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The stable isotopes of organic carbon (OC) and nitrogen, contents of OC and nitrogen for roots, bark, leaves, flowers and fruits of various mangrove species from Kisakasaka (Zanzibar) and Bagamoyo (mainland Tanzania) are used to assess (1) if some mangrove species are capable of fixing atmospheric nitrogen, (2) if there are differences between species in the same stand and in different stands and (3) if the mangrove signature is preserved in the sediments. Mean OC stable isotope results of various plant components range from −25.9‰ to −29.1‰ suggesting that mangrove trees in the two stands follow the C3 type of photosynthetic pathway. Mean nitrogen isotope values for various plant components range from −1.5‰ to 3.2‰ suggesting atmospheric nitrogen fixation by mangrove plants, but δ15N values approaching −3‰ that are more negative than typical diazotroph biomass exclude this possibility. Mangrove species thriving further inland are enriched in 13C and 15N relative to those thriving near the shoreline owing to complete utilization of available nutrients. Sediments beneath the mangrove forest are characterized by lower C/N ratio values and enrichment in 13C and 15N relative to plant material owing to mixing of nitrogenous rich material from adjacent area. High concentration of OC is found in bark and roots, while high nitrogen concentration is found in fruits and flowers only.

Biological rhythms with lunar components are common in nature. In the sea, the moon's gravitational pull on earth is the principal cause of the tides, which normally reach maximum amplitudes every new and full moon. Many populations synchronize spawning to this time. Some choose either the new or the full moon, implying that moonlight is important; but one lunar phase usually has higher tides than the other, and many species select the phase with the higher tide to improve the offshore transport of their progeny. However, tidal dominance by one lunar phase is not constant; it switches between new and full moon every seven months. We tested the influence of this 14-month “syzygy inequality cycle” (SIC) on lunar synchrony by sampling 11 populations of intertidal crabs at two locations in East Africa for 21 months. Eight populations synchronized larval release with the SIC. Tidal cues were more important than moonlight in entraining the reproductive rhythm, although two populations synchronized spawning to t...

Mangroves are among the most threatened ecosystems in the world and the coastal forests of East Africa are no exception to this trend. Although conservation, rehabilitation and sustainable management plans have been developed in various tropical regions, only a few locally based approaches have been launched along the Indian Ocean coast of Africa. In order to identify possible conditions for sustainable management of mangroves based on socio-economic and ecological considerations, we present a participatory approach designed to evaluate the relationships between mangroves and human activities and the use of multi-criterion analysis to identify management solutions. To achieve this goal, all the subjects involved in mangrove management (local communities, institutions and researchers) took an active part in the process. The research was carried out in three communities relying on mangrove swamps: Kisakasaka village on the island of Zanzibar, Mida Creek in Kenya, both relying on mangrove forests known to be endangered by over-exploitation, and Inhaca Island in Mozambique, where mangroves are more pristine. Families were the hub of the research and the importance of each of their economic activities was assessed. We then examined the methods by which mangroves are exploited by the local community and by other stakeholders working in the study areas. Our results show that the mangroves in Inhaca are exploited only for household needs and the pressure on the forests is still ecologically sustainable. In contrast, there is a well-established demand for mangrove products in Kisakasaka and Mida Creek and the mangroves represent an essential source of income for the families, resulting in an ecologically unsustainable rate of exploitation. Therefore, possible alternatives to the current management practices were identified in the two areas by means of a participatory approach. Multi-criterion analysis was then used to compare and discuss the alternatives in terms of social, economic and ecological criteria.

Numerous studies have highlighted the importance of crabs within mangrove ecosystems, but tested methods of enumeration are virtually non-existent. The aim of the present study was to assess the accuracy of quick, non-invasive methods for estimating the population density of three species of mangrove crabs in East Africa: Uca annulipes, Sesarma guttatum and Neosarmatium meinerti. Methods employed depended on the behaviour and habitat of each species. Counts of U. annulipes and N. meinerti individuals that were active on the surface (visual counts) and burrow counts (of U. annulipes) were compared to actual densities. Actual densities were determined by excavation of U. annulipes, and by multiplying counts of N. meinerti burrows by the mean number of crabs per burrow. Visual counts of marked individuals of S. guttatum were compared to known densities of marked crabs. For N. meinerti two visual counting methods were tested: (1) instant counts – crabs were counted every 5 min during 0.5 h and the highest count taken; and (2) continuous counts – all burrows were tagged, and those from which a crab emerged during 0.5 h were totalled. For U. annulipes on Inhaca Island (Mozambique) visual counts underestimated excavated numbers by 27%, whereas burrow counts overestimated numbers by 46%. These values were similar for both sexes, and on both spring and neap tides. Burrow counts and visual counts estimated absolute numbers of U. annulipes with equal consistency. At Mida Creek (Kenya) visual counts underestimated U. annulipes numbers by 37%, again with no differences between spring and neap tides. In Zanzibar 81% of N. meinerti burrows contained a crab. The instant visual count underestimated N. meinerti numbers by 41%, the continuous count, by 12%. Visual counts of S. guttatum in Zanzibar (Tanzania) underestimated crab numbers by 49%.

