Kenya's coastline has a total length of 1 420 km (Table 1). Of this, some 650 km, representing 45.7 percent of the total is found in Lamu District which, in addition to its very irregular coastline, has several islands within its boundaries. The northern end of the coast, from the Somali border down to the northern coast of Ngomeni, is in general characterized by mangroves and tidal flats. Vast tracts of mangroves are again found at the southern end of the coast from Gazi Bay down to the Tanzanian border. In between, the continental coastline from the south coast of Ngomeni to Chale Point, a length of 390 km, is mostly white sand beach or coralline - the type of coastline ideal for beach resorts and marine parks for which Kenya is noted. The only notable exception in the middle stretch of the coastline are Port Reitz and Port Tudor, which surrounds the island-city of Mombasa, as well as Mida and Kilifi Creeks. The shoreline of these creeks also consists of mangroves and tidal flats. In all, the total area of tidal swamps and flats in Kenya is estimated at 50 400 ha as shown in Table 2.
This figure, 50 400 ha, it should be noted, is way below the 1.3 million ha estimate of Sivalingam (1981), as cited by Balarin (1985). Sivalingam's figure may have been based on Bell and Canterbery (1976) who gave 1 296 million ha or the total area of Kenya's tidal swamps with potential for aquaculture, again cited by Balarin in the same report. It should be further noted that if these extremely high figures are correct, then Kenya's 1 420 km of coastline will have an average of 9 km of mangrove!
We could only speculate on the possible explanations for such a huge discrepancy. Either, the total forest area of Kenya which stands at 1.37 million ha (Kenya Yearbook 1982–83) has been misinterpreted to mean mangrove forests, or the 1.3 million figure is actually in square meters, which would be a discrepancy equivalent to four orders of magnitude, so that the actual hectarage would only be 130 000. The seasonal swamps of Tana River basin at 65 800 ha together with the tidal swamps of 50 400 ha adds up to 116 700 ha - a figure that is quite close to 130 000 ha.
An earlier figure of 44 203 ha given by Nikolic and Tang (1976) does not include the tidal swamps and flats of the Tana River District and some parts of the coast, but is close to our own estimate. The figure given by Kokwaro (1984) of 52 900 ha is even closer.
Kenya's coastline is part of the western shore of the Indian Ocean. Tidal fluctuation is semi-diurnal, that is, there are two highs and two lows during each 24-hour period. The tidal reference statoin for Kenyan waters is Kilindini Harbour at Mombasa. According to the tide tables for East African ports prepared by the Kenya Ports Authority, the time of flood and ebb occurs 40 minutes later than the reference station at Lamu and five minutes later at Malindi. All heights are based on the datum of the largest-scale admiralty charts for Port Kilindini.
The highest tide for 1986 occurs on 25 April at 4.02 m and the lowest on 3 November at 0.02 m below datum. This gives the 1985 range at 4.04 m. There should only be very slight variation in the annual range from year to year. Indeed the annual range is always taken to be equal to 4.0 m by the Fisheries Department. For 1986 we have determined the various standard mean heights as follows:
| Mean higher high water | - | 3.15 m |
| Mean lower low water | - | 0.65 m |
| Mean lower high water | - | 2.78 m |
| Mean higher low water | - | 0.80 m |
The average high tide is 2.96 m and the average low tide 0.71 m. Again, there should only be slight variations in these figures from year to year.
A general description of the tidal swamps and flats at the Majoreni-Vanga area has been given by Michieka et al. (1978) as follows, listed in the order of acending elevation:
With some minor differences the areas we visited conform to the above description. In none of the areas visited did we observe any Avicennia at the lowest zone. Instead we often find Rhizophora at the seaward margins, with solitary Sonneratia alba sometimes interspersed at irregular intervals. In Lamu where the Sonneratia appears to be more abundant than in other areas, these are often observed to form one solid row immediately before the Rhizophora, with their thick branches hanging almost horizontally by as far as three or four meters over the water, (See Figure 4).
