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Mirera, David OerstedORCID iD iconorcid.org/0000-0003-1556-096X
Publications (6 of 6) Show all publications
Mirera, D. O. & Moksnes, P.-O. (2015). Comparative performance of wild juvenile mud crab (Scylla serrata) in different culture systems in East Africa: effect of shelter, crab size and stocking density. Aquaculture International, 23(1), 155-173
Open this publication in new window or tab >>Comparative performance of wild juvenile mud crab (Scylla serrata) in different culture systems in East Africa: effect of shelter, crab size and stocking density
2015 (English)In: Aquaculture International, ISSN 0967-6120, E-ISSN 1573-143X, Vol. 23, no 1, p. 155-173Article in journal (Refereed) Published
Abstract [en]

Grow-out culture of mud crabs Scylla serrata in East Africa is at an earlier development phase and is dependent on wild seed crabs. We assessed three different culture systems (net cages, ponds and pens) in three treatments (shelter, size and density) to evaluate survival and growth in small-scale culture of mud crabs in Kenya. In small nursery cages, we assessed how availability of shelter, stocking density and size-class separation affected cannibalistic rates in small juveniles (20-80 mm internal carapace width) in 7-day experiments. The result indicated that shelter and size-class separation decreased cannibalism and mortality with 26 and 31 %, respectively, whereas no significant effect was found for different stocking densities. Earthen ponds and mangrove pens were used to compare growth and survival in long-term studies (2-4 months) in the presence and absence of shelter. Treatments with and without shelter yielded low overall recovery of crabs (4-26 %) indicating high mortality rates, and there was no significant effect of shelter or culture system on survival. In contrast, growth rate was high in both pens and ponds, but significantly lower in pen systems without shelter. Generally, the results indicated that cannibalism is the largest source of mortality in different culture systems (net cages, ponds and pens), and use of shelter and size grading of crabs improved survival significantly. In contrast, growth rates were high and comparable to natural growth in both pond and pen culture when shelter was provided. Using growth models to compare growth and survival in mud crabs from aquaculture studies in the literature, we show that shelter may have a stronger effect on growth than has been previously thought, whereas crab density appears to impact more on crab survival. Thus, improving survival in grow-out culture systems is a challenge that remains to be solved for small-scale mud crab culture in East Africa.

Keywords
Mud crab, Net cages, Ponds, Pens, Shelter, Density, Survival, Growth
National Category
Fish and Aquacultural Science Ecology
Research subject
Natural Science, Aquatic Ecology
Identifiers
urn:nbn:se:lnu:diva-40896 (URN)10.1007/s10499-014-9805-3 (DOI)000348538200013 ()2-s2.0-84939873246 (Scopus ID)
Available from: 2015-03-17 Created: 2015-03-17 Last updated: 2017-12-04Bibliographically approved
Moksnes, P.-O., Mirera, D. O., Björkvik, E., Hamad, M. I., Mahudi, H. M., Nyqvist, D., . . . Troell, M. (2015). Stepwise function of natural growth for Scylla serrata in East Africa: a valuable tool for assessing growth of mud crabs in aquaculture. Aquaculture Research, 46(12), 2938-2953
Open this publication in new window or tab >>Stepwise function of natural growth for Scylla serrata in East Africa: a valuable tool for assessing growth of mud crabs in aquaculture
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2015 (English)In: Aquaculture Research, ISSN 1355-557X, E-ISSN 1365-2109, Vol. 46, no 12, p. 2938-2953Article in journal (Refereed) Published
Abstract [en]

Predicting growth is critical in aquaculture, but models of growth are largely missing for mud crab species. Here, we present the first model of natural growth in juvenile and adult mud crabs Scylla serrata from East Africa using a stepwise growth function based on data on intermoult periods and growth at moult from field mark-recapture, pond and laboratory studies. The results showed a sigmoid growth pattern in carapace width and suggest that S.serrata in East Africa will reach 300g and sexual maturity similar to 9.9months after settlement, and a commercial size of 500g after 12.4months. Analyses of the literature identified several issues with the common praxis to compare standard growth measures between aquaculture studies with different initial size or growing periods. Using the new growth function to estimate the proportional difference between modelled and obtained growth as an alternative method, we show that growth rates of S.serrata cultured in cage systems, which are dominant in East Africa, was <40% of the estimated natural growth and growth obtained in pond systems. The analysis also indicated that growth rates of S.serrata in Southeast Asia was over 50% higher compared with similar culture systems in East Africa, and that different species of mud crabs had large differences in growth rates. This study shows that growth in the present mud crab aquaculture systems in East Africa is below their expected potential. Further work is needed to identify the factors behind this observation.

