Staff Publications

Staff Publications

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    'Staff publications' is the digital repository of Wageningen University & Research

    'Staff publications' contains references to publications authored by Wageningen University staff from 1976 onward.

    Publications authored by the staff of the Research Institutes are available from 1995 onwards.

    Full text documents are added when available. The database is updated daily and currently holds about 240,000 items, of which 72,000 in open access.

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    A trophic model to explore fishing effects for Eritrea's Red Sea coral reef fisheries
    Tsehaye, I.W. ; Nagelkerke, L.A.J. - \ 2015
    We assessed the potential for fisheries in the coral reefs of Eritrea from an ecosystem perspective using an Ecopath with Ecosim model. The model combined observed and literature data on the ecosystem and its inhabitants into a single framework, with the aim to gain a better insight into the structure and functioning of the ecosystem and assess potential impacts of fishing. Model outputs indicated that primary productivity in the system is mostly recycled through a detritus based food web, implying a bottom-up energy flow control. However, ecotrophic efficiencies were higher at higher trophic levels, suggesting that top-down mechanisms are also ecologically important. Despite an estimated high degree of primary productivity, the fishery yields from the coral reef ecosystem were relatively low, apparently because of lower ecotrophic efficiencies at the lower trophic levels. A retrospective simulation of trends in these fisheries using Ecosim showed that, given existing levels of fishing, as suggested by official statistics (0.114 tonnes km-2 year-1), levels of decline in abundance of reef-associated demersals inferred from CPUE analysis (ca. 25%) were unlikely. However, our model could reproduce past declines in yield when, based on anecdotal evidence on unreported catches, we assumed that existing fishing levels were five-fold official estimates. Simulation results showed that an optimal fishing strategy requires a slight reduction in annual catch of reef-associated fishes (to 0.32 tonnes km-2 year-1) and an increase in harvest of near-reef pelagics and large pelagics (to 0.19 and 0.39 tonnes km-2 year-1, respectively) from the putative levels.
    Exploring optimal fishing scenarios for the multispecies artisanal fisheries of Eritrea using a trophic model
    Tsehaye, I.W. ; Nagelkerke, L.A.J. - \ 2008
    Ecological Modelling 212 (2008)3-4. - ISSN 0304-3800 - p. 319 - 333.
    coral-reef ecosystems - red-sea - ecopath - biomass - monsoon - communities - temperature - populations - management - mortality
    This study represents the first attempt to assess the potential for fisheries in the artisanal Red Sea reef fisheries of Eritrea in an ecosystem context. We used an Ecopath with Ecosim model to integrate known aspects of the ecosystem and its inhabitants into a single framework, with the aim to gain a better insight into the structure and functioning of the ecosystem and to assess the impacts of fishing in these data-limited fisheries. Model outputs indicate that most of the primary productivity in the system is recycled through a detritus-based food web, implying a bottom-up energy flow control. Ecotrophic efficiencies were nevertheless higher at higher trophic levels, suggesting that top-down control mechanisms are also ecologically important. Notwithstanding a high degree of primary productivity, the fishery yields estimated for the coral reef ecosystem were relatively low, apparently because of lower ecotrophic efficiencies at the lower trophic levels. A retrospective simulation of trends in these fisheries using Ecosim showed that, given the existing fishing intensity as suggested by official statistics (0.114 tonnes km¿2 year¿1), fishery-induced decline in abundance and yield of reef-associated demersals of the magnitude (ca. 25%) suggested by a previous study are unlikely. However, the decline in yield observed in these fisheries could be reproduced in the simulation when the existing fishing level was assumed to be five-fold the official estimate, as suggested by anecdotal evidence on unreported catches. The simulation results show that an optimal fishing strategy could be achieved through a slight reduction in the annual catch of reef-associated fish (to 0.32 tonnes km¿2 year¿1) and an increase in the capture of near-reef pelagics and large pelagics (to 0.19 and 0.39 tonnes km¿2 year¿1, respectively) from the putative levels. We explored optimal fishing scenarios in view of both direct effects of fishing on target species and indirect effects of fishing on non-target organisms. Finally, this study demonstrates that, even if modeling is no substitute for rigorous assessment of individual fish stocks with more detailed data, it could improve our understanding of the impacts of fishing on the structure and functioning of coral reef ecosystems in such newly developing fisheries, particularly on groups that would otherwise be difficult to monitor.
