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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    Adaptive behaviour of fishers to external perturbations: simulation of the Tasmanian rock lobster fishery
    Hamon, K.G. ; Frusher, S.D. ; Little, L.R. ; Thebaud, O. ; Punt, A.E. - \ 2014
    Reviews in Fish Biology and Fisheries 24 (2014)2. - ISSN 0960-3166 - p. 577 - 592.
    jasus-edwardsii - fleet dynamics - climate-change - individual variation - australia - size - management - victoria - model
    The rock lobster, Jasus edwardsii, lies on a global "hotspot" for climate change in the southeastern Australian state of Tasmania. The short-term effects of climate change are predicted to lead to an increasing exploitable biomass in the south and declining biomass in the north of the state. The future of the fishery is highly uncertain due to climate change, but also due to insecurities linked to the market conditions. The market for Tasmanian rock lobster is driven by the demand of a single market, China, which absorbs 75 % of the catch. This study examines how fishers can adapt to external perturbations that affect the social and economic viability of the fleet and the ecological dynamics of the stock. Three fleet dynamic models of increasing complexity are used to investigate the effects of climate change and lobster price changes on the fishery. There could be local depletion leading to negative short-term profit for the fleet if it is static and the proportion of the total catch taken in each region of the fishery does not respond to climate-induced-changes. Better outcomes would occur if the fleet adapts dynamically to environmental conditions, and fishing effort follows stock abundance, which would counter-act the short-term effects of climate change. Only a model with explicit representation of economic drivers can fully capture the local economic and social impacts of large scale global perturbations.
    Lessons from long-term predator control: a case study with the red fox
    Kirkwood, R.J. ; Sutherland, D.R. ; Murphy, S. ; Dann, P. - \ 2014
    Wildlife Research 41 (2014). - ISSN 1035-3712 - p. 222 - 232.
    vulpes-vulpes-l - wallaby population-dynamics - penguins eudyptula-minor - south-eastern australia - phillip-island - rural britain - home-range - victoria - impact - density
    Context: Predator-control aims to reduce an impact on prey species, but efficacy of long-term control is rarely assessed and the reductions achieved are rarely quantified. Aims: We evaluated the changing efficacy of a 58-year-long campaign against red foxes (Vulpes vulpes) on Phillip Island, a 100-km2 inhabited island connected to the Australian mainland via a bridge. The campaign aimed to eliminate the impact of foxes on ground-nesting birds, particularly little penguins (Eudyptula minor). Methods: We monitored the success rate of each fox-control technique employed, the level of effort invested if available, demographics of killed foxes, the numbers of penguins killed by foxes and penguin population size. Key results: The campaign began as a bounty system that ran for 30 years and was ineffective. It transitioned into a coordinated, although localised, control program from 1980 to 2005 that invested considerable effort, but relied on subjective assessments of success. Early during the control period, baiting was abandoned for less effective methods that were thought to pose fewer risks, were more enjoyable and produced carcasses, a tangible result. Control was aided by a high level of public awareness, by restricted fox immigration, and by a clear, achievable and measurable target, namely, to prevent little penguin predation by foxes. Carcasses did prove valuable for research, revealing the genetic structure and shifts in fox demographics. The failure of the program was evident after scientific evaluation of fox population size and ongoing fox impacts. In 2006, the campaign evolved into an eradication attempt, adopting regular island-wide baiting, and since then, has achieved effective knock-down of foxes and negligible predation on penguins. Conclusions: Effective predator control was achieved only after employing a dedicated team and implementing broad-scale baiting. Abandoning widespread baiting potentially delayed effective control for 25 years. Furthermore, both predator and prey populations should be monitored concurrently because the relationship between predator abundance and impact on prey species is not necessarily density dependent. Implications: Critical to adopting the best management strategy is evaluating the efficacy of different methods independently of personal and public biases and having personnel dedicated solely to the task.
    When to declare successful eradication of an invasive predator?
