Southeast Monsoon Season Research Articles

Seasonal variation in mineral concentrations of four marine fish species retained by fishers in Vanga and Msambweni, Kenya

Marine fish is a rich source of minerals in the diet of humans, but seasonal variation in fish mineral concentrations is relatively unstudied. We investigated seasonal mineral concentrations among four marine fish species retained by small scale fishers in Msambweni and Vanga fishing villages in Kenya. The fish species were Siganus sutor (rabbitfish) in Msambweni and Decapterus macarellus (mackerel scad), Sphraena forsteri (bigeye barracuda) and Sphyraena obtusata (obtuse barracuda) in Vanga. Mineral concentrations were quantified in 120 fish specimens (60/season) and their mineral supplementation potential evaluated. The concentrations of Potassium (K), Magnesium (Mg), Zinc (Zn), Iron (Fe) and Iodine (I) were determined using Inductively Coupled Plasma Optical Emission Spectrometry. K, Mg, Zn and I levels in fish samples varied between seasons with higher concentrations during the wet and cool southeast monsoon (SEM) season (April to October). However, Fe concentration was significantly higher during the dry and warm northeast monsoon (NEM) (November to March). S. fosteri recorded the highest concentrations of Mg (72.29 mg/100g in SEM) and Fe (3.63 mg/100g in NEM). S. sutor was the richest source of K (410.21 mg/100g in SEM). These two species also recorded the highest concentration of I (57.6 μg/100g in SEM) while D. macarellus had the highest Zn concentration (5.84 mg/100g in SEM). Mineral concentrations were dependent on fish species as well as season, influencing the mineral intake in Msambweni and Vanga villages.

Seasonal and interannual variations of surface chlorophyll-a in the Karimata Strait

ABSTRACT This study aimed to evaluate the dynamics driving seasonal and interannual surface chlorophyll-a (chl-a) variability in the Karimata Strait (KS). The analysis shows that high chl-a concentrations were observed in the KS during December – February (northwest monsoon season) extending northwestward along the eastern coast of Peninsular Malaysia and northeastward along the northern coast of Borneo Island. This high chl-a concentration contrasts with the low chl-a concentration observed along the southern coast of Sumatra and Java during the same season. A substantial upwelling signal was identified in the central-eastern section of the South China Sea and along the northern coast of Java and the eastern coast of Sumatra from December to February. The positive wind stress curl forced downwelling in the southern section of the KS and the northern half of the Java Sea off the coast of Kalimantan. The elevated chl-a concentration in the KS during the northwest monsoon season could not be attributed to wind dynamics alone. The increase in surface chl-a concentration during the northwest monsoon season was associated with an increase in allochthonous nutrients from river discharges caused by increased precipitation over land. On an interannual timescale, a high chl-a concentration was observed during the southeast monsoon season when the La Niña event occurred in the tropical Pacific. The warm SST, associated with favourable downwelling winds, was observed over the entire the KS. It is suggested that anomalous chl-a that bloomed during the La Niña event was associated with anomalously high precipitation over land that transported nutrients to KS through river discharge.

Seagrass Blue Carbon Stock and Air–Sea CO2 Fluxes in the Karimunjawa Islands, Indonesia during Southeast Monsoon Season

