larger riverine trees are suitable for Macheiramphus alcinus and Circaetus fasciolatus, which probably breed there. The sand forest supports Hypargos margaritatus and Apalis ruddi, two restricted-range species. The forests are home to Cercotrichas signata, Cossypha dichroa, Batis fratrum, Lamprotornis corruscus and Nectarinia veroxii.

Non-bird biodiversity: Eight fish species of global conservation concern are known to have populations in the Kosi system, including Redigobius dewaali (LR/nt), Silhouettea sibayi (LR/nt) and Hypseleotris dayi (LR/nt).

Pressure/threats to key biodiversity
The area was proclaimed as a Nature Reserve under control of the KwaZulu-Natal Bureau of Natural Resources in 1987. Kosi is a large but relatively isolated estuarine system; this results in minimal transfer of estuarine organisms to and from other systems. If Kosi ceased to function, a barrier would be created obstructing movement of estuarine species from north to south of Kosi and vice versa. The Kosi system also provides the only recruitment area for several species of marine spat found along the KwaZulu-Natal coast, and it is undoubtedly the most pristine and best preserved large estuarine system in KwaZulu-Natal. Despite being slightly degraded, it still supports a great diversity of fish and other aquatic fauna. It is still a natural system in that humans, while making extensive use of the fish, have not yet substantially altered the system. Efforts are being made to increase the tourist use of the area in order to give greater benefit to the local communities; 25% of the gross revenue from visitors is given to the local tribal authority.

Afforestation and increased use of fertilizers in the catchment area pose threats to the system, by reducing water run-off and increasing nutrient-levels in the water that does reach the lake. Human population pressure is increasing as people attempt to get more land for habitation and cultivation. Pereskia aculeata and other non-native plants grow well, and need to be controlled before they overwhelm the indigenous vegetation.

Recommended citation
BirdLife International (2023) Important Bird Area factsheet: Kosi Bay system. Downloaded from on 01/10/2023.