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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Letters to the editor : Assembling the pieces of Lake Victoria's many food webs: Reply to Kolding
Downing, A.S. ; Nes, E.H. van; Janse, J.H. ; Witte, F. ; Cornelissen, J.J.L.M. ; Scheffer, M. ; Mooij, W.M. - \ 2013
Ecological Applications 23 (2013)3. - ISSN 1051-0761 - p. 671 - 675.
tilapia oreochromis-niloticus - nile tilapia - lates-niloticus - mwanza gulf - perch - kenya - l.
Stockholm University, Department of Systems Ecology, SE-10691, Stockholm, Sweden Aquatic Ecology and Water Quality Management Group, Department of Environmental Sciences, Wageningen University, P.O. Box 47, NL-6700 AA Wageningen, Netherlands Netherlands Environmental Assessment Agency, P.O. Box 303, 3720 AH Bilthoven, Netherlands Institute of Biology, Leiden University, Sylviusweg 72, 2300 RA Leiden, Netherlands Aquaculture and Fisheries Group, Department of Animal Sciences, Wageningen University, P.O. Box 338, 6700 AH Wageningen, Netherlands Department of Aquatic Ecology, Netherlands Institute of Ecology, P.O. Box 50, NL-6700 AB, Wageningen, Netherlands
Open access fisheries in African lakes: Do they need to be managed?
Kolding, J. ; Zwieten, P.A.M. van - \ 2012
When prey becomes predators: eutrophication and the transition of Lake Victoria’s fish community from a cichlid to a Nile perch dominated state
Kolding, J. ; Zwieten, P.A.M. van; Mkumbo, O.C. ; Hecky, R.E. ; Silsbe, G.M. - \ 2012
Status, trends and management of the Lake Victoria Fisheries
Kolding, J. ; Medard, M. ; Mkumbo, O. ; Zwieten, P.A.M. van - \ 2012
Lake Kariba – the evolution of a balanced fishery?
Zwieten, P.A.M. van; Nagelkerke, L.A.J. ; Kolding, J. ; Musando, B. ; Songore, N. - \ 2012
African inland fisheries have been observed to sustain high sustainable catches by harvesting a broad spectrum of species and sizes, despite their open-access nature and overall non-selective, mostly unregulated multi-gear fisheries. We use twenty years of multispecies data from experimental fisheries in a fished and an un-fished situation of Lake Kariba to examine whether structural changes in the fish community have occurred. The inshore fisheries on the Zambian side of the lake has had virtually no enforcement of regulations, and experienced high fishing intensity with changing fishing pattern towards increasingly smaller mesh sizes. This resulted in a higher exploitation level, higher yield and reduced stock sizes compared to moderately fished, regulated Zimbabwean side of the Lake, where large sections remained un-fished. Yet, the overall community and sizestructure of the fished situation remained intact, as inferred from (1) directional trends in species composition in the catch and in experimental surveys; (2) recruitment, length and catch rate indicators on a community level as well as main targeted species; and (3) changes in the overall and internal structure of the size spectrum. The Ecosystem Approach to Fisheries calls for an optimum fishing pattern with yield levels that infer the least structural changes on a fish community. As an example, Lake Kariba may indicate that this could be reached by an overall unselective multi-gear fishery.
Diversity, disturbance and succession in the fish community of Lake Kariba, Zimbabwe, from 1960 to 2001
Kolding, J. ; Songore, N. ; Nagelkerke, L.A.J. ; Zwieten, P.A.M. van - \ 2012
In: Book of abstracts of the 6th World Fisheries Congress, 07-11 May 2012, Edinburgh, Scotland. - - p. 76 - 76.
