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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Balanced harvest: concept, policies, evidence, and management implications
Zhou, Shijie ; Kolding, Jeppe ; Garcia, Serge M. ; Plank, Michael J. ; Bundy, Alida ; Charles, Anthony ; Hansen, Cecilie ; Heino, Mikko ; Howell, Daniel ; Jacobsen, Nis S. ; Reid, David G. ; Rice, Jake C. ; Zwieten, Paul A.M. van - \ 2019
Reviews in Fish Biology and Fisheries 29 (2019)3. - ISSN 0960-3166 - p. 711 - 733.
Ecological effect - Ecosystem approach to fishery - Ecosystem structure - Fishing intensity - Production - Selectivity - Sustainability

Balanced harvest has been proposed to reduce fishing impact on ecosystems while simultaneously maintaining or even increasing fishery yield. The concept has attracted broad interest, but also received criticisms. In this paper, we examine the theory, modelling studies, empirical evidence, the legal and policy frameworks, and management implications of balanced harvest. The examination reveals unresolved issues and challenges from both scientific and management perspectives. We summarize current knowledge and address common questions relevant to the idea. Major conclusions include: balanced harvest can be expressed in several ways and implemented on multiple levels, and with different approaches e.g. métier based management; it explicitly bridges fisheries and conservation goals in accordance with international legal and policy frameworks; modelling studies and limited empirical evidence reveal that balanced harvest can reduce fishing impact on ecosystem structure and increase the aggregate yield; the extent of balanced harvest is not purely a scientific question, but also a legal and social choice; a transition to balanced harvest may incur short-term economic costs, while in the long-term, economic results will vary across individual fisheries and for society overall; for its application, balanced harvest can be adopted at both strategic and tactical levels and need not be a full implementation, but could aim for a “partially-balanced” harvest. Further objective discussions and research on this subject are needed to move balanced harvest toward supporting a practical ecosystem approach to fisheries.

Small Fish and Food Security : Towards innovative integration of fish in African food systems to improve nutrition
Kolding, Jeppe ; Overa, Ragnhild ; Kjellevold, Marian ; Zwieten, P.A.M. van; Pucher, Johannes ; Yaro, Joseph ; Atter, Amy ; Njiru, James ; Taabu-Munyaho, Anthony - \ 2018
- 1 p.
Small Fish and Food Security : Towards innovative integration of small fish in African food systems to improve nutrition
Kolding, Jeppe ; Overa, Ragnhild ; Kjellevold, Marian ; Zwieten, P.A.M. van; Pucher, Johannes ; Yaro, Joseph ; Njiru, James ; Taabu-Munyaho, Anthony - \ 2018
In: WSFC 3rd world small-scale fisheries congress, Chiang Mai, Thailand, October 22-26, 2018. - - p. 57 - 57.
Complicit, but Not the Main Culprit: Nile Perch Was Not the Only Cause of the Haplochromine Collapse in Lake Victoria
Kolding, Jeppe ; Zwieten, P.A.M. van; Plank, Michael J. ; Hecky, Robert E. ; Bridgeman, Thomas B. ; Macintyre, Sally ; Seehausen, Ole ; Silsbe, Greg M. - \ 2018
- 1 p.
Small Fish and Food Security : Towards innovative integration of fish in African food systems to improve nutrition
Kolding, Jeppe ; Kjellevold, Marian ; Zwieten, P.A.M. van; Pucher, Johannes ; Yaro, Joseph ; Atter, Amy ; Njiru, James ; Taabu-Munyaho, Anthony - \ 2018
- 3 p.