    Rapid shifts in catch composition in the artisanal Red Sea reef fisheries of Eritrea
    Tsehaye, I.W. ; Machiels, M.A.M. ; Nagelkerke, L.A.J. - \ 2007
    Fisheries Research 86 (2007)1. - ISSN 0165-7836 - p. 58 - 68.
    marine food webs - multispecies fisheries - structural-change - fish communities - exploitation - ecosystem - size
    Shifts in catch composition were registered in the Eritrean artisanal fisheries, which were launched into a renewed development after the end of the independence war in 1991. Our analysis of catch and effort data showed that total fishing effort as well as total annual catch increased more than two-fold from 1996 to 2002. Yet, overall CPUE remained unchanged upon the expansion of the fisheries, suggesting that the fisheries are still at an early stage of development. However, at lower taxonomic level, we found that the CPUE for emperors (Lethrinidae), snappers (Lutjanidae), and sharks (Elasmobranchii) decreased, while the CPUE for barracudas (Sphyraenidae), jacks (Carangidae), and tunas (Scombridae) increased. Thus, the species composition of the catches shifted toward a smaller proportion of high-value demersals (from ca. 70% in 1996 to ca. 30% in 2002), implying a change in the underlying fish community structure. Changes in CPUE could not be attributed to changes in spatial effort allocation, or to changes in gear or boats used, ruling out changes in fishing strategies as likely causes. Yet the decline in CPUE at low fishing effort does not necessarily reflect a proportional decline in fish abundance, and could possibly be caused by ¿hyperdepletion¿. Nevertheless, the trends in CPUE are consistent with earlier findings that stocks of reef-associated demersal fishes are highly vulnerable to fishing and can decline even at the early stages of the fisheries. Therefore, the changes in CPUE could indeed represent true biomass changes, with potentially important ecological consequences. These effects could even become more severe if market conditions improve, leading to activation of latent fishing capacity, which was found to make up about 75% of the registered fleet.
    Monitoring fisheries in data-limited situations : a case study of the artisanal reef fisheries of Eritrea
    Tsehaye, I.W. - \ 2007
    Wageningen University. Promotor(en): Johan Verreth, co-promotor(en): Leo Nagelkerke. - [S.l.] : S.n. - ISBN 9789085047735 - 183
    visserij - vis vangen - kleine bedrijven - monitoring - gegevens verzamelen - simulatie - gevalsanalyse - eritrea - koraalriffen - visstand - visvangsten - aquatische ecosystemen - fisheries - fishing - small businesses - monitoring - data collection - simulation - case studies - eritrea - coral reefs - fish stocks - fish catches - aquatic ecosystems
    Elucidating trends in catch rate and composition is important to evaluate the impact of fishing on fish stocks, and thereby guide fisheries management action. Since major changes in fish community structure can take place even at the initial stages in the development of fisheries, the onset of reef fisheries in the Eritrean Red Sea provides a good opportunity to assess the effects of fishing on fish community structure in the early stages of fisheries. The artisanal reef fisheries of Eritrea were launched into a renewed development in 1991 after the end of a decades-long standstill, which was caused by the war for Eritrea’s independence. The main objective of this thesis was to evaluate the sustainability of existing fishing practices in the newly developing artisanal reef fisheries of Eritrea, while simultaneously exploring more effective ways of monitoring of fisheries and fish stocks. Analysis of official statistics from the artisanal fisheries showed that annual effort and annual catch increased upon the expansion of the fisheries, while the total catch rate remained unchanged over the years. However, disaggregation of catch rates into