We also failed to observe a distinctive zone of Ceriops at Lamu. Instead we find these interspersed with the Rhizophora and Avicennia where the two species meet. The Avicennia is clearly dominant in the higher areas beyond the Rhizophora zone. In many places the barren tidal flats do occur after the Avicennia stand but in some areas these may occur in the middle of the mangrove zone with the fringes occupied sparsely by some stunted Avicennia. Patches of low lying halophytes are often seen to grow on the tidal flats. Two types were observed. One with distinct ovoid leaf structures approximately 10–15 in diameter and another which was simply segmented, cylindrical branching stems. No large trees were found in any areas with the exception of Tana (which has an altogether different type of mangrove and will be discussed separately).
Lamu has the largest area of mangrove in Kenya covering some 28 400 ha. Although on a straight line basis the district extends only 138 km southwest from the Somalian border, its irregularity and numerous islands gives it a total coastline length of 650 km. The three biggest islands are Lamu, Pate and Manda. Except for the south or southeastern coast of the Islands of Lamu, Manda, Pate and the short stretches of the continental coastline directly exposed to the Indian Ocean, all of the Lamu's tidal plains are covered with mangrove forests of varying widths.
The extent of the mangrove is such that the channel which separates Manda Island from the continent has narrowed down to less than 100 m in most places and is too shallow for navigation even by small craft at low water. The structure of the mangrove forests follows the typical representation presented earlier - a narrow belt of Sonneratia (sometimes absent) in the seaward margins, a Rhizophora belt of varying width, finally an Avicennia belt at the highest reach of the tide. A clear space is found directly behind or in the midst of the Avicennia zone in most places.
The waters around the islands have a salinity level of 35–36 ppm. Within the mangroves and the channels the water has a rich, green colour indicating high productivity with transparency of no more than 2 m in most places. Evidence of this high productivity is the profuse growth of oysters in the prop roots and branches of the Rhizophora and Sonneratia. These oysters are small with shell lengths within the 2–4 cm range, although a few individuals growing in relatively less crowded situations were seen to have attained shell lengths of 6.0 cm. They are probably of the genus Saccostria. Except for the restaurants catering for tourists, where they are sometimes referred to as “tree oysters”, the oysters do not seem to be exploited by the local people even at a subsistence level.
The shallow water within the numerous creeks is notable for a very high incidence of P. monodon. Whereas in all other areas of Kenya as well as the Indo-Pacific region, P. monodon would rarely constitute more than five percent of penaeid fry catch, in Lamu the proportion of P. monodon can be as high as 80 percent according to information supplied by project KEN/80/018 staff.
The Lamu mangroves can be characterized by their relatively low elevation, with most areas less than 2.0 m above datum. The clear areas however are at least 2.75 m above datum with most within 3.0 to 3.5 m above datum. Only in Kimbo was a certain amount of clay-loam found.
All areas visited were invariably sandy with a very extensive amount of root material pervading the substrate even beyond 60 cm giving it very peaty characteristics. Upon excavation, dark, decomposing root material would normally be encountered below 50 cm exuding the characteristic swamp-gas smell upon exposure. Some areas have a misleading loamy appearance, but this was often limited to the upper 5 cm and would invariably be sandy and peaty upon excavation. A summary of field observations is presented in Table 3.
Access to Lamu from the mainland during the dry season which is most of the year, is not all that bad. The 225 km dirt road from Malindi to Mokowe - the take off point to Lamu, can be negotiated in 5 hours. The problem comes only during rainy season when the numerous drainage gullies along the Tana River Basin become torrential creeks and most of the low-lying roadways become impassable even to four-wheel drive vehicles. Within Lamu District itself boats are used to travel from point to point and from island to island through the narrow channels. Lamu is only 170 km from Mombasa by sea.
This district covers most of the area at the western side of Tana River from the national reserve area 350 km inland all the way to the Coast. The area of relevance to ths survey mission is only the small coastal plain which extends for only 12 km, where Tana River drains into the Indian Ocean after its long journey from the foot hills of Mts. Kenya and Aberdares.