    Rout, T.M. ; Kirkwood, R.J. ; Sutherland, D.R. ; Murphy, S. ; McCarthy, M. - \ 2014
    Animal Conservation 17 (2014)2. - ISSN 1367-9430 - p. 125 - 132.
    feral cat eradication - fox vulpes-vulpes - phillip-island - population - extinction - california - penguins - victoria
    Imperfect detection methods make it difficult to tell whether an invasive species has been successfully eradicated. However, management cannot continue indefinitely when individuals are no longer detected – at some point, efforts must be reduced or ceased entirely. The risks of mistakenly inferring that an eradication attempt has been successful can be high: the species can bounce back and even expand its range, causing environmental and economic damage, and rendering the initial eradication campaign redundant. This decision problem, balancing the risks of declaring eradication prematurely with the costs of continued management, is currently being contemplated by managers of the fox eradication programme on Phillip Island, in Victoria, Australia. We used a Bayesian catch-effort model to analyse data on the number of foxes removed and sighted using different methods. We estimate that there were 11 foxes remaining on Phillip Island as of end of June 2012. Baiting was the most effective method for removing foxes per person-hour invested, and spotlighting was the most effective method for sighting foxes without removal. We then projected forward into the future, assuming management effort continues at current levels, but no further foxes are detected (removed or sighted). Under this scenario, the mean estimate for the number of foxes remaining drops below a single fox after three years with no detections, and the probability that eradication has been successful is 0.69. This is the optimal time to declare eradication, given our estimated cost of declaring eradication prematurely. This framework indicates the minimum number of years for which management of foxes on the island must continue and allows decision makers to assess the trade-offs involved in any decision to declare eradication.
    Living with less water: development of viable adaptation options for Riverina irrigators
    Gaydon, D.S. - \ 2012
    Wageningen University. Promotor(en): Holger Meinke, co-promotor(en): Jan Vos; D. Rodriguez. - S.l. : s.n. - ISBN 9789461732323 - 225
    irrigatie - watergebruiksrendement - watervoorraden - bedrijfssystemen - participatieve methoden - simulatiemodellen - australië - victoria - new south wales - irrigation - water use efficiency - water resources - farming systems - participation methods - simulation models - australia - victoria - new south wales

    In Australia, the best use of limited national water resources continues to be a major political and scientific issue. Average water allocations for rice-cereal irrigation farmers in the Riverina region have been drastically reduced since 1998 as a consequence of high rainfall variability and prolonged periods of drought, together with political changes. This has severely impacted regional crop production during the last decade, threatening the livelihoods of many farmers and is in stark contrast to much of this region’s 100 year agricultural history, where water resources were available to farmers in steady abundance. The water ‘landscape’ has changed - bringing with it considerable social, economic and environmental consequences and forcing a rethink of how valuable water resources are best used under such variable, changed and changing conditions. This thesis presents details of investigations into on-farm adaptation options for rice-cereal farmers, using field experimentation, participatory engagement, and farming systems modelling as the major tools of research. Additionally, a major component of this work has been the development and testing of new modelling tools and decision-support structures.

    Well-tested cropping systems models that capture interactions between soil water and nutrient dynamics, crop growth, climate and management can assist in the evaluation of new agricultural practices. At the beginning of this research project, all available models were lacking in at least some major element required for simulation of rice-based cropping systems. The capacity to simulate C and N dynamics during transitions between aerobic and anaerobic soil environments was added into the APSIM model, to facilitate our need to model farming system scenarios which involved flooded rice in rotation with other crops and pastures. Thorough testing against international datasets was subsequently conducted. Photosynthetic aquatic biomass (PAB – algae) is a significant source of organic carbon (C) in rice-based cropping systems. A portion of PAB is capable of fixing nitrogen (N), and is hence also a source of N for crops. To account for this phenomenon in long term simulation studies of rice-based cropping systems, the APSIM model was modified to include new descriptions of biological and chemical processes responsible for loss and gain of C and N in rice floodwater.

    Using this improved APSIM model as a tool, together with participatory involvement of Riverina case-study farmers, it was demonstrated that the best on-farm cropping and irrigation strategies in years of high water availability were substantially different to those when water supplies were low. The strategies leading to greatest farm returns vary on a season-by-season basis, depending primarily on the water availability level. Significant improvements in average farm profits are possible by modifying irrigation strategies on a season-by-season basis.

