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Seaweed in the tropical seascape: Importance, problems and potential
Stockholm University, Faculty of Science, Department of Ecology, Environment and Plant Sciences. Stockholms universitetsbibliotek.ORCID iD: 0000-0003-1359-703X
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.
Keyword [en]
Seaweed, Macroalgae, Eucheumoids, Non-indigenous, Haplotype, Fish, Nursery, Epifauna, Diversity, Seaweed farming, Zanzibar, East Africa, Tropical
National Category
Ecology
Research subject
Marine Ecotoxicology
Identifiers
URN: urn:nbn:se:su:diva-129256ISBN: 978-91-7649-396-0 (print)OAI: oai:DiVA.org:su-129256DiVA: diva2:920632
Public defence
2016-06-08, Vivi Täckholm (Q-salen), NPQ-huset, Svante Arrhenius väg 20, Stockholm, 13:00 (English)
Supervisors
Note

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: 2017-02-24Bibliographically approved
List of papers
1. Extensive spread of farmed seaweeds causes a shift from native to non-native haplotypes in natural seaweed beds
Open this publication in new window or tab >>Extensive spread of farmed seaweeds causes a shift from native to non-native haplotypes in natural seaweed beds
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2015 (English)In: Marine Biology, ISSN 0025-3162, E-ISSN 1432-1793, Vol. 162, no 10, 1983-1992 p.Article in journal (Refereed) Published
Abstract [en]

Seaweed farming has been the cause of introductions of non-indigenous seaweed species and genotypes throughout the world. In Zanzibar, Tanzania, foreign genotypes of Eucheuma denticulatum were introduced for farming purposes in 1989, and in recent years a spread of non-indigenous haplotypes has been reported. The current study aimed to investigate the presence and extent of introduced and native haplotypes of E. denticulatum as well as their relative frequencies, to obtain the severity of the spread of cultivated seaweed and the current state of the native populations. The results show that all investigated sites are dominated by the introduced South-east Asian haplotypes, even where seaweed farming has never occurred. As the frequencies of East African haplotypes are remarkably low, this shows a shift from native to introduced E. denticulatum. This shift may, at least in part, be caused by earlier overharvest of natural seaweed populations, and indicates a cryptic invasion of the introduced haplotypes at the potential cost of the recovery of the native haplotype populations.

National Category
Biological Sciences
Research subject
Marine Ecotoxicology; Marine Ecology
Identifiers
urn:nbn:se:su:diva-122757 (URN)10.1007/s00227-015-2724-7 (DOI)000362322200005 ()
Funder
Sida - Swedish International Development Cooperation Agency, SWE2010-052
Available from: 2015-11-16 Created: 2015-11-10 Last updated: 2017-12-01Bibliographically approved
2. Tropical seaweed beds as important habitats for juvenile fish in an East African seascape
Open this publication in new window or tab >>Tropical seaweed beds as important habitats for juvenile fish in an East African seascape
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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
Identifiers
urn:nbn:se:su:diva-129253 (URN)
Funder
Sida - Swedish International Development Cooperation Agency, SWE2010-052
Available from: 2016-04-18 Created: 2016-04-18 Last updated: 2016-05-16
3. Tropical seaweed beds are important habitats for mobile invertebrate epifauna
Open this publication in new window or tab >>Tropical seaweed beds are important habitats for mobile invertebrate epifauna
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(English)Manuscript (preprint) (Other academic)
Abstract [en]

Marine macrophyte habitats in temperate regions are known to provide productive habitats for numerous organisms, with their abundant and diverse invertebrate epifaunal assemblages constituting important linkages between benthic primary production and higher trophic levels. While it is commonly also recognized that certain vegetated habitats in the tropics, such as seagrass meadows, can harbour diverse epifaunal assemblages and may constitute important feeding grounds to fish, little is known about the epifaunal assemblages associated with tropical seaweed beds.

We investigated the abundance, biomass and taxon richness of the mobile epifaunal community of tropical East African seaweed beds, as well as the abundance of invertivorous fish, and compared it with that of closely situated seagrass meadows, to establish the ecological role of seaweed beds as habitat for epifauna as well as potential feeding grounds for fish. The results showed that seaweed beds had a higher abundance of mobile epifauna (10565±5954 vs 3742±2788 per m2) than seagrass meadows, as well as a higher biomass (35.9±46.8 vs 1.9±2.1 g per m2) and taxon richness (32.7±11.8 vs 19.1±6.3 taxa per sample). Additionally, the high abundance of invertivorous fish found in seaweed beds indicates that they act as important feeding grounds to several fish species in the region.

National Category
Biological Sciences
Research subject
Marine Ecotoxicology; Marine Ecology
Identifiers
urn:nbn:se:su:diva-129254 (URN)
Funder
Sida - Swedish International Development Cooperation Agency, SWE2010-052
Available from: 2016-04-18 Created: 2016-04-18 Last updated: 2016-05-16
4. The introduction of South East Asian seaweed and its ecological implications; Can native East African Eucheuma denticulatum and Kappaphycus alvarezii be a potential alternative for farming?
Open this publication in new window or tab >>The introduction of South East Asian seaweed and its ecological implications; Can native East African Eucheuma denticulatum and Kappaphycus alvarezii be a potential alternative for farming?
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(English)Manuscript (preprint) (Other academic)
Abstract [en]

The seaweed farming of eucheumoids in East Africa is solely based on introduced South East Asian (SEA) haplotypes of the carrageenophytes Eucheuma and Kappaphycus. As overexploitation of natural seaweed resources lead to a decline in harvest and export, commercial seaweed farming was started using highly productive SEA strains of the same genus introduced from the Philippines to Zanzibar in 1989.  Initially, productivity was high, the sector grew rapidly and seaweed farming soon became an important livelihood. Today, the industry faces various challenges such as decreased productivity and high rates of diseases and epiphytic infestations. Continuous introduction of foreign stock for cultivation vitalization might not be the solution, as escapees of SEA Eucheuma denticulatum have been found spreading into natural environments around Zanzibar with uncertain ecological consequences.  We suggest that indigenous haplotypes of E. denticulatum and Kappaphycus alvarezii should be re-evaluated for farming potential,  for increasing the genetic diversity and hence resilience within stocks.

This study is a first step towards a reassessment of farming potential of East African (EA) haplotypes. Molecularly identified haplotypes of E. denticulatum and K. alvarezii were tested in in-situ farming conditions in Zanzibar, and growth rates, grazing and epiphytes were compared between EA and SEA haplotypes. Results show, except for an overall decreased growth compared to previous studies, that growth rate was site dependent and that SEA Eucheuma haplotypes have a higher growth rate (1.3 ±1.8 - 3.6 ±1.9% per day) compared to EA haplotypes (0.2 ±1.0 - 2.0 ±0.4% per day). No significant differences were found in grazing rate between native and introduced Eucheuma haplotypes, while native Kappaphycus was more prone to grazing. 

In conclusion the farming potential for native E. denticulatum, is not rejected but underlines that there is an urgent need of continued search for native East African seaweed resources and a further identification of their desirable traits. If successful, this would enable East African seaweed industry to further expansion and secure its ecological and economical sustainability.

National Category
Biological Sciences
Research subject
Marine Ecotoxicology; Marine Ecology
Identifiers
urn:nbn:se:su:diva-129255 (URN)
Funder
Sida - Swedish International Development Cooperation Agency, SWE2010-052
Available from: 2016-04-18 Created: 2016-04-18 Last updated: 2016-05-16

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