Research focusing on seagrass ecosystems as carbon storage has been conducted in various Indonesian waters. However, an essential aspect that remains unexplored is the simultaneous analysis of blue carbon storage in seagrass alongside carbon dioxide (CO2) flux values, particularly within Karimunjawa waters. This study aims to assess the organic carbon stock and sea–air CO2 flux in seagrass ecosystems in Karimunjawa. Our hypothesis posits that although seagrass ecosystems release CO2 into the water, their role as blue carbon ecosystems enables them to absorb and accumulate organic carbon within seagrass biomass and sediments. This investigation took place in Karimunjawa waters, encompassing both vegetated (seagrass meadows) and unvegetated (non-seagrass meadows) areas during August 2019, 2020, and 2022. Over this period, the organic carbon stock in seagrass and sediment displayed an increase, rising from 28.90 to 35.70 gCorg m−2 in 2019 and from 37.80 to 45.25 gCorg m−2 in 2022. Notably, the expanse of seagrass meadows in Karimunjawa dwindled by 328.33 ha from 2019 to 2022, resulting in a total carbon stock reduction of the seagrass meadows of 452.39 tC to 218.78 tC. Sediment emerges as a pivotal element in the storage of blue carbon in seagrass, with sedimentary organic carbon outweighing seagrass biomass in storage capacity. The conditions in Karimunjawa, including a high A:B ratio, low dry bulk density, and elevated water content, foster a favorable environment for sediment carbon absorption and storage, facilitated by the waters’ CO2 emission during the southeast monsoon season. Notably, our findings reveal that CO2 release within vegetated areas is lower compared to unvegetated areas. This outcome underscores how seagrass ecosystems can mitigate CO2 release through their adeptness at storing organic carbon within biomass and sediment. However, the presence of inorganic carbon in the form of calcium carbonate introduces a nuanced dynamic. This external source, stemming from allochthonous origins like mangroves, brown algae like Padina pavonica, and calcareous epiphytes, leads to an increase in sedimentary organic carbon stock of 53.2 ± 6.82 gCorg m−2. Moreover, it triggers the release of CO2 into the atmosphere, quantified at 83.4 ± 18.26 mmol CO2 m−2 d−1.

Seasonal variability of eddy kinetic energy in the Banda Sea revealed by an ocean model: An energy budget perspective

Seasonal eddy kinetic energy (EKE) variability in the Banda Sea during 1993–2014 is studied from an energy budget perspective, based on the outputs of Ocean Forecasting Australian Model version 3. High EKE is confined within the upper 300 m of the western Banda Sea with the largest intensity exceeding 3 × 103 J/m2 in the northwest monsoon (NWM) season. In this strong EKE region during NWM, eddies derive almost two thirds of their kinetic energy from the direct wind power input (WP), with additional contributions from the barotropic (BT) and baroclinic instability (BC) of the background flow. Both WP and BT modulate the EKE seasonality and drive the peak energy during NWM, while BC strengthens in the southeast monsoon (SEM) season because of the intensified baroclinicity of the upper circulation. The westerly wind bursts associated with the Madden-Julian Oscillation (MJO), together with steep topography, facilitate more cyclonic eddy generation events in the western Banda Sea during NWM. During SEM, EKE becomes relatively moderate across the Banda Sea but with a regional peak to the southwest of the Buru Island, which can be attributed to island wake effect. Over the Banda Sea, WP, BT and BC contribute 74% (42%), 14% (19%) and 12% (39%) of total energy to EKE during NWM (SEM), respectively. The majority of EKE generated is diverged horizontally by pressure work and dissipated by turbulent viscosity, with dissipation depleting the most. This study highlights the importance of both monsoon and MJO wind forcing in generating EKE variability along the pathway of the Indonesian Throughflow.

Phytoplankton abundance and diversity in the Northern Banda Sea

Although phytoplankton of the Banda Sea has been studied since 1970s, its information is still limited. In order to provide a better understanding of phytoplankton diversity, during the Expedition of Jala Citra 2 Banda 2022, phytoplankton sampling was done in the surface waters at five stations in the northern Banda Sea. The purpose of this study was to analyze the abundance and species composition of phytoplankton during the southeast monsoon season. Phytoplankton samples were taken at different mixing layers, kept them into sample bottle and were identify in the laboratory of Deep Sea Research Centre BRIN Ambon. Chemical and physical parameters were measured simultaneously. A total of 64 species of phytoplankton were found and they composed of diatom which the most diversified group with 48 species, followed by dinoflagellate with 16 species. Among diatoms, most dominant species were contributed by Thalassionema nitzschiodes (3640-1,266,573 cell/m3) and Thalassionema fraunfeldii (2800-288,570 cell/m3) followed by Chaetoceros sp and Skeletonema costatum. These dominant species composition were different from previous reports. Most of these species were found in a chain. For the dinoflagellates Ceratium longipes (280-5,231 cell/m3) dan Ceratium vulvur (280-1.960 cell/m3) were identified as the dominant species. The cell count of total phytoplankton varied from 280-1,266,573 cell/m3. Spesies diversity index was calculated using Shannon’s formula (H’) and dominance index (D) range from 1.089-2.552 and 0.117-0.418, respectively. Water temperature, salinity, pH and dissolved oxygen ranged from 26.7-29.5°C, 33-34 ppt, 7.85-8.14, and 5.57-7.03 mg/l, respectively.

