Phase III – Small-scale community-based, grow-out aquacultures of mud crabs Scylla serrata as a sustainable livelihood in East Africa

Small-scale aquaculture of mud crabs is progressively developing as an alternative source of income for coastal communities in East Africa. In the last decades various NGOs have promoted crab fattening in cages as the culture method. However, crab culture is dependent on collection of small adult crabs as seeds, and mangrove snails (Terebralia rupestris) as feed, and it is unclear if it would be sustainable if expanded to larger scales. The goal of the present project was to assess if small-scale, grow-out culture of mud crabs, in which very small juvenile crabs are grown in ponds until they reach market size, has a better potential to develop into a sustainable alternative livelihood. In view of the above project assessed the following: (1) the socio-economic opportunities and constraints for this activity, (2) if small juvenile mud crabs can be collected in high numbers without negative impact on wild populations, (3) if and how much sustainable crab-feed resources are available in local communities and (4) if profitable grow-out aquaculture methodologies can be developed in local communities.

Targeted beneficiaries include the local communities, policy and decision makers and the scientific community in general. The project was mainly implemented in Kenya and Tanzania and that include for socio-economic and ecological assessments, however, market surveys for mud crab were extended to Mozambique.

Using a multidisciplinary approach, parallel field studies and surveys were carried out in Tanzania and Kenya to assess opportunities and constraints of this activity from both an ecological and socio-economic context. Conducted activities included understanding local communities’ social and cultural acceptance, interests, experience, skills and attitudes towards crab farming, assessing markets and price range with regard to mud crab farming, assessing the impact on the local juvenile mud crab population from seed-crab collection, understanding how crab farming contributes to overall wellbeing of all community members.

Furthermore, studies on population dynamics of T. palustris (food for mudcrab) was investigated together with the availability of local food resources for crab feed. The suitability of some available local food waste resources as potential food for small juvenile mud crabs in grow-out cultures were carried out in a laboratory growth experiment. Other laboratory experiments carried out include; assessment of how size-grading of crabs and refuge habitats in cultures could affect cannibalism rates, assessment of the effect of crab density and shelter and evaluation of grow-out culture methods from South East Asia in an East African setting, comparison of cost and profit margins of grow-out and fattening aquaculture of mud crabs in Kenya and Tanzania and cost-revenue analyses for production of 450g commercially sized crabs (i.e. 500 g) with the two culture methods.

Three years of multidisciplinary studies in this project had identified both possibilities and constraints for the development of a small-scale, grow-out aquaculture of mud crabs in East Africa. The established obstacles for developing small-scale crab farms include theft, lack of funds for the initial investment, lack of training and low crab prices. The project found that several groups of people were interested to participate, including women.

Although ecological studies demonstrated that high numbers of small juvenile mud crabs could be collected for grow-out aquaculture with minimal impact on the environment and local mud crab populations, analyses of the sustainability of grow-out mud crab farms demonstrated that both the availability of small seed-crabs and local sources of crab feed put clear limits on the maximum size of grow-out aquaculture farms in coastal communities, which need to be taken into account if these activities should be promoted.

Cultivation studies showed that small juvenile crabs could survive and grow in pond cultures, and show high growth rates on alternative feed sources available in coastal communities that were not used for human consumption. However, full-scale pond trials showed variable growth and high losses of crabs when crabs were larger, demonstrating that the pond culture methods need to be developed further before it can be recommended for coastal communities in East Africa.

Field experiments demonstrated that natural mortality was higher for small juveniles compared to larger crabs, suggesting that it is more sustainable to use small seed-crabs than larger ones. Growth and survival of crabs in fattening cultures were found to be lower than in nature. This activity is therefore less productive and sustainable than traditional fishing of crabs.

Furthermore, market analyses showed that prices for mud crabs were over 50% lower in Tanzania than in other East African countries, and that most of the profit goes to middlemen and exporters. In addition, cost-revenue analyses of crab-fattening demonstrated a very poor profitability when realistic labour costs were taken into account. Thus, crab fattening does not appear to constitute a sustainable or profitable alternative livelihood. Cost-revenue analyses of grow-out aquacultures demonstrated small profit margins, and a positive return of the investment only if prices above $US 3.4 per kg are obtained, which is achievable in Kenya and Mozambique, but difficult in Tanzania due to the low prices.

The results of this project has clearly shown that the pond culture methods need to be developed further before it can be recommended for coastal communities in East Africa. Furthermore, the project has shown that new markets and market-chains for crab farmers need to be developed particularly in Tanzania, in order to improve their profits.

Future studies can focus on finding more information with respect to feed and impacts on growth performance of larger sizes of crabs.