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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Cod monitoring : results 2016, quarter 1
Trapman, B.K. ; Machiels, M.A.M. - \ 2016
IJmuiden : Wageningen Marine Research (Wageningen Marine Research rapport C105/16) - 19
cod - fisheries - monitoring - fish catches - north sea - dutch waters - kabeljauw - visserij - visvangsten - noordzee - nederlandse wateren
Monitoring cod catches of the Dutch demersal fleet in 2015
Trapman, B.K. ; Machiels, M.A.M. - \ 2016
IJmuiden : Wageningen Marine Research (Wageningen Marine Research rapport C106/16) - 30
cod - fisheries - fish catches - monitoring - demersal fisheries - netherlands - kabeljauw - visserij - visvangsten - demersale visserij - nederland
Wettelijke Onderzoek Taken WOT-05 Visserijonderzoek : werkafspraken en werkplan 2017
Verver, S.W. - \ 2016
IJmuiden : Stichting Wageningen Research, Centrum voor Visserijonderzoek (CVO) (CVO rapport 16.009) - 103
visserij - onderzoek - wetgeving - planning - nederland - fisheries - research - legislation - netherlands
Dit rapport beschrijft het werkplan voor 2017 voor het Wettelijke taken programma Visserij onderzoek van Wageningen UR. Wageningen UR voert voor het Ministerie van Economische Zaken (EZ) een aantal programma’s met Wettelijke Onderzoek Taken (WOT) uit. Binnen programma WOT- 05 worden Wettelijke Onderzoek Taken uitgevoerd die betrekking hebben op het beheer van de visserij op zee, in Nederlandse kust- en binnenwateren en de aquacultuur. Het werkplan is een uitwerking van de Uitvoeringsovereenkomst en werkafspraken tussen het Ministerie van EZ en de Stichting Wageningen Research, onderdeel van Wageningen UR, voor diensten vanwege wettelijke taken op het terrein van visserijonderzoek voor de periode 2011-2015. In september 2015 is de Uitvoeringsovereenkomst voor WOT-05 met 1 jaar verlengd tot 31 december 2016. Voor 2017 wordt deze Uitvoeringsovereenkomst verlengd of vernieuwd. De Uitvoeringsovereenkomst en de bijbehorende werkafspraken vormen de basis voor de jaarlijkse werkplannen. Daarnaast zijn in deze overeenkomst afspraken gemaakt over de KennisBasis (KB-WOT) die specifiek aan dit programma is gekoppeld. KB-WOT is bedoeld voor het in stand houden en ontwikkelen van de specifieke kennisinfrastructuur die nodig is om het programma te kunnen uitvoeren. Voor de inzet van KB-WOT visserijonderzoek wordt jaarlijks een afzonderlijk werkplan gemaakt. In brede zin heeft de inhoud van het programma betrekking op de advisering van het visserijbeleid en het verzamelen van gegevens die daarvoor nodig zijn. Tevens wordt via de internationale organisaties meegewerkt aan de advisering voor het beheer van de visbestanden in internationale wateren.
Fishing over the sides or over the stern: does it matter : comparison of two fishing methodologies in the Wadden Sea Demersal Fish Survey
Chen, C. ; Bolle, L.J. ; Boois I.J. de, Ingeborg - \ 2016
IJmuiden : Wageningen Marine Research (Wageningen Marine Research report C122/16) - 27
fisheries - fishing methods - fishing gear - demersal fisheries - wadden sea - visserij - vismethoden - vistuig - demersale visserij - waddenzee
Since 1972, the Demersal Fish Survey (DFS) in the Wadden Sea has been carried out with the RV Stern. Within a few years this vessel will be replaced by another vessel as a result of the current ship replacement policy of Rijkswaterstaat Rijksrederij. It is not yet clear which vessel will replace RV Stern. In the search for a new vessel the main question is if fishing over the sides is needed, or if fishing over the stern is possible without a major effect on catch efficiency. Especially in shallow waters, catch efficiency may be affected by fishing over the stern, because of the current created by the ship’s propeller. To be able to continue the use of the long and valuable time-series the shift in vessel should not lead to a different gear efficiency.
Vangstsamenstelling per tuigcategorie : herziening contingentenstelsel visserij in Nederland in het kader van de aanlandplicht
Helmond, A.T.M. ; Steins, N.A. - \ 2016
IJmuiden : IMARES Wageningen UR (IMARES rapport C107/16) - 62
vangstsamenstelling - visvangsten - visserij - quota - vistuig - nederland - catch composition - fish catches - fisheries - quotas - fishing gear - netherlands
In het nieuwe Gemeenschappelijke Visserijbeleid is vastgelegd dat vissers uiterlijk eind 2019 verplicht zijn de volledige vangst van alle soorten waarvoor vangstquota gelden, mee te nemen naar de wal (aanlandplicht). Tot nu moesten vissers soorten waarvoor ze geen vangstrechten hadden of niet marktwaardig waren, juist terug zetten (discarden). Dit betekent dat het huidige Nederlandse co-management systeem voor quotabeheer moet worden afgestemd op de aanlandplicht. In Nederland wordt het Europese quotabeleid sinds eind jaren ’70 ingevuld via een stelsel van individuele contingenten. Een contingent is een (overdraagbaar) aandeel dat een visserijbedrijf heeft in het nationale quotum voor die soort. Op dit moment zijn er contingenten voor schol, tong, kabeljauw, wijting, haring, makreel, horsmakreel, blauwe wijting en grote zilversmelt. Vissers die geen of onvoldoende contingenten voor een soort hebben, mogen deze niet aan boord houden of aanlanden. De nieuwe aanlandplicht verplicht deze groep vissers echter wel tot aanlanden. Het Ministerie van Economische Zaken (EZ) bereidt daarom een herziening van het contingentenstelsel voor. Een van de maatregelen die wordt verkend is een uitvaarverbod. Dit houdt in een visser niet met een specifiek vistuig mag uitvaren als hij geen of onvoldoende contingent heeft voor de soorten die naar verwachting een substantieel onderdeel uitmaken van de vangsten met dat betreffende vistuig.
Fleet dynamics in a changing policy environment
Batsleer, J. - \ 2016
Wageningen University. Promotor(en): Adriaan Rijnsdorp, co-promotor(en): Jan Jaap Poos. - Wageningen : Wageningen University - ISBN 9789462579521 - 171
fleet dynamics - fisheries - fishery policy - fishery management - sustainability - pleuronectiformes - vlootdynamica - visserij - visserijbeleid - visserijbeheer - duurzaamheid (sustainability)

