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John Kathena - One of the best experts on this subject based on the ideXlab platform.
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migration distribution and population stock structure of shallow water hake merluccius capensis in the benguela current Large Marine Ecosystem inferred using a geostatistical population model
Fisheries Research, 2016Co-Authors: Teunis Jansen, Paulus Inekela Kainge, Deon Durholtz, Tore Stromme, Uffe Hogsbro Thygesen, Margit R Wilhelm, John Kathena, Tracey P Fairweather, Kasper Kristensen, Sarah C PaulusAbstract:Abstract Shallow-water hake (Merluccius capensis) is of considerable ecological and economic importance in the Benguela Current Large Marine Ecosystem in South Africa and Namibia. Optimal management of the resource is currently constrained by the limited understanding of migration patterns and population (stock) structure. We combined data from multiple demersal trawl surveys from the entire distribution area to estimate growth rate, mortality and spatial and temporal patterns of M. capensis. Analyses were conducted using the geostatistical model GeoPop. The complexity of the model and the amount of data required a new level of soft- and hardware performance. This was achieved by utilizing Template Model Builder and high-end computational hardware (Amazon Elastic Compute Cloud, EC2). The data and the model enabled us to follow the distribution and infer movements of M. capensis from the recruitment/nursery areas, through the juvenile phase and the adults’ migration to the spawning areas outside/upstream of the nursery areas. This revealed some previously unknown migration patterns and indicated natal homing and the existence of three primary population components in the region, namely the Walvis (central and northern Namibia), the Orange (Southern Namibia—Northern SA) and the Agulhas (Southern part of SA) components. Our results also indicated substantial regional differences in mortality. We recommend that fisheries assessment, advice and management take consideration of these aspects of the distribution and population (stock) structure of M. capensis in the Benguela Current Large Marine Ecosystem.
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spawning patterns of shallow water hake merluccius capensis and deep water hake m paradoxus in the benguela current Large Marine Ecosystem inferred from gonadosomatic indices
Fisheries Research, 2015Co-Authors: Teunis Jansen, Paulus Inekela Kainge, Deon Durholtz, Tore Stromme, Margit R Wilhelm, John Kathena, Larvika Singh, Victoria Ndinelago ErasmusAbstract:Abstract We use gonad- and body-weight data from 54,000 samples of Merluccius capensis and Merluccius paradoxus collected in all months of the years between 1991 and 2013 to infer peak spawning periods and areas in the Benguela Current Large Marine Ecosystem. We develop and apply a new gonadosomatic index threshold for identification of spawning individuals. Spawning M. capensis were observed throughout the study area, mainly in areas of about 100 m bottom depth. The highest proportions of spawning M. capensis females in the northern Benguela region were observed off central Namibia between 24.0 and 26.0°S. In the southern Benguela, peaks in the proportions of spawning M. capensis were observed in two areas off the South African West Coast (31.0–32.5°S and 34.5–36.0°S), whereas spawning females off the South African South Coast (east of 20°E) appeared to be more evenly distributed in space. Seasonality differed between areas. In the northern Benguela, the main spawning season of M. capensis appeared to be the austral winter (July–September, peaking in August), while off the South African South Coast, the main spawning season is suggested to be in summer (around January). Between these two extremes, on the western Agulhas bank in the southern Benguela, spawning peaks were observed in both summer and winter. These peaks Largely coincided with peaks in phytoplankton production that are linked to upwelling conditions in the region. Hake condition decreased subsequent to the development of the gonads. The annual spawning cycle differed between small and Large M. capensis. The current October–closure of the fishery in Namibia may not match the peak spawning in August/September and may need to be shifted to earlier in the year. Spawning M. paradoxus were mainly found in areas of 200–650 m bottom depths. In the northern Benguela, spawning M. paradoxus were observed as far north as 25°S in August. The proportion of spawning females peaked between 34.5°S and 36.5°S off the West Coast, and between 23.0°E and 26.5 °E off the South Coast. It was suggested that M. paradoxus spawn throughout the year off the South African coast, with increased intensity around March and August–October. The finding of multiple spawning seasons and areas of both M. capensis and M. paradoxus strongly suggest multiple stocks (reproductive units).
