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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    Natuurkwaliteit en biodiversiteit van de Nederlandse zoute wateren
    Wortelboer, F.G. ; Arts, F. ; Bos, O.G. ; Brasseur, S.M.J.M. ; Brinkman, A.G. ; Craeymeersch, J.A.M. ; Deerenberg, C.M. ; Dijkman, E.M. ; Lindeboom, H.J. ; Meesters, H.W.G. ; Mesel, I.G. de; Reijnders, P.J.H. ; Quirijns, F.J. ; Hofstede, R. ter; Breukelen, S. van; Densen, W.L.T. van; Duin, W.E. van; Overzee, H.M.J. van; Witbaard, R. - \ 2010
    Den Haag/Bilthoven : Planbureau voor de Leefomgeving (Achtergrondstudies / Planbureau voor de Leefomgeving (PBL) ) - 240
    vissen - vogels - bodemfauna - biodiversiteit - mariene gebieden - natuurwaarde - aquatische ecosystemen - noordzee - inventarisaties - waddenzee - voordelta - fishes - birds - soil fauna - biodiversity - marine areas - natural value - aquatic ecosystems - north sea - inventories - wadden sea - voordelta
    Van de biodiversiteit in de Nederlandse zoute wateren is momenteel nog ongeveer 40% aanwezig van de biodiversiteit die zou bestaan in een oorspronkelijke, meer natuurlijke situatie. Deze conclusie is gebaseerd op een grote hoeveelheid gegevens over kenmerkende soorten en eigenschappen van de Nederlandse mariene ecosystemen
    Differences in causal reasoning about resource dynamics and consequences for the participatory debate on North Sea fisheries
    Verweij, M.C. ; Densen, W.L.T. van - \ 2010
    Marine Policy 34 (2010)6. - ISSN 0308-597X - p. 1144 - 1155.
    management - knowledge - misperceptions - plaice - trawl
    Fishermen, scientists, policy makers, and staff of environmental NGOs (ENGOs) have significantly different understandings of the processes that determine developments in fish stocks. These perception differences hinder the participatory debate on why fish stocks change and which management measures are effective. In this study, differences in causal reasoning about processes between fishermen, policy makers, ENGO-staff, and scientists were examined, regarding four case studies within the management of the fishery on North Sea plaice. First, it appeared that all parties, besides scientists, had difficulty reasoning about long-term effects because of comprehension problems with stock dynamics, and because of short-term economical interests. Second, there were differences in how parties deal with natural variation and interconnectedness of natural and anthropogenic influences. Stock assessment scientists work with single-species models, reducing complexity by using assumptions that rule out variation, in order to inform policy makers about the effect of one isolated management measure. Fishermen on the other hand, relying on information from their daily lives at sea, emphasize complexity and interconnectedness, and the impact of the ever-changing and unpredictable nature. ENGO-staff appeared reluctant to reason about single species and broaden the debate to the ecosystem-level, while emphasizing the effect of man. As a consequence of the diverging perceptions, much time in multi-stakeholder settings is lost on repetitive discussions, mainly on the relative importance of 'nature' versus 'man'. No wonder that policy makers feel lost, and experience processes as very complex. Concluding, to handle these perception differences, there is need for a directive process coordinator, and a more creative informative role for fisheries scientists. Together with all participants, they should map all expectancies and lines of reasoning at the beginning of the debate. This scheme can be relied on during subsequent meetings, in which perceptions can adequately be positioned.
    The tower of Babel: Different perceptions and controversies on change and status of North Sea fish stocks in multi-stakeholder settings
    Verweij, M.C. ; Densen, W.L.T. van; Mol, A.P.J. - \ 2010
    Marine Policy 34 (2010)3. - ISSN 0308-597X - p. 522 - 533.