Using fishers’ local ecological knowledge for management of small-scale fisheries in data-poor regions: Comparing seasonal interview and field observation records in East Africa

Fishers, scientists and policy makers need to describe, understand and “agree on” variations in fish catches caused by exploitation and climate change for effective fisheries management. To achieve this, relevant data with sufficient spatiotemporal resolution is a necessity. In regions of the Global South, such as the Western Indian Ocean (WIO), fish catch data useful for management is scarce or non-existing. Still, the potential of local ecological knowledge to provide such information has not been fully utilised in these regions. In this study, we evaluated fishers’ local ecological knowledge (based on interviews) against detailed seasonal fish catch variability data based on catch per unit effort (CPUE) records. Because of the importance of the monsoon seasons for marine resource variability, differences in fish catches during the northeast (NE) and southeast (SE) monsoon seasons were investigated. Fishers’ perceptions generally agreed with catch data records, both showing that the NE monsoon season generally provides higher catch rates than the SE monsoon season. The fishers’ perceptions at two of the landing sites (Nyamisati and Shangani) contradict the recorded observations by showing highest fish catches during the SE monsoon season. It was clear, however, that fishers’ perceptions in these two sites focused on the most valuable target species (prawn and tuna in Nyamisati and Shangani, respectively) rather than total catches. In this particular case, fishers’ perceptions facilitated the significance of taking target species into consideration. The findings of this study highlight the importance of integrating local ecological knowledge into scientific research to help understand the complex dynamics of coastal fisheries and improve the management of data-poor fisheries.

Distinct mechanisms of chlorophyll-a blooms occur in the Northern Maluku Sea and Sulu Sill revealed by satellite data

Chlorophyll-a is the predominant phytoplankton pigment responsible for determining primary productivity. In the present study, we used satellite-based data of chlorophyll-a, surface wind, and precipitation from 2003 to 2019 to investigate the variability of chlorophyll-a in the northern Maluku Sea and the Sulu Sill and examine its generating mechanism. We found that the chlorophyll-a bloom in the northern Maluku Sea occurs during the southeast monsoon season, while in the Sulu Sill, the chlorophyll-a concentration is higher than that in the northern Maluku Sea and occurs throughout the year. In the northern Maluku Sea, the chlorophyll-a bloom is generated by coastal upwelling. The maximum southerly wind during the southeast monsoon generates the strongest offshore Ekman Mass Transport (EMT) in the northern Maluku Sea triggering coastal upwelling. However, the power spectra analysis of satellite-derived chlorophyll-a shows strong peaks and amplitudes at both fortnightly (MSf) and monthly (Mm) frequencies, indicating that tidal mixing is an important generating mechanism for chlorophyll-a blooms in the Sulu Sill. Shallow bathymetry in the Sulu Sill may aid tidal mixing in effectively transporting nutrients from the near bottom to the sea surface, increasing chlorophyll-a concentration.

Retained fish catches of artisanal fishers is dependent on fishing area, season and fishing gear type: A case study from the south coast of Kenya