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spatio temporal trends in diversity of demersal fish species in the benguela current Large Marine Ecosystem region
Fisheries Oceanography, 2015Co-Authors: Dawit Yemane, John Kathena, Samuel K. Mafwila, Silvi E. Nsiangango, Stephen P KirkmanAbstract:Biodiversity is changing at an unprecedented rate on a global scale as a complex response to several anthropogenic changes, in addition to the background natural environmental variability and cycles. In the Benguela Current Large Marine Ecosystem region (BCLME), aspects of demersal fish species diversity were studied between 1985 and 2010 in three countries (Angola, Namibia, and South Africa) using scientific survey data (depending on country). The study was aimed to address multi-tiered objectives: to document patterns of diversity (as measured by the different indices of diversity) in the three countries; to relate these patterns to various explanatory variables (depth, latitude, longitude, bottom temperature, and total catch) using a generalized additive model; and to compare the patterns of diversity and importance of potential drivers of biodiversity across the BCLME region. Results showed contrasting diversity patterns between the three countries in terms of both the form and strength of the relationship with different sets of predictors. Species richness S, Pielou's evenness index J', and Shannon–Wiener index H' were shown to increase with increasing depth in all three countries. In addition, both the overall and depth-dependent latitudinal pattern in S showed local and regional level bottlenecks in biodiversity, and the latitudinal pattern of S was similar to the pattern of bottom temperature. Overall, the species richness of demersal fish species (S) was found to increase as one moved towards the equator, in line with similar studies that reported an increase in richness with latitude.
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assessing changes in the distribution and range size of demersal fish populations in the benguela current Large Marine Ecosystem
Reviews in Fish Biology and Fisheries, 2014Co-Authors: Dawit Yemane, John Kathena, Silvi E. Nsiangango, Stephen P Kirkman, Bjorn Erik Axelsen, Toufiek SamaaiAbstract:Distributional change, expressed as range expansion or contraction , has been observed in many Marine populations and related to changes in the environment. The extent of such distributional changes is also expected to increase in response to future climate change. The Benguela Current Large Marine Ecosystem (BCLME) which adjoins the south-western coast of Africa is a global Marine hotspot with long-term warming occurring over a Large area. The area is also an important centre of Marine food production for three countries—South Africa, Namibia and Angola and is considered to be vulnerable to future climate change or increased climate variability. In this study we analysed change in distribution and range size of several demersal fish species in the BCLME over the period 1985–2010, including both commercial and non-commercial fish populations. Some of the observed changes in distribution and range size correspond to what is expected with increased warming whereas others appear to the contrary. Overall the results of the study highlight the complex nature of the response of fish population to climate change.
Tore Stromme - One of the best experts on this subject based on the ideXlab platform.
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migration distribution and population stock structure of shallow water hake merluccius capensis in the benguela current Large Marine Ecosystem inferred using a geostatistical population model
Fisheries Research, 2016Co-Authors: Teunis Jansen, Paulus Inekela Kainge, Deon Durholtz, Tore Stromme, Uffe Hogsbro Thygesen, Margit R Wilhelm, John Kathena, Tracey P Fairweather, Kasper Kristensen, Sarah C PaulusAbstract:Abstract Shallow-water hake (Merluccius capensis) is of considerable ecological and economic importance in the Benguela Current Large Marine Ecosystem in South Africa and Namibia. Optimal management of the resource is currently constrained by the limited understanding of migration patterns and population (stock) structure. We combined data from multiple demersal trawl surveys from the entire distribution area to estimate growth rate, mortality and spatial and temporal patterns of M. capensis. Analyses were conducted using the geostatistical model GeoPop. The complexity of the model and the amount of data required a new level of soft- and hardware performance. This was achieved by utilizing Template Model Builder and high-end computational hardware (Amazon Elastic Compute Cloud, EC2). The data and the model enabled us to follow the distribution and infer movements of M. capensis from the recruitment/nursery areas, through the juvenile phase and the adults’ migration to the spawning areas outside/upstream of the nursery areas. This revealed some previously unknown migration patterns and indicated natal homing and the existence of three primary population components in the region, namely the Walvis (central and northern Namibia), the Orange (Southern Namibia—Northern SA) and the Agulhas (Southern part of SA) components. Our results also indicated substantial regional differences in mortality. We recommend that fisheries assessment, advice and management take consideration of these aspects of the distribution and population (stock) structure of M. capensis in the Benguela Current Large Marine Ecosystem.