    fisheries management - mixed fisheries - information - knowledge - anecdotes - advice - risk
    Fishermen, scientists, national policy makers, and staff of environmental NGOs (ENGOs) hold different perceptions about temporal patterns in fish stocks. Perception differences are problematic in multi-stakeholder settings, because they elicit controversies and unbalanced disputes. These hinder effective participation, a prerequisite for ‘good governance’ and effective management of sustainable fisheries. This study shows that perceptions of change (‘does the stock increase or decrease?’) and of current status of a fish stock (‘is it doing well or not?’) are influenced by the capturing and processing of information, rather than by interests alone. We focused on the Dutch North Sea fishery on plaice and sole and examined (1) availability and accessibility of information on temporal patterns of these stocks and (2) perception differences between all parties. A first explanation for these differences is the use of different parameters as a measure for stock size. Fishermen focus on catch rates or catch-per-unit-effort (relative stock size), whereas scientists, policy makers, and ENGO-staff mainly use scientific assessments of spawning stock biomass (absolute stock size). Between-group perception differences are further explained by spatial aggregation levels of information, lengths of time series evaluated, and by modes of comparison to qualify the current status of fish stocks. Awareness of information differences and the development of shared information use and processing may release some of the tensions in multi-stakeholder settings debating fisheries management. However, comprehension problems amongst all parties on how spawning stock biomass is reconstructed and how it relates to catch rates in the fishery may pose an enduring barrier
    Kernbegrippen visserijbeheer en overzicht toestand visbestanden in Europa
    Densen, W.L.T. van; Hintzen, N.T. - \ 2010
    IJmuiden : IMARES (Rapport / IMARES Wageningen UR no. C006/10) - 63
    visbestand - visstand - visserij-ecologie - noordzee - oostzee - fishery resources - fish stocks - fisheries ecology - north sea - baltic sea
    Dit rapport informeert over de biologische achtergronden van het Europese visserijbeleid en over de visbestanden waarop dat beleid van toepassing is. Van de belangrijkste bestanden per ecoregio, zoals Noordzee en Oostzee, is weergegeven hoe groot de paaistand en de visserijdruk zijn.
    Wordt de Noordzee leeggevist? integendeel: visserijsterfte op voorzorgsniveau
    Densen, W.L.T. van - \ 2009
    Visserijnieuws 29 (2009)44. - ISSN 1380-5061 - p. 6 - 6.
    On expansion, natural productivity and control 50 years North Sea fisheries and management
    Densen, Wim van - \ 2009
    Toezicht op minimum maaswijdte
    Machiels, M.A.M. ; Marlen, B. van; Densen, W.L.T. van - \ 2008
    IJmuiden : IMARES (Rapport / IMARES Wageningen UR C001/08) - 16
    visserij - vis vangen - uitrusting - vistuig - netten - regelingen - wetgeving - bemonsteren - Nederland - fisheries - fishing - equipment - fishing gear - nets - regulations - legislation - sampling - Netherlands
    De vangst en sterfte van vis wordt hoofdzakelijk bepaald door de maaswijdte (of maasopening) in de kuil van het gebruikte visnet. Meting van maaswijdte is niet eenvoudig en de huidige regelgeving is aan kritiek onderhevig. Dit heeft geleid tot de ontwikkeling van een nieuw meetprotocol en meetinstrument. De visserij op tong met 80 mm en een minimummaat van 24 cm is gevoelig voor een juiste vaststelling van de maaswijdte van de kuil. Dit rapport behandelt statistische en juridische aspecten van de huidige regelgeving en de eventuele invoering van de nieuwe OMEGA-maaswijdtemeter.
