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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Feature filtering and selection for dry matter estimation on perennial ryegrass: A case study of vegetation indices
Alckmin, G.T. ; Kooistra, L. ; Lucieer, A. ; Rawnsley, R. - \ 2019
In: ISPRS Geospatial Week 2019, 10–14 June 2019, Enschede, The Netherlands. - ISPRS (International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences - ISPRS Archives ) - p. 1827 - 1831.
Biomass - Collinearity - Dry Matter - Feature Selection - Machine Learning - Pasture - Perennial Ryegrass - Vegetation Indices

Vegetation indices (VIs) have been extensively employed as a feature for dry matter (DM) estimation. During the past five decades more than a hundred vegetation indices have been proposed. Inevitably, the selection of the optimal index or subset of indices is not trivial nor obvious. This study, performed on a year-round observation of perennial ryegrass (n Combining double low line 900), indicates that for this response variable (i.e. kg.DM.ha−1), more than 80% of indices present a high degree of collinearity (correlation > |0.8|.) Additionally, the absence of an established workflow for feature selection and modelling is a handicap when trying to establish meaningful relations between spectral data and biophysical/biochemical features. Within this case study, an unsupervised and supervised filtering process is proposed to an initial dataset of 97 VIs. This research analyses the effects of the proposed filtering and feature selection process to the overall stability of final models. Consequently, this analysis provides a straightforward framework to filter and select VIs. This approach was able to provide a reduced feature set for a robust model and to quantify trade-offs between optimal models (i.e. lowest root mean square error – RMSE Combining double low line 412.27 kg.DM.ha−1) and tolerable models (with a smaller number of features – 4 VIs and within 10% of the lowest RMSE.).

FeedCalculator, een app om rantsoenen voor vis te berekenen
Bosma, R.H. ; Veluw, S. van; Meijer, P. - \ 2019
Aquacultuur 34 (2019)1. - ISSN 1382-2764 - p. 20 - 23.
De Nederlandse sociale onderneming Single Spark heeft een smartphone app ontwikkeld waarmee kleine boeren en bedrijven zelf rantsoenen voor hun dieren kunnen berekenen (zie foto’s onder). Dat geeft hen de mogelijkheid goedkopere grondstoffen, zoals hun eigen oogstresten, te gebruiken en toch een optimaal rantsoen te voeren. Nadat het wereldwijd voor kippen en varkens was uitgerold, is de app in 2018 getest voor meerval en tilapia in resp. Nigeria en Benin.
Slimme visnetten voor de toekomst (KennisOnline in beeld 2019)
Molenaar, P. - \ 2019
Wageningen University & Research
Om te kijken wat een vis doet in een visnet plaatsen onderzoekers van Wageningen University & Research camera’s in het visnet. Hierdoor kunnen ze erachter komen op welke plekken ze het beste ontsnappingsluikjes in het net kunnen maken voor vissen die de visser niet wil vangen. Dit is belangrijk omdat slimme netten bij kunnen dragen aan de beperking van ongewenste bijvangst.
Modes of Eating and Phased Routinisation: Insect-Based Food Practices in the Netherlands
House, Jonas - \ 2019
Sociology : the Journal of the British Sociological Association 53 (2019)3. - ISSN 0038-0385 - p. 451 - 467.
consumer acceptance - edible insects - entomophagy - insects - novel foods - routinisation - theories of practice

Sociological research on sustainable consumption has seen widespread application of theories of practice (‘practice theories’) as a means of transcending the limitations of epistemologically individualistic ‘behaviour change’ approaches. While in many ways the central insights of practice theories vis-a-vis consumption are now well established, this article argues that the approach holds further insights for sociological analysis of food consumption in general, and of novel foods in particular. Based on empirical research with consumers of a range of insect-based convenience foods in the Netherlands, this article introduces two practice-theoretic concepts – ‘modes of eating’ and ‘phased routinisation’ – which contribute to sociological theorisations of how food practices are established, maintained, interdepend and change. Beyond its theoretical contribution, the article substantively extends research literatures on the introduction, uptake and normalisation of insect-based and other novel foods.