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Collaboration between the Nansen Programme and the Large Marine Ecosystem Programmes
Environmental Development, 2016Co-Authors: Gabriella Bianchi, Åsmund Bjordal, Kwame A. Koranteng, Merete Tandstad, Birane Sambe, Tore StrommeAbstract:Abstract Norwegian development cooperation in fisheries has been taken place since the early 1950s and the Nansen Programme, now operating as the EAF-Nansen Project, has been one of its major representatives. The Nansen Programme has cooperated with developing nations, institutions and national and regional projects over the last forty years, representing an important mechanism for development assistance in the field of fisheries for the Norwegian Agency for Development Cooperation (Norad). The scope of the cooperation has changed over the years to respond to emerging needs and challenges in partner countries. Since the Large Marine Ecosystem (LME) concept appeared in the global Marine science and governance agenda, the Nansen Programme was seen as an important partner for the regional projects in Africa that emerged from the concept. The Nansen Programme has collaborated with the Agulhas and Somali Currents LME (ACSLME) project, the Benguela Current LME (BCLME) project and its successor the Benguela Current Commission (BCC) as well as the Canary Current LME (CCLME) project and the Guinea Current LME (GCLME) project. In South Asia, the Nansen Programme has also collaborated with the Bay of Bengal LME (BoBLME) project. This paper chronicles the nature of the collaboration and the mutual benefits accruing to all the parties.
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spawning patterns of shallow water hake merluccius capensis and deep water hake m paradoxus in the benguela current Large Marine Ecosystem inferred from gonadosomatic indices
Fisheries Research, 2015Co-Authors: Teunis Jansen, Paulus Inekela Kainge, Deon Durholtz, Tore Stromme, Margit R Wilhelm, John Kathena, Larvika Singh, Victoria Ndinelago ErasmusAbstract:Abstract We use gonad- and body-weight data from 54,000 samples of Merluccius capensis and Merluccius paradoxus collected in all months of the years between 1991 and 2013 to infer peak spawning periods and areas in the Benguela Current Large Marine Ecosystem. We develop and apply a new gonadosomatic index threshold for identification of spawning individuals. Spawning M. capensis were observed throughout the study area, mainly in areas of about 100 m bottom depth. The highest proportions of spawning M. capensis females in the northern Benguela region were observed off central Namibia between 24.0 and 26.0°S. In the southern Benguela, peaks in the proportions of spawning M. capensis were observed in two areas off the South African West Coast (31.0–32.5°S and 34.5–36.0°S), whereas spawning females off the South African South Coast (east of 20°E) appeared to be more evenly distributed in space. Seasonality differed between areas. In the northern Benguela, the main spawning season of M. capensis appeared to be the austral winter (July–September, peaking in August), while off the South African South Coast, the main spawning season is suggested to be in summer (around January). Between these two extremes, on the western Agulhas bank in the southern Benguela, spawning peaks were observed in both summer and winter. These peaks Largely coincided with peaks in phytoplankton production that are linked to upwelling conditions in the region. Hake condition decreased subsequent to the development of the gonads. The annual spawning cycle differed between small and Large M. capensis. The current October–closure of the fishery in Namibia may not match the peak spawning in August/September and may need to be shifted to earlier in the year. Spawning M. paradoxus were mainly found in areas of 200–650 m bottom depths. In the northern Benguela, spawning M. paradoxus were observed as far north as 25°S in August. The proportion of spawning females peaked between 34.5°S and 36.5°S off the West Coast, and between 23.0°E and 26.5 °E off the South Coast. It was suggested that M. paradoxus spawn throughout the year off the South African coast, with increased intensity around March and August–October. The finding of multiple spawning seasons and areas of both M. capensis and M. paradoxus strongly suggest multiple stocks (reproductive units).
