Phytoplankton distribution along a salinity gradient in two Kenyan saltworks (Tana and Kurawa)

The current study assessed the diversity and abundance of phytoplankton genera in two Kenyan saltworks (Tana and Kurawa) in March and September 2021. Water samples were obtained from ponds with salinities ranging from 30 to 200 ppt by filtering 40 l of water using a 20-μm phytoplankton net. Seventy-six genera of phytoplankton were identified. Genera richness, evenness and diversity decreased with increasing salinity while phytoplankton abundance increased with increasing salinity. Higher phytoplankton densities were observed in the Tana than in the Kurawa saltworks. Ponds of <100 ppt were dominated by Dinophyceae and Bacillariophyceae which accounted for >90 % of the phytoplankton community. Ponds of salinities >100 ppt were dominated by Cyanophyceae which accounted for >90 % of the phytoplankton community. From the results it was concluded that Kenyan saltworks host diverse phytoplankton genera whose richness decreases with increasing salinity and varies with seasons. The present data describes variation of phytoplankton assemblages in salt ponds between two selected seasons, but several samplings throughout the year would be more appropriate to describe variations of phytoplankton with season in these salt ponds

Reproductive biology and body condition of exploited populations of Emperor Angelfish, Pomacanthus imperator (Bloch, 1787) along the Kenyan Coast

A substantial proportion (in terms of value and quantity) of the Pomacanthus imperator catch on the Kenyan coast are traded, and the species is also harvested as food in the artisanal fishery. However information on their reproductive biology is scanty. The overall sex ratio differed significantly from 1:1 according to chi-square test (p<0.05). Sizes at maturity (L50) were estimated to be 25cm and 28cm TL for females and males respectively. Fecundity was estimated to be in the range of 17,790-266,472 with a Mean ±SE of 79,353±11,747, and was linearly related to total length and ovary weight. March-April was the main period of reproductive activity based on gonad somatic indices and monthly proportion of mature individuals. The LWR indicated isometric growth both in males and females of P. imperator, as the allometric coefficient b values were not significantly different from the expected isometric value of 3 (Student’s t-test; p=0.12). Relative condition factor did not vary significantly between the months sampled. The reproductive parameters obtained from this study provide some baseline information for management of this species which has proven to be highly vulnerable to depletion due to overfishing.

Hook size selectivity in the artisanal handline fishery of Shimoni fishing area, south coast, Kenya

Selectivity of five handline fishing hook sizes was determined following Holt’s 1963 model using data that was collected during January to June, 2016. A total of 966 fish specimens comprising of 65 species belonging to 23 families were sampled. Fish abundance was low for large sized hooks while catch rate was higher. Species diversity was higher during the northeast monsoon season and at the study sites of Mpunguti and Nyuli. However, species diversity decreased with increase in hook size. All hook sizes had a higher selection for mature Lethrinus borbonicus while hook size No. 8 selected immature Lethrinus lentjan. Hook sizes No. 9 and 10 selected mature L. lentjan and Lethrinus rubrioperculatus, hook size No. 15 selected immature L. lentjan, L. rubrioperculatus and Aprion virescens, while hook size No. 16 selected immature A. virescens and L. rubrioperculatus. Species similarity was higher for fish caught by hook sizes No. 16 and 15, and No. 8 and 9, while those captured by hook size No. 10 differed from those caught by other hook sizes. The larger hook size No. 8 is recommended for the sustainable exploitation species in the artisanal handline fishery in Shimoni fishing area. Future work needs to consider the effects of bait type and size and the stock status of the fish under exploitation

Conditions and from factor of the five threatened fishes from the Jamuna (Brahmaputra River Distributary) River, Northern Bangladesah

The small indigenous species namely Ailiichthys punctata (Day 1871), Botia lohachata (Chaudhuri 1912), Chanda nama (Hamilton 1822), Laubuca laubuca (Hamilton 1822) and Mystus cavasius (Hamilton 1822) comprise an important component of riverine fisheries of Bangladesh. But the natural populations are decreasing fast due to high fishing pressure as well as environmental degradation. Therefore, data and information is needed to avert the alarming decline and initiate conservation measures for these important fishes of the Jamuna River. This study describes the condition- (Fulton’s and Relative weight) and form-factor (a3.0) of the five threatened fishes from the Jamuna River, a distributary of the Brahmaputra River in northern Bangladesh. A total of 919 specimens from five species in five families used for this study were caught by traditional fishing gear during March 2010 through February 2011. For each individual, the total (TL), fork (FL) and standard (SL) length were measured by digital slide calipers while individual body weight (BW) was measured using a digital balance. The Fulton’s condition factor (KF) showed significant variations (p0.05) indicating a balanced habitat with food availability relative to the presence of predators. The calculated minimum and maximum form factor (a3.0) was 0.0062 for A. punctata and 0.0158 for B. lohachata, respectively. To the best of our knowledge, this study presents the first reference on KF, WR and a3.0 factors for these threatened species in Bangladesh. The results would be useful for sustainable management and conservation of the limited stocks in the Brahmaputra River ecosyste

