| 1. |
- Jahnke, Marlene, et al.
(författare)
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Population genetic structure and connectivity of the seagrass Thalassia hemprichii in the Western Indian Ocean is influenced by predominant ocean currents
- 2019
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Ingår i: Ecology and Evolution. - : John Wiley & Sons. - 2045-7758. ; 9:16, s. 8953-8964
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Tidskriftsartikel (refereegranskat)abstract
- This study is the first large-scale genetic population study of a widespread climax species of seagrass, Thalassia hemprichii, in the Western Indian Ocean (WIO). The aim was to understand genetic population structure and connectivity of T. hemprichii in relation to hydrodynamic features. We genotyped 205 individual seagrass shoots from 11 sites across the WIO, spanning over a distance of similar to 2,700 km, with twelve microsatellite markers. Seagrass shoots were sampled in Kenya, Tanzania (mainland and Zanzibar), Mozambique, and Madagascar: 4-26 degrees S and 33-48 degrees E. We assessed clonality and visualized genetic diversity and genetic population differentiation. We used Bayesian clustering approaches (TESS) to trace spatial ancestry of populations and used directional migration rates (DivMigrate) to identify sources of gene flow. We identified four genetically differentiated groups: (a) samples from the Zanzibar channel; (b) Mozambique; (c) Madagascar; and (d) the east coast of Zanzibar and Kenya. Significant pairwise population genetic differentiation was found among many sites. Isolation by distance was detected for the estimated magnitude of divergence (D-EST), but the three predominant ocean current systems (i.e., East African Coastal Current, North East Madagascar Current, and the South Equatorial Current) also determine genetic connectivity and genetic structure. Directional migration rates indicate that Madagascar acts as an important source population. Overall, clonality was moderate to high with large differences among sampling sites, indicating relatively low, but spatially variable sexual reproduction rates. The strongest genetic break was identified for three sites in the Zanzibar channel. Although isolation by distance is present, this study suggests that the three regionally predominant ocean current systems (i.e., East African Coastal Current, North East Madagascar Current, and the South Equatorial Current) rather than distance determine genetic connectivity and structure of T. hemprichii in the WIO. If the goal is to maintain genetic connectivity of T. hemprichii within the WIO, conservation planning and implementation of marine protection should be considered at the regional scale-across national borders.
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| 2. |
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| 3. |
- Mgeleka, Said S. S., et al.
(författare)
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Population genetics of the hound needlefish Tylosurus crocodilus (Belonidae) indicate high connectivity in Tanzanian coastal waters
- 2023
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Ingår i: Marine Biology Research. - : Taylor & Francis. - 1745-1000 .- 1745-1019. ; 19:4-5, s. 261-270
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Tidskriftsartikel (refereegranskat)abstract
- The hound needlefish Tylosurus crocodilus (Belonidae) is a highly demanded fish in the local markets of Tanzania, but the growing coastal population threatens its sustainability. As belonids are highly migratory fishes utilising various parts of the seascape, increased fishing pressure may disrupt connectivity patterns on different spatiotemporal scales and disaggregate populations. Using the COI gene, this study assessed the genetic population structure, connectivity patterns, and historical demography of T. crocodilus collected in seven sites spread along Tanzanian coastal waters. Results showed fourteen haplotypes with low overall nucleotide and haplotype diversity. Pairwise F-ST comparisons revealed no significant differences among the sampled sites, except for the northernmost site (Tanga) and an island in the south (Songosongo). Analysis of molecular variance (AMOVA) revealed a non-significant genetic structure among populations (F-ST = 0.01782), suggesting the fishery across Tanzanian waters exploits the same population. Moreover, there was no correlative relationship between genetic and pairwise geographic distances, rejecting the isolation by distance hypothesis. However, neutrality tests and mismatch distribution analysis revealed that recent demographic expansion might exist. Empirical evidence of panmixia suggests high genetic connectivity. In combination with low genetic diversity, management should be directed to actions that prevent genetic diversity loss and the effect of genetic drift on populations.
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| 4. |
- Silas, Mathew Ogalo, et al.