Teunis Jansen - One of the best experts on this subject based on the ideXlab platform.
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migration distribution and population stock structure of shallow water hake merluccius capensis in the benguela current Large Marine Ecosystem inferred using a geostatistical population model
Fisheries Research, 2016Co-Authors: Teunis Jansen, Paulus Inekela Kainge, Deon Durholtz, Tore Stromme, Uffe Hogsbro Thygesen, Margit R Wilhelm, John Kathena, Tracey P Fairweather, Kasper Kristensen, Sarah C PaulusAbstract:Abstract Shallow-water hake (Merluccius capensis) is of considerable ecological and economic importance in the Benguela Current Large Marine Ecosystem in South Africa and Namibia. Optimal management of the resource is currently constrained by the limited understanding of migration patterns and population (stock) structure. We combined data from multiple demersal trawl surveys from the entire distribution area to estimate growth rate, mortality and spatial and temporal patterns of M. capensis. Analyses were conducted using the geostatistical model GeoPop. The complexity of the model and the amount of data required a new level of soft- and hardware performance. This was achieved by utilizing Template Model Builder and high-end computational hardware (Amazon Elastic Compute Cloud, EC2). The data and the model enabled us to follow the distribution and infer movements of M. capensis from the recruitment/nursery areas, through the juvenile phase and the adults’ migration to the spawning areas outside/upstream of the nursery areas. This revealed some previously unknown migration patterns and indicated natal homing and the existence of three primary population components in the region, namely the Walvis (central and northern Namibia), the Orange (Southern Namibia—Northern SA) and the Agulhas (Southern part of SA) components. Our results also indicated substantial regional differences in mortality. We recommend that fisheries assessment, advice and management take consideration of these aspects of the distribution and population (stock) structure of M. capensis in the Benguela Current Large Marine Ecosystem.
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spawning patterns of shallow water hake merluccius capensis and deep water hake m paradoxus in the benguela current Large Marine Ecosystem inferred from gonadosomatic indices
Fisheries Research, 2015Co-Authors: Teunis Jansen, Paulus Inekela Kainge, Deon Durholtz, Tore Stromme, Margit R Wilhelm, John Kathena, Larvika Singh, Victoria Ndinelago ErasmusAbstract:Abstract We use gonad- and body-weight data from 54,000 samples of Merluccius capensis and Merluccius paradoxus collected in all months of the years between 1991 and 2013 to infer peak spawning periods and areas in the Benguela Current Large Marine Ecosystem. We develop and apply a new gonadosomatic index threshold for identification of spawning individuals. Spawning M. capensis were observed throughout the study area, mainly in areas of about 100 m bottom depth. The highest proportions of spawning M. capensis females in the northern Benguela region were observed off central Namibia between 24.0 and 26.0°S. In the southern Benguela, peaks in the proportions of spawning M. capensis were observed in two areas off the South African West Coast (31.0–32.5°S and 34.5–36.0°S), whereas spawning females off the South African South Coast (east of 20°E) appeared to be more evenly distributed in space. Seasonality differed between areas. In the northern Benguela, the main spawning season of M. capensis appeared to be the austral winter (July–September, peaking in August), while off the South African South Coast, the main spawning season is suggested to be in summer (around January). Between these two extremes, on the western Agulhas bank in the southern Benguela, spawning peaks were observed in both summer and winter. These peaks Largely coincided with peaks in phytoplankton production that are linked to upwelling conditions in the region. Hake condition decreased subsequent to the development of the gonads. The annual spawning cycle differed between small and Large M. capensis. The current October–closure of the fishery in Namibia may not match the peak spawning in August/September and may need to be shifted to earlier in the year. Spawning M. paradoxus were mainly found in areas of 200–650 m bottom depths. In the northern Benguela, spawning M. paradoxus were observed as far north as 25°S in August. The proportion of spawning females peaked between 34.5°S and 36.5°S off the West Coast, and between 23.0°E and 26.5 °E off the South Coast. It was suggested that M. paradoxus spawn throughout the year off the South African coast, with increased intensity around March and August–October. The finding of multiple spawning seasons and areas of both M. capensis and M. paradoxus strongly suggest multiple stocks (reproductive units).