Maden voor de vis! Dat is niet mis!
Haenen, Olga - \ 2019
Convergent evolution of plant parasitism among nematodes, and the relevance of comparative genomics to improve our understanding of (in)compatible host-parasite interactions
Steenbrugge, Joris van; Holterman, M.H.M. ; Sterken, M.G. ; Elsen, S.J.J. van den; Smant, G. ; Helder, J. - \ 2019
Parasitism is a wanted life strategy among nematodes: among the  16,000 described terrestrial species about 25% parasitizes plants, and another 25% animals. Plant parasitism has arisen independently in four different major clades, and even within individual clades plant parasitism arose multiple times. Massive convergent evolution combined with a limited number of informative morphological characters made it nearly impossible to robustly deduce systematic relationships. Wide-scale use of multiple molecular markers allowed for the generation of a more stable overall framework, and currently we have a reasonably good idea about inter-clade and intra-clade relationships among plant parasitic nematodes. Among the 4,100 described plant parasites slightly more than a dozen species are responsible for the major part of the economic losses (worldwide estimated at about €110 billion annually). Here, we will pinpoint the phylogenetic relationships between these high-impact species mainly based on small subunit (SSU) ribosomal DNA sequences. In a next step, we generated PacBio-based reference genomes for a number of major cyst and root knot nematode species to serve as a reference for comparison of re-sequenced field populations. We are currently investigating whether genetic variants can be found that are associated with (a)virulence vis-à-vis a given resistance (R) gene included in crop cultivars. Most R proteins are activated by effector variants present in a subset of the populations of a given species only. We are currently investigating whether effector variants (or tightly associated markers) can be found that are associated with (a)virulence. To illustrate our approach, progress on the effectiveness of resistant potato varieties in relation to the genetic constitution of potato cyst nematode populations will be presented.
Linking Terrestrial LiDAR Scanner and Conventional Forest Structure Measurements with Multi-Modal Satellite Data
Mulatu, Kalkidan ; Decuyper, Mathieu ; Brede, Benjamin ; Kooistra, Lammert ; Reiche, Johannes ; Mora, Brice ; Herold, Martin - \ 2019
Forests 10 (2019)3. - ISSN 1999-4907 - 19 p.
Obtaining information on vertical forest structure requires detailed data acquisition and analysis which is often performed at a plot level. With the growing availability of multi-modal satellite remote sensing (SRS) datasets, their usability towards forest structure estimation is increasing. We assessed the relationship of PlanetScope-, Sentinel-2-, and Landsat-7-derived vegetation indices (VIs), as well as ALOS-2 PALSAR-2- and Sentinel-1-derived backscatter intensities with a terrestrial laser scanner (TLS) and conventionally measured forest structure parameters acquired from 25 field plots in a tropical montane cloud forest in Kafa, Ethiopia. Results showed that canopy gap-related forest structure parameters had their highest correlation (|r| = 0.4 − 0.48) with optical sensor-derived VIs, while vegetation volume-related parameters were mainly correlated with red-edge- and short-wave infrared band-derived VIs (i.e., inverted red-edge chlorophyll index (IRECI), normalized difference moisture index), and synthetic aperture radar (SAR) backscatters (|r| = −0.57 − 0.49). Using stepwise multi-linear regression with the Akaike information criterion as evaluation parameter, we found that the fusion of different SRS-derived variables can improve the estimation of field-measured structural parameters. The combination of Sentinel-2 VIs and SAR backscatters was dominant in most of the predictive models, while IRECI was found to be the most common predictor for field-measured variables. The statistically significant regression models were able to estimate cumulative plant area volume density with an R2 of 0.58 and with the lowest relative root mean square error (RRMSE) value (0.23). Mean gap and number of gaps were also significantly estimated, but with higher RRMSE (R2 = 0.52, RRMSE = 1.4, R2 = 0.68, and RRMSE = 0.58, respectively). The models showed poor performance in predicting tree density and number of tree species (R2 = 0.28, RRMSE = 0.41, and R2 = 0.21, RRMSE = 0.39, respectively). This exploratory study demonstrated that SRS variables are sensitive to retrieve structural differences of tropical forests and have the potential to be used to upscale biodiversity relevant field-based forest structure estimates.