(författare)
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Adaptive capacity and coping strategies of small-scale coastal fisheries to declining fish catches: Insights from Tanzanian communities
- 2020
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Ingår i: Environmental Science and Policy. - : Elsevier BV. - 1462-9011 .- 1873-6416. ; 108, s. 67-76
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Tidskriftsartikel (refereegranskat)abstract
- Small-scale fishing communities are expected to adapt to fish catch fluctuations linked to global environmental change. Notwithstanding, impacts from severe climate events and overexploitation of fisheries resources can compromise functions and resilience of ecosystems and associated species, and thereby jeopardize long-term population trend stability and fisheries productivity. To date, most assessments and vulnerability studies of fisheries-dependent populaces have focused on global, regional and national levels, while studies at village and community levels, where adaptive planning in the context of climate- and environmental changes is important, are less common. Based on data from official fishery records over a three-decadal period (1984–2016) and recent interviews with artisanal fishermen (319 fishers from eight communities) along the Tanzanian coast, we assessed small-scale fisheries with regard to (i) long-term trends in fishery landings, (ii) long-term alterations in fishing gear use, and (iii) fishers’ perceptions on how they have been coping and adapting to fluctuating fish landings. We further investigated (iv) the adaptive capacity of a wide range of coastal villages by assessing the fishers’ responses to an anticipated future scenario of a major (50 %) decline in landings from the current fisheries catch levels. The long-term trend records of fish landings showed a remarkable ∼50 % reduction in terms of both catch per vessel and catch per fisher from 1984 to 2016. According to the interviews, the majority of fishers (75 %) have changed fishing grounds from nearshore to offshore areas during the last decade, owing to a general perception that nearshore areas have suffered major reduction in fish stocks (due to overfishing and environmental changes related to extreme climate- or weather events), while offshore areas were considered still productive. The change in location of fishing grounds is probably a result of the clear switch in major gear type utilization from beach seine to ring net that occurred over the last decades. With a further progressive decline in fishery catches to a predictive level of 50 % of the current catch level, there is a general perception that artisanal fishers will continue fishing because alternative livelihoods (like crop farming, which employs more than 65 % of the population) have suffered similar negative impact. These findings highlight the need for building adaptive capacity in local coastal communities to develop alternative coping strategies for the impacts of climate- and environmental changes. © 2020 Elsevier Ltd
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| 6. |
- Silas, Mathew Ogalo, et al.
(författare)
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Growth, mortality, exploitation rate and recruitment pattern of Octopus cyanea (Mollusca: Cephalopoda) in the WIO region : A case study from the Mafia Archipelago, Tanzania
- 2021
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Ingår i: Western Indian Ocean Journal of Marine Science. - : African Journals Online (AJOL). - 0856-860X .- 2683-6416. ; 20:1, s. 71-79
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Tidskriftsartikel (refereegranskat)abstract
- Octopus cyanea is a commercially important cephalopod in the Western Indian Ocean (WIO) region, but scientific information to inform management strategies for the species is limited. A study was conducted in 2014, 2015, 2017 and 2018 to investigate biological parameters including growth, mortality, exploitation rates and recruitment patterns in the sea around Mafia Archipelago, Tanzania. Virtual population analysis (VPA) indicated differential mortality between two sampling sites; the lowest and highest fishing mortality of F = 1.5yr-1 and F = 2.7yr-1 were observed in Bwejuu (Dorsal mantle length, DMT = 18-20 cm) and Jibondo (DMT = 8-12 cm) fishing villages, respectively. The maximum exploitation rate (Emax), which gives the maximum relative yield per recruit, was estimated at 0.380 and 0.379 for Jibondo and Bwejuu, respectively. The exploitation rates E 0.5, which corresponded to 50% of the unexploited stock relative biomass per recruit, were estimated at 0.248 for Jibondo and 0.247 for Bwejuu. These values differ greatly from the exploitation rates of 0.53 and 0.41 for Jibondo and Bwejuu, respectively, suggesting that the stock of O. cyanea is probably being overfished both in terms of yield per recruit and biomass per recruit. The stock-recruit pattern was observed to be continuous year-round, with the peak being between May and July. Since the peak in recruitment of both areas coincides with the south-east monsoon (SE Monsoon) and the level of maximum sustainable yield has been overshot, it is recommended that management plans are implemented that will reduce effort while increasing biomass, for example, implementing temporal octopus fishery closures at a village level.