The artisanal fishery is one of the sources of livelihood along the Kenya coast. Catches of retained fish of artisanal fishers between two fishing areas on the south coast of Kenya were compared. Artisanal fishers usually retain a portion of their daily catch which is reserved for family consumption while the rest is traded for cash to meet other family needs. This study hypothesized that retained fish catches were influenced by fishing area, fishing gear type and season. Daily fish landings and retained fish catches among artisanal fishers were monitored between August 2020 to July 2021 covering the South East Monsoon (SEM) and North East Monsoon (NEM) in Msambweni and Vanga fishing areas. A total of 60 fishers (30 for each area) were randomly selected and their retained catch monitored on a daily basis. The overall total fish landings were 23.3 tons and 210 tons in Msambweni and Vanga, respectively. The mean catch rate at Msambweni was 3.3 ± 0.12 kg.fisher-1day-1 in the NEM season and 2.2 ± 0.09 kg.fisher-1day-1 in the SEM season. Fish catch rates in Vanga was 8.5 ± 0.28 kg.fisher-1day-1 in the NEM season and 7.9 ± 0.29 kg.fisher-1day-1 in the SEM season. A total of 0.98 tons (only 4.2 %) of fish was retained versus 22.32 tons (95.8 %) not retained by the artisanal fishers in Msambweni, while in Vanga a total of 1.64 tons (only 0.78 %) was retained versus 208.36 tons (99.22 %) not retained. Retained fish catch varied significantly with season and type of fishing gear used (p < 0.05). Among the retained species in Msambweni, Siganus sutor made the highest contribution (53 %), followed by Lethrinus mahsena, (5.08 %). The species Euthynnus affinis and Sphyraena obtusata were mostly retained in Vanga with a contribution of 10.4 % and 10.2 %, respectively. The same retained fish species represented the most abundant species in the overall fish landings, however sizes were significantly smaller than those not retained by fishers (p < 0.05). Results of non-Metric Multidimensional Scaling (nMDS) ordination showed differences in species composition between fishing areas, seasons and fishing gear types with implication on fishers’ retained catches.

A Biological‐Parameter‐Optimized Modeling Study of Physical Drivers Controlling Seasonal Chlorophyll Blooms off the Southern Coast of Java Island

AbstractA physical‐biogeochemical model consists of many key parameters; theoretically, 10,000 or more numerical experiments are required to obtain a set of optimal biological parameters. The corresponding computing resources are too expensive when running a three‐dimensional and high‐resolution physical‐biogeochemical model. In this study, we improve the performance of a three‐dimensional physical‐biogeochemical model via the conditional nonlinear optimal perturbation method for southern Java Island, where chlorophyll blooms occur seasonally. Based on the improved model results, we calculated the mixed‐layer nutrient budget, thereby characterizing how each physical process transports nutrients into the surface oligotrophic layer and then stimulates the chlorophyll blooms during the southeast monsoon season. Nutrients that upwell along the submarine slope are found in narrow regions along the coast and spread into most of the study area by local wind‐induced offshore transport. The contribution of nonlinearity (advection) is essential to modulating the upslope movement of nutrients off the coast. If local winds are neglected in the model, onshore transport induced by the pressure gradient force in the zonal direction (alongshore) and subsequent downwelling would be dominant, and chlorophyll blooms would diminish in most regions. The role of the South Java Coastal Current varies under the joint influence of remote and local wind forcings. The Indonesian Throughflow from the Lombok Strait also contributes to surface phytoplankton growth along its path off the south of Java Island.

Seasonal Occurrence and Relative Abundance of Marine Fish Larval Families over Healthy and Degraded Seagrass Beds in Coastal Kenya

Seagrass beds provide critical nursery habitats and spawning grounds for new generations of fish. The habitats are under threat from human activities and climate change, and with that, an important ocean service is lost that limits fish production. The present study investigates patterns in the larval occurrence and abundance in seagrass meadows at two locations with varying degrees of seagrass fragmentation. Monthly ichthyoplankton sampling was conducted during the northeast monsoon (NEM) and southeast monsoon (SEM) seasons in 2019 and 2020. A total of 42 larval fish families belonging to 37 genera and 21 species were identified. Dominant families were Labridae (29.5%), Blenniidae (28.7%), Gobiidae (26.0%), Engraulidae (23.3%) and Scaridae (22.3%). Canonical Correspondence Analysis and regression analysis revealed water temperature, dissolved oxygen and pH as the most important abiotic variables driving taxonomic composition of larval assemblages, while zooplankton and chlorophyll-a were the most important biotic factors. Fish larvae were more abundant in healthy seagrass habitats as compared to degraded ones. However, despite some loss in functionality, the degraded sites equally played a role in supporting some species, including Gobiidae and Blenniidae. Seasonality influenced larval abundance at the two sites, with a peak in mean abundance coinciding with the NEM season. Interannual variability in fish larval abundance was observed at both sites, indicating that factors controlling larval production varied between the years. This study demonstrates the important role of seagrass meadows in the replenishment of fish stocks and supportive evidence for their management and conservation.