    The opportunities for Riverina farmers to exploit their irrigation water resources also extend beyond the farm gate. Currently there is considerable confusion amongst farmers on how to evaluate and compare on-farm and off-farm water options. Direct selling of water seasonally on the open market and even permanent sale of irrigation water entitlements are possibilities. In response to this confusion, a new conceptual framework was developed that enables quantitative comparisons between various options. The framework is based on a method regularly employed in the financial world for share portfolio analysis. Simulation modelling provided risk-return characteristics for on-farm options, and helped to elucidate circumstances under which off-farm options were viable.

    A modified version of alternate wet-and-dry water management for Australian rice-growing conditions (delayed continuous flooding, DCF) was investigated via a 2 year field experiment – aimed at reducing irrigation water requirement and increasing water productivity (WP). We demonstrated up to a 17% increase in WP, and field data was generated on system performance for a range of discrete irrigation strategies. The APSIM model was then parameterized, calibrated and validated before being used to extrapolate findings from the two year experimental period to a much broader climatic record (55 years), allowing detailed investigation of optimal management strategies and a more realistic estimation of likely long-term gains in water productivity, and associated risks, from this new rice irrigation practice. Best practice guidelines were developed, and the potential impact of a changing climate on both optimal practice and likely benefits was assessed.

    This thesis concludes by synthesising the approaches taken - addressing the question of whether improved rice irrigation practices, seasonally-flexible cropping and irrigation strategies and off-farm exploitation options, can in combination address the challenges of reduced irrigation water allocations in Australia’s Riverina region. Evidence is presented that the answer is yes under certain circumstances, but that limits to change exist beyond which the investigated on-farm adaptations are not enough. The thesis also proposes that the concepts and methods developed during this project are globally applicable and useful in the design of farming system adaptation options.

    Keywords: irrigation, limited water resources, farming systems modelling, participatory engagement.

    Simulation of efficiency impact of drainage water reuse: case of small-scale vegetable growers in North West Province, South Africa
    Speelman, S. ; Haese, M.F.C. D'; Haese, L. D' - \ 2011
    Agrekon 50 (2011)1. - ISSN 0303-1853 - p. 89 - 101.
    data envelopment analysis - productive efficiency - technical efficiency - irrigation - benchmarking - victoria - schemes - farms - spain - dea
    This paper focuses on estimating the effect of drainage water reuse on the technical efficiency of small-scale vegetable growers in South Africa applying a data envelopment analysis (DEA). In the semi-arid North West Province of South Africa water scarcity and the soon to be implemented water charges have urged farmers in small-scale irrigation schemes to evaluate the efficiency of their water use. Data on 60 farmers were used to estimate the level of technical efficiency and the effect that drainage water re-use could have on efficiency levels. This effect of water reuse was simulated by a 5, 10, 15 and 20 per cent reduction in water use at farm level. A Malmquist productivity index was calculated to evaluate the effect of these reductions. The main finding was that under current farming conditions many farmers operated at suboptimal levels of technical efficiency. While a reduction in water use evidently increased factor productivity for most farms, the effect clearly varied strongly between farms. This confirms the need to take a systems approach for this type of evaluations.
    Intra-lake stable isotope ratio variation in selected fish species and their possible carbon sources in Lake Kyoga (Uganda): implications for aquatic food web studies
    Mbabazi, D. ; Makanga, B. ; Orach-Meza, F. ; Hecky, R.E. ; Balirwa, J.S. ; Ogutu-Ohwayo, R. ; Verburg, P.H. ; Chapman, L. ; Muhumuza, E. - \ 2010
    African Journal of Ecology 48 (2010)3. - ISSN 0141-6707 - p. 667 - 675.