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| 7. |
- Silas, Mathew Ogalo, 1982-
(författare)
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Patterns of small-scale coastal fisheries and local fisheries management in Tanzania : adaptation to a changing climate
- 2022
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Coastal habitats like mangroves, seagrass meadows, coral reefs, and adjacent offshore waters constitute an important part of the tropical coastal seascape. They provide conducive environmental conditions for fish and other marine animals, which serve as food and income for coastal communities around the world. Despite the beneficial ecosystem services generated, these habitats are under immense pressure from anthropogenic and climate change impacts that destroy habitats and reduce their capacity to support fisheries. This leaves coastal communities in a challenging situation, where people need to adapt to changing fish stocks. This thesis aimed to understand the past, current, and future patterns of small-scale fisheries in Tanzania and how fishing communities can respond to human activities and climate change. The thesis assesses and integrates fish catch data (from 2013 and 2014) and fishers’ perceptions. Archived fish landing data from 1950 to 2016 were used to understand the role of coastal marine ecosystems for small-scale fisheries and to investigate how local fishers adapt to environmental changes and fishing pressure. To address the goals of the thesis, Paper I investigated seasonal (gear-based) patterns of fish catch in coastal marine habitats. Paper II explored the influence of environmental conditions and monsoon seasons on fish catch. Papers III and IV examined the effects of climate change on small-scale fisheries, and Paper IV also addressed the response of fishing villages to a declining catch trend in the country. The findings show that the coastal seascape (from nearshore to offshore waters) is vital in sustaining fish stocks, while the productivity of coastal habitats varies with seasons. The highest catch rates were observed in the offshore environment during the northeast monsoon season (Paper I), which is characterised by weak winds and high sea surface temperature. In contrast, the catches of important target species, such as tuna and prawn, were found high in nearshore habitats during the rougher southeast monsoon season (Paper II), which is characterised by strong winds and low sea surface temperature. The fishers’ perceptions revealed similar patterns of high catches of tuna and prawn during the southeast monsoon season in nearshore waters (Paper II). This emphasises the relevance of incorporating fishers’ perceptions into research to better understand the complex dynamics of small-scale fisheries. A comparative assessment of climate change impacts on long-term fisheries productivity of two target stocks showed clear species-specific effects of climate change. For instance, pelagic mackerel (Rastrelliger kanagurta) was found sensitive to increased precipitation, as a lowered salinity may negatively affect the maturing phase (Paper III). Demersal prawn, on the other hand, was positively related to increased sea surface temperature (Paper III). Importantly, the findings of Paper IV show that most small-scale fishers will continue to fish despite declining catches. This suggests a need of building adaptive capacity among local coastal communities, which could serve as alternative coping mechanisms to the impacts of climate change and other stressors (Paper IV).In conclusion, this thesis shows how shallow-water habitats and adjacent offshore waters in the coastal seascape, environmental conditions and seasonal weather patterns, and stressors such as climate change and fishing, play essential roles in determining fish catch patterns and the behaviour of fishers in the western Indian Ocean region. The findings of the thesis demonstrate the benefits of integrating fish catch records and local knowledge facilitating applied research and coastal resource management.
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| 8. |
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| 9. |
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| 10. |
- Silas, Mathew Ogalo, 1982-, et al.
(författare)
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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
- 2023
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Ingår i: Fisheries Research. - : Elsevier. - 0165-7836 .- 1872-6763. ; 264
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 11. |
- Wilson, Robert J., et al.
(författare)
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Large projected reductions in marine fish biomass for Kenya and Tanzania in the absence of climate mitigation
- 2021
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Ingår i: Ocean and Coastal Management. - : Elsevier BV. - 0964-5691 .- 1873-524X. ; 215
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Tidskriftsartikel (refereegranskat)abstract
- Climate change is projected to cause significant reductions in global fisheries catch during the 21st Century. Yet, little is understood of climate change impacts on tropical fisheries, which support many livelihoods, as is the case in the Western Indian Ocean region (WIO). Here, we focus on two central WIO countries - Kenya and Tanzania and run a multi-species fish model (Size Spectrum Dynamic Bio-climate Envelope Model; SS-DBEM) for 43 species of commercial and artisanal importance, to investigate the effects of climate change. We include both national Exclusive Economic Zones (EEZs) as domains. The model was forced by data from a biogeochemical model (NEMO-MEDUSA), run under the high emissions scenario Representative Concentration Pathway (RCP) 8.5, until the end of the 21st century. Impacts of fisheries and climate change were investigated by running SSDBEM under five scenarios of fishing pressures to predict a range of possible future scenarios. Fishing pressure was represented as the Maximum Sustainable Yield (MSY), expressed as MSY0, MSY1, MSY2, MSY3 and MSY4 representing fishing mortality of 0, 1, 2, 3 and 4 times MSY, respectively. Large reductions in average fish biomass were projected over the 21st Century, with median reductions of fish species biomass of 63-76% and 56-69% for the Kenyan and Tanzanian EEZs respectively across the fishing scenarios. Tunas were particularly impacted by future climate change, with the six modelled species exhibiting biomass reductions of at least 70% in both EEZs for all fishing scenarios during the 21st Century. Reductions in fish biomass were much more severe during the second half of the 21st Century, highlighting the benefits to tropical fisheries of global action on climate change.
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