Assessment of vertical and horizontal distribution of meiobenthos along a salinity gradient in the Tana and Sabaki Estuaries, north coast Kenya

Meiobenthos respond to variability in salinity gradients in estuarine habitats and are thus suitable organisms for ecological studies. The vertical and horizontal distribution of the meiofauna community structure of two major estuaries (Sabaki and Tana) on the north coast of Kenya were compared. The aim was to draw a meiofaunal dataset inventory of the two estuaries and to examine how salinity gradient, seasonality and sediment characteristics influence their structure. Replicate samples were collected from three sampling stations along the salinity gradient of each river estuary. A total of 3,556 individuals belonging to 26 taxa were recorded. Based on seasons and across stations, the upper surface (0-5 cm) layer recorded the highest meiobenthic density (90 ± 42 ind.10 cm-2), followed by 46 ± 23 ind.10 cm-2 (5-10 cm) and 30 ± 8 ind.10 cm-2 in the deepest sediment layer (10-15 cm) studied. The southeast monsoon season recorded the highest mean density (160 ± 22 ind.10 cm-2) compared to the northeast monsoon season (22 ± 12 ind.10 cm-2) for both estuaries. Results of the non-Metric Multidimensional Scaling technique revealed distinct seasonal composition in meiobenthos but not between the estuaries. Results of the 2-way ANOSIM test confirmed no significant differences in meiobenthic composition between the estuaries (p = 0.712). However, seasonal difference was significant (p = 0.001) with higher densities for nematoda (166 ± 99 ind.10 cm-2 and 56 ± 29 ind.10 cm-2) recorded in Tana and Sabaki, respectively during the southeast monsoon season. At least 7 taxa out of a total of 26 were present in both estuaries. Salinity gradient, season and sediment depth were found to influence the meiobenthic densities and taxa composition.

Seasonality in Phytoplankton Species Composition and their Influence on Small Pelagic Fish along the Western Pemba Channel

The influence of phytoplankton biomass on small pelagic fish groups was studied at three sites along the Western Pemba Channel, to assess the contribution of seasonality. Sampling took place at the three sites in the coastal waters of Tanga, Tanzania. The study used in-situ data from July 2016 to January 2017, remotely-sensed chlorophyll-a data from August 2016 to August 2017 and 2019–2020, and small pelagic fish catch data from the same periods as the remotely-sensed data. The dominant groups at all the three sites were diatoms, dinoflagellates, and cyanobacteria, with diatoms having the highest number of species compared to the other two. In comparison to the southeast monsoon season, phytoplankton species and biomass were significantly higher during the northeast monsoon season. The most plentiful fish were anchovies, which had high peaks in both seasons, followed by sardinella, while mackerel had the lowest catch. Chlorophyll-a and anchovy catches showed a positive correlation; however, the relationship was not significant (r = 0.47, df = 11, p = 0.12). Both variables showed the highest peaks in October, while other fish groups showed very weak and insignificant positive or negative correlations. These findings suggest that factors other than phytoplankton biomass contribute to controlling small pelagic fish availability.
 Keywords: Phytoplankton; monsoon seasons; chlorophyll-a; Pemba Channel; small pelagic fish.

Reproductive seasonality and fecundity of the spiny lobsterPanulirus homarus homarus(Linnaeus, 1758) (Decapoda: Achelata: Palinuridae) in Kenya: implications for sustainable management