    trophic structure - east-africa - mwanza gulf - nitrogen - diet - delta-n-15 - victoria - ecosystem - specialization - delta-c-13
    The stable isotopes of nitrogen (delta 15N) and carbon (delta 13C) provide powerful tools for quantifying trophic relationships and carbon flow to consumers in food webs; however, the isotopic signatures of organisms vary within a lake. Assessment of carbon and nitrogen isotopic signatures in a suite of plants, invertebrates, and fishes in Lake Kyoga, indicated significant variation between two sites for delta 13C (paired t = 6.305; df = 14, P <0.001 and delta 15N paired t = 1.292; df = 14; P <0.05). The fish fauna in Bukungu was generally more 13C enriched (mean delta 13C = -16.37 +/- 1.64 parts per thousand) than in Iyingo (mean delta 13C = -20.80 +/- 2.41 parts per thousand) but more delta 15N depleted (mean delta 15N = 5.57 +/- 0.71 parts per thousand) than in Iyingo (mean delta 15N = 6.92 +/- 0.83 parts per thousand). The simultaneous shifts in phytoplankton and consumer signatures confirmed phytoplankton as the major source of carbon for the food chain leading to fish. Limited sampling coverage within lakes may affect lake wide stable isotope signatures, and the same error is transferred into trophic position estimation. Consideration of potential intra-lake spatial variability in isotope ratios and size is essential in evaluating the spatial and trophic structure of fish assemblages.Resume Les isotopes stables d'azote (delta 15N) et de carbone (delta 13C) sont des outils interessants pour quantifier les relations trophiques et le flux de carbone vers les consommateurs de chaines alimentaires; cependant, la signature isotopique des organismes varie au sein d'un meme lac. L'evaluation des signatures isotopiques du carbone et de l'azote dans une suite de plantes, d'invertebres et de poissons du lac Kyoga indiquait une variation significative entre deux sites pour delta 13C (test t apparie = 6.305; df = 14; P <0.05). La faune piscicole de Bukungu etait generalement plus enrichie en delta 13C (moyenne de delta 13C = -16.37 +/- 1.64 parts per thousand) qu'a Iyingo (moyenne de delta 13C = -20.80 +/- 2.41 parts per thousand) mais plus depourvue de delta 15N (moyenne de delta 15N = 5.57 +/- 0.71 parts per thousand) qu'Inyingo (moyenne de delta 15N = 6.92 +/- 0.83 parts per thousand). Les glissements simultanes des signatures du phytoplancton et des consommateurs confirmaient que le phytoplancton est la source principale de carbone de la chaine alimentaire qui aboutit aux poissons. Une couverture limitee de l'echantillonnage dans les lacs peut affecter la signature des isotopes stables de tout le lac, et cette meme erreur est reportee dans l'estimation de la situation trophique. Il est essentiel de tenir compte de la variabilite spatiale possible des taux et de la taille des isotopes dans les lacs lorsque l'on evalue la structure spatiale et trophique des assemblages de poissons.
    Adaptive radiation of Lake Tana's (Ethiopia) Labeobarbus species flock (Pisces, Cyprinidae)
    Graaf, M. de; Dejen, E. ; Osse, J.W.M. ; Sibbing, F.A. - \ 2008
    Marine and Freshwater Research 59 (2008)5. - ISSN 1323-1650 - p. 391 - 407.
    late pleistocene desiccation - tropical fish assemblage - cichlid fishes - barbs barbus - east-africa - reproductive segregation - ecological divergence - victoria - speciation - origin
    Studying species flocks (e.g. Darwin¿s finches, Caribbean anoline lizards, East African cichlid fishes) has proven to be highly successful in understanding the forces driving speciation. The only known, intact species flock of cyprinid fishes, the 15 Labeobarbus species in Lake Tana (Ethiopia), includes eight piscivorous species. Piscivory is a rare specialisation among the highly successful (>2000 species) but mostly benthivorous Cyprinidae. The extent and mechanisms of diversification of this remarkable Labeobarbus species flock, particularly among the unexpected piscivorous species, are still largely unknown. In the present study we demonstrate that all 15 Labeobarbus species are segregated to a great extent along spatial, trophic and/or temporal dimensions. The spatial distribution, diet (prey species but not prey size), time of active feeding and predation techniques differed significantly among the eight piscivores. Lake Tana¿s cyprinids displayed their retained potential for ecological diversification and speciation, including the uncommon specialisation of piscivory. The latter is probably a result of the absence of common African specialist piscivores in Lake Tana. We suggest that the evolution of Lake Tana¿s Labeobarbus species flock at this stage is predominantly structured by ecological selection models. The labeobarbs most likely underwent sequential stages of radiation and speciation: habitat divergence followed by trophic divergence.
    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.
