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Integrated study of the Sabaki River Mouth

Sand dunes at the Sabaki River Mouth
Sand dunes at the Sabaki River Mouth
The Sabaki River Mouth is one of the most important bird areas on the Kenyan coast and has been identified as a globally important site under the ‘congregations’category of the Important Bird Area (IBA) criteria. The sandbanks at the mouth of the river are roosting sites for tens of thousands of gulls and terns, while the intertidal mudflats provide a feeding ground for thousands of shorebirds and lesser numbers of other waterbirds.

The river mouth includes intertidal mud and sand flats as well as salt marshes, dunes, seasonal and permanent freshwater pools, mangroves and scrub. Tides are semidiurnal with a range of 2-4 m. The area experiences a tropical monsoon climate with south east trades prevailing from April to October and north-east monsoon from November to March.

Together with a botanical inventory, the studies described below were funded by the Royal Netherlands Embassy in Nairobi as part of an Integrated study of the internationally important migratory bird populations at Sabaki River Mouth, Malindi, Kenya. This was a project run by A Rocha Kenya to produce a management plan for the area to ensure its long-term conservation.

1. Low tide counts of water birds

Birds were counted once a month over the low tide period from April 2004 to February 2005, usually on neap tides which left less of the intertidal area exposed, making the birds more concentrated and easier to count. The volunteer counters followed a set route along the river bank, mudflats, sand bars and open beach of the Indian Ocean. Nearly 70 waterbird species were counted including pelicans, cormorants, herons, storks, flamingos, ducks, geese, waders, gulls, terns and kingfishers; 40 of these species were recorded in sufficient numbers to assess seasonal trends.

Greenshank , Tringa nebularia, a Palearctic migrant to the Sabaki
Greenshank , Tringa nebularia, a Palearctic migrant to the Sabaki
One of the key results of the study was that Palearctic migrants (including birds that breed in the arctic regions of north-west Europe and Russia as well as others from Central Asia) make use of the estuary during the northern winter. Birds return to the estuary or stop off for short periods to feed as they fly further south, building in numbers as early as July, peaking in September and December. The number of species using the estuary also peaked in September, reflecting this arrival from the north.

2. Macro-invertebrate communities in the intertidal mudflats

The Sabaki River (called Athi or Galana River further upstream) is the second largest river in the country with a catchment area of approximately 40,000 km2. During the wet season (April-July) the river is depositing an increasing amount of sediment at the estuary due to deforestation and agricultural intensification on hillsides within its watershed. Such depositions can kill benthic invertebrates and therefore reduce the amount of food for feeding birds. This baseline survey aimed to (i) identify and quantify the macro-invertebrate communities present in areas of high shorebird density; (ii) consider the effects of periodic exposure to high levels of sedimentation; (iii) identify future research priorities and management recommendations for the Sabaki River Mouth in order to preserve shorebird habitat.

Macro-invertebrates were sampled in 30 sediment cores collected on 14 and 15 June 2005. Samples were sieved through a 600 micro-metres sieve and preserved in 70% isopropyl alcohol with estuarine water and then identified as close to species level as possible. Cores for sediment analysis were taken adjacent to each macro-invertebrate core. The mean number of taxa was extremely low, not exceeding an average of 1.66 species at any site. Thedominant species were Nereid polychaete worms with up to 196 individuals detected in one individual core: the complete dominance of Nereid polychaetes in all samples seems to be unrecorded at any other site worldwide and would seem to indicate stress in the benthic community.

Counting birds at low tide at the Sabaki River Mouth
Counting birds at low tide at the Sabaki River Mouth
Repeated deposition of sediment during floods will increase the likelihood of macro invertebrate destruction. Subsequent recovery will be by a few opportunistic species able to adapt quickly to such events; those taxa able to cope best may include the Nereid polychaetes, hence their observed high numbers at Sabaki. Catchment protection is the most appropriate way of ensuring the long-term livelihood of the macro-invertebrate communities and the bird populations that rely on them.

3. Entomological survey

This baseline survey was undertaken from the seashore inland for a few kilometres across the dune system, covering both sides of the estuary, using pit-fall traps, sweep netting, light trapping, hand gathering and direct observation and recording. Dry season sampling was undertaken 19-27 December 2004, wet season sampling was done 3-5 April, 27 and 31 May 2005.

The high diversity of insects recorded form an important part of this ecosystem and the surrounding inland area. The large diversity of pollinators, mainly bees and wasps, has significance for the nearby human population, as they are providing free pollination of fruit and vegetable crops in an area that is becoming increasingly unsuitable for ground dwelling solitary bees and wasps because of frequent tillage. A high number of ground beetles (Carabidae) was also noteworthy as a family often used to bio-monitor terrestrial ecosystems: future surveys can use the baseline data on ground beetles to monitor their comparative diversity and abundance to assess the general health of the terrestrial habitats at Sabaki.

Project leader: Colin Jackson

Partners: Royal Netherlands Embassy, Nairobi, Ornithology Department of National Museums of Kenya, Nature Kenya

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