Keywords
mud crab, moult increment, intermoult period, segmented growth, aquaculture
National Category
Fish and Aquacultural Science
Research subject
Ecology, Aquatic Ecology
Identifiers
urn:nbn:se:lnu:diva-47377 (URN)10.1111/are.12449 (DOI)000363594500011 ()2-s2.0-84945494423 (Scopus ID)
Available from: 2015-11-24 Created: 2015-11-24 Last updated: 2019-01-23Bibliographically approved
Mirera, D. O. (2014). Capture-based mud crab (Scylla serrata) aquaculture and artisanal fishery in East Africa- Practical and ecological perspectives: Mud crab ecology and aquaculture. (Doctoral dissertation). Vaxjo: Linnaeus University Press
Open this publication in new window or tab >>Capture-based mud crab (Scylla serrata) aquaculture and artisanal fishery in East Africa- Practical and ecological perspectives: Mud crab ecology and aquaculture
2014 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Mud crab Scylla serrata is a crustacean that spends most of its life cycle in the mangrove environment throughout its range. Fishery and aquaculture of this crab are significant economic activities in coastal areas in the tropics and sub-tropics because of the meat quality and nutritional value. However there is a significant shortage of information on the ecology, fishery and aquaculture of these crabs in sub-Saharan Africa. This impacts the development of a sustainable aquaculture and fishery for the benefit of coastal communities. The present study analyses various aspects of mud crab ecology, fishery, aquaculture and social economics in East Africa using multidisciplinary approaches. The results are given in seven papers based on field and laboratory studies. The study established for the first time that high intertidal mangrove back-flats constitute a key habitat for the earliest instars of S. serrata (4 -30 mm CW). It also showed that diurnal tidal migration behaviour occurs in small juveniles that migrate to sub-tidal habitats during the day, possibly due to variable predation risks. Monthly sampling of juveniles in Kenya and Tanzania indicated continuous recruitment throughout the year. The large numbers of juvenile crabs along mangrove fringes indicate that these habitats could serve as sites suitable for collection of juvenile crabs for aquaculture. However, these areas must also be managed and protected to support the recruitment to the wild crab populations. An assessment of the crab fishery indicated that artisanal crab fishers possess significant traditional knowledge mainly inherited from their parents that enabled them to exploit the resource. Such knowledge could be useful for the development of the aquaculture and in management of the fishery. Mud crab fishing was found to be a male dominated activity, and fishers on foot practiced fishing in burrows at spring low tides. Interviews indicated that the average size of marketable crabs has declined over the years and a weak management system was observed with most fishers operating without a license. Due to the knowledge required regarding the local conditions, fishers are unable to shift to new areas. Furthermore fishers and could not fish at neap tides. Such limitations provide a “natural closure” of the fishery. Also foot fishers cover fairly limited distances in their daily operations, an aspect that can be utilized to effect site-specific management for the fishery if necessary. Laboratory and field experiments indicated that cannibalistic interactions are heavily influenced both by size differences of crabs and the availability of shelter but no significant effect was found for different stocking densities. Such information is of direct importance for crab farmers in East Africa, where seed from the wild are of multiple sizes and there is a need to grade juvenile crabs and provide shelter at stocking to ensure maximum survival. Experimental studies in earthen pond and mangrove pen cultures indicated high mortality rates. Comparing growth in earthen pond and mangrove pen systems indicated that growth rates were generally high in both systems, but significantly lower in pen systems without shelter, suggesting that shelter may have a stronger effect on growth than has been previously thought. Similar to artisanal mud crab fishery, an assessment of small-scale mud crab farming by organized community groups in Kenya indicated low level of women participation. A good knowledge of the market existed among the mud crab farming groups where hotels and exporters offered the highest prices. However there is a need for national policies to be directed to support small-scale aquaculture development by ensuring training and capacity building for women, operation and management of groups, data management and provision of user rights for communities working in the mangrove environment. Market analyses showed that the common market size of crabs in East Africa ranged between 500-1000 g and are thus larger than in Southeast Asia where the average size is reported at 300 g. Prices for mud crabs were over 50 % lower in Tanzania than in other East African countries and most of the profit was earned by middlemen and exporters. Cost revenue analysis showed that it would be more profitable to farm smaller commercial crabs, and develop a market for 300 g crabs to increase the profitability of crab farming in East Africa. Also, the same analyses found that farming large crabs in individual cages, which is the dominant culture form in East Africa today, had very low profitability due to high labor costs and low growth rates. Using a step-wise function of natural growth it was shown that growth rates of S. serrata cultured in cages was 40 % of the growth rates obtained in experimental pond and pen cultures, which were similar to natural growth. Therefore the good performance of grow-out cultures of juvenile mud crabs in earthen ponds and mangrove pens showed a potential to develop into a profitable and sustainable intervention. However, more work is needed to improve survival in culture systems and address the identified limitations of crab seeds and feed to enable development of sustainable mud crab aquaculture in East Africa.