Paulus Inekela Kainge - One of the best experts on this subject based on the ideXlab platform.
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fisheries yields climate change and Ecosystem based management of the benguela current Large Marine Ecosystem
Environmental development, 2020Co-Authors: Paulus Inekela Kainge, C D Van Der Lingen, Stephen P Kirkman, Johannes N Kathena, Virgilio Estevao, Uatjavi Uanivi, Anja Van Der Plas, Jean Githaigamwicigi, Azwianewi B Makhado, Lavinia NghimwatyaAbstract:Abstract Fisheries resources of the Benguela Current Large Marine Ecosystem (BCLME) are critically important to the socio-economic well-being of the people of Angola, Namibia and South Africa. Their high degree of environmental vulnerability underscores the importance of maintaining a healthy Ecosystem by increasing its resilience to the impacts of anthropogenic and natural forcing. Conflicting objectives in fisheries management processes between the biological sustainability of the stocks and socio-economic gains of the fishers and the economy in general, are some of the challenges facing the BCLME. In this paper, an overview and description of the fisheries yields from major resources of the BCLME is provided, using time series catch data, with discussions on how these are affected by climate variability and/or change. Data were sourced from previous syntheses, reports on the state of fisheries resources, and various national databases. Results indicate that, although major fisheries yields in the BCLME are currently much lower than their historical past, catches have in general been stable during the past two decades. Currently, there appears to be no clear evidence of the impacts of climate change on the fisheries resources in the BCLME, although some changes have been observed, but such impacts may be masked by highly variable and complex nature of this LME. However, through the Benguela Current Convention (BCC), the three coastal states are focused on ensuring that the shared commercially important fish stocks are jointly monitored, assessed and managed, through the implementation of adaptive fisheries management plans for priority shared fish resources based on Ecosystem approach to fisheries (EAF) principles.
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migration distribution and population stock structure of shallow water hake merluccius capensis in the benguela current Large Marine Ecosystem inferred using a geostatistical population model
Fisheries Research, 2016Co-Authors: Teunis Jansen, Paulus Inekela Kainge, Deon Durholtz, Tore Stromme, Uffe Hogsbro Thygesen, Margit R Wilhelm, John Kathena, Tracey P Fairweather, Kasper Kristensen, Sarah C PaulusAbstract:Abstract Shallow-water hake (Merluccius capensis) is of considerable ecological and economic importance in the Benguela Current Large Marine Ecosystem in South Africa and Namibia. Optimal management of the resource is currently constrained by the limited understanding of migration patterns and population (stock) structure. We combined data from multiple demersal trawl surveys from the entire distribution area to estimate growth rate, mortality and spatial and temporal patterns of M. capensis. Analyses were conducted using the geostatistical model GeoPop. The complexity of the model and the amount of data required a new level of soft- and hardware performance. This was achieved by utilizing Template Model Builder and high-end computational hardware (Amazon Elastic Compute Cloud, EC2). The data and the model enabled us to follow the distribution and infer movements of M. capensis from the recruitment/nursery areas, through the juvenile phase and the adults’ migration to the spawning areas outside/upstream of the nursery areas. This revealed some previously unknown migration patterns and indicated natal homing and the existence of three primary population components in the region, namely the Walvis (central and northern Namibia), the Orange (Southern Namibia—Northern SA) and the Agulhas (Southern part of SA) components. Our results also indicated substantial regional differences in mortality. We recommend that fisheries assessment, advice and management take consideration of these aspects of the distribution and population (stock) structure of M. capensis in the Benguela Current Large Marine Ecosystem.