    Vijftig jaar visserij en beheer op de Noordzee
    Densen, W.L.T. van; Overzee, H.M.J. van - \ 2008
    Wageningen : Wettelijke Onderzoekstaken Natuur & Milieu (WOt-rapport 81) - 112
    noordzee - visserij - visserijbeheer - visserijbeleid - visbestand - mariene gebieden - aquatische ecologie - north sea - fisheries - fishery management - fishery policy - fishery resources - marine areas - aquatic ecology
    Dit rapport geeft een overzicht van de inspanningen en uitkomsten van de Noordzeevisserij in de afgelopen 50 jaar en van het daarbij gevoerde beheer. Het richt zich uiteindelijk op de vraag of dat beheer op een begrijpelijke manier effectief is gebleken. Sinds de jaren vijftig is de visserij door drie fasen gegaan. Eerst heeft een expansie plaatsgevonden op een golf van technische ontwikkelingen en de hogere productiviteit van de visstand. Daarna vond een periode plaats van bijsturing (paaistand) via risicomijdend beheer en daarbij behorende voorzorgniveaus voor de visserijsterfte. Op dit moment is er inkrimping en structuurverandering; actief via beheer dat gericht is op een veel lagere visserijdruk en passief via het economisch renderen dat steeds afhankelijker is geworden van de stijgende olieprijs. Visserijbeheer is een complexe zaak. Zonder overzichtelijke informatie over visstand en visserij verliest het debat daarover zich in weinig productieve versimpelingen. Trefwoorden: Noordzeevisserij, visserijdruk; visserijbeheer, visserijbeleid, Noordzee
    Oplossingsrichtingen bij het verminderen van scholdiscards in de 80 mm visserij.Ten behoeve van het Klein Beheersoverleg van 3 december 2007
    Densen, W.L.T. van; Quirijns, F.J. ; Helmond, A.T.M. van - \ 2008
    IJmuiden : IMARES (Rapport / Wageningen IMARES C064/08) - 24
    boomkorvisserij - vis vangen - vistuig - netten - discards - schol - beam trawling - fishing - fishing gear - nets - discards - plaice
    Tong- en schol met de 80mm boomkor leidt tot vangst van schol vanaf 17cm. De aanvoermaat voor schol is 27cm, dit heeft tot gevolg dat schol tussen de 17 en 27cm wordt gediscard. Het merendeel van deze scholmaat houdt zich op in het zuidelijke gebied van de Noordzee, precies het verspreidingsgebied van de lucratieve tong. Deze platvissoort heeft een kleinere aanvoermaat (24cm) en heeft een flexibelere lichaamsbouw dan schol. Om optimaal te kunnen profiteren van deze hoogwaardige soort is het gebruik van een 80mm maaswijdte in de boomkorvisserij noodzakelijk. Deze ongelukkige combinatie van aanvoermaten en verspreiding van beide platvissoorten maakt het discarden van ondermaatse schol tot een structureel probleem. Daarbij komt dat sinds 1995 door verschillende factoren, een verschuiving van de visserijinspanning van het noorden naar het zuiden heeft plaatsgevonden. Dit rapport geeft een overzicht van mogelijke maatregelen om het percentage scholdiscards te verminderen. Effectiviteit en implementeerbaarheid van deze maatregelen worden, waar mogelijk, onderbouwd met resultaten en uitkomsten van wetenschappelijke studies.
    Effecten van een verhoging van de miniummaat (MLS) voor tong
    Densen, W.L.T. van; Smit, J. ; Quirijns, F.J. ; Oostenbrugge, H. van; Machiels, M.A.M. - \ 2008
    IJmuiden : IMARES (Rapport / Wageningen IMARES nr. C045/08) - 20
    visserij - tong (vis) - lengte - visserijbeleid - visvangsten - discards - fisheries - dover soles - length - fishery policy - fish catches - discards
    De aanvoer van Noordzee tong in relatie tot de vangst wordt bepaald door de minimum maat (MLS) van deze vissoort. Nu bedraagt de MLS 24 cm en gevangen tong die kleiner is dan deze MLS moet overboord gezet worden. In dit rapport worden de consequenties van een mogelijke verhoging van de MLS voor tong naar 27 cm geïnventariseerd.
    Serial correlation and inter-annual variability in relation to the statistical power of monitoring schemes to detect trends in fish populations
    Nagelkerke, L.A.J. ; Densen, W.L.T. van - \ 2007
    Environmental Monitoring and Assessment 125 (2007)1-3. - ISSN 0167-6369 - p. 247 - 256.
    management - size
    We studied the effects of inter-annual variability and serial correlation on the statistical power of monitoring schemes to detect trends in biomass of bream (Abramis brama) in Lake Veluwemeer (The Netherlands). In order to distinguish between `true¿ system variability and sampling variability we simulated the development of the bream population, using estimates for population structure and growth, and compared the resulting inter-annual variabilities and serial correlations with those from field data. In all cases the inter-annual variability in the field data was larger than in simulated data (e.g. for total biomass of all assessed bream ¿¿=¿0.45 in field data, and ¿¿=¿0.03¿0.14 in simulated data) indicating that sampling variability decreased statistical power for detecting trends. Moreover, sampling variability obscured the inter-annual dependency (and thus the serial correlation) of biomass, which was expected because in this long-lived population biomass changes are buffered by the many year classes present. We did find the expected serial correlation in our simulation results and concluded that good survey data of long-lived fish populations should show low sampling variability and considerable inter-annual serial correlation. Since serial correlation decreases the power for detecting trends, this means that even when sampling variability would be greatly reduced, the number of sampling years to detect a change of 15%·year¿1 in bream populations (corresponding to a halving or doubling in a six-year period) would in most cases be more than six. This would imply that the six-year reporting periods that are required by the Water Framework Directive of the European Union are too short for the existing fish monitoring schemes.