Place, publisher, year, edition, pages
Vaxjo: Linnaeus University Press, 2014. p. 79
Series
Linnaeus University Dissertations ; 159/2014
Keywords
Mud crab, ecology, aquaculture, economics
National Category
Fish and Aquacultural Science
Research subject
Ecology, Aquatic Ecology
Identifiers
urn:nbn:se:lnu:diva-32399 (URN)978-91-87427-70-1 (ISBN)
Public defence
2014-02-20, Fullriggaren, Landgången 4, Kalmar, 09:00 (English)
Opponent
Supervisors
Available from: 2014-02-25 Created: 2014-02-19 Last updated: 2017-02-14Bibliographically approved
Mirera, D. O., Ochiewo, J. & Munyi, F. (2014). Social and economic implications of small-scale mud crab (Scylla serrata) aquaculture: the case of organised community groups. Aquaculture International, 22(4), 1499-1514
Open this publication in new window or tab >>Social and economic implications of small-scale mud crab (Scylla serrata) aquaculture: the case of organised community groups
2014 (English)In: Aquaculture International, ISSN 0967-6120, E-ISSN 1573-143X, Vol. 22, no 4, p. 1499-1514Article in journal (Refereed) Published
Abstract [en]

Small-scale mud crab aquaculture was introduced in East Africa in late 1990s as a mangrove friendly aquaculture to improve the livelihood of coastal communities through organised community groups (OCGs). The OCGs approach was a strategy aimed at ensuring benefits to the village and regulating access to the open access resource (land in mangrove forests). A social and economic assessment was conducted at Majaoni, Makongeni, Ihaleni and Dabaso along the Kenyan coast. The paper looks at the social and economic characteristics of the groups, their management systems, contribution of mud crab aquaculture to the livelihoods of local communities and policy issues related to the interventions. There is wider involvement of the mijikenda community in mud crab farming; however, the level of women participation is low. A strong market link exists between the groups and the different market outlets, where hotels and exporters offer the highest prices. Regional price variations existed and may need networking between the groups. The paper concludes that national policies may need to be redirected to support small-scale aquaculture development and ensuring capacity building for women, operation and management of groups, provision of extension services, data management and provision of user rights for communities working in the mangrove environment.

Keywords
Aquaculture, Community based, Mud crab, Small-scale, Social and economic
National Category
Ecology
Research subject
Natural Science, Aquatic Ecology
Identifiers
urn:nbn:se:lnu:diva-36827 (URN)10.1007/s10499-014-9762-x (DOI)000339106200020 ()2-s2.0-84903879811 (Scopus ID)
Available from: 2014-09-10 Created: 2014-09-10 Last updated: 2017-12-05Bibliographically approved
Mirera, D. O. & Moksnes, P.-O. (2013). Cannibalistic interactions of juvenile mud crabs Scylla serrata: the effect of shelter and crab size. African Journal of Marine Science, 35(4), 545-553
Open this publication in new window or tab >>Cannibalistic interactions of juvenile mud crabs Scylla serrata: the effect of shelter and crab size
2013 (English)In: African Journal of Marine Science, ISSN 1814-232X, E-ISSN 1814-2338, Vol. 35, no 4, p. 545-553Article in journal (Refereed) Published
Abstract [en]

