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Fish community patterns in Tanzanian mangrove creeks
Stockholm University, Faculty of Science, Department of Zoology. (Ecology)
2009 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Human beings have exploited the biota of mangrove systems for centuries and fish continue to be one of the main products harvested from these habitats. The assumption of mangroves functioning as recruitment areas for juvenile fish from neighbouring habitats such as seagrass beds and coral reefs is a common argument for conservation and management of mangrove ecosystems. Yet, mangrove systems are increasingly in decline worldwide primarily because of destructive anthropogenic activities. In the present thesis, fish assemblages in various mangrove creeks in Tanzania were investigated. Specifically, it dealt with the distribution, abundance and trophic structure of fishes in space and time as predicted by habitat characteristics and environmental conditions occurring naturally and following human-induced changes of clearing for construction of ponds for salt production and fish farming. In addition, the variation in population density and genetic connectivity of the economically important striped mullet (Mugil cephalus) was examined. In undisturbed creeks the fish abundance and assemblage composition showed inconsistent spatial patterns within and among different creeks that appear to be strongly influenced by species-specific responses to dominant bottom substrate types and to some extent to water depth and clarity. Seasonality had little impact on most fish assemblage variables. The genetic analysis showed mixed demes of Mugil cephalus and significant separation between Zanzibar Island and Tanzania mainland. Anthropogenic effects were found on the structural and functional organisation of fish assemblages resulting in lower densities, species numbers, diversity and number of trophic groups in the reservoirs compared to less disturbed sites. These changes in fish assemblage composition seem to be caused by an increase in salinity and water temperature as well as by the loss of refuge and feeding grounds. The findings in this thesis will contribute to coastal management planning for conservation and suggests integration of various activities (like fish farming and salt production) in order to maximise the economical use of deforested areas.

Place, publisher, year, edition, pages
Stockholm: Department of Zoology, Stockholm University , 2009. , 38 p.
Keyword [en]
Mangrove ecosystems, fish assemblages, Mugil cephalus, population genetics, spatial and seasonal variability, human disturbance, East Africa
National Category
Natural Sciences
Research subject
Animal Ecology
Identifiers
URN: urn:nbn:se:su:diva-27168ISBN: 978-91-7155-879-4 (print)OAI: oai:DiVA.org:su-27168DiVA: diva2:212771
Public defence
2009-05-27, William-Olssonsalen, Geovetenskapens hus, Svante Arrhenius väg 8 A, Stockholm, 10:00 (English)
Opponent
Supervisors
Available from: 2009-05-06 Created: 2009-04-23 Last updated: 2009-05-05Bibliographically approved
List of papers
1. Spatial and seasonal variation of fish assemblages in mangrove creek systems in Zanzibar (Tanzania)
Open this publication in new window or tab >>Spatial and seasonal variation of fish assemblages in mangrove creek systems in Zanzibar (Tanzania)
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2010 (English)In: Estuarine, Coastal and Shelf Science, ISSN 0272-7714, E-ISSN 1096-0015, Vol. 89, no 4, 277-286 p.Article in journal (Refereed) Published
Abstract [en]

Spatial and seasonal variations of fish assemblage composition were studied in three non-estuarine mangrove creeks of Zanzibar (Tanzania). Fish were collected monthly for one year at three sites (lower, intermediate and upper reaches) in each creek using a seine net (each haul covering 170 m(2)). Density, species number and diversity of fish were all higher at sites with dense cover of macrophytes (seagrass and macroalgae) than over unvegetated sandy sites. In general, fish assemblages mainly comprised juveniles of a few abundant taxa, e.g. Mugil cephalus, Mugilidae spp. and Leiognathus equulus at sites with mud substratum and Germs oyena, Lethrinus harak and Sillago sihama at sites dominated by macrophytes. Multivariate analyses revealed significant separations in fish assemblage composition within the two creeks where the bottom substratum differed among sites. Overall, season seemed to have little effect on density, species number, diversity index (H') and assemblage structure of fish. Water condition variables were also relatively stable across the season, although a short-term fluctuation primarily induced by decreased salinity, occurred during the heavy rains in April and May. Fish assemblage structure was not significantly affected by any of the abiotic factors tested. However, significant regressions were found between the other fish variables and environmental variables, but since these associations were mostly species-specific and generally inconsistent, we suggest that the overall distribution patterns of fish were mainly an effect of particular substrate preferences of fish species rather than contemporary water conditions.

