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The non-linear relationship between body size and function in parrotfihes
Stockholm University, Faculty of Science, Department of Systems Ecology. Stockholm University, Stockholm Resilience Centre.
Stockholm University, Faculty of Science, Department of Systems Ecology. Stockholm University, Stockholm Resilience Centre.
Stockholm University, Faculty of Science, Department of Systems Ecology. Stockholm University, Stockholm Resilience Centre.
2008 (English)In: Coral reefs (Print), ISSN 0722-4028, E-ISSN 1432-0975, Vol. 27, no 4, 967-974 p.Article in journal (Refereed) Published
Abstract [en]

Parrotfishes are a group of herbivores that play an important functional role in structuring benthic communities on coral reefs. Increasingly, these fish are being targeted by fishermen, and resultant declines in biomass and abundance may have severe consequences for the dynamics and regeneration of coral reefs. However, the impact of overfishing extends beyond declining fish stocks. It can also lead to demographic changes within species populations where mean body size is reduced. The effect of reduced mean body size on population dynamics is well described in literature but virtually no information exists on how this may influence important ecological functions. The study investigated how one important function, scraping (i.e., the capacity to remove algae and open up bare substratum for coral larval settlement), by three common species of parrotfishes (Scarus niger, Chlorurus sordidus, and Chlorurus strongylocephalus) on coral reefs at Zanzibar (Tanzania) was influenced by the size of individual fishes. There was a non-linear relationship between body size and scraping function for all species examined, and impact through scraping was also found to increase markedly when fish reached a size of 15-20 cm. Thus, coral reefs which have a high abundance and biomass of parrotfish may nonetheless be functionally impaired if dominated by small-sized individuals. Reductions in mean body size within parrotfish populations could, therefore, have functional impacts on coral reefs that previously have been overlooked.

Place, publisher, year, edition, pages
2008. Vol. 27, no 4, 967-974 p.
National Category
Ecology
Identifiers
URN: urn:nbn:se:su:diva-15616DOI: 10.1007/s00338-008-0394-3ISI: 000260616400027OAI: oai:DiVA.org:su-15616DiVA: diva2:182136
Available from: 2008-12-06 Created: 2008-12-06 Last updated: 2012-04-24Bibliographically approved
In thesis
1. Exploring the resilience in coral reefs
Open this publication in new window or tab >>Exploring the resilience in coral reefs
2009 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Considering the unprecedented global decline of coral reefs concerns about their future existence are well-justified. Safeguarding ecological resilience (i.e. the capacity of ecosystems to absorb disturbance without changing their identity) has become a prime goal for management in order to combat further degradation of coral reefs. This thesis uses the concept of ecological resilience as the theoretical framework to analyze vulnerability of coral reefs exposed to human interventions. This thesis consists of four papers. Papers 1-3 are based on field data from Zanzibar Island, Tanzania, whereas Paper 4 is a synthesis that explores the use of the resilience concept in coral reefs, putting the first papers into a broader context.

Paper 1 investigates the distribution and estimate the status of functional groups of coral, fish and sea urchins on five coral reefs outside the western coast of  Zanzibar Island. The study provides a first ecological “baseline” that may help detect future degradation and evaluate the effects of impending management interventions. The results show that reefs with high accessibility, i.e. close to shore and open to fisheries, have lower abundance and diversity of functional groups of both coral and fish compared to more remote or protected reefs. Paper 2 analyzes the impact of artisanal fishing on three key functional groups of herbivorous (grazers, scrapers and bioeroders). The study shows a negative correlation between fishing pressure and fish biomass, abundance, and diversity. The study also demonstrates a negative influence of fishing on the demographic structure of functional groups. Paper 3 focuses on the scraping function (i.e. the capacity of fish to remove algae and open up bare substratum for coral larval settlement) and investigates how body size of individual fishes influences the function. The results reveal a non-linear relationship between body size and scraping function and suggest that fishes start to have a significant impact on the function only after reaching a certain size. The results from Paper 1-3 suggest that human interventions (fishing in particular) can have profound impacts on the distribution and composition of functional groups which influence the vulnerability of coral reefs. Paper 4 provides an overview of the divergent uses of the resilience concept and proposes a range of empirical indicators which can be helpful when assessing coral reef resilience, such as functional groups, the ratio of “good” and “bad” colonizers of space, demographic skewness, discontinuities, the distribution of local phase shifts in the seascape and estimates of potential space availability against grazing capacity.