    Regeneration after 8 years in artificial canopy gaps in mountain ash (Eucalyptus regnans F. Muell.) forest in south-eastern Australia
    Meer, P.J. van der; Dignan, P. - \ 2007
    Forest Ecology and Management 244 (2007)1-3. - ISSN 0378-1127 - p. 102 - 111.
    seedling establishment - acacia-mearnsii - f muell. - growth - management - victoria - site - size - tree - disturbance
    We report on a study of regeneration of Mountain Ash (Eucalyptus regnans) forest in S.E. Australia in artificially created canopy gaps (0.01¿2 ha) and clearfelled coupes (4¿27 ha) with different seedbed treatments. Treatments were applied in 1988, 1989, and 1990. Our results are based on measurements 8 years after harvesting. Gap size had a significant influence on sapling height and diameter growth. The effect was substantial for the canopy openings larger than 2 ha, but inconsistent across the smaller gap sizes. Mean height of the largest diameter saplings was significantly greater in the clearfells (13.2 m) than in the 2 ha gaps (10.7 m), while mean heights in the smaller gaps ranged from 6 to 8.8 m. Mean sapling diameters (at 1.3 m) in clearfells and 2 ha gaps were 9.6 and 7.9 cm respectively compared with 3.8¿5.8 cm for the smaller gaps. There was lower stocking in all treatments for the third regeneration year (1990), when regeneration operations were delayed by high summer and autumn rainfall. Stocking ranged from 12 to 33% for the smallest gaps, increasing with increasing gap size to 34¿64% for 2 ha gaps and 48¿76% for clearfells. Results were consistent with a previous study 3 years after treatment, indicating that stocking at 8 years is still dominated by initial stand establishment rather than by stand development processes. There was no significant influence of seedbed preparation on any of the variables tested at year 8; this was in contrast with earlier findings at year 3 when seedling height was significantly higher on burnt sites compared with mechanically disturbed sites. The growth of non-eucalypt competitors was also influenced by gap size and year of regeneration treatment. The main competitor species differed between larger and smaller clearings and between sites. Results indicate that using a silvicultural system based on smaller gaps (
    Vulnerability to a small-scale commercial fishery of Lake Tana's (Ethiopia) endemic Labeobarbus compared with African catfish and Nile tilapia: An example of recruitment-overfishing?
    Graaf, M. de; Zwieten, P.A.M. van; Machiels, M.A.M. ; Lemma, E. ; Wudneh, T. ; Dejen, E. ; Sibbing, F.A. - \ 2006
    Fisheries Research 82 (2006)1-3. - ISSN 0165-7836 - p. 304 - 318.
    oreochromis-niloticus - species flock - reproductive segregation - perch upsurge - barbs barbus - cyprinidae - victoria - impact
    In 1986 a motorised, commercial gillnet fishery was introduced in Lake Tana, Ethiopia's largest lake (3050 km2) in addition to the artisanal, predominantly subsistence fishery conducted from reedboats. The three main species groups targeted by this fishery are a species flock of endemic, large Labeobarbus spp., African catfish (Clarias gariepinus) and Nile tilapia (Oreochromis niloticus). The commercial gillnet fisheries was monitored during 1991-1993 and in 2001 (CPUE, effort allocation, catch composition, yield) and the development of the stocks of the three species groups was determined using an experimental trawl program during 1991-1993 and 1999-2001. In 1991-1993 the vast majority of fishing activities took place in the southern Bahar Dar Gulf (71%). Each species group contributed roughly one third to the total catch. In 2001, 41% of the effort was allocated to the north-eastern shores of Lake Tana. Both the CPUE and the contribution of O. niloticus to the total catch had doubled. However, the proportion of large specimen (>50 cm TL C. gariepinus; >20 cm FL O. niloticus) in the trawl surveys decreased significantly. A three-fold decline in abundance of the anadromous Labeobarbus species occurred. The 15 large labeobarb species are long-lived, ecologically specialised endemics. The seven riverine spawning Labeobarbus species form aggregations in the river mouths in August-September, during which period they are targeted by the commercial gillnet fishery. A sharp decrease in abundance by ca. 75% of the migratory riverine spawning Labeobarbus species in the sublittoral and pelagic zones of the lake, areas where no fishing takes place and the collapse of juvenile Labeobarbus (between 5 and 18 cm FL: by 90%) during the 1990s suggest recruitment-overfishing. To prevent the possible extinction of the unique Labeobarbus species flock all fishing effort should be severely restricted near the river mouths and on the upstream spawning areas during August-September (peak breeding period) to protect the vulnerable spawning aggregations
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