Relationship between MODIS EVI and LAI across time and space
Alexandridis, Thomas K. ; Ovakoglou, Georgios ; Clevers, Jan G.P.W. - \ 2019
Geocarto International (2019). - ISSN 1010-6049 - 15 p.
The Leaf Area Index (LAI) is used as input in hydrological and bio-chemical models for the estimation of water-cycle characteristics, agricultural primary production and other processes. Vegetation Indices (VIs) are used to monitor vegetation state and health. Considering that easily computed VIs can be used for the estimation of LAI, this study applied a regression analysis between MODIS Enhanced Vegetation Index (EVI) and LAI data in five sites around the world. A linear model was found to provide a good description of the LAI–EVI relationship across all examined vegetation types and times. Medium accuracy models were improved when variability of time and vegetation type were considered, indicating that these parameters highly affect the LAI–EVI relationship. Sensitivity of EVI to LAI was higher in periods of high biomass production. Regression analysis between LAI–EVI showed a stronger relationship for the study sites characterized by dry and warm tropical climatic conditions.
Speaking Power to “Post-Truth”: Critical Political Ecology and the New Authoritarianism
Neimark, Benjamin ; Childs, John ; Nightingale, Andrea J. ; Cavanagh, Connor Joseph ; Sullivan, Sian ; Benjaminsen, Tor A. ; Batterbury, Simon ; Koot, Stasja ; Harcourt, Wendy - \ 2019
Annals of the American Association of Geographers 109 (2019)2. - ISSN 2469-4452 - p. 613 - 623.

Given a history in political ecology of challenging hegemonic “scientific” narratives concerning environmental problems, the current political moment presents a potent conundrum: how to (continue to) critically engage with narratives of environmental change while confronting the “populist” promotion of “alternative facts.” We ask how political ecologists might situate themselves vis-à-vis the presently growing power of contemporary authoritarian forms, highlighting how the latter operates through sociopolitical domains and beyond-human natures. We argue for a clear and conscious strategy of speaking power to post-truth, to enable two things. The first is to come to terms with an internal paradox of addressing those seeking to obfuscate or deny environmental degradation and social injustice, while retaining political ecology’s own historical critique of the privileged role of Western science and expert knowledge in determining dominant forms of environmental governance. This involves understanding post-truth, and its twin pillars of alternative facts and fake news, as operating politically by those regimes looking to shore up power, rather than as embodying a coherent mode of ontological reasoning regarding the nature of reality. Second, we differentiate post-truth from analyses affirming diversity in both knowledge and reality (i.e., epistemology and ontology, respectively) regarding the drivers of environmental change. This enables a critical confrontation of contemporary authoritarianism and still allows for a relevant and accessible political ecology that engages with marginalized populations likely to suffer most from the proliferation of post-truth politics. Key Words: authoritarianism, environmental policy, political ecology, post-truth, science.

Robust soil mapping at the farm scale with vis–NIR spectroscopy
Ramirez-Lopez, L. ; Wadoux, A.M.J.C. ; Franceschini, M.H.D. ; Terra, F.S. ; Marques, K.P.P. ; Sayão, V.M. ; Demattê, J.A.M. - \ 2019
European Journal of Soil Science 70 (2019)2. - ISSN 1351-0754 - p. 378 - 393.