In 1969 Eugene Odum presented his ‘Strategy of Ecosystem Development’ and suggested a series of ecological attributes for measuring ecological succession and stability. The work has now become classical and widely cited but unfortunately rarely tested empirically. Man-made Lake Kariba in Southern Africa is a grand-scale ecological laboratory where the development in the fish community has been monitored continuously for more than 40 years through nearly weekly experimental gillnet catches from a permanent station. The changes in fish species diversity over the four-decade period the lake has existed are described, and related to biotic and abiotic factors to understand the mechanisms behind the dynamics. The results show that fish species succession in Lake Kariba took approximately three decades to stabilize. Overall fish diversity has steadily increased, but inter-annual variations are significantly negatively correlated to mean annual lake level changes and to the abundance of the main teleost predator, the tigerfish (Hydrocynus vittatus). These two factors, one abiotic bottom-up and one biotic top-down, can be regarded as key disturbances that play a regulatory role. From the data it was possible to test eight of Odum’s indicators for ecological succession, such as standing biomass, P/B ratio, net production, size of organisms, diversity and resistance. All eight tested confirmed his predictions. The changes in these attributes indicate that Lake Kariba, although fluctuating, is becoming increasingly mature and stable. In agreement with the intermediate disturbance hypothesis there is a negative relationship between fish productivity and diversity, and nutrient inputs (indexed by flushing rates) appear to be the most important factor for regulating this
Relative lake level fluctuations and their influence on productivity and resilience in tropical lakes and reservoirs
Kolding, J. ; Zwieten, P.A.M. van - \ 2012
Fisheries Research 115-116 (2012). - ISSN 0165-7836 - p. 99 - 109.
potential fish production - water-level - morphoedaphic index - standing crop - great-lakes - fisheries - yield - phytoplankton - ecosystems - africa
Lakes and reservoirs are traditionally characterised from static morphological or chemical parameters such as depth and dissolved solids, while the dynamic impact of shifting water supplies has received little attention. There is increasing evidence, however, that the hydrodynamic regime in tropical water bodies plays a significant role in the injection and re-suspension of nutrients, and consequently has a strong influence on the biological communities and productivity. Lake level fluctuations can therefore be used as a proxy for bottom up driven processes. The application of a relative fluctuation index (RLLF) and its relationship with fish yields in a range of tropical lakes and reservoirs in Asia and Africa is reviewed. The RLLF is a simple empirical indicator defined as the mean amplitude of the annual or seasonal lake level fluctuations divided by the mean depth of the lake or reservoir, times 100. It builds on the classic morpho-edaphic index (MEI) for lakes and the more recent dynamic flood pulse concept (FPC), originally developed for rivers and floodplains. The RLLF index can be used as a predictive indicator for classifying lakes and reservoirs from stable to pulsed systems, and thereby their potential resilience to external disturbances. The index also has a strong log-linear relationship with the fish productivity. Shallow lakes and man-made reservoirs in general have the highest lake level changes, but also the highest fish yield per unit area, and even extreme fluctuations (amplitude higher than mean depth) seem only to accelerate the biological processes. The influence of water level changes on aquatic productivity should be taken into account when assessing environmental impacts within and outside man-made reservoirs.
Reconsidering the Consequences of Selective Fisheries
Garcia, S.M. ; Kolding, J. ; Rice, J. ; Rijnsdorp, A.D. - \ 2012
Science 335 (2012)6072. - ISSN 0036-8075 - p. 1045 - 1047.