The Tana River contributes significantly to Kenya's development. Four hydro-electric power stations built along its course now generate 340 megawatts. A fifth one to be operational by 1988 will bring up its power output to 480 megawatts. Thousands of hectare of arid and semi-arid lands have been made productive with irrigation from the Tana River with an area up to 20,000 ha being targeted.
Tana River also plays an important role in fisheries. Upstream, at the 45 km long Masinga Dam which has a storage capacity of 1560 million cubic meters of water, almost a ton of fish, mostly tilapia and carp, are now being landed by several subsistence fishermen. Ungwana Bay where it discharges, is a major trawling area where shrimps such as P. indicus and P. monodon are caught together with other commercially valuable fish such as groupers and snappers.
Tana is estimated to have dry-season flow of 80 m2 per sec. But twice a year, in May-June and November-December, the river overflows, flooding an area of some 65,000 ha. The flood though ranges only from 30 to 60 cm in depth. However, flooding is expected to diminish in the near future because of the construction of dams in the upper Tana and the use of Tana water for major irrigation schemes. It is sometimes assumed by some that a big flood occurs at Tana River once every ten years, as is often the case in many other rivers elsewhere. We cannot find published account of such an event for Tana River. Neither could we get any confirmation from local informants who have lived in the vicinity of the river mouth since birth. On the contrary, what is reported instead by a Kenya Soil Survey team for an irrigation study is a ten-year low during which the dry season flow drops down to 30 m2 per second from the usual 80 m3 per second.
From the extent of the mangrove growth it could be infered from charts that seawater probably reaches as far as 7 km upstream from the river mouth during high water. Salinity measurements at three different sections of the river on 21 March 1986 show that with the incoming tide (from 13.00 to 13.50 h) even with an expected rise of only 2.09 m (14.08 h), the river water was still slightly saline (4 ppm at 1.0 m) at a point 4 km away from the river mouth (Figure 5). Vertical salinity profiles at three sections along the river (Figure 6) shows that statification is more pronounced towards the river mouth although the upper 1.5 m layer is still fairly well mixed as a result of wave action.
The mangrove forest lining the banks of Tana is totally different from those that are found elsewhere along the Coast. Instead of Sonneratia or Rhizophora, a beautiful, pure stand of evergreen rising straight as high as 25 m, was encountered, which must be Heritiera littoralis said to abound on the Tana below Kau where the river enters the ocean, as described by Kokwaro (1984). The spaces between the trees are void of any undergrowth but pneumatophores cover the ground. Here hippopotamus tracks and droppings were encountered.
Beyond the evergreen zone, which extends only up to a distance of 15 to 20 m from the river's edge, the vegetation again changes. Here some large Rhizophora were found mixed with other tree species which probably consist of the two Xylocarpus species found in Kenya mangrove, which together with Heritiera littoralis are the only three species known to favour areas with a good admixture of freshwater. In some areas along the bank, large sand ridges rising about 0.7 m above ground were encountered within this zone extending landward for as far as 120 m. These are indicated in the topographic maps as barren sand flats but actually have a low density cover consisting of shrubs and thorn trees.
Beyond the sand ridge, the ground resumes its usual elevation. Here the vegetation would still consist of the large trees described previously as occuring after the evergreen zone. Further on towards the high ground, at elevation above 3.5 m the Avicennia becomes dominant.
The soil at the mangrove area appears to be almost homogeneous from the surface down to 60 cm. Result of manipulative tests show them to be a clay to sandy clay in nature, and pH measurements on fresh dried samples range from 6.1 to 7.64 (See Table 4).
The River Tana has changed its course as it meandered over time, with the mouth shifting northeastward along the coast. It has left behind two abandoned courses. The older course with a discharge point at Mto Kilifi is about 41 km southwest of the present mouth of Kipini; and the latter course, Mto Tana about 11 km further east of Mto Kilifi. The latter change was reportedly aided by the construction of the “Bellazoni Canal” by the Sultan of Witu in 1865, which connected the original Tana with the short river Ozi which opens at Kipini.