Sustainable agriculture practices are often hampered by the prohibitive costs associated with the generation of fine-resolution soil maps. Recently, several papers have been published highlighting how visible and near infrared (vis–NIR) reflectance spectroscopy may offer an alternative to address this problem by increasing the density of soil sampling and by reducing the number of conventional laboratory analyses needed. However, for farm-scale soil mapping, previous studies rarely focused on sample optimization for the calibration of vis–NIR models or on robust modelling of the spatial variation of soil properties predicted by vis–NIR spectroscopy. In the present study, we used soil vis–NIR spectroscopy models optimized in terms of both number of calibration samples and accuracy for high-resolution robust farm-scale soil mapping and addressed some of the most common pitfalls identified in previous research. We collected 910 samples from 458 locations at two depths (A, 0–0.20 m; B, 0.80–1.0 m) in the state of São Paulo, Brazil. All soil samples were analysed by conventional methods and scanned in the vis–NIR spectral range. With the vis–NIR spectra only, we inferred statistically the optimal set size and the best samples with which to calibrate vis–NIR models. The calibrated vis–NIR models were validated and used to predict soil properties for the rest of the samples. The prediction error of the spectroscopic model was propagated through the spatial analysis, in which robust block kriging was used to predict particle-size fractions and exchangeable calcium content for each depth. The results indicated that statistical selection of the calibration samples based on vis–NIR spectra considerably decreased the need for conventional chemical analysis for a given level of mapping accuracy. The methods tested in this research were developed and implemented using open-source software. All codes and data are provided for reproducible research purposes. Highlights: Vis–NIR spectroscopy enables an increase in sampling density with little additional cost. Guided selection of vis–NIR calibration samples reduced the need for conventional soil analysis. Error of spectroscopic model prediction was propagated by spatial analysis. Maps from the vis–NIR augmented dataset were almost as accurate as those from conventional soil analysis.

Meer zeehonden, minder vis in Waddenzee
Aarts, Geert - \ 2019
Feasibility of Unmanned Aerial Vehicle Optical Imagery for Early Detection and Severity Assessment of Late Blight in Potato
Franceschini, Marston Héracles Domingues ; Bartholomeus, Harm ; Apeldoorn, Dirk Frederik Van; Suomalainen, Juha ; Kooistra, Lammert - \ 2019
Remote Sensing 11 (2019)3. - ISSN 2072-4292 - 47 p.
Assessment of disease incidence and severity at farm scale or in agronomic trials is frequently performed based on visual crop inspection, which is a labor intensive task prone to errors associated with its subjectivity. Therefore, alternative methods to relate disease incidence and severity with changes in crop traits are of great interest. Optical imagery in the visible and near-infrared (Vis-NIR) can potentially be used to detect changes in crop traits caused by pathogen development. Also, cameras on-board of Unmanned Aerial Vehicles (UAVs) have flexible data collection capabilities allowing adjustments considering the trade-off between data throughput and its resolution. However, studies focusing on the use of UAV imagery to describe changes in crop traits related to disease infection are still lacking. More specifically, evaluation of late blight (Phytophthora infestans) incidence in potato concerning early discrimination of different disease severity levels has not been extensively reported. In this article, the description of spectral changes related to the development of potato late blight under low disease severity levels is performed using sub-decimeter UAV optical imagery. The main objective was to evaluate the sensitivity of the data acquired regarding early changes in crop traits related to disease incidence. For that, UAV images were acquired on four dates during the growing season (from 37 to 78 days after planting), before and after late blight was detected in the field. The spectral variability observed in each date was summarized using Simplex Volume Maximization (SiVM), and its relationship with experimental treatments (different crop systems) and disease severity levels (evaluated by visual assessment) was determined based on pixel-wise log-likelihood ratio (LLR) calculation. Using this analytical framework it was possible to identify considerable spectral changes related to late blight incidence in different treatments and also to disease severity level as low as between 2.5 and 5.0% of affected leaf area. Comparison of disease incidence and spectral information acquired using UAV (with 4–5 cm of spatial resolution) and ground-based imagery (with 0.1–0.2 cm of spatial resolution) indicate that UAV data allowed identification of patterns comparable to those described by ground-based images, despite some differences concerning the distribution of affected areas detected within the sampling units and an attenuation in the signal measured. Finally, although aggregated information at sampling unit level provided discriminative potential for higher levels of disease development, focusing on spectral information related to disease occurrence increased the discriminative potential of the data acquired.
Physiological response of temperature shocks in turbot and sole
Foss, Atle ; Imsland, Albert Kjartan Dagbjartarson ; Vis, Hans van de; Abbink, Wout ; Lambooij, Bert ; Roth, Bjørn - \ 2019
Journal of Applied Aquaculture 31 (2019). - ISSN 1045-4438 - p. 34 - 47.