La pêche dans les zones arides d’Afrique Subsaharienne: le poisson vient avec la pluie : Favoriser la résilience dans les zones arides pour la sécurité alimentaire et la nutrition des populations qui dépendent de la pêche
Kolding, Jeppe ; Zwieten, Paul van; Marttin, Felic ; Poulain, Florence - \ 2017
Rome : Organisation des Nations Unies pour l'Alimentation et l'Agriculture (Circulaire sur les pêches et l’aquaculture 1118, FIAF/C1118 (Fr)) - ISBN 9789252092193 - 64
Fisheries in the drylands of Sub-Saharan Africa “Fish come with the Rains” : Building resilience for fisheries-dependent livelihoods to enhance food security and nutrition in the drylands
Kolding, Jeppe ; Zwieten, P.A.M. van; Marttin, Felix ; Poulain, Florence - \ 2016
FAO Fisheries and Aquaculture Circular (2016)C1118. - ISSN 2070-6065 - 64 p.
Dryland areas cover more than half of sub-Saharan Africa and are home to nearly 50 percent of
its populations, who depend on agriculture (including livestock, crops and fisheries) as their main
livelihood strategy. Sporadic and irregular rainfall patterns are the most important environmental
driver for these regions and water, in particular surface water, is the primary element of scarcity in drylands. Generally, dryland water bodies are unstable and strongly pulsed ecosystems owing to intermittent and largely unpredictable precipitation. Such systems are characterized by very productive and highly resilient, small opportunistic fish species with “boom and bust” fluctuation adapted to strong environmental disturbances, and are therefore difficult to overfish. As a result of high productivity, they can sustain very high yields in years of good rains, but being largely short-lived they also respond rapidly to environmental changes in hydrological regimes, which means that alternating periods of low productivity are inevitable. The focus of this review is to both document the general resilience of many fish resources to climatic variability − including their underestimation in livelihood importance, particularly in protracted crisis situations − and to enhance the potential supply of fish from dryland areas through improved use of the available water bodies, and in particular small reservoirs. The important role that small water bodies play in supplying essential micronutrients and protein to rural communities has largely been overlooked since the termination of the FAO/ALCOM (Aquaculture for Local Community Development) programme in 1998, although they are more productive on a per unit area basis than the large lakes and reservoirs and, when pooled, constitute a much larger area of water. Most of the fish production, however, is consumed locally and goes unrecorded in official catch statistics. By refocusing attention on the fish productivity of small water bodies and reservoirs in drylands, and in particular by integrating fisheries with developments in water harvesting, irrigation and improved water storage facilities, the potential to increase the role played by fish in the diets of dryland people, and to provide improved livelihood opportunities is great. The overall conclusion is that the potential for increasing fish production in dryland areas is significant, that the resources are highly resilient and productive, but that the general and increased unpredictability of the rainfall required to sustain surface water bodies creates uncertainties in annual production. That must be counteracted by an adaptive and diversified livelihood strategy.
The Nile perch invasion in lake Victoria : Cause or consequence of the haplochromine decline?
Zwieten, Paul A.M. van; Kolding, Jeppe ; Plank, Michael J. ; Hecky, Robert E. ; Bridgeman, Thomas B. ; Macintyre, Sally ; Seehausen, Ole ; Silsbe, Greg M. - \ 2016
Canadian Journal of Fisheries and Aquatic Sciences 73 (2016)4. - ISSN 0706-652X - p. 622 - 643.

We review alternative hypotheses and associated mechanisms to explain Lake Victoria’s Nile perch (Lates niloticus) takeover and concurrent reduction in haplochromines through a (re)analysis of long-term climate, limnological, and stock observations in comparison with size-spectrum model predictions of co-existence, extinction, and demographic change. The empirical observations are in agreement with the outcomes of the model containing two interacting species with life histories matching Nile perch and a generalized haplochromine. The dynamic interactions may have depended on size-related differences in early juvenile mortality: mouth-brooding haplochromines escape predation mortality in early life stages, unlike Nile perch, which have miniscule planktonic eggs and larvae. In our model, predation on the latter by planktivorous haplochromine fry acts as a stabilizing factor for co-existence, but external mortality on the haplochromines would disrupt this balance in favor of Nile perch. To explain the observed switch, mortality on haplochromines would need to be much higher than the fishing mortality that can be realistically reconstructed from observations. Abrupt concomitant changes in algal and zooplankton composition, decreased water column transparency, and widespread hypoxia from increased eutrophication most likely caused haplochromine biomass decline. We hypothesize that the shift to Nile perch was a consequence of an externally caused, climate-triggered decrease in haplochromine biomass and associated recruitment failure rather than a direct cause of the introduction.