At the eastern bank of the old Tana is a seasonal flood plain. Within this area a partially irrigated commercial estate of about 4,000 ha was in operation from 1912 to 1931. The Bellazoni or Sadami estate as it was known was bounded by dykes presumably to protect it from the seasonal floods. The estate was used for growing cotton, perak rubber, coconut, maize, cassava and rice all of which were reported to be doing well. The estate was reportedly abondoned because of the 1931 economic slump. Investigations by the Kenya Soil Survey (Wokambi et al, 1976) as well as previous studies indicte that parts of the estate are saline with the soil becoming acidified upon drying. Previous data cited by the Kenya Soil Survey also suggest that at least two thirds of the area (southern part) is unsuitable for cultivation due to high salinity.
We have included an account of the Bellazoni estate from secondary data sources because it appears interesting to us from the point of view of coastal aquaculture. Much as we would have wanted to, we were unable to visit the area due to its inaccessibility which characterizes most of the Tana River flood plains. According to the Kenya Soil Survey report most of the flood plain areas were inaccessible even by landrover, with helicopter being the only way to cover the area. If there was an access road when the estate was still in operation, it must have been obliterated and overgrown by now. The available charts certainly do not show any roadway to the area. The only other alternative would have been to use a fairly large boat suitable for negotiating the choppy Indian Ocean waters, but this fortunately was not available.
Between the Mto Tana and Mto Kilifi is a vast expanse of flood plain fronted along the coast by higher scrublands. But within the Mto Kilifi mouth are some 2 000 ha of tidal flats and swamps. The northern half of this is still within the Tana River District. This is the site of the abandoned Tana Saltworks which is now being proposed for new saltworks development (See Figure 7). The proposed project will have semi-intensive or intensive shrimp ponds and Artemia ponds integrated with the saltworks. Here the elevation ranges from 2.5 to 5.0 m. The top soil within the tidal flat area varies from silty-sand to silty loam. The area has been subjected to a topographic and soil survey by a private engineering firm for feasibility study purposes. Based upon 45 bore holes, the survey indicates that the surface varies from silty sand to silty loam with a layer of clay-loam undulating at depths ranging from 0 to 100 cm. In some areas the clay layer rises all the way to the surface while in others the sand layer goes all the way down to the maximum auger depth of 110 cm. Chemical analysis of fresh dried soil shows a pH of 6.96.
The Kilifi District coastline starts at Mto Kilifi and extends southward to Mtwapa Creek for a total length of 265 km. Most of the coastline is sandy beaches such as found in Malindi. The areas of interest to this survey are limited to the tidal flats and swamps within the Mto Kilifi to Ngomeni segment, Mida Creek and Kilifi Creek, but reconnaissance at various points in between was also made in order to identify possible hatchery sites.
The mangrove areas within Mto Kilifi to Ngomeni are relatively higher than those of the Lamu area, with elevation ranging from 2.5 to 3.8 m (versus Lamu's 0.92 to 2.0 m). Dominant species are Rhizophora at the lower area and Avicennia at the upper reaches of the tide. The tidal flats are behind the mangrove zone and have elevations ranging from 3.5 to 4 m. These are the areas which have been developed as salt farms. Many of these salt farms intrude into the mangrove area and also use up part of the scrub land immediately above the highest tide level. There are now eight companies operating within the area and together they occupy 7 922 ha as shown in Table 5. The salt farms have occupied most of the tidal flats and swamps within the area shown in Figure 8 with only small patches of mangroves left intact. The pilot shrimp farm of Project KEN/80/018 has been constructed within one of such saltworks.
The soil types in the salt farm and adjoining areas range from sandy loam to clay loam wih the topsoil in some areas consisting of silty sand. Soil pH on fresh dried samples range from 6.1 to 7.86. (See Table 6). As stated earlier a more systematic study on tidal flats and mangrove soils by the Kenya Soil Survey show potential sulfate acidity even when fresh samples would give a pH reading equal to or above 7.0.