Hypothermia - live chilling - sole - temperature - turbot

In the present study, selected temperature drops were examined to investigate the effects of live chilling on stress and welfare in turbot and sole. This study demonstrated that rapid temperature drops from 11°C–12°C and 18°C–18.7°C to 4°C–0°C in turbot resulted in higher blood sodium and potassium at 0°C (164 mmol l−1, 4.4 mmol −1) compared to 4°C (153 mmol l−1, 3.1 mmol −1), indicating osmoregulatory disturbance. A rapid temperature drop from 18°C to 0°C in Senegal sole also resulted in higher blood sodium and potassium at 0°C (164 mmol l−1, 4.8 mmol −1) compared to the control group at 18°C (157 mmol l−1, 3.2 mmol −1). Based on present findings, we conclude that immersion in ice water will have a negative effect on the animal osmoregulatory capacity, and we recommend that turbot and sole are stunned before slaughter.

Predicting soil microplastic concentration using vis-NIR spectroscopy
Corradini, Fabio ; Bartholomeus, Harm ; Huerta Lwanga, Esperanza ; Gertsen, Hennie ; Geissen, Violette - \ 2019
Science of the Total Environment 650 (2019). - ISSN 0048-9697 - p. 922 - 932.
Microplastics - Near-infrared spectroscopy - Soil pollution - Spectroradiometer - Vis-NIR

Microplastic accumulation in soil may have a detrimental impact on soil biota. The lack of standardized methods to identify and quantify microplastics in soils is an obstacle to research. Existing techniques are time-consuming and field data are seldom collected. To tackle the problem, we explored the possibilities of using a portable spectroradiometer working in the near infrared range (350–2500 nm) to rapidly assess microplastic concentrations in soils without extraction. Four sets of artificially polluted soil samples were prepared. Three sets had only one polymer polluting the soil (low-density polyethylene (LDPE), polyethylene terephthalate (PET), or polyvinyl chloride (PVC)). The fourth set contained random amounts of the three polymers (Mix). The concentrations of microplastics were regressed on the reflectance observed for each of the 2150 wavelengths registered by the instrument, using a Bayesian approach. For a measurement range between 1 and 100 g kg−1, results showed a root-mean-squared-deviation (RMSD) of 8, 18, and 10 g kg−1 for LDPE, PET, and PVC. The Mix treatment presented an RMSD of 8, 10, and 5 g kg−1 for LDPE, PET, and PVC. The repeatability of the proposed method was 0.2–8.4, 0.1–5.1, and 0.1–9.0 g kg−1 for LDPE, PET, and PVC, respectively. Overall, our results suggest that vis-NIR techniques are suitable to identify and quantify LDPE, PET, and PVC microplastics in soil samples, with a 10 g kg−1 accuracy and a detection limit ≈ 15 g kg−1. The method proposed is different than other approaches since it is faster because it avoids extraction steps and can directly quantify the amount of plastic in a sample. Nevertheless, it seems to be useful only for pollution hotspots.

Reisverslag kustsurvey EGSII : Juni 2018
Hal, Ralf van; Dijkman Dulkes, André - \ 2018
IJmuiden : Wageningen Marine Research (Wageningen Marine Research rapport C076.18) - 21
In de periode 18 tot 22 juni 2018 heeft Wageningen Marine Research in opdracht van Rijkswaterstaat een multidisciplinaire survey uitgevoerd in de vooroever bij Schiermonnikoog. De survey was opgezet om de verspreiding en abundantie van vis in de vooroever te bepalen en gegevens te verzamelen van (a)biotische factoren die deze verspreiding bepalen. Het doel is deze gegevens te gebruiken om mogelijke effecten van zandsuppleties in de vooroever op vis te voorspellen. De dit jaar uitgevoerde bemonsteringen zijn een aanvulling op de werkzaamheden uitgevoerd in 2017. In de vooroever van Schiermonnikoog waren zes raaien met zeven monsterlocaties van strand tot 10 meter diepte gepland. De geplande bemonsteringsactiviteiten op deze locaties waren voor vis: boomkor, zegen en akoestisch; voor benthos en sediment: steekbuizen, boxcore en 3d-stereocamera; voor zoöplankton: WP2-net; en voor omgevingsvariabelen: CTD, Secchi-schijf en multimeter. In de betreffende week is de uitvoering door met name de weersomstandigheden beperkt gebleven tot de diepste 4 stations op 5 van de 6 raaien. Er zijn twee ondiepe stations bemonsterd met de boomkor, maar dit was vanwege de weersomstandigheden geen succes. Dit alles is gedaan van maandag tot woensdag, de weersomstandigheden maakte bemonsteren op donderdag en vrijdag onmogelijk. Woensdag is er geprobeerd ook raai 6 te bemonsteren, maar tijdens een boomkortrek op deze raai kwam het tuig vast te zitten en kwam dit uiteindelijk zwaar beschadigd boven. Door de tijd die dit kostte is het niet gelukt de verdere bemonstering op raai 6 uit te voeren. Aanvullend is op 27 juni vanaf het strand van Schiermonnikoog een groot deel van de ondiepe stations bemonsterd. Daarmee is het grootste deel van het programma toch bemonsterd.