AbstractThe scalloped spiny lobster Panulirus homarus homarus (Linnaeus, 1758) is one of the most commercially important lobsters exploited in Kenya. Despite its local importance and wide distribution, detailed knowledge on its reproduction in East Africa is lacking. The study reported herein sought to determine its spawning season based on seasonal variations in female gonadosomatic index (GSI), temporal distribution of ovarian maturity stages, and the proportion of ovigerous females in the monthly samples captured off Mambrui, Kenya. The study also assessed size composition, sex ratio, fecundity, and reproductive potential. A total of 2,454 lobsters were collected during the study period (January 2000-December 2001), covering a size range of 26–115 mm carapace length (CL). Neither the overall sex ratio nor the monthly sex distributions deviated significantly from the expected ratio of 1:1, except for a few months when males and females alternately dominated the samples with no discernible seasonal trends. Analysis of the cyclical changes in reproductive activity shows strong spawning seasonality in synchrony with the Northeast monsoon season in both years. Simultaneous breeding of a wide size range of females (54–90 mm CL) commenced in November and intensified over the following months until April. Outside the Northeast monsoon season, only 10 females, out of the total 517 sexually mature females sampled during the 2000 and 2001 Southeast monsoon seasons, bore eggs, and all were large individuals (>66 mm CL) encountered in the 2001 samples. Fecundity was positively and linearly related to female size (CL) and best described by the equation E = 10,400 CL – 472,040 (R2 = 0.929, N = 59). Estimated fecundity varied between 81,000 and 474,000 eggs per spawn in female lobsters (54 mm–91 mm CL). Female lobsters between 66 mm and 80 mm CL contributed 62% of the estimated total egg production of the population. The widespread retention of egg-bearing females in a fishery where the main fishing season falls during the spawning season, can have detrimental effects on the spawning stock abundance and egg production with increased risk of recruitment failure and stock collapse.

The use of statistical models in identifying skipjack tuna habitat characteristics during the Southeast Monsoon in the Bone Gulf, Indonesia

Abstract. Safruddin, Hidayat R, Farhum SA, Zainuddin M. 2022. The use of statistical models in identifying skipjack tuna habitat characteristics during the Southeast Monsoon in the Bone Gulf, Indonesia. Biodiversitas 23: 2231-2237. Skipjack tuna (Katsuwonus pelamis) is the main fishery product in the Bone Gulf, Indonesia, since it has high economic value and is widely accepted by the market. The migration of skipjack tuna can be described by oceanographic dynamics, influenced by seasonal changes. This study aimed to explore the relationship between the oceanographic factors and the distribution and abundance of skipjack tuna obtained by pole and line fishing in the Bone Gulf, Indonesia. The research was conducted during the southeast monsoon season (April-September) from 2015 until 2018. Three oceanographic parameters were used to understand this relationship, i.e., Sea Surface Temperature (SST), Sea Surface Chlorophyll-a (SSC), and water depth. Generalized Additive Models (GAMs) and Generalized Linear Models (GLMs) used to predict the spatial pattern of the skipjack tuna showed that all considered parameters significant influenced the distribution of skipjack tuna in the study area. The fish are mostly distributed from coastal to offshore areas, ranging between 50 to 2000 meters and even wider. The concentration tends to be high within the range of 29.5 to 30.5°C of SST, 0.25 to 0.35 mgm-3 of SSC, and 500 to 1500 meters of water depth. The research also found that the skipjack tuna migration to the Bone Gulf may have followed the current water interruption during the southeast monsoon, for preferred warmer temperatures and lower primary productivity waters in the offshore areas. This factor may represent the optimal habitat of skipjack tuna during the period and can be the basis for determining the skipjack fishing schedule in the Bone Gulf.

Evaluation of the Kenya long line pelagic fishery: Temporal variation in fishing effort and catch rates

Pelagic fishes support a vital part of the local and export production and trade at global, regional, and local levels. In the Western Indian Ocean, pelagic fishes are mainly exploited by purse-seiners and long-line fishing vessels operating in national exclusive economic zones and international waters. Catch data from these vessels is often not adequately evaluated to determine the variation of catches and the state of the exploited stocks. This study evaluates the Kenya long line fishery between 2016 and 2020 to determine catch rates, temporal variation over time, and the key species’ maximum sustainable yield (MSY). The catches were dominated by Xiphias gladius (44.7%), Thunnus albacares (24.4%), Thunnus obesus (10.9%), and Prionace glauca (7.7%). Catch rates were significantly different among years, months, vessels, and depths for all species except for X. gladius and P. glauca, whose catch rates did not significantly differ at different depths. The catch rates for X. gladius, T. obesus, and T. albacares were higher during the northeast monsoon season, while P. glauca had higher catch rates during the southeast monsoon season. The maximum sustainable yield (MSY) was higher than the annual catches for the four species. We recommend catch rates be monitored over a more extended period for trend comparison and future work to consider discards encountered during fishing. We recommend seasonal closures for management, especially for the vessels targeting tunas and swordfish to conduct fishing during the NEM season when the catch rates are higher. There is a need for future work to focus on hook selectivity for the long-line fisheries along the Kenya EEZ.

