Western Indian Ocean Journal of Marine Science

Influence of Covid-19 restrictions on the status of mangrove vegetation in coastal Kenya

Mumini Dzoga — 2024-09-11

Mangrove forest ecosystems provide essential services to tropical coastal communities, includ- ing as a source of food, fuel and building materials. During the Covid-19 pandemic human movements were restricted at all levels, including locally in coastal communities. The impact of these measures on marine ecosystems such as mangroves were not well understood. A household survey was conducted in a mangrove-dependent community at Mtwapa Creek to evaluate their perceptions on the rate of mangrove degradation, factors contributing to man- grove degradation, and species that were most exploited during the pandemic period. Multiple Correspondence Analysis (MCA) was used to determine the association of Covid-19 with eco- nomic activities. Due to the restrictions on human interactions and travel during the pandemic, some natural ecosystems thrived, but in contrast accelerated destruction of mangrove forests occurred along Mtwapa Creek. The MCA indicated significant association between mangrove degradation rate and unemployment, charcoal burning, and reduced farming activities. A Chi- square test confirmed significant degradation of mangrove forests. Loss of income sources dur- ing the pandemic led to an increase in exploitation of Rhizophora mucronata, Ceriops tagal and Avicennia marina. These species were mainly harvested for construction and charcoal burning. Mangrove management measures could not be effectively enforced during the pandemic.

Seasonal difference in marine litter along the coast of Dar es Salaam, Tanzania

Florence Jovinary Peter — 2024-10-10

Marine litter accumulation poses a significant threat to marine ecosystems and biodiversity. Marine litter and the influence of river discharge on marine litter accumulation during wet and dry seasons along the coast of Dar es Salaam was investigated. Litter samples collected between 2019 and 2021 from transects on beaches and by nets spanned across two rivers were counted, dried and weighed, and litter counts and weight (kg) calculated by area (litter/m2) and volume (litter/m3). Over three years, a total of 127,658 marine litter items weighing 470 kg were sampled from beaches, and 1,365 items from rivers, in nine different litter categories. Most of the litter (70 %) was produced locally and the marine litter density decreased by 61 % over the three years of the study. Litter densities at beaches and rivers were significantly greater during the wet than the dry season, attributed to the transport of litter in rivers to the ocean, especially plastics, which had a higher density than other riverine litter. It is concluded that high local plastic pro- duction and poor waste disposal systems require an intervention, and that a policy to minimize and reduce marine litter should be developed and implemented.

Seasonal Dynamics of Trichodesmium and Phytoplankton Communities in the Eastern Arabian Sea: Winter Vs Spring-inter monsoon

Anima Tirkey — 2024-10-09

The filamentous marine cyanobacterium Trichodesmium forms extensive blooms in tropical seas, where it plays an important role in nitrogen fixation. This study investigated Trichodesmium-associated phytoplankton in the Eastern Arabian Sea (EAS) during bloom and nonbloom conditions. Trichodesmium blooms over the continental shelf primarily comprised of T. erythraeum and T. thiebautii during the Spring Inter Monsoon (SIM) and T. erythraeum during the Winter Monsoon (WM). T. erythreaum blooms have rarely been reported from the north-east Arabian Sea during WM. Trichomes comprised of 143-1552 cells during the WM, with filaments ranging from 1339 to 13163 µm in length. Non-diazotrophic groups like diatoms and dinoflagellates were associated with Trichodesmium during both seasons within the bloom waters. The study found a novel symbiotic relationship between the heterocystous diazotroph Richelia intracellularis and Nitzschia sicula during WM blooms. This research expands knowledge of lesser-known phytoplankton species and their associations in the EAS.

Remote sensing of coral reef habitats in Madagascar using Sentinel-2 satellite images

Aina Le Don Nomenisoa — 2024-10-18

Publicly available Sentinel-2 satellite imagery was used to map the coral reef systems of Toliara in Madagascar, to standardize methods for monitoring reef health and guiding management decisions. Fieldwork conducted between March and December 2021 used georeferenced photoquadrats to assess benthic structure. The satellite image classification was based on the Object-Based Image Analysis (OBIA) and machine learning algorithms, with k-NN achieving the highest overall accuracy at 83 %, followed by the Bayes classifier (79 %), DT (68 %), RT (67 %) and SVM (42 %). The analysis identified distinct surface areas occupied by seagrass (21 km2), sand (73 km2), rubble (21 km2), coral (10 km2) and algae (6 km2). Comparative assessment with the Allen Coral Atlas underscored the importance of aligning satellite image analysis with in-situ data. The study emphasized the role of selecting appropriate classifier algorithms for precise mapping and stressed the importance of local data collection for accurate habitat mapping. It also showcased the successful application of OBIA with satellite imagery and field data for coral reef mapping, providing insights into habitat health and spatial changes essential for effective conservation.

Economic viability of seaweed and sea cucumber culture using integrated multitrophic aquaculture systems in Zanzibar, Tanzania

Markus Bernard — 2024-11-07

The viability of co-culturing seaweed and sea cucumbers in Zanzibar, Tanzania, was assessed using integrated multitrophic aquaculture (IMTA) systems with oyster and recirculating aquaculture systems with milkfish. Seaweed production was affected by the ice-ice disease, resulting in specific growth rates of 1.32 to -1.96 % d-1. Nitrogen content in the seaweed thallus and inorganic nutrient in the water indicated that seaweed could potentially be used as a biofilter. An economic analysis showed the economic viability of IMTA systems, co-culturing seaweed Kappaphycus alvarezii, sea cucumber Holothuria scabra and oyster Pinctada margaritifera, achieving a maximum benefit-cost ratio of 1.61 over one year. Interviews with local farmers on Zanzibar showed willingness to accept new aquaculture systems, but also highlighted constraints, including a lack of infrastructure, funds and expertise.

Uncovering the relationship between light intensity and Tripneustes gratilla (collector sea urchin): implications for aquaculture

Shamira Payet — 2024-11-07

The sea urchin species Tripneustes gratilla has high aquaculture potential. It is known for using debris to cover itself, likely to avoid light, which suggests that high light levels may impact its production. This study aimed to validate these concerns through four experiments. The first experiment assessed if T. gratilla preferred opaque or transparent covering materials, finding no evidence that shading ability of the material influenced their choice (F(2, 33) = 0.27, p = 0.765). The second experiment investigated the impact of shade on the number of covering materials collected, showing that higher solar radiation correlated with increased material collection per urchin (H(3) = 7.844, p = 0.049). The third experiment evaluated righting response time as a fitness trial, finding that high light intensity reduced fitness to urchins acclimated to low light (H(2) = 5.615, p = 0.020). The fourth long-term experiment demonstrated that T. gratilla can acclimatize to higher light intensities, with no significant differences in fitness, mortality, disease susceptibility, growth, and gonadosomatic index (p > 0.128). While this suggests that shading may not be a strict requirement for T. gratilla aquaculture, it is recommended for consideration. These findings underscore the importance of acclimatization when altering light conditions for T. gratilla.