The aim of this work was to determine the abundance of crabs inmangrove communities along a latitudinal gradient along the eastern coastof Africa from 4°S to 32°S. Surveys were made atMombasa (Kenya), Zanzibar (Tanzania), Maputo (Mozambique) and in theTranskei (South Africa). Crabs were estimated at three designated levelsin the mangroves by visual census using a common protocol, and numberswere converted to biomass.Even after standardising the selection of sites and methods of censusthere was still extensive variability in the data, emphasising the complexheterogeneity of mangrove ecosystems. Lunar phase (full versus new moonsprings) did not have a consistent effect on results, but shore height hadseveral effects. Total crab biomass was similar in the two lower shore strataexamined, but about twice as high at the top-Avicennia level. Theratio of grapsid biomass:ocypodid biomass also changed with height: fromnear unity in the lower mangrove, to 0.14 in the middle strata, but to 15at the top.There was no consistent latitudinal trend in total crab numbers, but totalcrab biomass increased from north to south. In addition there was aconsistent and marked change in the grapsid biomass:ocypodid biomassratio: this swung from 0.65 at Mombasa to 6.8 in the Transkei. This hasimplications for the transfer of primary production through the food chain. Grapsids are important macrophagous feeders on the leaves and other partsof mangroves, whereas ocypodids are microphagous deposit feeders.

The mangrove vegetation occupies a key position in the livelihood of the people of Zanzibar. About 45% of the total population live along the coast and the majority are involved in the extraction of mangrove wood products and fishing. Between 80% and 90% of the local fishing is concentrated in waters close to mangrove-vegetated areas, creeks and bays. The area under mangrove vegetation, approximately 20,000 ha, is nevertheless only 6% of the total land area and necessarily needs to be used in a sustainable way. Recent efforts to link research findings from studies of aspects of the biology and ecology of the mangroves of Zanzibar and the dependent fisheries, with public awareness programs designed to educate and sensitize the local community, have generated a lot of positive interest in participatory management of the mangrove resources. Awareness of the ecological links between mangrove ecosystems and resources such as fish is key to winning management support. The instillation of a sense of ownership of the resources by the local population and their legal empowerment so that they are able to institute and execute control measures on their resources are also essential components in the process aimed at achieving full community support for the sustainable utilization and management of common resources. Local communities, for example at Kisakasaka village, are beginning to protect and to plan better ways of exploiting their mangrove resources. Based on biological knowledge gleaned through awareness campaigns, they are carrying out mangrove restoration programs with positive results. This short overview discusses some aspects of the biology and ecological roles of the mangroves of Zanzibar and what has happened to help change the attitude of the local coastal community towards the mangrove resources.

The grazing behaviour of Diadema setosum Leske and Heliocidaris erythrogramma Lamarck, on species of marine macro-algae has been studied in the laboratory. D. setosum showed strong preference to Galaxaura oblongata (Soland.) Lamx., while H. erythrogramma exhibited strong preference to Eucheuma striatum Forsk.) Schmitz. A generalized ranking of the algae used in the experiment from most to least preferred for both urchins included: Galaxaura oblongata, Padina gymnospora (Kutz.) Vick., Halymenia venusta (Forsk.) Borgesen, Dictyota sp., Eucheuma striatum, Gracilaria crassa (Forsk.) Harvey and Cystoseira trinodis (Forsk.) G. Agardh for Diadema setosum and Eucheuma striatum, Galaxaura oblongata, Dictyota sp., Padina gymnospora, Hormophysa triquetra (C.Ag) Kutz., Gracilaria crassa and Hypnea cornuta (Lamouroux) J. Agardh, for Heliocidaris erythrogramma. There was sustained avoidance for Sargassum aquifolium (Turn.) J. Agardh, Turbinaria sp., Boodlea composita (Harv.) Brand and Chaetomorpha crassa (C. Agardh) by both urchins. The agronomic implication of the feeding habits of the herbivores is indicated.