Mangroves are considered as an important source of nutrition to aquatic consumers, which, in return, sustain the fish population. However, there is still controversy about the importance of mangroves to fish communities, and furthermore, not all mangroves fulfill every function ascribed to them. The Persian Gulf presents the northernmost mangroves in the Indo-Pacific region, thriving under extreme environmental conditions such as very low rainfall and high salinities. Very few studies have addressed fish distribution patterns of mangroves in this arid region. The main objective of this study is to describe and compare the fish community structure and food web dynamics of mangrove and non-mangrove intertidal creeks in a low-rainfall system in Qeshm Island, Persian Gulf, Northern Indian Ocean (26.8°N, 55.75°E). Therefore, this thesis addresses temporal changes in mangrove fish assemblages related to the tidal and diel cycles in the meso-tidal mangrove of Qeshm Island. Also, the food webâ s dynamics were studied to determine the role of mangroves in fish food webs. Moreover, due to the arid climate along the northern Indian Ocean, many neighboring creeks are devoid of mangroves but still provide habitat for fish. This provided the necessary conditions for a natural experiment to compare fish assemblage structures and trophic diversity between creeks that are mangrove lined, and creeks without mangroves across seasons. For that, fish were collected with respect to the different combinations of tidal amplitude (and lunar phase) and day light conditions as well as seasons (winter, summer). Also, various potential food sources were collected for à ´13C and à ´15N measurements from both habitats across seasons. The findings show the impact of environmental variables (e.g. tide and temperature) in structuring fish assemblages in Qeshm intertidal creeks. The interaction between tide and time of day emerged as an important factor in organizing mangrove fish assemblages in this meso-tidal arid region. The highest fish biomass, abundance, and diversity were observed during spring tide night. High inundation during spring tides coinciding with the darkness provided the most favorable conditions for fish. When mangrove fish communities were compared with the non-mangroves, seasonality appeared as a more important factor than habitat. Extreme summer water temperatures (>33°C) likely limit the fish abundances and biomass. Fish assemblages of mangrove and non-mangrove intertidal creeks were equally diverse and dominated by the same fish families (e.g. Mugilidae). Small-sized fish dominated catches in both habitats. Also, there were no consistent differences in fish abundance and biomass for mangrove vs. non-mangrove fish collections. Community trophic diversity, measured with size-corrected standard ellipse areas (SEAc), also showed no significant difference across habitats. Instead, strong seasonal patterns were observed for the food sources and fish communities isotopic niche sizes. These findings demonstrate that the structuring effect of seasonality is more important than habitat in determining the fish distribution patterns in this region. Mangrove-derived organic matter contributed to a maximum of 36% to the fish tissue, whereas organic matter produced by microphytobenthos and plankton plays a major role in the diets of the most abundant fish species, with contributions of 64 - 100%. Thus, the fish food webâ s support of mangrove and non-mangrove intertidal creeks seems to be energetically driven by pelagic and benthic food pathways in this study area. Furthermore, the dominance of detritivores in both mangrove and non-mangrove intertidal creeks suggests the importance of benthic food webs in the intertidal creeks at Qeshm Island. The outcomes of this dissertation contribute to the global understanding of the importance of mangrove and non-mangrove intertidal creeks for fish food webs and fisheries support in a low-rainfall coastal ecosystem. The findings imply some potentially interesting consequences on habitat conservation and indicate that the abundance of small fish is not always higher in complex vegetation. Like mangrove creeks, the bare creeks are utilized by a wide variety of fish, suggesting greater attention should be drawn to these habitats