Fisheries, the inverted food pyramid
Kolding, J. ; Bundy, A. ; Zwieten, P.A.M. van; Plank, M. - \ 2016
ICES Journal of Marine Science 73 (2016)6. - ISSN 1054-3139 - p. 1697 - 1713.
A global assessment of fishing patterns and fishing pressure from 110 different Ecopath models, representing marine ecosystems throughout the world and covering the period 1970–2007, show that human exploitation across trophic levels (TLs) is highly unbalanced and skewed towards low productive species at high TLs, which are around two TLs higher than the animal protein we get from terrestrial farming. Overall, exploitation levels from low trophic species were <15% of production, and only 18% of the total number of exploited groups and species were harvested >40% of their production. Generally, well-managed fisheries from temperate ecosystems were more selectively harvested at higher exploitation rates than tropical and upwelling (tropical and temperate) fisheries, resulting in potentially larger long-term changes to the ecosystem structure and functioning. The results indicate a very inefficient utilization of the food energy value of marine production. Rebuilding overfished components of the ecosystem and changing focus to balancing exploitation across a wider range of TLs, i.e. balanced harvesting, has the potential to significantly increase overall catches from global marine fisheries.
Maximizing fisheries yields while maintaining community structure
Kolding, J. ; Jacobsen, N.S. ; Andersen, K.H. ; Zwieten, P.A.M. van - \ 2016
Canadian Journal of Fisheries and Aquatic Sciences 73 (2016)4. - ISSN 0706-652X - p. 644 - 655.
Under the Ecosystem Approach to Fisheries an optimal fishing pattern is one that gives the highest possible yield while causing the least structural impact on the community. Unregulated, open access African inland fisheries have been observed to sustain high catches by harvesting a broad spectrum of species and sizes, often in conflict with current management regulations in terms of mesh and gear regulations. Using a size and trait-based model we explore whether such exploitation patterns are commensurable with the Ecosystem Approach to Fisheries, by comparing the impacts on size spectrum slope and yield with the different size limit regimes employed in the Zambian and Zimbabwean side of man-made Lake Kariba. Long-term multispecies data under fished and unfished conditions are used to compare and validate the model results. Both model and observations show that the highest yields and low structural impact on the ecosystem are obtained by targeting small individuals in the community. These results call for a re-evaluation of the size based management regulations that are ubiquitous in most fisheries.
The Optimal Fishing Pattern
Kolding, J. ; Law, R. ; Plank, M. ; Zwieten, P.A.M. van - \ 2016
In: Freshwater Fisheries Ecology / Craig, J.F., New York : Wiley-Blackwell - ISBN 9781118394427 - p. 524 - 540.
Conventional fisheries management encourages highly selective fishing patterns for various purposes, such as increase relative yield, reduce unwanted bycatch, protect various species or sizes and rebuild ecosystems. Recent empirical and theoretical studies, however, show increasing evidence that selective removals of targeted components have unintended adverse effects on stocks, fish communities and the ecosystem. Based on case studies from artisanal African freshwater fisheries, and results from dynamic size-based models, this chapter supports the renewed suggestion that an application of a more balanced fishing pattern will mitigate adverse effects and enhance food security better than increased selectivity. Contrary to common belief, small-scale unregulated artisanal fisheries, with a high diversity of seasonally adapted fishing methods, are probably the closest empirical examples we have of an optimal exploitation pattern with the least disruptive effects on the structure of the ecosystem. As such they are among the best examples of an ecosystem approach to fisheries that we have.