 

Place, publisher, year, edition, pages
Stockholm: Department of Systems Ecology, Stockholm University, 2009. 50 p.
Keyword
Coral Reefs, Resilience, Functional Groups, Vulnerability, Overfishing, Ecosystem Indicators, Tanzania, Zanzibar
National Category
Ecology
Research subject
Natural Resources Management
Identifiers
urn:nbn:se:su:diva-26659 (URN)978-91-7155-857-2 (ISBN)
Public defence
2009-05-08, William-Olssonsalen, Geovetenskapens hus, Svante Arrhenius väg 8 A, Stockholm, 10:00 (English)
Opponent
Supervisors
Available from: 2009-04-17 Created: 2009-04-06 Last updated: 2009-04-06Bibliographically approved
2. Fish for Food and Ecosystem Function: Fisheries, Trade and Key Ecosystem Processes in Coral Reefs
Open this publication in new window or tab >>Fish for Food and Ecosystem Function: Fisheries, Trade and Key Ecosystem Processes in Coral Reefs
2012 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Fish is a key source of food and income to millions of people living along tropical coastlines. They also play essential roles underpinning key ecosystem processes in coral reefs. For example, herbivorous fish keep algae in check that otherwise may outcompete corals, reducing the reef’s social-ecological values. New fishing methods and globalization have turned fish into global commodities, threatening the ecological resilience of many reefs. This thesis addresses the delicate balance between social and ecological aspects associated with fisheries and trade with reef fish in Zanzibar, Tanzania. Paper I examines how parrotfish contribute to keeping reefs free from algae and shows a non-linear relationship between fish body size and function, hence illustrating that maintaining large fish individuals on coral reefs is particularly important. Gear-based management (GBM) has been suggested as an alternative to no-take marine protected areas (MPAs) to sustain ecosystem processes without threatening fishers’ livelihood. Paper II investigates fishers’ use of different gears and how these gears select different functional groups of fish. Results indicate that gear restrictions could be used to reduce fishing pressure on particularly important groups of fish, but also that such an approach is far from simple. MPAs and GBM are both restrictions targeting fishers, but with increasing global demand for reef fish, it is neither realistic nor fair to place the sole burden of sustainable fisheries on them. Papers III & IV analyze the value chain of fish, from fishers to traders and consumers. Results show that the Zanzibar fishery targets a wide range of fish species and sizes, intended for different markets. The thesis concludes that a sound understanding of the functional properties of fish and more nuanced approaches to regulate fishing may contribute to the management of reef fisheries, but that sustainable solutions will require that measures are taken far beyond the oceans.

Place, publisher, year, edition, pages
Stockholm: Department of Systems Ecology, Stockholm University, 2012. 58 p.
Keyword
fish, food, key ecosystem processes, functional groups, coral reefs, trade, gear, small-scale fisheries, social-ecological systems, Zanzibar, Tanzania, Western Indian Ocean
National Category
Natural Sciences
Research subject
Natural Resources Management
Identifiers
urn:nbn:se:su:diva-75102 (URN)978-91-7447-469-5 (ISBN)
Public defence
2012-05-15, Nordenskiöldsalen, Geovetenskapens hus, Svante Arrhenius väg 12, Stockholm, 09:30 (English)
Opponent
Supervisors
Note

At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 2: Manuscript. Paper 4: Submitted.

Available from: 2012-04-23 Created: 2012-04-05 Last updated: 2012-04-17Bibliographically approved

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