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How do seaweed farms influence fishery catches in a seagrass-dominated setting in Chwaka Bay, Zanzibar?
Stockholm University, Faculty of Science, Department of Systems Ecology.
2006 (English)In: Aquatic Living Resources, ISSN 0990-7440, E-ISSN 1765-2952, Vol. 19, no 2, 137-147 p.Article in journal (Refereed) Published
Abstract [en]

Seaweed farming is often depicted as a sustainable form of aquaculture, contributing to poverty reduction and financial revenues in producer countries. However, farms may negatively affect seagrasses and associated organisms (e.g. invertebrate macrofauna) with possible effects on the flow of ecosystem goods and services to coastal societies. The present study investigates the influence of a seaweed farm, and the farmed seaweed Eucheuma denticulatum in particular, on fishery catches using a traditional fishing method ("madema" basket traps) in Chwaka bay (Zanzibar, Tanzania). The results suggest that a seaweed farm, compared to a seagrass bed, had no influence on catch per unit effort (no. of individuals per catch, or catch weight) or no. of species per catch, but significantly affected catch composition (i.e. how much that was caught of which species). The two species contributing most to differences between the sites were two economically important species; the herbivorous seagrass rabbit fish Siganus sutor, which was more common in the seaweed site and is known to graze on the farmed algae; and the benthic invertebrate feeder chloral wrasse Cheilinus chlorourus, more common in the seagrass site. Compared to vegetation-free bottoms, however, the catches were 3-7 times higher, and consisted of a different set of species (ANOSIM global R > 0.4). As traps placed close to the seaweeds fished three times more fish than traps placed on sand patches within the seaweed farm, the overall pattern is attributed to the presence of submerged vegetation, whether seagrass or seaweed, probably as shelter and/or food for fish. However, qualitative differences in terms of spatial and temporal dynamics between seagrass beds with and without seaweed farms, in combination with other factors such as institutional arrangements, indicate that seaweed farms cannot substitute seagrass beds as fishing grounds.

Place, publisher, year, edition, pages
2006. Vol. 19, no 2, 137-147 p.
Keyword [en]
Artisanal fisheries / Fish trap catches / Seaweed farming / Aquaculture / Rhodophyta / Meadows / Seagrass loss / Indian Ocean
National Category
URN: urn:nbn:se:su:diva-22737DOI: 10.1051/alr:2006013OAI: diva2:189342
Part of urn:nbn:se:su:diva-1061Available from: 2006-05-18 Created: 2006-05-18 Last updated: 2010-12-14Bibliographically approved
In thesis
1. Humans and Seagrasses in East Africa: A social-ecological systems approach
Open this publication in new window or tab >>Humans and Seagrasses in East Africa: A social-ecological systems approach
2006 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The present study is one of the first attempts to analyze the societal importance of seagrasses (marine flowering plants) from a Natural Resource Management perspective, using a social-ecological systems (SES) approach. The interdisciplinary study takes place in East Africa (Western Indian Ocean, WIO) and includes in-depth studies in Chwaka Bay, Zanzibar, Tanzania. Natural and social sciences methods were used. The results are presented in six articles, showing that seagrass ecosystems are rich in seagrass species (13) and form an important part of the SES within the tropical seascape of the WIO. Seagrasses provide livelihoods opportunities and basic animal protein, in from of seagrass associated fish e.g. Siganidae and Scaridae. Research, management and education initiatives are, however, nearly non-existent. In Chwaka Bay, the goods and ecosystem services associated with the meadows and also appreciated by locals were fishing and collection grounds as well as substrate for seaweed cultivation. Seagrasses are used as medicines and fertilizers and associated with different beliefs and values. Dema (basket trap) fishery showed clear links to seagrass beds and provided the highest gross income per capita of all economic activities. All showing that the meadows provide social-ecological resilience. Drag-net fishery seems to damage the meadows. Two ecological studies show that artisanal seaweed farming of red algae, mainly done by women and pictured as sustainable in the WIO, has a thinning effect on seagrass beds, reduces associated macrofauna, affects sediments, changes fish catch composition and reduces diversity. Furthermore, it has a negative effect on i.a. women’s health. The two last papers are institutional analyses of the human-seagrass relationship. A broad approach was used to analyze regulative, normative and cultural-cognitive institutions. Cooperation and conflict take place between different institutions, interacting with their slow or fast moving characteristics, and are thus fundamental in directing the system into sustainable/unsustainable paths. Ecological knowledge was heterogeneous and situated. Due to the abundance of resources and high internal control, the SES seems to be entangled in a rigidity trap with the risk of falling into a poverty trap. Regulations were found insufficient to understand SES dynamics. “Well” designed organizational structures for management were found insufficient for “good” institutional performance. The dynamics between individuals embedded in different social and cultural structures showed to be crucial. Bwana Dikos, monitoring officials, placed in villages or landing sites in Zanzibar experienced four dilemmas – kinship, loyalty, poverty and control – which decrease efficiency and affect resilience. Mismatches between institutions themselves, and between institutions and cognitive capacities were identified. Some important practical implications are the need to include seagrass meadows in management and educational plans, addressing a seascape perspective, livelihood diversification, subsistence value, impacts, social-ecological resilience, and a broad institutional approach.

Place, publisher, year, edition, pages
Stockholm: Institutionen för systemekologi, 2006. 62 p.
seagrasses, social-ecological systems, institutions, seaweed farming, artisanal fisheries, common-pool resources, natural resource management, Zanzibar, Tanzania, East Africa, Western Indian Ocean
urn:nbn:se:su:diva-1061 (URN)91-7155-228-6 (ISBN)
Public defence
2006-06-09, Nordenskiöldsalen, Geovetenskapens hus, Svante Arrhenius väg 8 C, Stockholm, 13:00
Available from: 2006-05-18 Created: 2006-05-18Bibliographically approved

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