Where there is water there is fish – Small-scale inland fisheries in Africa: dynamics and importance
Kolding, J. ; Zwieten, P.A.M. van; Mosepele, K. - \ 2015
In: A History of Water : Vol 3 Water and Food: From hunter-gatherers to global production in Africa London : I.B. Tauris - ISBN 9781780768717
Balanced Harvest in the Real World. Scientific, Policy and Operational Issues in an Ecosystem Approach to Fisheries
Garcia, S.M. ; Bianchi, G. ; Charles, A. ; Kolding, J. ; Rice, J. ; Rochet, M.J. ; Zhou, S. ; Delius, G. ; Reid, D. ; Zwieten, P.A.M. van; Atcheson, M. ; Bartley, D. ; Borges, L. ; Bundy, A. ; Dagorn, L. ; Dunn, D. ; Hall, M. ; Heino, M. ; Jacobsen, B. ; Jacobsen, N.S. ; Law, R. ; Makino, M. ; Martin, F. ; Skern-Mauritzen, M. ; Suuronen, P. ; Symons, D. - \ 2015
Gland, Switzerland : IUCN - 94 p.
The concept of the Ecosystem Approach has entered the fishery harvesting discussions both from fishery perspectives (Reykjavik Declaration; FAO 2003 Annex to the Code of Conduct and from the principles of the Ecosystem Approach adopted by the CBD in 1995. Both perspectives establish the need to maintain ecosystem structure and functioning, whether for sustainable use of biodiversity (CBD) or simply to keep exploited ecosystems healthy and productive (fisheries). In response, the “Balanced Harvest” (BH) concept was suggested by a group of scientists brought together by the IUCN Fisheries Experts Group during the CBD CoP 10 in 2010. The meeting and the BH concept as consolidated there highlighted some of the collateral ecological effects of current fishing patterns and unbalanced removals of particular components of the food web, stimulating a critical rethinking of current approaches to fisheries management. The meeting on “Balanced Harvest in the real world - Scientific, policy and operational issues in an ecosystem approach to fisheries” (Rome, September 29-October 2, 2014) examined the progress made since 2010 on a number of fronts. It considered questions related to the scientific underpinning of the BH concept, including theory, modelling, and empirical observations. It began to explore the economic, policy and management implications of harvesting in a more balanced way.
Coupled human and natural system dynamics as key to the sustainability of Lake Victoria’s ecosystem services
Downing, A.S. ; Nes, E.H. van; Balirwa, J.S. ; Beuving, J. ; Bwathondi, P.O.J. ; Chapman, L.J. ; Cornelissen, I.J.M. ; Cowx, I.G. ; Goudswaard, P.C. ; Hecky, R.E. ; Janse, J.H. ; Janssen, A.B.G. ; Kaufman, L. ; Kishe-Machumu, M.A. ; Kolding, J. ; Ligtvoet, W. ; Mbabazi, D. ; Medard, M. ; Mkumbo, O.C. ; Mlaponi, E. ; Munyaho, A.T. ; Nagelkerke, L.A.J. ; Ogutu-Ohwayo, R. ; Ojwang, W.O. ; Peter, H.K. ; Schindler, D.E. ; Seehausen, O. ; Sharpe, D. ; Silsbe, G.M. ; Sitoki, L. ; Tumwebaze, R. ; Tweddle, D. ; Wolfshaar, K.E. van de; Dijk, J.W.M. van; Donk, E. van; Rijssel, J.C. van; Zwieten, P.A.M. van; Wanink, J. ; Witte, F. ; Mooij, W.M. - \ 2014
Ecology and Society 19 (2014)4. - ISSN 1708-3087
cyprinid rastrineobola-argentea - perch lates-niloticus - nile perch - east-africa - water hyacinth - mwanza gulf - oreochromis-niloticus - morphological-changes - introduced predator - biological-control
East Africa’s Lake Victoria provides resources and services to millions of people on the lake’s shores and abroad. In particular, the lake’s fisheries are an important source of protein, employment, and international economic connections for the whole region. Nonetheless, stock dynamics are poorly understood and currently unpredictable. Furthermore, fishery dynamics are intricately connected to other supporting services of the lake as well as to lakeshore societies and economies. Much research has been carried out piecemeal on different aspects of Lake Victoria’s system; e.g., societies, biodiversity, fisheries, and eutrophication. However, to disentangle drivers and dynamics of change in this complex system, we need to put these pieces together and analyze the system as a whole. We