Mida creek and Kilifi creek are almost identical in area and both have narrow openings to the Indian Ocean. But the terrain around Mida slopes gently upward while that of Kilifi rises abruptly to as high as 10.0 m within 1 to 2 km from its shoreline. Consequently Kilifi has a much narrower tidal zone and only has a third of Mida's 1 800 ha mangrove and tidal flats. At Mida the much narrower opening prevents a faster turnover of water; consequently the water inside the lagoon is reportedly always higher in salinity than the open sea. At the time of the visit the salinity measured 40 ppm, while that of Kilifi only 37 ppm.
Mida was the first site selected for Project KEN/80/018 where five test ponds were reportedly dug before being given up to two years later. Digging at three different locations, one within a clear area close to the lagoon, another inside the mangrove area where one of the test ponds was dug, and the third within the highest edge of the tidal flat close to the roadway, all yielded sand only, even at almost 1 m below ground level. An attempt to build salt ponds within the area in the mid-sixties was reportedly given up due to the sandy nature of the substrate.
At Kilifi the open tidal flats were also observed to be sandy. However, a small saltworks is operating within the area, with dyke material which appears to be of a different nature from the natural substrate and which must have been transported from higher grounds close by.
Kwale is the southernmost coastal district, while Mombasa is an urban, commercial/industrial district. The two districts are treated as one in this report because one of the major tracts of mangrove and tidal flats straddles across the boundary of the two districts.
The island city of Mombasa is situated between two interconnecting but virtually separate lagoons: Port Tudor and Port Reitz. The narrow channel which connects Port Tudor to the Indian Ocean is the site of Mombasa Harbour and that of Port Reitz, Kilindini Habour. Tidal flats and swamps cover almost half of the northern portion of Port Tudor, most of the northwestern “lobe” and southern and western “fingers” of the bigger and irregularly-shaped Port Reitz, part of which is within Kwale District. Together the tidal flats and swamps in the lagoons are estimated to cover some 3 000 ha of which about 75 per cent are in Port Reitz.
Mombasa harbour is designated only for dhows but Kilindini harbour is a major East African port serving also land-locked Uganda, and even Somalia and Sudan. There is therefore considerable shipping activity at Kilindini, and with it all the oil slicks, flotsam and jetsam normally associated with such activity. Such pollution, however, appears to be restricted to the immediate vicinity of the harbour and there is no evidence inside Port Reitz that the oil slicks are a serious threat. The oysters are thriving on the mangrove prop roots and the mangroves bear no dark smudges or other evidence of being affected by the harbour pollution. Aside from an oil refinery at Port Reitz and a meat-processing plant at Port Tudor, there appears to be no major industry that could seriously threaten the waters around Mombasa.
At Port Reitz, the soil within the mangroves ranges from clay loam to silty clay loam. In the open tidal flats the topsoil is often sandy but the subsoil in many places was found to be clay loam at varying depths. Elevation ranges from 1.81 m within the mangrove area to 3.93 m within the open tidal flats. Results of soil analysis are shown in Table 7, together with those of Port Tudor.
At Port Tudor, no open tidal flats were encountered. The open areas shown on the topographic map appear to have been covered by new growth, primarily of Rhizophora. Within the mangrove the topsoil varies from silty loam to clay loam up to 5–10 cm, changing to loam or sandy loam with depth. The subsoils are all slightly peaty. The 800 ha mangrove area within the western finger of Port Tudor consists of very soft silty mud incapable of supporting the weight of a person. Only the mangrove root system beneath prevents one from sinking completely.
The tidal flat and swamps within the Shimoni-Vanga area were found to have sandy and peaty substrates. At Shimoni the peaty-sand substrate was found to be very shallow (30 cm) and was underlain by a bed of coralline material. An open tidal flat at Majorine behind the Rhizophora zone yielded greyish clay material binding the sand particles at 35 cm below the surface. At Bodo an abandoned small-scale saltworks (about 5 ha) was encountered at the upper reaches of the tidal flat. Here, clay was encountered 5 cm below the sandy top soil within the enclosed pond area, but outside the pond area the soil area was invariably sandy with light mottles of clay material. Dark organic matter appeared at 25 cm but the soil was still sandy. The field observations are summarized in Table 8.