Longline visserij op schol : een pilot onderzoek naar effectiviteit van longlines voor de visserij op schol
Neitzel, Sophie ; Molenaar, Pieke - \ 2018
IJmuiden : Wageningen Marine Research (Wageningen Marine Research rapport C003/19) - 29
Dit onderzoek is uitgevoerd binnen het Kennisdesk Onderzoek in het kader van EZ-programma's. Omdat de visgronden voor de sleepnetvisserij in de toekomst beperkter worden door o.a. aanleg van nieuwe windmolenparken waar sleepnetvisserij verboden is, wil de visserij sector een economisch rendabel alternatief ontwikkelen waarmee de doelsoorten van de huidige Nederlandse sleepnetvisserij gevangen kunnen worden. Dit pilot project richt zich daarom op het ontwikkelen van een longlinevisserij op schol als een low impact vistuig als alternatief voor de bodemberoerende sleepnetvisserij. In deze studie is de vangst efficiëntie van verschillende longlines (haak- en lijn types) met een aantal aassoorten onderzocht Hiervoor is er voorafgaand aan de experimenten op zee een labtest uitgevoerd naar de geschiktheid van verschillende haaktypen voor het vangen van schol. Er is gekeken naar de maximale bekopening van verschillende lengteklassen schol, en welke type en grootte haak geschikt zijn voor deze lengteklassen. Vervolgens zijn verschillende lijnen en haaktypen geselecteerd die getest zijn tijdens een tweetal experimenten op zee. Tijdens de eerste reis zijn er tien schollen gevangen maar heeft de bemanning slechts één etmaal kunnen testen vanwege slechte weersomstandigheden. Deze resultaten zijn verder niet meegenomen in de analyses. De tweede reis, waarbij een waarnemer van WMR aanwezig was, is uitvoerig getest met dertien longlines en zeven aassoorten. Tijdens deze reis werden slechts twee schollen gevangen. Verschillende factoren kunnen mogelijke oorzaken zijn voor het niet vangen van schollen. Zo is bijvoorbeeld weinig bekend over het aasgedrag van schol. Om een antwoord te kunnen formuleren op de vraag of longlinevisserij op schol rendabel kan zijn, is het aan te bevelen om eerst onderzoek te doen naar het aasgedrag van schol voordat er nieuwe experimenten gedaan worden. Ook kan er in de toekomst worden gekeken naar het geschikt maken van de longlines voor het vangen van andere marktwaardige vis als zeebaars, kabeljauw of tarbot.