did so by first building a qualitative model of the lake’s social-ecological system. We then investigated the model system through a qualitative loop analysis, and finally examined effects of changes on the system state and structure. The model and its contextual analysis allowed us to investigate system-wide chain reactions resulting from disturbances. Importantly, we built a tool that can be used to analyze the cascading effects of management options and establish the requirements for their success. We found that high connectedness of the system at the exploitation level, through fisheries having multiple target stocks, can increase the stocks’ vulnerability to exploitation but reduce society’s vulnerability to variability in individual stocks. We describe how there are multiple pathways to any change in the system, which makes it difficult to identify the root cause of changes but also broadens the management toolkit. Also, we illustrate how nutrient enrichment is not a self-regulating process, and that explicit management is necessary to halt or reverse eutrophication. This model is simple and usable to assess system-wide effects of management policies, and can serve as a paving stone for future quantitative analyses of system dynamics at local scales.
Sustainable fishing of inland waters
Kolding, J. ; Zwieten, P.A.M. van - \ 2014
Journal of Limnology 73 (2014)s1. - ISSN 1129-5767 - p. 132 - 148.
size-based indicators - multispecies fisheries - marine ecosystems - celtic sea - body-size - exploitation - community - abundance - patterns - fluctuations
Sustainability in fisheries has over the past decades evolved from a single species maximization concept to covering ecosystem and biodiversity considerations. This expansion of the notion, together with increased evidence that the targeted removal of selected components of the fish community may have adverse ecological consequences, poses a serious dilemma to the conventional fisheries management approach of protecting juveniles and targeting adults. Recently, the idea of balanced harvest, harvesting all components in the ecosystem in proportion to their productivity, has been promoted as a unifying solution in accordance the ecosystem approach to fisheries, but this will require a fundamental change to management. In this paper, we review theoretical background, and practicalities of securing high yielding fisheries in inland waters, with empirical examples freshwater fisheries which satisfy the extended objectives of minimal impact on community and ecosystem structure. We propose framework of ecological indicators to assess these objectives.
Status, trends and management of the Lake Victoria Fisheries
Kolding, J. ; Medard, M. ; Mkumbo, O. ; Zwieten, P.A.M. van - \ 2014
In: Inland fisheries evolution and management. Case studies from four continents / Welcomme, R.L., Valbo Jørgensen, J., Halls, A.S., Rome : FAO (FAO Fisheries and Aquaculture Technical Paper 579) - ISBN 9789251081259 - p. 49 - 62.
Is catch a function of effort or is effort a function of catch?
Kolding, J. ; Zwieten, P.A.M. van - \ 2013
In: Proceedings of the MARE conference people and the sea VII: Maritime Futures, June 26-28 2013, Amsterdam, The Netherlands. - - p. 106 - 106.
Balanced Harvesting
Zwieten, P.A.M. van; Kolding, J. - \ 2013
Catching the right size of fish: are size regulations harmful for small scale fisheries?
Zwieten, P.A.M. van; Kolding, J. - \ 2013
In: Proceedings of the MARE conference people and the sea VII: Maritime Futures, June 26-28 2013, Amsterdam, The Netherlands. - - p. 106 - 106.
Lake Kariba revisited: an update on fish community developments from 1960 to 2012
Kolding, J. ; Songore, N. ; Nagelkerke, L.A.J. ; Ndhlovu, N. van; Zwieten, P.A.M. van - \ 2013
In: Abstracts of the Fifth International Conference of the Pan African Fish and Fisheries Association (PAFFA5): African Fish and Fisheries: Diversity, Conservation and Sustainable Management, 16 – 20 September 2013, Bujumbura, Burundi,. - - p. 96 - 96.
