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Tropical seaweed beds as important habitats for juvenile fish in an East African seascape
Stockholm University, Faculty of Science, Department of Ecology, Environment and Plant Sciences.ORCID iD: 0000-0003-1359-703X
Stockholm University, Faculty of Science, Stockholm University Baltic Sea 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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(English)Manuscript (preprint) (Other academic)
Abstract [en]

Seaweed beds within tropical seascapes have received little attention as potential fish habitat despite being a prominent feature within these systems. Other tropical shallow-water habitats such as seagrass meadows and mangroves are relatively well studied and are commonly recognised as important nurseries for several species of coral reef fishes. However, there are indications that structural complexity may be more important for the juvenile fish community than the habitat type itself, which implies that other shallow habitats with high structural complexity, like seaweed beds, could also be important for fish recruitment. This study therefore investigated the role of seaweed beds as fish habitat, particularly for juveniles, in the Western Indian Ocean by comparing their fish assemblages to that of closely situated seagrass beds.

Fish assemblages were assessed by visual census in belt transects, where fish were identified and their length estimated, and habitat variables were estimated for each transect.

Total fish abundance was found to be similar between seaweed and seagrass habitats, while abundance of juvenile fishes was higher in seaweed beds than in seagrass meadows (25.0±13.7 vs 10.1±10.3 per transect), with no differences in subadult and adult fish abundance. Species richness was higher in seaweed beds than in seagrass meadows (11.2±3.1 vs 8.2±3.9 per transect), and seaweed beds also had higher juvenile abundance of commercially important (19.6±12.3 vs 7.6±8.9 per transect) and coral reef associated fish species (21.1±13.0 vs 3.9±5.3 per transect) than did seagrass meadows. The total fish assemblages, as well as juvenile family communities, differed between seaweed and seagrass habitat, with the fish communities of seaweed beds being less variable than those of seagrass meadows. These results highlight that tropical seaweed beds are important as juvenile fish habitats, and underscore the need to widen the view of the shallow tropical seascape and incorporate seaweed beds in management actions. 

National Category
Biological Sciences
Research subject
Marine Ecotoxicology; Marine Ecology
URN: urn:nbn:se:su:diva-129253OAI: diva2:920611
Sida - Swedish International Development Cooperation Agency, SWE2010-052
Available from: 2016-04-18 Created: 2016-04-18 Last updated: 2016-05-16
In thesis
1. Seaweed in the tropical seascape: Importance, problems and potential
Open this publication in new window or tab >>Seaweed in the tropical seascape: Importance, problems and potential
2016 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The increasing demand for seaweed extracts has led to the introduction of non-native seaweeds for farming purposes in many tropical regions. Such intentional introductions can lead to spread of non-native seaweeds from farming areas, which can become established in and alter the dynamics of the recipient ecosystems. While tropical seaweeds are of great interest for aquaculture, and have received much attention as pests in the coral reef literature, little is known about the problems and potential of natural populations, or the role of natural seaweed beds in the tropical seascape.

This thesis aims to investigate the spread of non-native genetic strains of the tropical macroalga Eucheuma denticulatum, which have been intentionally introduced for seaweed farming purposes in East Africa, and to evaluate the state of the genetically distinct but morphologically similar native populations. Additionally it aims to investigate the ecological role of seaweed beds in terms of the habitat utilization by fish and mobile invertebrate epifauna. The thesis also aims to evaluate the potential of native populations of eucheumoid seaweeds in regard to seaweed farming.

The initial results showed that non-native E. denticulatum is the dominating form of wild eucheumoid, not only in areas in close proximity to seaweed farms, but also in areas where farming has never occurred, while native eucheumoids are now scarce (Paper I). The low frequency of native E. denticulatum in seaweed beds, coupled with a low occurrence of reproductive structures, indicates that the effective population size may be low, which in turn may be a threat under changing environmental conditions. These results, combined with indications that seaweeds may be declining in East Africa, illustrates the need for attaining a better understanding of the ecological role of tropical seaweed habitats. The studies on the faunal communities of seaweed beds showed that they are species rich habitats, with high abundances of juvenile fish and mobile epifauna (Paper II and III), strongly indicating that these habitats should be considered for future seascape studies and management actions. Productivity in East African seaweed farming is decreasing, and as the current cultivation is based on a single non-indigenous haplotype, a more diverse genetic base has been suggested as a means to achieve a more productive and sustainable seaweed farming. Although our results show that East African E. denticulatum has a lower growth rate than the currently used cultivar (Paper IV), the several native haplotypes that are present in wild populations illustrates that, though a demanding endeavour, there is potential for strain selection within native populations.

Place, publisher, year, edition, pages
Stockholm: Department of Ecology, Environment and Plant Sciences, Stockholm University, 2016. 49 p.
Seaweed, Macroalgae, Eucheumoids, Non-indigenous, Haplotype, Fish, Nursery, Epifauna, Diversity, Seaweed farming, Zanzibar, East Africa, Tropical
National Category
Research subject
Marine Ecotoxicology
urn:nbn:se:su:diva-129256 (URN)978-91-7649-396-0 (ISBN)
Public defence
2016-06-08, Vivi Täckholm (Q-salen), NPQ-huset, Svante Arrhenius väg 20, Stockholm, 13:00 (English)

At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 2: Manuscript. Paper 3: Manuscript. Paper 4: Manuscript.

Available from: 2016-05-16 Created: 2016-04-18 Last updated: 2016-05-17Bibliographically approved

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Tano, Stina
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