Keyword
Mangrove creeks, Fish communities, Non-estuarine, Substratum heterogeneity, East Africa
National Category
Fish and Aquacultural Science
Research subject
Animal Ecology
Identifiers
urn:nbn:se:su:diva-27395 (URN)10.1016/j.ecss.2010.08.002 (DOI)000283002100004 ()
Projects
Sida-SAREC Marine Bilateral Programme between Sweden and Tanzania
Available from: 2009-04-30 Created: 2009-04-30 Last updated: 2017-12-13Bibliographically approved
2. Distribution patterns of the striped mullet (Mugil cephalus) in mangrove creeks of Zanzibar, Tanzania
Open this publication in new window or tab >>Distribution patterns of the striped mullet (Mugil cephalus) in mangrove creeks of Zanzibar, Tanzania
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(English)Manuscript (Other academic)
Abstract [en]

Spatial and seasonal variations in density of the striped mullet Mugil cephalus were investigated in four mangrove creeks in Zanzibar (Tanzania) during a one-year cycle. Fish were collected monthly in the lower, intermediate and upper reaches of each creek using a beach seine net (each haul covering 170 m2). All individuals collected were juveniles with a mean size of 2 to 16 cm (standard length). The density of juvenile mullets inhabiting mangrove creeks differed significantly among the different creeks, but the patterns within creeks were consistent, with higher densities upstream in three of the creeks. In general, small-sized juvenile mullets (2-10 cm) were more abundant in the upper reaches compared to the lower and intermediate sites in most creeks. Seasonal patterns were fairly weak, although high mullet densities were observed during the period of heavy rains (from March to May). Principal component analysis (PCA) showed that a muddy bottom with microphytobenthos was likely important to explain high mullet densities, although site-specific variables like low water depth and water clarity may also be important. Our findings suggest that the densities of juvenile striped mullet vary among sites and creeks in response to refuge availability from turbid shallow waters and the accessibility of food resources from benthic microalgae.

Keyword
Mugil cephalus, Population density, Marine embayment, Spatial and seasonal variation, Zanzibar Island
National Category
Fish and Aquacultural Science
Research subject
Animal Ecology
Identifiers
urn:nbn:se:su:diva-27396 (URN)
Available from: 2009-05-01 Created: 2009-05-01 Last updated: 2010-01-14Bibliographically approved
3. Population genetics structure of juvenile Mugil cephalus around Zanzibar and Bagamoyo (Tanzania) reveals multiple genetic demes
Open this publication in new window or tab >>Population genetics structure of juvenile Mugil cephalus around Zanzibar and Bagamoyo (Tanzania) reveals multiple genetic demes
(English)Manuscript (Other academic)
Abstract [en]

There is a growing demand for wild caught juvenile fish to supply the market for aquaculture. However, little is known about the genetic effects of juvenile collection from wild populations. There are a number of imminent threats to both aquaculture systems and wild populations. Juvenile collection from a single population can for example reduce population’s evolutionary potential as well as the disease resistance within an aquaculture pond. In this study, we investigated the local genetic structure of juvenile Mugil cephalus collected from six sites around Bagamoyo (Tanzanian mainland) and Zanzibar Island, East Africa. Fish were caught in low tide using a seine net. All fish collected were juveniles with a total length ranging between 7 and 14 cm (mean length of about 10 cm). Samples were analyzed using Amplified Fragment Length Polymorphism (AFLP), and the Bayesian assignment test implemented in the STRUCTURE 2.2 software was applied to detect if sites were composed of several genetic demes. Our results indicate that all sites contain several different genetic demes suggesting that juvenile collection from a single site may neither harm the genetic diversity of wild M. cephalus nor reduce its disease resistance within an aquaculture system. By collecting juvenile fish from a single site one will in effect harvest juveniles from several genetic lineages.