managing fisheries - by-catch - management - ecosystem - fish - conservation - community - evolution - atlantic - capture
Concern about the impact of fishing on ecosystems and fisheries production is increasing (1, 2). Strategies to reduce these impacts while addressing the growing need for food security (3) include increasing selectivity (1, 2): capturing species, sexes, and sizes in proportions that differ from their occurrence in the ecosystem. Increasing evidence suggests that more selective fishing neither maximizes production nor minimizes impacts (4–7). Balanced harvesting would more effectively mitigate adverse ecological effects of fishing while supporting sustainable fisheries. This strategy, which challenges present management paradigms, distributes a moderate mortality from fishing across the widest possible range of species, stocks, and sizes in an ecosystem, in proportion to their natural productivity (8), so that the relative size and species composition is maintained
Review of tropical reservoirs and their fisheries : the cases of Lake Nasser, Lake Volta and Indo-Gangetic Basin reservoir
Zwieten, P.A.M. van; Bene, C. ; Kolding, J. ; Brummett, R. ; Valbo-Jorgensen, J. - \ 2011
Rome : Food and Agriculture Organization of the United Nations (FAO fisheries and aquaculture technical paper 557) - 148
reservoirs - visserij - meren - tropen - india - egypte - ghana - fisheries - lakes - tropics - egypt
Scientists: Juvenile Tuna Can Be Fished
Zwieten, P.A.M. van; Kolding, J. - \ 2011
atuna.com
Decades of scientific fishery management advice and volumes of simulation studies may not have been effective at all and might even have damaged stocks and productivity. That is the conclusion of the fishery biologist professor Paul van Zwieten of the Dutch Wageningen University, together with his Danish colleague Jeppe Kolding of the University of Bergen, in a recently published scientific paper. According to Kolding and Van Zwieten the existing models that result in bans on fishing juveniles and quota regulations don’t fit reality and are ‘ecologically vacuous’. Kolding and Van Zwieten base their conclusion on a study of inland small scale fisheries in Africa. “But you can apply the basic conclusion to al fisheries, including tuna”, says Mr. Van Zwieten. “The catch of juvenile tuna in Indonesia might be not as harmful as has been thought. And even the policy of avoiding bycatch might be contra productive.” The conclusions add to the heated debate in circles of marine biologists that question the existing wisdom from single species management models that have been applied in the last 50 years. According to Kolding and Van Zwieten generations of fisheries biologist have been taught the Yield-per-Recruit models to the point that ‘indiscriminate’ fishing methods are by default synonymous with destructive fishing practices. Killing juveniles, as happens in small scale fisheries, has been condemned as a form of depleting the stocks , “so dogmatic that it doesn’t even warrant verification”. “With the increasing focus on discarded bycatch problems in single species industrial fisheries the issue of selectivity has been further highlighted, and much research is devoted to develop increasingly selective fishing methods and exclusion devices”, write Kolding and Van Zwieten. The result of all this is that modern objective for industrial fisheries has become a highly selective kill on targeted species and sizes. But data prove a totally different and more complex reality. Populations experimentally harvested on small sizes produce after only four generations nearly twice as much yield as the populations where only large specimens were harvested. This is clearly in contrast to what the existing model predicts. “Fisheries scientific advice to management, however, is largely oblivious to these evolutionary and ecological studies and continues to reiterate the standard recipe from Yield-per-Recruit models”, write Kolding and Van Zwieten. According to several data, indiscriminate fishing methods might not always be bad from an ecologically point of view as long as it forms part of a fishery that fishes all different age levels in proportion to their natural production. The scientist point out that Lake Kainji - one of the most productive lakes in Africa - since 1996 experienced a 60 % reduction in effort due to banning beach seines and introducing small mesh sizes and mandatory licensing. The only visible result was a corresponding 60 % decrease in yield and no positive response in the individual catch rates as the models assume. It turned a high biologically productive into a less productive system. Instead the most productive inland fisheries like Lake Victoria, are also the most intensely exploited. Open access fisheries prove to result in a certain stable average amount of individual catch. This situation is not unique for African inland fisheries, also investigations in Newfoundland inshore cod fisheries did find similar results. According to both scientists the existing fishery management is based on the long existing misbelieve among ecologists that ecosystems are closed entities in a process towards equilibrium. Human interventions such as fishing are therefore regarded as an external disturbance that affect the productivity of the system. But new dynamic ecology questions this view. Instead it regards ecosystems in a constant and ever changing state of disequilibrium with chaotic fluctuations due climatic variation or human interventions. Another serious misconception is that fish is treated like live stock. But there has been the consistent lack of relationship between adults and recruitment in fisheries science. That indicates that the life history of fish is probably closer to insects and plants. “There is increasing evidence that only show negative ecological effects of adult size selectivity. Everything else being equal, we can safely deduce that the less we select on species and sizes, the more the original composition and structure of a fish community will remain the same”, conclude the biologists. According to Van Zwieten these kind of new fishery management views can also be applied in fisheries like yellowfin tuna and skipjack. For heavily overfished stocks like bigeye tuna, the fishing pressure should nonetheless substantially be reduced regardless the new insights.