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.
Sustainable harvesting of mixed fisheries, the small scale perspective
Kolding, J. ; Zwieten, P.A.M. van - \ 2008
The origin and effect of mesh size regulations and why they may have to be rethought.
Kolding, J. ; Zwieten, P.A.M. van - \ 2008
The state, trends and threats of the Lake Victoria Fisheries and Environment
Kolding, J. ; Zwieten, P.A.M. van; Mkumbo, O.C. ; Silsbe, G.M. ; Hecky, R.E. - \ 2008
The small scale perspective: Value, resilience, fishing patterns, and the need for open access. Key-note address presented at the Fisheries Management for Ecosystem Health workshop at the American Fisheries Society 138th Annual meeting. August 17-21, 2008. Ottawa, Canada.
Kolding, J. ; Zwieten, P.A.M. van - \ 2008
Are the Lake Victoria fisheries threatened by exploitation or eutrophication? Disentangling social and ecological drivers of ecosystem change (SEDEC).
Zwieten, P.A.M. van; Kolding, J. ; Mkumbo, O.C. ; Silsbe, G.M. ; Hecky, R.E. ; Nagelkerke, L.A.J. - \ 2008
In: Abstracts of the 4th International Conference of the Pan-African Fish and Fisheries Association (PAFFA). September 22-26, 2008. Addis Abeba. Ethiopia.. - - p. 141 - 141.
Shifts in taxonomic and trophic patterns of fish communities in African lakes in relation to (human-induced) pressures
Nagelkerke, L.A.J. ; Sibbing, F.A. ; Zwieten, P.A.M. van; Graaf, M. de; Kolding, J. - \ 2008
In: Abstracts of the 4th International Conference of the Pan African Fish and Fisheries Association. - Addis Ababa, Ethiopia : - p. 61 - 61.
Relative lake level changes and fish productivity in African lakes and reservoirs
Kolding, J. ; Zwieten, P.A.M. van - \ 2007
Eutrophication threatens the size structure of the Nile perch stocks in Lake Victoria
Kolding, J. ; Zwieten, P.A.M. van - \ 2007
The state, trends and threats of the Lake Victoria fisheries and environment
Kolding, J. ; Zwieten, P.A.M. van; Hecky, R.E. ; Silsbe, G.M. ; Mkumbo, O.C. - \ 2007
Lake Victoria Nile perch threatened by exploitation or eutrophication?
Kolding, J. ; Zwieten, P.A.M. van - \ 2006
Lake Victoria Nile perch fishery threatened by exploitation or eutrophication?
Kolding, J. ; Zwieten, P.A.M. van - \ 2006
Improving productivity in tropical lakes and reservoirs
Kolding, J. ; Zwieten, P.A.M. van - \ 2006
Cairo : WorldFish Center (Aquatic Ecosystems and Fisheries Review Series 1. Theme 3 of CPWF. 1) - ISBN 9771730878 - 139
meren - visproductie - visserijbeheer - reservoirs - tropen - aquatische ecosystemen - lakes - fish production - fishery management - reservoirs - tropics - aquatic ecosystems
Regional Synthesis Report on fisheries research and management. States, trends and processes
Kolding, J. ; Zwieten, P.A.M. van; Manyala, J. ; Okedi, J. ; Mgaya, Y.D. ; Orach-Meza, F. - \ 2005
Maun,Wageningen,Dar Es Salaam : Lake Victoria Environmental Management Program (LVEMP) - 216 p.
Management, co-management or no-management : major dilemmas in the sustainable utilisation of SADC freshwater fisheries
Jul-Larsen, E. ; Kolding, J. ; Overa, R. ; Nielsen, J.R. ; Zwieten, P.A.J. van - \ 2002
In: FAO Fisheries : FAO Fisheries, Rome 2002 / FAO Technical Paper. - Rome : FAO, 2002
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