In the culture of mud crab Scylla serrata, cannibalism is often the greatest cause of mortality. A laboratory study was conducted to compare the influence of size class differences and shelter on cannibalism and limb loss in juvenile mud crabs (20-70 mm internal carapace width; ICW). Four size classes of juvenile crab (A: 21-30 mm, B: 31-40 mm, C: 41-50 mm and D: 51-70 mm ICW) were tested in all possible combinations using four different substrata with varying degree of shelter (seaweed, plastic strings, bamboo tubes and open sand substratum) in 48 h trials. Results suggest that cannibalistic interactions are heavily influenced both by size differences of crabs and the availability of shelter. Cannibalism on the smallest size class (20-30 mm ICW) increased about 10 times in the presence of the largest crab (51-70 mm ICW) compared with treatment with only same-size crabs (control treatment). Shelter provided little refuge for the smallest crabs, whereas cannibalism in larger size classes decreased by >50% in all the shelters compared with the sand substratum. The findings suggest that both size-grading and provision of shelter could minimise cannibalism in the culture of mud crabs.

Keywords
cannibalism, mortality, size combination, size grading, substrata
National Category
Ecology
Research subject
Natural Science, Evolutionary Biology
Identifiers
urn:nbn:se:lnu:diva-32078 (URN)10.2989/1814232X.2013.865677 (DOI)000328940500008 ()2-s2.0-84891293405 (Scopus ID)
Available from: 2014-02-05 Created: 2014-02-05 Last updated: 2017-12-06Bibliographically approved
Mirera, D. O., Ochiewo, J., Munyi, F. & Muriuki, T. (2013). Heredity or traditional knowledge: Fishing tactics and dynamics of artisanal mangrove crab (Scylla serrata) fishery. Ocean and Coastal Management, 84, 119-129
Open this publication in new window or tab >>Heredity or traditional knowledge: Fishing tactics and dynamics of artisanal mangrove crab (Scylla serrata) fishery
2013 (English)In: Ocean and Coastal Management, ISSN 0964-5691, E-ISSN 1873-524X, Vol. 84, p. 119-129Article in journal (Refereed) Published
Abstract [en]

Globally, artisanal fisheries are challenged by the combined impacts of overfishing, ecosystem degradation, climate change and lack of management intervention. Little is known of how traditional knowledge/skills held by fishers influence ability to exploit marine resources and whether such information could be incorporated into management practices. Failure to understand fishers' traditional knowledge/skills and behaviour may undermine the success of fisheries management measures. Use of traditional knowledge in exploitation of mangrove crabs (Scylla serrata) on local fishing grounds is assessed. The study used complementary approaches (field experiments and fisher-based surveys) to investigate fishing tactics employed by artisanal crab fishers. Also time series data from the ministry of fisheries was used to assess annual trends in production and value of the fishery. Fishers with calibrated GPS fished in different areas and catch per unit effort (CPUE), fishing time, distance and efficiency were assessed. Key informant and semi-structured interviews and direct observation were conducted among mud crab fishers to establish knowledge/skill requirement, fishing tactics and entry into the fishery. The results indicate that mud crab fishing is a male dominated activity. Fishing is done at spring low tide by foot fishers in burrows mainly with rare use of baited traps and lift nets at ankle height water along the intertidal mangrove front boundaries or channels. Fisher's follow specific tracks that are strongly guarded by individual fishers. Fishing for adult crabs showed deeper and further movement in the mangrove forest unlike juvenile crab collection. Fisher's in new areas collected few crabs due to low efficiency compared to well known areas. Entry into the fishery is minimal and skills are inherited from parents or grand parents with limited transfer being obtained from colleagues. The average size of marketable crabs has declined over the years while season and tide have remained major challenges for production and market determination in the fishery. Fisher-based surveys overestimated CPUE and sizes of smallest crabs caught while it underestimated fishing time and distance moved. A weak management system was observed with most fishers operating without a license and lacking a synchronized landing system. Inability of fishers to shift to new areas, "natural closures" and limited distance covered by foot fisher's support site-specific management for the fishery.

National Category
Biological Sciences
Research subject
Natural Science, Aquatic Ecology
Identifiers
urn:nbn:se:lnu:diva-30995 (URN)10.1016/j.ocecoaman.2013.08.002 (DOI)000326364100012 ()2-s2.0-84883530091 (Scopus ID)
Available from: 2013-12-06 Created: 2013-12-06 Last updated: 2017-12-06Bibliographically approved
Organisations
Identifiers
ORCID iD: ORCID iD iconorcid.org/0000-0003-1556-096X

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