Fish community patterns in Tanzanian mangrove creeks

Sammanfattning: 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.