The cases of Lake Nasser, Lake Volta and Indo-Gangetic Basin reservoirs
Zwieten, P.A.M. van; Béné, C. ; Kolding, J. ; Brummett, R. ; Valbo-Jorgensen, J. - \ 2011
FAO Fisheries and Aquaculture Technical Paper (2011)557. - ISSN 2070-7010
The tragedy of our legacy: how do global management discourses affect small-scale fisheries in the South?
Kolding, J. ; Zwieten, P.A.M. van - \ 2011
Forum for Development Studies 38 (2011)3. - ISSN 0803-9410 - p. 267 - 297.
Modern fisheries management discourse is supported by two fundamental narratives that have global impacts. One is the fear of open access regimes, and the other is the condemnation of catching under-sized and immature fish. These narratives have existed for more than half a century and originate from the common property theory and the maximum yield per recruit theory. Our aim is to critically discuss and evaluate these narratives which have been developed within the context of scientific management of single-species industrial fisheries. We will show that the underlying assumptions can be seriously wrong and particularly absurd in fluctuating multi-species, multi-gear artisanal fisheries. Fishing effort in small scale fisheries is often largely regulated by natural production, like other top predators, and many targeted fish stocks and fish communities display a high degree of resilience. Furthermore, in spite of common belief, small scale unregulated, non-selective, adaptive fishing patterns could be healthier and far more ecosystem conserving than the current imposed single species management strategies. Many of these fisheries are serving as a ‘social security system’ – a common good and thereby function as a ‘last resort’ for economic mishap. Limiting open access will undermine the role of small scale fisheries to provide insurance, particularly for the poorest and least advantaged. The immense pressure to adapt to modern fisheries management thinking and economic theory is based on flawed assumptions and will not only have negative social effects, but also negative biological effects.
Non-selective open access fisheries in African lakes: Are they as bad as we think?
Kolding, J. ; Zwieten, P.A.M. van - \ 2011
Selecting indicators to assess the fisheries of Lake Malawi and Lake Malombe: Knowledge base and evaluative capacity
Zwieten, P.A.M. van; Banda, M. ; Kolding, J. - \ 2011
Journal of Great Lakes Research 37 (2011)1. - ISSN 0380-1330 - p. 26 - 44.
african great-lakes - cichlid fishes - reference points - east-africa - management - perspective - tanganyika - diversity - history - nyasa
The provision of management information on the fisheries of Lakes Malawi and Malombe has been characterised by top–down controlled single species steady-state assessment techniques originating from single gear industrial fisheries but applied to an open access highly diverse and adaptive small-scale multispecies and multi-gear fishery. The result has largely been an unhappy marriage with uncertainties blamed more on the data than the process, although the data collection generally is detailed and comprehensive on catch and effort parameters. An extensive literature review of primary and grey literature on ecosystem drivers, exploitation pressures, and fish population and community states shows that Malawi has the necessary knowledge base for expanding their assessment into multi-causal and exploratory indicator-based methods that can assist in better understanding and more disciplined use of existing data and monitoring systems. Selection and ranking of a suite of indicators focusing on the major fisheries in the Southeast arm of Lake Malawi and Lake Malombe were done by a group of Malawian fisheries researchers and management advisers, thereby testing a framework of scoring criteria assessing an indicator's acceptability, observability, and relatedness to management. Indicators that are close to raw observational data and that require limited permutations and few assumptions appear to be preferable in the Malawian context. CPUE-based assessments can improve the utility of data and information in communicating developments and processes and evaluate fisheries management policies
The fishing pattern of open access, non-regulated African freshwater fisheries: The paradoxical gap between theory and practice.