Beroepsvisserijenquête IJsselmeer en Markermeer : Praktijkkennis over de toestand van commerciële visbestanden in het jaar 2017 gebundeld
Rijn, J. van; Beier, U. ; Steenbergen, J. - \ 2018
IJmuiden : Wageningen Marine Research (Wageningen Marine Research rapport C107/18) - 91
Naast het formele advies over de biologische toestand van vier schubvissoorten, aal en spiering, vindt er sinds 2016 een enquête plaats onder beroepsvissers. Het doel van de zogenaamde beroepsvisserij enquête is het formeel inzichtelijk maken van het praktijkperspectief over de biologische toestand van belangrijke commerciële vissoorten op het IJsselmeer en Markermeer, te weten: aal, baars, snoekbaars, blankvoorn, brasem, Chinese wolhandkrab, bot en spiering. Daarnaast worden er algemene vragen over de visser, bijvangst per tuig en de bedrijfssituatie gesteld. In de enquête van 2017 zijn de respondenten gevraagd een onderscheid te maken tussen de biologische toestand van de visbestanden in het IJssel- en Markermeer. Met de feedback van drie vissers en de opdrachtgever is de enquête in december 2017 gefinaliseerd en verzonden naar vissers die volgens de logboekregistratie actief zijn geweest in 2017. Van de 56 verstuurde enquêtes zijn er 25 exemplaren teruggestuurd, hiermee is een responspercentage van 45% behaald. Van deze respondenten zijn er 24 die voltijd vissen. De voltijdvissers hebben gemiddeld 32 jaar ervaring en visten in 2017 gemiddeld 37 weken. De visserij op de acht soorten vindt verdeeld over alle gebieden plaats waarbij het zuidelijk deel van het Markermeer, IJmeer en de wateren rond Flevoland duidelijk minder vaak gebruikt worden voor visserij. Uit het aantal soorten dat er per respondent bevist wordt, lijkt er een verdeling te zijn tussen meer specialistische vissers die gericht op 2-3 soorten vissen en wat meer de generalisten die gericht op 5-7 soorten vissen. Op aal, snoekbaars en wolhandkrab vindt er een duidelijk gerichte visserij plaats. Baars en blankvoorn worden zowel gevangen in gerichte visserij als door bijvangst. Bij de vragen over de biologische toestand in 2017 vergeleken met 2016 viel op dat de respondenten voor de meeste soorten gematigd positief waren over de hoeveelheid maatse en ondermaatse individuen. Over aal en spiering is men duidelijk positief en over bot is men iets negatiever dan gemiddeld. Van brasem ziet men vooral kleine individuen en voor blankvoorn vooral grote, terwijl men voor de overige soorten alle maten individuen ziet. Wat opvalt is dat men voor aal, in beide meren, een lagere natuurlijke sterfte ziet ten opzichte van 2016 en voor het Markermeer bij baars en snoekbaars juist een hogere natuurlijke sterfte dan in 2016. Bij de voorspelling van de hoeveelheid volwassen vis in 2018 t.o.v. 2017 valt het op dat men erg optimistisch is over aal, spiering, baars (in het IJsselmeer) en Chinese wolhandkrab (in het Markermeer). Zonder de vergelijking te maken met 2017 was de respondenten hun voorspelling over de marktwaardige individuen overwegend optimistisch met uitzondering van bot, brasem en snoekbaars in het IJsselmeer. De niet-marktwaardige bijvangst lijkt overwegend lager te zijn dan vorig jaar voor de twee soorten fuiken en onveranderd voor de staand want en hoekwant visserij. Over de bedrijfssituatie is men erg optimistisch. Er bleek volgens de respondenten geen duidelijk onderscheid in de biologische toestand van de bestanden tussen de twee meren. In het vervolg is dit onderscheid duidelijker te bevragen door een open vraag per soort te stellen in plaats van de huidige dubbele vraagstelling. Om de kwaliteit van de resultaten te waarborgen is het aan te raden vooraf een verkorte proef enquête te laten invullen door een paar respondenten en hier feedback over te vragen. Het is aanbevolen om de resultaten van de enquête ook dit jaar in een focusgroep discussie te bespreken.
Jurjen Vis, Diaconie. Vijf eeuwen armenzorg in Den Haag
Teeuwen, N.M. - \ 2018
Bijdragen en Mededelingen betreffende de geschiedenis der Nederlanden = BMGN : Low Countries Historical Review 133 (2018)2. - ISSN 0165-0505 - 2 p.
Drone brengt habitat jonge vis in Uiterwaarden in kaart
Stoffers, Twan - \ 2018
Drone brengt habitat jonge vis in uiterwaarden in kaart
Stoffers, Twan - \ 2018
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