Phase V – Larval fish production and dispersal in critical habitats of coastal East Africa (FLAPSEA)

The FLAPSEA project focused on studying fish larvae in seagrass ecosystems and their adjacent habitats along the coast of East Africa. The primary objective was to explore how coastal habitat degradation, driven by climate change and urban development, threatens the vital food-provisioning services associated with fish production in the region. To  achieve this goal, the project outlined four key objectives and work packages: Critical Habitat Conditions: Identifying the essential habitat conditions that drive fish recruitment and the factors influencing fish larvae production (Work Package 1 – WP1); Dispersal Potential: Assessing the dispersal potential of fish larvae from seagrass habitats to adult fish spawning areas (WP2); Economic Impacts and Vulnerability: Predicting future economic impacts and identifying the most vulnerable coastal areas (WP3).; Improving Management Strategies: Providing scientific insights to enhance management and conservation strategies for coastal ecosystems in East Africa (WP4).

Field sampling took place in targeted “healthy” and “degraded” seagrass habitats in Kenya and Tanzania from June 2019 to January 2021, covering both the Southeast Monsoon (SEM) and Northeast Monsoon (NEM) seasons. In Kenya, study sites included Watamu on the north coast and Diani on the south coast, while in Tanzania, sites were located in Mwarongo and Kigombe, Tanga.

During our investigations, we measured various environmental parameters, including plankton samples, chlorophyll “a,” and physico-chemical variables (nutrients, temperature, salinity, pH, and dissolved oxygen). Seagrass assessments involved evaluating shoot density and percent cover in both healthy and degraded areas. Underwater Visual Census (UVC) techniques and fish juvenile sampling were employed in Kenya to understand patterns of juvenile recruitment within seagrass habitats.

Our findings revealed significant seasonal variations in salinity and temperature, with higher levels observed during the NEM. Additionally, chlorophyll “a” concentrations were generally greater during corresponding SEM conditions across both countries, alongside a noticeable difference in nutrient levels (ammonia, phosphates, and nitrates) between seasons.

We identified four major groups of phytoplankton, with Bacillariophyceae (diatoms) emerging as the most abundant and diverse, comprising a total of 150 species. Our comparative assessments indicated no significant differences in species diversity between the seasons or sites in both countries (p>0.05).

Zooplankton densities exhibited seasonally dependent trends, with higher levels recorded during the NEM in Kenya but the opposite observed in Tanzania. A statistical analysis demonstrated significant differences in zooplankton taxonomy density among sites in Kenya, highlighting the complexity affecting fish larvae distributions. Notably, we found positive correlations between chlorophyll “a” levels, zooplankton abundance, and fish larvae in Kenya, while fish eggs displayed a significant negative correlation with phytoplankton density.

While fish larval abundance was generally higher in healthy sites across both countries, the differences were not statistically significant. The most prevalent fish families included Blenniidae, Engraulidae, Gobiidae, Mullidae, and Scaridae, emphasizing the crucial nursery functions of seagrass habitats.

UVC assessments in Kenya indicated greater fish densities and species richness during the NEM relative to SEM, with thriving fish populations in healthier seagrass sites. Dominant species observed included Archamia mozambiquensis, Lutjanus fulviflamma, and Siganus sutor, which are associated with reef environments and likely seek refuge in seagrass areas for feeding purposes.

Our analyses of seagrass metrics revealed that healthy sites exhibited greater coverage, shoot density, and overall biomass compared to degraded environments. Notably, Thalassodendron ciliatum and T. hemprichii were the dominant seagrass species. Some sites, like Watamu, displayed high species diversity, suggesting a degree of ecological resilience.

Socio-economic studies indicated declining stocks of significant species such as rabbit fish and thumbprint emperor, attributed to overfishing, climate change, and seagrass loss. All surveyed landing sites reported reductions in seagrass cover, often linked to an increase in sea urchin populations, unsustainable fishing practices, and coastal erosion. Control efforts included trapping sea urchins for use as bait by fishermen.

FLAPSEA has accomplished several key milestones, including the submission of six research papers (with one currently under review) and substantive capacity-building initiatives resulting in two BSc graduates, two PhD recipients, and two MSc graduates from various universities.

Challenges were encountered, notably travel restrictions affecting Swedish students involved in WP2, which have since been lifted, allowing the continuation of research by December 2021, including genetic sampling. A stakeholder workshop is planned for December, where policy briefs will be distributed to relevant national and county offices.

Additionally, FLAPSEA established a flexible data-sharing protocol that facilitated seamless collaboration across work packages and institutions in the three participating countries.