Kolding, J. ; Zwieten, P.A.M. van - \ 2010
Management or mis-management of African flood pulse fisheries. Wetlands in a Flood Pulsing Environment:Effects on and responses in biodiversity, ecosystem functioning and human society
Kolding, J. ; Zwieten, P.A.M. van - \ 2010
Floodplains are among the most productive aquatic ecosystems, which can be attributed to the advantages described in the ¿flood pulse concept¿. However, many lakes and most reservoirs also have a fluctuating hydrology associated with river inflows, and significant relationships between fish productivity and fluctuations in water levels - serving as a proxy for nutrient loading - are often found. Depending on the amplitude of the fluctuations, lakes and reservoirs can vary from stable to highly pulsed systems which have important implications for the life history of the fish populations, and consequently for their productivity and resilience to exploitation. Inland fisheries in Africa constitute a ’social security system’ - a common good that requires common access. They often serve as the ’last resort’ when everything else fails, and for this they have been seriously undervalued. Small scale fisheries, by their nature, are largely regulated by the natural production and display a high degree of adaptability and resilience. In spite of common belief, the non regulated, non-selective, adaptive fishing patterns are healthier and far more ecosystem conserving than current management strategies based on gear restrictions and size limitations. The immense pressure to adapt to modern management thinking based on economic theory is based on flawed assumptions and will, under present conditions, not only have negative social effects, but also negative biological consequences on aquatic ecosystems.
Selectivity and open access: Sustainable harvesting in small scale inland fisheries.
Zwieten, P.A.M. van; Kolding, J. ; Bene, Ch. - \ 2010
Recent theoretical development on the fishing patterns and management of small-scale fisheries.
Kolding, J. ; Zwieten, P.A.M. van - \ 2010
The tragedy of our legacy: How global management discourses affect rational exploitation of small scale fisheries
Kolding, J. ; Zwieten, P.A.M. van - \ 2009
Are the Lake Victoria fisheries threatened by exploitation or eutrophication? Towards an ecosystem-based approach to management
Kolding, J. ; Zwieten, P.A.M. van; Mkumbo, O. ; Silsbe, G. ; Hecky, R. - \ 2008
In: The Ecosystem Approach to Fisheries / Bianchi, G., Skjoldal, H.R., CAB International and FAO - ISBN 9781845934149 - p. 309 - 355.
Lake Victoria’s ecosystem has shown fundamental changes over its past recorded history in terms of nutrient loadings, productivity, faunal composition and fisheries. As yet, however, no attempt has been made to link the driving processes of eutrophication and fisheries to understand the feedback observed in fish stocks, food webs, exploitation patterns and trade. Single- and multi-species stock assessments, based on steady-state models with effort (and/or predation) as the only driver – still used in the region to advise on management – uniformly indicate overfished stocks of Nile perch that are in danger of collapse. These current views of overfishing are not validated by empirical observations. This chapter presents a holistic integrated ecosystem approach which combines a phenomenological analysis of key processes with a comprehensive set of simple indicators, covering physical, biological and human development, where directionality in time is made explicit to understand ongoing processes in the changing ecosystem. This new approach results in: (i) no signs of overfishing in any of the verifiable indicators; and (ii) biological production increasing over time together with effort and yield as a function of increased eutrophication. The results indicate that continued eutrophication presents a much graver risk to the resource base and thus livelihoods of Lake Victoria’s coastal populations than fishing pressure. Lake Victoria can serve as an interesting case study for the inherent risk of using traditional fish stock assessment in changing ecosystems, and for the development of holistic monitoring systems for ecosystem-based management.
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