    Utilization of seagrass habitats by juvenile groupers and snappers in Banten Bay, Banten Province, Indonesia
    Nuraini, S. ; Carballo, E.C. ; Densen, W.L.T. van; Machiels, M.A.M. ; Lindeboom, H.J. ; Nagelkerke, L.A.J. - \ 2007
    Hydrobiologia 591 (2007)1. - ISSN 0018-8158 - p. 85 - 98.
    unvegetated habitats - florida bay - fishes - recruitment - patterns - meadows - diel - abundance - selection - victoria
    Coastal development in Banten Bay, Indonesia, decreased seagrass coverage to only 1.5% of its surface area. We investigated the importance of seagrass as habitat for juvenile groupers (Serranidae) and snappers (Lutjanidae), by performing beam trawl hauls on a weekly basis in two seagrass locations and one mudflat area, and monthly trawl hauls in three different microhabitats (dense, mixed and patchy seagrass) in one of the seagrass locations. We studied the effects of location and microhabitat, as well as temporal patterns (diel, weekly and monthly) on the probability of occurrence and abundance of the most abundant grouper (Orange-spotted grouper, Epinephelus coioides) and snapper (Russell¿s snapper, Lutjanus russellii). We found that both species were almost exclusively found in seagrass locations, with a preference for microhabitats of high complexity (dense and mixed microhabitats). L. russellii had a higher probability of catch and abundance during the night, most probably because of its ability to avoid the beam trawl during daytime sampling. In addition there was an effect of week and month on the presence and abundance of both species, but patterns were unclear, probably because of high fishing pressure on juvenile groupers and snappers by push net fishermen. Groupers and snappers mainly fed on abundant shrimps, and to a lesser extent on fish. Moreover, juveniles find protection against predators in seagrass, which confirmed the critical role of quantity and quality of seagrass areas for juvenile groupers and snappers in Banten Bay.
    Benefits and organization of cooperative research for fisheries management
    Johnson, T.R. ; Densen, W.L.T. van - \ 2007
    ICES Journal of Marine Science 64 (2007)4. - ISSN 1054-3139 - p. 834 - 840.
    illex-illecebrosus - falkland islands - mortality - atlantic - stocks - squid - cod
    Drawing on research in the northeastern USA and northwestern Europe, a description is given of how cooperative research is organized and a statement made of how involving fishers in research can contribute to better fisheries management. The focus is on improving stock assessments through the collection of better fishery-dependent and -independent data and through efforts to address bycatch through gear-selectivity studies. Direct benefits of cooperative research include increased quantity and quality of data, inclusion of fishers' knowledge in science and management, improved relevance of research to fisheries management, and reduced costs of science. Indirect benefits are the buy-in of science and management by industry and improved relationships and trust between fishers and scientists (and managers). These indirect benefits are best achieved under conditions of transparency and communication. In some cases, cooperative research also provides income to the industry and supports the maintenance of fishing infrastructure. Most important, cooperative research improves capacity-building and establishes intellectual property rights within the fishing industry, and it encourages innovative approaches to management, such as adaptive and ecosystem-based approaches. Finally, guidelines for making cooperative research more effective are outlined
    Improving communication from managers to fisheres in Europe and the US
    Densen, W.L.T. van; McCay, B.J. - \ 2007
    ICES Journal of Marine Science 64 (2007)4. - ISSN 1054-3139 - p. 811 - 817.
    resources - collapse - science
    Communication problems need to be solved if managers are to be more persuasive about the need for limitations on fishing, to protect and restore fish populations. The context is widespread scepticism about the effectiveness of management on both sides of the Atlantic. That scepticism is fuelled by assessment bias as seen in the case of the northern cod of Newfoundland, and by failure to take into account differences in perceptions of stock size and fishing mortality, differences in causal reasoning about fishing pressure and environmental factors influencing stock size; and differences in the capacity to read and understand the mostly graphic information that underlies and is often used to explain management decisions. This analysis is based on interviews and observations in the European Union and the northeastern USA.