Composition and structure of the mangrove fish and crustacean communities of Vanga Bay, Kenya

Mangroves support coastal fisheries, particularly by providing nurseries for juvenile fish. However, much remains unknown about the fish and crustacean communities in mangroves and about the lifecycles and behaviours of individual species. This study is the first to describe the fish and crustacean communities in the Vanga mangrove ecosystem, the largest mangrove forest and the most important marine fisheries landing site in southern Kenya. Using fyke nets, 14 mangrove creek sites were sampled once every 3 months during spring tides, between September 2015 and September 2017. The sampling period covered the north east (NEM) and south east (SEM) monsoon seasons. A total of 1,879 fishes and 1,132 crustaceans were caught, represented by 59 and 16 species, respectively. Over 95 % of the fishes caught were juveniles with 50 % of both fish and crustacean species being of commercial importance. About 70 % of the catch was dominated by only 6 fish species with Yarica hyalosoma and Acropoma japonicum contributing 44 % of the total abundance. Penaeus semisulcatus and Penaeus indicus dominated the crustaceans. Fish abundances differed significantly between seasons, with the SEM catches almost three times higher than those in the NEM, while crustaceans displayed no clear seasonality. The study documents for the first time the fish and crustacean communities in Vanga mangroves and confirms their importance as juvenile habitat for commercial species. The importance of mangrove and near mangrove habitat as nursery habitat for both fish and crustaceans emphasizes the need for conservation particularly of seaward edges

Vulnerability of fishery-based livelihoods to climate variability and change in a tropical island: insights from small-scale fishers in Seychelles

While climate shocks and stressors are not unique to developing countries, their impacts are expected to be most acute here due to limitations in the assets and infrastructure required for adaptation. This study assesses the vulnerability of fishery-based livelihoods to the impacts of climate variability and change across two major islands in Seychelles based on 80 household surveys and three shared dialogue workshops (SDWs) with small-scale fishers. Results showed that the percentage of fishers’ households that depend mainly on fisheries as a source of income was 95% and 97% for Mahe and Praslin respectively with alternative income streams along the fishing value chain such as transportation, fish mongers and processor. Fishers on Mahe Island had a dependency ratio index that was slightly higher than those on Praslin. Overall, fishing households on Mahe showed greater vulnerability on socio-demographic profile index compared to their counterparts on Praslin. However, greater livelihood diversification was recorded for householders on Mahe than Praslin as fishers earned income from tourism-related activities such as guest house, car rental, boat ride, and sales of coconuts as mentioned during the SDWs. Fishers on Mahe struggled to find fish for 3 months during the southeast monsoon season compared to 2 months for those on Praslin given that access is granted to fish in the lagoon during this season. More importantly, the voluntary closure of some fishing zones between the months of November to April on Praslin is a sustainability strategy that witnessed an increase in size and number for both rabbit and parrotfish. Further studies are needed in two key areas as follows: the role of subsidies and sustainable fisheries management, and a value-chain approach to vulnerability of small-scale fishers within the fishery sector in Seychelles.