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spawning patterns of shallow water hake merluccius capensis and deep water hake m paradoxus in the benguela current Large Marine Ecosystem inferred from gonadosomatic indices
Fisheries Research, 2015Co-Authors: Teunis Jansen, Paulus Inekela Kainge, Deon Durholtz, Tore Stromme, Margit R Wilhelm, John Kathena, Larvika Singh, Victoria Ndinelago ErasmusAbstract:Abstract We use gonad- and body-weight data from 54,000 samples of Merluccius capensis and Merluccius paradoxus collected in all months of the years between 1991 and 2013 to infer peak spawning periods and areas in the Benguela Current Large Marine Ecosystem. We develop and apply a new gonadosomatic index threshold for identification of spawning individuals. Spawning M. capensis were observed throughout the study area, mainly in areas of about 100 m bottom depth. The highest proportions of spawning M. capensis females in the northern Benguela region were observed off central Namibia between 24.0 and 26.0°S. In the southern Benguela, peaks in the proportions of spawning M. capensis were observed in two areas off the South African West Coast (31.0–32.5°S and 34.5–36.0°S), whereas spawning females off the South African South Coast (east of 20°E) appeared to be more evenly distributed in space. Seasonality differed between areas. In the northern Benguela, the main spawning season of M. capensis appeared to be the austral winter (July–September, peaking in August), while off the South African South Coast, the main spawning season is suggested to be in summer (around January). Between these two extremes, on the western Agulhas bank in the southern Benguela, spawning peaks were observed in both summer and winter. These peaks Largely coincided with peaks in phytoplankton production that are linked to upwelling conditions in the region. Hake condition decreased subsequent to the development of the gonads. The annual spawning cycle differed between small and Large M. capensis. The current October–closure of the fishery in Namibia may not match the peak spawning in August/September and may need to be shifted to earlier in the year. Spawning M. paradoxus were mainly found in areas of 200–650 m bottom depths. In the northern Benguela, spawning M. paradoxus were observed as far north as 25°S in August. The proportion of spawning females peaked between 34.5°S and 36.5°S off the West Coast, and between 23.0°E and 26.5 °E off the South Coast. It was suggested that M. paradoxus spawn throughout the year off the South African coast, with increased intensity around March and August–October. The finding of multiple spawning seasons and areas of both M. capensis and M. paradoxus strongly suggest multiple stocks (reproductive units).
Johannes N Kathena - One of the best experts on this subject based on the ideXlab platform.
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fisheries yields climate change and Ecosystem based management of the benguela current Large Marine Ecosystem
Environmental development, 2020Co-Authors: Paulus Inekela Kainge, C D Van Der Lingen, Stephen P Kirkman, Johannes N Kathena, Virgilio Estevao, Uatjavi Uanivi, Anja Van Der Plas, Jean Githaigamwicigi, Azwianewi B Makhado, Lavinia NghimwatyaAbstract:Abstract Fisheries resources of the Benguela Current Large Marine Ecosystem (BCLME) are critically important to the socio-economic well-being of the people of Angola, Namibia and South Africa. Their high degree of environmental vulnerability underscores the importance of maintaining a healthy Ecosystem by increasing its resilience to the impacts of anthropogenic and natural forcing. Conflicting objectives in fisheries management processes between the biological sustainability of the stocks and socio-economic gains of the fishers and the economy in general, are some of the challenges facing the BCLME. In this paper, an overview and description of the fisheries yields from major resources of the BCLME is provided, using time series catch data, with discussions on how these are affected by climate variability and/or change. Data were sourced from previous syntheses, reports on the state of fisheries resources, and various national databases. Results indicate that, although major fisheries yields in the BCLME are currently much lower than their historical past, catches have in general been stable during the past two decades. Currently, there appears to be no clear evidence of the impacts of climate change on the fisheries resources in the BCLME, although some changes have been observed, but such impacts may be masked by highly variable and complex nature of this LME. However, through the Benguela Current Convention (BCC), the three coastal states are focused on ensuring that the shared commercially important fish stocks are jointly monitored, assessed and managed, through the implementation of adaptive fisheries management plans for priority shared fish resources based on Ecosystem approach to fisheries (EAF) principles.
