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Establishing suitable salt tolerant all-male tilapia strains with fast growth rate is a crucial requirement for successful euryhaline farming required for controlling prolific breeding and as an adaptation to the effect of climate-induced drought. This study assessed the percentage of tilapia male hybrids, their growth performance and survival rate produced by crossing female Nile tilapia (Oreochromis niloticus) and male Rufiji tilapia (Oreochromis urolepis urolepis) at different salinities. The fingerlings were reared for 70 days in triplicate concrete tanks (1 m3), each stocked with 15 fish weighing 1.62 ± 0.03 g, 1.69 ± 0.02 g and 1.41 ± 0.06 g exposed to 2, 15 and 35 ppt salinities, respectively. Acclimatization was done by adding seawater to freshwater at a rate of 2 ppt per day for 18 days. Fish sexing was done by visual assessment of the genital papilla followed by rearing the hybrids for six months to observe reproduction. Results showed that, the cross between O. niloticus (female) and O. urolepis urolepis (male) produced hybrids that were all-males without any reproduction within six months post hybridization. The hybrids reared at a salinity of 2 ppt had higher growth performance than those reared at 15 and 35 ppt. Similarly, hybrids cultured at 15 ppt had significantly higher growth performance than those reared at 35 ppt. All the hybrids produced had survival rates of 100%. This study indicated that, all-male tilapia produced by crossing between O. niloticus (female) and O. urolepis (male) can be used by fish farmers as alternative species for mariculture development to control prolific breeding and as an adaptation strategy to climate change.

An investigative field survey was performed to gather baseline data on locally available feed ingredients and fish farming practices in different regions of Tanzania. More than 80% of respondents relied on locally available feed ingredients as a major feed supplement for their cultured fish, with maize bran being the most commonly used feed ingredient in all regions. Crude protein content in most analyzed local feed ingredients was medium-high, while crude fat content was high in some animal and agricultural by-products, and medium-low in other ingredients. Most respondents were males and the majority of fish farms were owned by individuals. Earthen pond was the most common fish farming system in all regions except Dar es Salaam. Semi-intensively mixed-sex tilapia monoculture was the dominating fish farming practice. The results of the survey presented provide a good platform for future development of culture systems and feeding strategies for tilapia in Tanzania.

Pollution, turbidity and coral bleaching history, as well as coral diversity, vary along the Tanzanian coastline. Prior to this study, it was not known whether exposure to such environmental variation might have influenced the diversity and distribution of Symbiodinium along this coastline. Such information can provide insight into whether Tanzanian reef-building corals develop adaptation to current trends of climate change. Here, 10 reef-building coral samples were collected from different reefs along the Tanzanian coast with different micro-environments and bleaching histories. The ITS-2 region of ribosomal DNA was employed in the characterisation of Symbiodinium harboured by reef-building corals. DGGE fingerprints and DNA sequences showed that most coral species host a single Symbiodinium type, which is maintained throughout the coast regardless of local environmental differences. We present polymorphic symbioses in Acropora spp., Millepora sp. and Galaxea fascicularis in mainly turbid and warm environ...

Coastal vegetative habitats are known to be highly productive environments with a high ability to capture and store carbon. During disturbance this important function could be compromised as plant photosynthetic capacity, biomass, and/or growth are reduced. To evaluate effects of disturbance on CO2 capture in plants we performed a five-month manipulative experiment in a tropical seagrass (Thalassia hemprichii) meadow exposed to two intensity levels of shading and simulated grazing. We assessed CO2 capture potential (as net CO2 fixation) using areal productivity calculated from continuous measurements of diel photosynthetic rates, and estimates of plant morphology, biomass and productivity/respiration (P/R) ratios (from the literature). To better understand the plant capacity to coping with level of disturbance we also measured plant growth and resource allocation. We observed substantial reductions in seagrass areal productivity, biomass, and leaf area that together resulted in a negative daily carbon balance in the two shading treatments as well as in the high-intensity simulated grazing treatment. Additionally, based on the concentrations of soluble carbohydrates and starch in the rhizomes, we found that the main reserve sources for plant growth were reduced in all treatments except for the low-intensity simulated grazing treatment. If permanent, these combined adverse effects will reduce the plants’ resilience and capacity to recover after disturbance. This might in turn have long-lasting and devastating effects on important ecosystem functions, including the carbon sequestration capacity of the seagrass system.

Seagrass species diversity and abundance were studied in East African back-reef lagoons with contrasting groundwater-outflow rates. The selection of the lagoons was based on a groundwater flow model. A total of 10 seagrass species was observed at all sites together. Sites with a higher groundwater outflow displayed a lower species diversity than sites with a lower groundwater outflow. Thalassodendron ciliatum dominated at sites with high groundwater outflow rates, while Thalassia hemprichii showed higher coverage at sites with low groundwater outflow. Porewater salinities were up to 5 psu lower at locations with predicted high groundwater-outflow rates indicating supply of freshwater. The reduction in porewater salinity at groundwater outflow sites is relatively low, which makes it unlikely that a difference in optimum salinity for growth is the main factor causing reduced diversity at these sites. Nitrogen-stable isotope signatures of seagrass leaves showed a significant increase with increased groundwater outflow rates. This suggests that the nitrogen source for these plants was, at least in part, groundwater. Differences in competition for nitrogen may explain the observed pattern in species diversity and abundance. To establish a substantive link between the observed reduced seagrass diversity or enhanced δ15N values of T. ciliatum leaves on the one hand and increased groundwater outflow rates on the other, further exploration through detailed measurements of groundwater outflow rates and groundwater nitrogen isotopic composition are needed., This research is part of the GROFLO project which was supported by grant no. IC18-CT96-0065 of the Commission of the European Communities within the framework of the INCO Programme. This is NIOO-CEMO publication no. 2854

Flooding and salinity fluctuations are common in mangrove systems. Sometimes these events are long-lasting, persisting several months. With an increased frequency of heavy rainfalls and terrestrial run-off, subsequent floods have been associated with massive mangrove mortality and failure to regenerate in the region. Owing to climate change, these events are expected to be more common in the future. We investigated how three weeks of submergence in water of different salinities affected the photosynthetic rates in seedlings of three common mangroves: Bruguiera gymnorrhiza (L.) Lamk.; Avicennia marina (Forssk.) Vierh.; and Heritiera littoralis Dryand. We found that photosynthesis and survival rates declined with increasing salinity and submergence time for all species. Prolonged submergence caused a significant decline in photosynthetic rates (as electron transport rate – ETR) for B. gymnorrhiza (P = 0.021) and H. littoralis (P = 0.002), whereas significant effects of both salinity (P = 0.003) and submergence (P = 0.023) were observed between species. Maximum diurnal values of ETR declined in the order of A. marina > B. gymnorrhiza > H. littoralis. After submergence, survived seedlings were tended normally, watered twice a day with freshwater. Three seedlings of B. gymnorrhiza from freshwater and 33% seawater treatments and of A. marina from freshwater treatment displayed signs of recovery for the first 3–5 days, but after that they died. We conclude that submergence time and water salinity will affect the performance of mangrove areas, such that areas experiencing prolonged submergence with flooding dominated by saline water might be most severely impacted.