Seasonal Variability of Waves Within the Indonesian Seas and Its Relation With the Monsoon Wind

The previous studies have simulated the variability of the wave within the Indonesian seas which showed that the variability of wave follows the seasonal pattern. However, their analysis only consider the influence of local wind forcings. The bias and error of their simulated wave were also unclear. In the present study, we investigate the variability of wave within the Indonesian seas and its relation with the surface wind speed using the combination of reanalysis and remote sensing data with high accuracies. We split the analysis into swell and wind wave to obtain the influence of local and remote wind forcings. We show that at the inner seas (i.e., the South China Sea, Java Sea, Flores Sea, Banda Sea and Arafura Sea), the variability of significant wave height (SWH) is majorly influenced by the variability of the speed of monsoon wind. The maximum SWH during Northwest monsoon (NWM) season is located at the South China Sea while during Southeast monsoon (SEM) season is at Arafura Sea. This indicates that the wind wave (sea) is dominant at the inner seas. At the open seas (i.e., Pacific Ocean and Indian Ocean) the variability of SWH less corresponds to the the speed of monsoon wind. The remote wind forcings control the wave variability in the open ocean area. This indicates that swell is dominant at the open seas. In general, the magnitude of SWHswell is also more than SWHsea within the Indonesian seas.

Impact of a ring net fishery in the inshore marine waters of Kilifi on the reproductive biology of six pelagic fish species

Ring nets are modified purse seines used for pelagic fishing along the Kenya coast. This method is however associated with potential negative environmental implications in inshore and shallow fishing grounds. Biological impacts on selected pelagic species caught in the marine waters of Kilifi were investigated during the Southeast Monsoon season (SEM) in April, May and September 2014, and the Northeast Monsoon season (NEM) in October and November 2014, and March 2015. Amblygaster sirm, Rastrelliger brachysoma, Sardinella gibbosa, Rastrelliger kanagurta, Hemiramphus far and Sphyraena obtusata were the most abundant species caught by the ring nets and comprised 68.9 % of fish catch composition. Analysis of ring net impacts on the fish sizes and maturity status were conducted for the NEM and SEM seasons. The catch composition for A. sirm in October comprised of 53.3 % juveniles and 53.4 % juveniles for S. obtusata in November. High percentages of H. far (78.3%) and S. obtusata (58.9 %) were captured in September. Incidences of undersize fish for A. sirm, S. gibbosa, R. kanagurta and R. brachysoma were observed across the seasons. Sex ratios for A. sirm, R. brachysoma, R. kanagurta and S. obtusata in the NEM season deviated from a normal ratio of 1:1 with more males observed. Females were dominant in the catch for H. far (1: 1.28) but the ratio was not different for S. gibbosa (χ² = 5.564, df = 1, p = 0.21). Males were dominant for A. sirm (1: 0.8) and S. gibbosa (1: 0.7) in the SEM season but the ratios were not different for H. far, S. obtusata, R. brachysoma and R. kanagurta. Differences in sex ratios were attributed to fish migrations and reproductive processes. Ring nets fished relatively offshore during the NEM season and targeted mainly spawning aggregations in stage IV. Immature fish in stage I and II which comprised of juveniles were harvested within sheltered inshore waters in the SEM season. Use of ring nets to target juveniles and spawning aggregations may disrupt recruitment processes. To enhance sustainable management of the ring net fishery, there is a need to develop harvesting strategies based on the information on stock status of the target fish.

Seasonal variability of vertical patterns in chlorophyll-a fluorescence in the coastal waters off Kimbiji, Tanzania

A study on the vertical pattern of chlorophyll-a (Chl-a) fluorescence was undertaken in the Mafia Channel offKimbiji, Tanzania. Data was collected during the Southeast Monsoon (SEM) and Northeast Monsoon (NEM) seasons. There was higher Chl-a concentration of 0.1 to 1.1 mgm-3 in the surface layer off Kimbiji to about 50 m depth due to the presence of mixed layer depth (MLD) which allowed water mixing in the layer. A deep Chl-a maximum was recorded at around 40 m depth during the NEM and between 40 and 70 m in the SEM. Surface water between longitude 39.9°E and 40.2°E had low Chl-a from the surface to about 50 m depth due to poor nutrient input. The NEM had an insignificantly higher Chl-a value than the SEM (p > 0.05) which differed from other studies in which Chl-a was higher during the SEM than the NEM, than, the Chl-a concentration was higher at the surface during the SEM than during the NEM. Satellite data showed higher Chl-a in the SEM than NEM, localized along the Mafia Channel. During the SEM season the wind pushes higher Chl-a water from the Mafia Channel towards the north and leads to a higher concentration at Kimbiji.