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hake species merluccius capensis and m paradoxus assessment in the benguela current Large Marine Ecosystem
Environmental development, 2016Co-Authors: Uffe Hogsbro Thygesen, Johannes N Kathena, A H Nielsen, Casper Willestofte BergAbstract:Abstract An important resource of the Benguela Current Large Marine Ecosystem (BCLME) is the hake fisheries, and accurate assessments of hake stocks are essential. Traditionally, the two species M. capensis and M. paradoxus have been assessed combined as a single stock, since commercial catch data does not distinguish the two species, but differences in biology and ecology suggest that this could lead to a biased assessment. Here, we apply three state space models to assess BCLME hake stocks off Namibia: Two independent single-species assessments, one for each species, and one combined assessment which treats the two species as a single stock. Catch-at-age information from commercial fishery and annual swept-area biomass survey estimates conducted during 1998–2012 were used, together with information on maturity at age, stock mean weight-at-age and natural mortality. The results demonstrate that the estimated spawning stock biomass of M. capensis has been increasing in recent years while estimated fishing mortalities are higher on M. paradoxus . These observations are not visible in the species-combined assessment. However, the species-combined estimates of fishing mortality and biomass have less uncertainty than the sum of the single-species estimates. Hence, the choice between species-combined and species-specific assessment is an example of a generic trade-off between bias and variance in assessments of structured stocks: Combining data and treating the two species as one decreases the variance by enlarging the data base but introduces a bias originating from difference in rates between the two species, when their relative abundances change in time.
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regime shifts in demersal assemblages of the benguela current Large Marine Ecosystem a comparative assessment
Fisheries Oceanography, 2015Co-Authors: Stephen P Kirkman, Silvi E. Nsiangango, Dawit Yemane, Bjorn Erik Axelsen, L J Atkinson, Johannes N Kathena, Larvika Singh, Toufiek SamaaiAbstract:Using long-term survey data, changes in demersal faunal communities in the Benguela Current Large Marine Ecosystem were analysed at community and population levels to provide a comparative overview of the occurrence and timing of regime shifts. For South Africa, the timing of a community-level shift observed in the early 1990s, and of a lesser shift observed in the mid-2000s, corresponded well with the results of other studies that showed environmental, community-level or population-level changes at similar times, suggesting that environmental forcing had played a role. Several population-level shifts were detected for Namibia; these and a regime shift in the overall community identified for this country corresponded well to the timing of severe environmental perturbations and an extensive regime shift in the pelagic Ecosystem of this area. However, the interpretation of these shifts was confounded by changes in sampling gear; closer scrutiny of the types of species affected and the direction of shifts (increase/decrease) in relation to the timing and nature of sampling gear modifications, revealed that the observed shifts were potentially an artefact of gear changes. This highlighted the importance of accounting for changes in sampling protocols during the analysis and interpretation of long-term data. For Angola, a community level shift in the mid-2000s and population-level changes for a few species (mainly positive), could not have been influenced by gear changes which took place mainly before the onset of the time series under consideration. However, no clear environmental or anthropogenic changes that could have influenced these shifts were obvious.