different taxonomic and ecological categories showed varying trends in catch rates of the different groups of fishes, resulting in a clear shift in catch composition over the years. These results are not unlikely given the supposedly large amounts of unreported catches, which are mostly taken illegally to Yemen. This is more so given the highly selective nature of species targeting strategies in the artisanal fisheries. These findings are also consistent with other studies suggesting that stocks of reef-associated demersal species are highly vulnerable to fishing and can decline even at the early stages of fisheries. Furthermore, the results from this study show that the impacts of fishing in the artisanal fisheries could even become more severe if market conditions improve leading to the activation of latent fishing capacity. That having been said, even though the changes in catch rate could be interpreted as reflecting changes in the underlying fish community structure, they could also be partly due to shifts in fish targeting strategies driven by socio-economic circumstances. The relationship between catch rate and effort in the artisanal fisheries could be elucidated more accurately by standardizing catch rates for systematic variations among fishing vessels, which would otherwise cause non-random variations in catch rate unrelated to abundance. The effectiveness of the monitoring program in these fisheries could also be improved through aggregation of catch data. The catch data from the artisanal fisheries could be aggregated into three distinct ecological categories, without considerable loss of information on trends in the catch rate of individual species. These categories included reef-associated demersals, open-water pelagics and near-reef pelagics. Aggregation of catch data into these ecological categories was found to improve the statistical power to detect trends. Moreover, besides improving statistical power, aggregate catch rates could provide better insight into community-level responses to exploitation. An Ecopath with Ecosim model was used to verify if the fishery-induced changes in yield and abundance of fish implied by official statistics are likely, given the existing levels of fishing intensity. This model was also used to explore optimal harvesting strategies for the artisanal fisheries by examining the ecosystem effects of fishing. A retrospective simulation of trends in catch rate (yield) using Ecosim showed that, given the existing fishing intensity as suggested by official statistics, a decline in yield or abundance of reef-associated demersals of the magnitude suggested by trends in catch rate are unlikely. The decline in yield observed in these fisheries could however be reproduced in an Ecosim simulation when the existing fishing level was assumed to be fivefold the official estimate, as suggested by anecdotal evidence on unreported catches. The simulation results also suggest that an optimal fishing strategy could be achieved in the artisanal fisheries through a slight reduction in the annual catch of reef-associated fishes and an increase in the capture of near-reef pelagics and large pelagics from the putative levels of fishing.
    Sampling fisheries with high levels of accepted relative error may lead to over-estimation of the statistical power of trend detection
    Nagelkerke, L.A.J. ; Tsehaye, I.W. - \ 2006
    The effect of individual species variability on the use of the total catch as an indicator of trends in a multispecies fisheries in the Red Sea
    Tsehaye, I.W. ; Nagelkerke, L.A.J. - \ 2006
    Sampling fisheries with high levels of accepted relative error may lead to over-estimation of the statistical power of trend detection
    Nagelkerke, L.A.J. ; Tsehaye, I.W. - \ 2006
    The effect of individual species variability on the use of the total catch as an indicator of trends in a multispecies fisheries in the Red Sea
    Tsehaye, I.W. ; Nagelkerke, L.A.J. - \ 2006
    Re-allocation of sampling effort can optimize the monitoring of artisanal fisheries and the fish community changes in the Eritrean Red Sea
    Tsehaye, I.W. ; Nagelkerke, L.A.J. ; Machiels, M.A.M. - \ 2004
    Rapid shifts in catch composition in the artisanal reef fisheries of Eritrea
    Tsehaye, I.W. ; Nagelkerke, L.A.J. ; Machiels, M.A.M. - \ 2004
    In: Abstracts of the International Symposium on Quantitative Ecosystem Indicators for Fisheries Management, Paris, France, 31 March-3 April 2004 - p. 69 - 69.
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