    Influences of discourse on decision-making in EU fisheries management: the case of North Sea cod (Gadus morhua)
    Delaney, A.E. ; McLay, H.A. ; Densen, W.L.T. van - \ 2007
    ICES Journal of Marine Science 64 (2007)4. - ISSN 1054-3139 - p. 804 - 810.
    Does public debate have an influence on decision-making in European fisheries management? Our premise is that for fisheries management to be viewed as legitimate, stakeholders should be included in the process. Because the first step towards stakeholder participation is letting their voices be heard, we analysed the public debate surrounding the management of North Sea cod, focusing on two aspects: the mixed fisheries problem of total allowable catch management and the causes of cod decline. Using standard social-science methodologies including discourse analysis, participant observation, and qualitative interviews, we suggest that the public debate has not had a direct effect on year-to-year management decisions. Still, it can be argued that the debate has influenced the attitude of those involved in the management system and, therefore, has had an impact on the changes in the management and advisory system that are taking place, although these take place at different levels and speeds than expected.
    Policy and knowledge in fisheries management: a policy brief
    Schwach, V. ; Bailly, D. ; Christensen, A.S. ; Delaney, A.E. ; Degnbol, P. ; Densen, W.L.T. van; Holm, P. ; McLay, A. ; Nielsen, K.N. ; Pastoors, M.A. ; Reeves, S.A. - \ 2007
    ICES Journal of Marine Science 64 (2007)4. - ISSN 1054-3139 - p. 798 - 803.
    north-sea cod
    The EU project Policy and Knowledge in Fisheries Management investigated the use of biological knowledge in various parts of the fisheries system, using North Sea cod as a case study. The project examined the way scientific advice was generated from technical and institutional perspectives, as well as the way claims about science appeared in both policy-setting and in public debate through the press. The results suggested that many people involved in the system want a new way to reflect about science in management. People from all major stakeholder groups are calling for a more interactive system of producing a common knowledge base. Such a system could bring uncertainty from its current marginal role as the leftovers of certainty to the heart of the science process. It would require stakeholders to help address uncertainty and to negotiate a more realistic placement of burden of proof.
    5 Years of F-Project: a final report
    Quirijns, F.J. ; Keeken, O.A. van; Densen, W.L.T. van - \ 2007
    IJmuiden : IMARES (Rapport / Wageningen IMARES nr. C058/07) - 44
    visserij - bepaling - berekening - samenwerking - visvangsten - visstand - fisheries - determination - calculation - cooperation - fish catches - fish stocks
    Het F-project was een samenwerkingsproject tussen de beheerder, de visserij en het onderzoek en vond plaats van 2002-2007. Het project had tot doel om de kwaliteit en transparantie van het visserijadvies voor het beheer van Noordzee tong en schol te verbeteren. Er waren drie onderdelen gedefinieerd: F1) verbeteren van bestandsschattingen; F2) verbeteren van gebruik van commerciële vangstgegevens; en F3) verbeteren communicatie. Dit document geeft een overzicht van alle producten die in het kader van het F-project zijn gemaakt.
    5 Jaar F - Project
    Densen, W.L.T. van; Quirijns, F.J. - \ 2007
    IJmuiden : IMARES (Rapport / Wageningen IMARES nr. C057/07) - 21
    visserij - visserijbeheer - visserijbeleid - vissers - onderzoekers - communicatie - bevolkingsafname - mortaliteit - quota's - nederland - visstand - visvangsten - fisheries - fishery management - fishery policy - fishermen - research workers - communication - population decrease - mortality - quotas - netherlands - fish stocks - fish catches
    Rond 2000 verslechterde de legitimiteit van het beheer omdat de visserijsector grote moeite had met de soms grote bijstellingen in de schattingen voor het bestand (omlaag) en de visserijsterfte F (omhoog). Dat gold vooral voor schol en kabeljauw. De schommelingen in die bestandsschattingen kwam niet overeen met het meer stabiele beeld dat de vissers hadden van ontwikkelingen in de visstand op basis van hun vangstsucces. Het leidde tot een verslechtering van de communicatie tussen sector en onderzoek, waarbij de vissers de onderzoekers verantwoordelijk hielden voor de kwaliteit van het beheer. Het F-project kreeg de opdracht de situatie te helpen verbeteren door te zien: - waar de bestandsschattingen beter konden - hoe meer rekening viel te houden met informatie uit de visserij en - hoe de communicatie tussen de partijen was te verbeteren.
