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Cascade effects and sea-urchin overgrazing: An analysis of drivers behind the exploitation of sea-urchin predators for management improvement
Stockholm University, Faculty of Science, Department of Ecology, Environment and Plant Sciences.
Stockholm University, Faculty of Science, Department of Physical Geography. Stockholm University, Faculty of Science, Stockholm Resilience Centre.
Stockholm University, Faculty of Science, Department of Ecology, Environment and Plant Sciences.
Stockholm University, Faculty of Science, Department of Ecology, Environment and Plant Sciences.
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2015 (English)In: Ocean and Coastal Management, ISSN 0964-5691, E-ISSN 1873-524X, Vol. 107, 16-27 p.Article in journal (Refereed) Published
Abstract [en]

Marine ecosystems generate a wide variety of goods and services, but are globally deteriorating due to multiple drivers associated with anthropogenic activities. Intense fishing pressure can lead to changes in structure and function of marine food webs. Particularly overfishing of predatory species at high trophic levels can cause cascading effects leading to ecosystem degradation, affecting both marine organisms and people dependent on them. In the Western Indian Ocean region, intensive fishing takes place and degradation of coral reefs and seagrass beds has been documented. One reason behind this degradation is overgrazing by increasing numbers of sea urchins. An essential step towards better management is to thoroughly understand the drivers leading to such changes in ecosystems. Against this background, the general aim of this study was to gain understanding about whether sea urchin predators in the WIO region are fished, and to identify the drivers behind the fishing of these species. The study had four objectives: (i) to document if and how predatory fish eating sea urchins are caught in smallscale fisheries, (ii) to assess if, and if so why, sea urchin predators are targeted species, (iii) to assess if and to what degree local ecological knowledge (LEK) on ecological complexity involving sea urchins and their predators (e.g. trophic cascades) is present among local fishers, and (iv) to identify fishers' suggestions for management that can reduce problems linked to sea urchin overgrazing. The results show that all investigated species of sea urchin predators are fished by local small-scale fishers. Most sea urchin predators are not actively targeted, are not popular local food fish, and have minor use and economic importance for fishers. This stands in sharp contrast to their ecological keystone role by controlling sea urchin populations. The fishers' awareness and LEK were weak and partly lacking. Management suggestions targeted mostly the symptoms of food web changes rather than the drivers behind them.

Based on the results we suggest that management of degraded ecosystems, as a result of food web changes, should encompass a wide variety of strategies and scales. Specific suggestions for sea urchin predator management are education of local stakeholders on destructive gear effects and food web complexity, further investigations of catch- and release fishing as well as the use of selective gears.

Place, publisher, year, edition, pages
2015. Vol. 107, 16-27 p.
Keyword [en]
food web changes, fisheries management, overgrazing, small-scale fisheries, social-ecological drivers, LEK, trophic cascades, Kenya, Zanzibar
National Category
Biological Sciences
Research subject
Marine Ecotoxicology
Identifiers
URN: urn:nbn:se:su:diva-141046DOI: 10.1016/j.ocecoaman.2015.01.010OAI: oai:DiVA.org:su-141046DiVA: diva2:1085455
Available from: 2017-03-29 Created: 2017-03-29 Last updated: 2017-03-31Bibliographically approved
In thesis
1. Fishing for sustainability: Towards transformation of seagrass-associated small-scale fisheries
Open this publication in new window or tab >>Fishing for sustainability: Towards transformation of seagrass-associated small-scale fisheries
2017 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Small-scale fisheries employ many millions of people around the world, and are particularly important in developing countries, where the dependency on marine resources is high and livelihood diversification options are scarce. In many areas of the world however, small-scale fisheries are at risk which threatens the food security and wellbeing of coastal people. Small-scale fisheries management has in many cases been insufficient and new comprehensive approaches are recommended to achieve social-ecological sustainability in the long-term. The aim of this thesis is to analyze empirically how social-ecological elements of seagrass-associated small-scale fisheries in the Western Indian Ocean region can be addressed for a transformation from the current mostly degraded state to more sustainable social-ecological systems and secure future livelihoods. The main method used was semi-structured interviews with local fishers. The main findings show the crucial contributions seagrass-associated small-scale fisheries make to food security and income generation and highlight the need to acknowledge the social-ecological importance of seagrasses in the seascape (Paper I). A discrepancy between low societal gains of the fishing of sea urchin predator fish species and their crucial importance in the food web (in controlling sea urchin populations and the associated grazing pressure on seagrasses) was identified (Paper II). These results suggest catch-and-release practice of sea urchin predator fish species, which could contribute to more balanced predator – sea urchin – seagrass food webs in the long run. The use of illegal dragnets was identified as a major threat to local seagrass meadows (Paper IV). Institutional elements influencing the use of such destructive dragnet were identified to be normative, cultural-cognitive and economic, which constitutes an institutional misfit to the current emphasis on regulative elements in a hierarchical manner (Paper III). Concerning future co-management initiatives, gear restrictions and education were the favoured management measures among all fishers (Paper IV). A majority of fishers were willing to participate in monitoring and controls, and most fishers thought they themselves and their communities would benefit most from seagrass-specific management. These findings highlight the need for actions on multiple scales, being the local-, management-, policy- and governance levels. The suggested actions include: education and exchange of ecological and scientific knowledge, gear management including the cessation of dragnet fishing, strengthening of local institutions, an active participation of fishers in enforcement of existing rules and regulations and an introduction of adequate alternative livelihood options.

Place, publisher, year, edition, pages
Stockholm: Department of Ecology, Environment and Plant Sciences, Stockholm University, 2017. 69 p.
Keyword
small-scale fisheries, co-management, social-ecological systems, ecosystem services, institutions, transformation, seagrass meadows, destructive gear, food-web interactions
National Category
Ecology
Research subject
Marine Ecotoxicology
Identifiers
urn:nbn:se:su:diva-141052 (URN)978-91-7649-762-3 (ISBN)978-91-7649-763-0 (ISBN)
Public defence
2017-05-05, Vivi Täckholmsalen, NPQ-huset, Svante Arrhenius väg 20, Stockholm, 09:00 (English)
Opponent
Supervisors
Note

At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 1: Manuscript. Paper 4: Manuscript.

Available from: 2017-04-10 Created: 2017-03-29 Last updated: 2017-04-03Bibliographically approved

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Wallner-Hahn, Sieglindde la Torre-Castro, MaricelaEklöf, Johan S.Gullström, Martin
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