Keyword
Mugil cephalus, Population genetic structure, deme, Western Indian Ocean, aquaculture
National Category
Fish and Aquacultural Science
Research subject
Animal Ecology
Identifiers
urn:nbn:se:su:diva-27397 (URN)
Projects
Sida-SAREC Marine Bilateral Programme between Sweden and Tanzania
Available from: 2009-05-01 Created: 2009-05-01 Last updated: 2010-01-14Bibliographically approved
4. Fish assemblages in Tanzanian mangrove creek systems influenced by solar salt farm constructions
Open this publication in new window or tab >>Fish assemblages in Tanzanian mangrove creek systems influenced by solar salt farm constructions
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2009 (English)In: Estuarine, Coastal and Shelf Science, ISSN 0272-7714, E-ISSN 1096-0015, Vol. 82, no 2, 193-200 p.Article in journal (Refereed) Published
Place, publisher, year, edition, pages
Amsterdam Netherlands: Elsevier, 2009
Keyword
Mangrove, creek systems, fish assemblages, human disturbance, salt farms, Tanzania
National Category
Fish and Aquacultural Science
Research subject
Animal Ecology
Identifiers
urn:nbn:se:su:diva-27394 (URN)10.1016/j.ecss.2008.12.010 (DOI)000265011300004 ()
Projects
Sida-SAREC Marine Bilateral programme between Sweden and Tanzania
Available from: 2009-04-30 Created: 2009-04-30 Last updated: 2017-12-13Bibliographically approved
5. Influence of mangrove deforestation on trophic organization of fish assemblages in creek systems
Open this publication in new window or tab >>Influence of mangrove deforestation on trophic organization of fish assemblages in creek systems
(English)Manuscript (Other academic)
Abstract [en]

The mangrove biotopes in Tanzania are under increasing pressure from domestic uses and changes in land-use for aquaculture and solar salt farms. To investigate the impacts of man-made activities on trophic structure of mangrove-associated fish species, sampling of fish from various trophic groups was performed in mangrove creeks. Trophic organization and stable isotope signatures (δ 13C and δ 15N) of fish in undisturbed areas of mangrove creeks were compared with clear-cut areas of mangrove as well as with reservoirs for saltworks or fish farms constructed after mangrove clearing. Results showed significantly higher densities, species numbers, diversity (H’) and numbers of trophic groups in undisturbed sites compared to both types of disturbed sites. Overall, omnivorous fish comprised the most abundant feeding guild, with the highest number of individuals found in the cleared sites followed in order by the uncleared sites and the reservoirs. The feeding guild zoobenthivores/piscivores was the most diverse group, with the highest species richness in the undisturbed areas. Multivariate analysis showed that assemblage structure of omnivores in the reservoirs was separated from those in the uncleared and cleared sites, while zoobenthivores/piscivores differed between uncleared sites and the disturbed areas (cleared sites and reservoirs). Stable isotope ratios of δ13C and δ15N values in fish tissue muscles indicate significant diet shifts between undisturbed and disturbed mangrove creek systems, although the effects are species-specific. Our findings suggest that mangrove deforestation combined with land-use changes, such as salt- or fish farm constructions, has a greater impact on the trophic structure of fish in mangrove creeks than mangrove deforestation only. Hence, the extent and severity of disturbance seem to be important in predicting fish assemblage composition.

Keyword
Mangrove, Fish, Functional groups, Anthropogenic disturbance, Tanzania, Western Indian Ocean.
National Category
Fish and Aquacultural Science
Research subject
Animal Ecology
Identifiers
urn:nbn:se:su:diva-27398 (URN)
Projects
Sida-SAREC Marine Bilateral Programme between Sweden and Tanzania
Available from: 2009-05-01 Created: 2009-05-01 Last updated: 2014-10-13Bibliographically approved

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