    Sustainable use of flatfish resources: addressing the credibility crisis in miced fisheries management
    Rijnsdorp, A.D. ; Daan, N. ; Dekker, W. ; Poos, J.J. ; Densen, W.L.T. van - \ 2007
    Journal of Sea Research 57 (2007)2-3. - ISSN 1385-1101 - p. 114 - 125.
    north-sea plaice - beam trawl fleet - fishing vessels - competitive interactions - effort allocation - science - area - cod - dynamics - policy
    Many flatfish species are caught in mixed demersal trawl fisheries and managed by Total Allowable Catch (TAC). Despite decades of fisheries management, several major stocks are severely depleted. Using the Common Fisheries Policy (CFP) as an example, the failure of mixed-fisheries management is analysed by focussing on: the management system; the role of science; the role of managers and politicians; the response of fisheries to management. Failure of the CFP management could be ascribed to: incorrect management advice owing to bias in stock assessments; the tendency of politicians to set the TAC well above the recommended level; and non-compliance of the fisheries with the management regulations. We conclude that TAC management, although apparently successful in some single-species fisheries, inevitably leads to unsustainable exploitation of stocks caught in mixed demersal fisheries as it promotes discarding of over-quota catch and misreporting of catches, thereby corrupting the basis of the scientific advice and increasing the risk of stock collapse. This failure in mixed demersal fisheries has resulted in the loss of credibility of both scientists and managers, and has undermined the support of fishermen for management regulations. An approach is developed to convert the TAC system into a system that controls the total allowable effort (TAE). The approach takes account of the differences in catch efficiency between fleets as well as seasonal changes in the distribution of the target species and can also be applied in the recovery plans for rebuilding specific components of the demersal fish community, such as plaice, cod and hake.
    Spatial and seasonal patterns in the feeding habits of juvenile Lates niloticus (L.) in the Mwanza Gulf of Lake Victoria
    Katunzi, E.F.B. ; Densen, W.L.T. van; Wanink, J.H. ; White, F. - \ 2006
    Hydrobiologia 568 (2006)1. - ISSN 0018-8158 - p. 121 - 133.
    ontogenic niche shifts - nile perch - east-africa - nyanza-gulf - rastrineobola-argentea - haplochromine cichlids - largemouth bass - life-history - fish - predation
    Flexibility in the feeding habits of juvenile Nile perch (1¿30 cm total length) was studied from September 1988 to September 1989 at four sites (depth range: 1¿25 m) in the Mwanza Gulf of Lake Victoria. During this period haplochromine cichlids were virtually absent in the area. We looked at the combined effects of predator size, season and habitat. Stomach content analysis showed that with increase in size, the diet of Nile perch shifted from zooplankton and midge larvae, to macro-invertebrates (shrimps and dragonfly nymphs) and fish. At a size of 3¿4 cm Nile perch shifted from size-selective predation on the largest cyclopoids to predation on the largest, less abundant, calanoids. Zooplanktivory ended at a size of ca. 5 cm. Although an ontogenetic shift in the diet of juvenile Nile perch was obvious at all sampling stations, the contribution of prey types appeared to be habitat related. With increasing water depth the frequency of occurrence in the diet of most prey types decreased, but that of shrimps increased. At the entrance of the gulf (20¿25 m deep) shrimps were the main food source throughout the year. Halfway the gulf (12¿16 m), Nile perch showed seasonality in their feeding behaviour. Shrimps were taken there especially during the rainy season (January to May) when their densities at this station were high, whereas cannibalism prevailed during the rest of the year. In an environment with Nile perch and dagaa as alternative prey, shrimps were taken almost exclusively. They could be regarded as a key prey for Nile perch between 5 and 30 cm
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