Vertical habitat use by black and striped marlin in the Western Indian Ocean

Black marlin Istiompax indica and striped marlin Kajikia audax are large, fast-swimming, oceanic apex predators. Both species are increasingly exploited by fisheries with varied gear encounter rates at different depths, causing concern for their status. Here, we examined vertical habitat use by 34 black and 39 striped marlin caught off Kenya, using pop-up satellite tags to compare their diving behaviours. Tags recorded depth and temperature time-series for a mean (±SD) of 43 ± 53 days per track. Marlin dived extensively moving up to ~14 vertical km in cumulative dives per day in addition to a daily mean of ~50 km in horizontal movements. Both species had similar maximum depths (460-470 m). Striped marlin dived deeper more frequently than black marlin, and also spent more of their time at the water surface (top 5 m: 50.7 vs. 32.3% in black marlin). Most striped marlin had a normal diel vertical migration dive pattern over their track (61.5% of individuals), while ~35% of black marlin showed a crepuscular pattern, diving particularly deep at dusk and dawn. Striped marlin spent almost twice as much time (7.4%) inside the oxygen minimum zone (-1 dissolved oxygen) than black marlin (4%). The extensive use of surface waters by striped marlin may be a behavioural response to re-oxygenate and/or warm up after dives into cold or oxygen-poor waters. Two free-jumping events immediately before tag detachment demonstrated why it is challenging to keep tags attached to these highly active fishes. Their vertical habitat use shows that both species are highly susceptible to capture in regional drift gillnet and longline fisheries

Vertical habitat use by black and striped marlin in the Western Indian Ocean

Black marlin Istiompax indica and striped marlin Kajikia audax are large, fast-swimming, oceanic apex predators. Both species are increasingly exploited by fisheries with varied gear encounter rates at different depths, causing concern for their status. Here, we examined vertical habitat use by 34 black and 39 striped marlin caught off Kenya, using pop-up satellite tags to compare their diving behaviours. Tags recorded depth and temperature time-series for a mean (±SD) of 43 ± 53 days per track. Marlin dived extensively moving up to ~14 vertical km in cumulative dives per day in addition to a daily mean of ~50 km in horizontal movements. Both species had similar maximum depths (460-470 m). Striped marlin dived deeper more frequently than black marlin, and also spent more of their time at the water surface (top 5 m: 50.7 vs. 32.3% in black marlin). Most striped marlin had a normal diel vertical migration dive pattern over their track (61.5% of individuals), while ~35% of black marlin showed a crepuscular pattern, diving particularly deep at dusk and dawn. Striped marlin spent almost twice as much time (7.4%) inside the oxygen minimum zone (-1 dissolved oxygen) than black marlin (4%). The extensive use of surface waters by striped marlin may be a behavioural response to re-oxygenate and/or warm up after dives into cold or oxygen-poor waters. Two free-jumping events immediately before tag detachment demonstrated why it is challenging to keep tags attached to these highly active fishes. Their vertical habitat use shows that both species are highly susceptible to capture in regional drift gillnet and longline fisheries

Woody plant species diversity of the coastal forests of Kenya: filling in knowledge gaps in a biodiversity hotspot

The coastal forests of Kenya are global biodiversity hotspots known for rich plant diversity and endemism. They exist as fragmented forest islands, and their current conservation status and quantitative trends in plant diversity are understudied. We investigated these knowledge gaps by providing a comprehensive literature review and comparing to field data collected using standardized sampling protocol. Our goals were to build a robust basis for future analyses, biodiversity monitoring, and to understand the role of fragment area in determining species richness. We recorded a total of 937 woody species belonging to 88 families in 30 forest patches from reviewed and sampled data. Species richness per site from literature review was affected by biases in data scarcity, forest size and variation in sampling methods. In general, large forests reserves of Shimba hills and Arabuko exhibited a high number of cumulative species compared to smaller forest patches. Species-area relationship showed a significant proportion of species richness per forest was determined by forest area, according to Arrhenius model. This study is the first to review forest patch woody plant species diversity knowledge gaps in the coastal forests of Kenya, and the resulting comparison provides the first quantitative overview and foundation of these forests

Natural resource-use in the Lower Tana River Delta based on household surveys and remote sensing of land cover and land use patterns

Household survey data and spatially explicit Sentinel-2 satellite images of land cover and land use during the dry and wet seasons were used to investigate livelihood strategies in the Lower Tana River Delta in Kenya, where food security and economic activity rely almost exclusively on ecosystem goods and services. Land cover classification of satellite imagery successfully showed seasonal changes to estuary-related vegetation and habitats from which land use cycles could be inferred. Flood-recession agriculture and part-time fishing were the most common activities at Ozi village, some 10 km upstream from the estuary mouth, whereas full-time fishing dominated activities at Kipini town, where the Tana Estuary discharges into Ungwana Bay. Seasonality of fishing at Kipini depended on favourable sea conditions, arrival of migrant fishers and peaks in shrimp abundance. Seasonality of agriculture at Ozi depended on the freshwater flooding regime, visible in satellite images as an inverse relationship of areas covered by wetlands and cultivated lands. The predominance of fishing assets at Kipini indicated specialization, which underlies a socio-economic network of fish processing, marketing, distribution and logistical support services. In contrast, mixed farming assets and traditional fishing gear at Ozi reflected more diversified farmer-fisher livelihoods, as a risk avoidance strategy. Key outcomes of this study were that land cover and land use were strongly seasonal, that coastal and upstream communities in the Tana Estuary relied on different combinations of ecosystem goods and services, and that livelihood strategies at the two locations differed fundamentally. Combining social, spatial and ecological data to describe socio-ecological systems typical of the Tana Estuary provided a broad platform for shared resource management strategies