The European Common Fisheries Policy has received much criticism. In the first place for failing to implement effective management measures aimed at rebuilding and maintaining fish stocks at a sustainable level. In addition, it is said current fisheries policy fails to integrate the wider ecosystem effects of fishing into policy making, such as reducing pressure on non-target species and habitats. Especially discarding (i.e. throwing back unwanted catches at sea) in commercial fisheries has received an increased amount of negative attention. Public campaigns stressed discarding is a wasteful and disruptive practice as natural resources are extracted from the ecosystem only to be thrown back to sea dead. In response, the European Commission agreed to enforce a discard ban for European fisheries, obliging all European fishing vessels to retain and land their entire catch, including small sized individuals, of all species subject to quota management.

This thesis explores how management measures can mitigate the adverse effects of fishing to support the development of an ecosystem-based approach to fisheries management. I evaluate how fleet dynamics, i.e. the decision of individual skippers on when and where to fish, while competing for similar resources, may influence the performance of management measures. In particular the thesis focusses on how location choice and discard decisions made by individual fishers in a mixed fishery are influenced by management, resource distribution and technological innovations. Using a generic dynamic state variable model (DSVM), specific applications are built to address the behavioural dynamics for the Dutch North Sea flatfish fisheries and the French mixed demersal fisheries in the Channel in response to new management regulations and economic opportunities.

The first research chapter of this thesis (Chapter 2) explores how a combination of quota management with a discard ban may improve the regulation of fishing mortality for a depleted stock that is exploited in a mixed fishery. Our finding show that under a discard ban, when properly enforced, individual fishers reallocate fishing effort away from areas and weeks with high catch rates of the quota constrained species, reducing over-quota discarding and thus contributes to the conservation of vulnerable species. However, discard reduction measures which coincide with a reduction in the economic performance of the fishery may jeopardise compliance as fishers may trade-off economic gains of non-compliance against the costs.

Fishers will discard marketable fish when quota are exhausted (over-quota discarding) or by trying to optimize their economic return by discarding size or age classes with the lowest economic value (high-grading). In chapter 3 observations of over-quota discarding and high-grading are reviewed to gain insight in the conditions under which discard decisions may occur. The review suggest that high-grading occurs under different management systems for a large variety of fisheries worldwide. In addition, outcomes of the simulation model illustrate fishers have the ability to strategically plan their fishing activities to optimize the composition of their catch taking account of the availability of quota and seasonal price variations. As a result, the size composition of the high-graded catch differs from the landed catch. Difficulties in accounting for this difference may undermine the accuracy of the stock assessments and quality of scientific advice.

Chapter 4 addresses the question how a ban on discarding may promote the transition towards more selective fishing gears. Model results suggest that under a discard ban, fishing activities are restricted and reallocated away from areas and weeks of high catches of small fish constrained by quota. Activities are allocated to areas and weeks where a maximum revenue can be realised landing other species and economically more valuable length classes of the quota restricted species. When using more selective fishing gear, fishing activities will be less restricted. Fewer small fish are landed which would otherwise be counted against the quota, fetching a low price and reduce the economic value of the landings. Hence, there is an economic advantage and fishers can continue fishing for a longer period of time, including areas where a higher density of small fish as well as more economically valuable fish co-occur. As such, if properly enforced, a discard ban can incentivise the use of more selective gear to reduce the catch of undersized fish.

Trawl fisheries targeting demersal fish and shellfish cause mortality on target and non-target species, but impact benthic ecosystems. Fisheries managers mainly resort to technical management measures, such as gear restrictions or spatial measures to mitigate these impacts. Chapter 5 explores the potential of a habitat credit system as an alternative management approach to achieve sustainable exploitation of target species while minimizing the impact on the benthic ecosystem. Results show that a habitat credit system has the potential to reduce the benthic impact and maintain profitable fisheries as vessels can adjust their behaviour by reallocating fishing activities to make optimal use of the available credits.

Fishers can adapt and change their behaviour in relation to imposed constraints, which can lead to unintended and unexpected consequences of fisheries management. To date, the effect of behavioural adaptations of individual fishers on the success of fisheries management is often overlooked by policymakers. The models developed in this thesis provide a strong basis to explore possible unexpected effects of management measures resulting from the adaptive behaviour of fishers to these measures. Outcomes of this thesis draw attention to the importance of making fleet dynamics an integral part of fisheries management and the need to develop innovative analytical methodologies which deliver sufficiently robust insights into complex socioeconomic and ecosystem issues to improve the basis of decision making.

Fishing intensity around the BBL pipeline
Hintzen, Niels - \ 2016
IJmuiden : Wageningen Marine Research (Wageningen Marine Research rapport C102/16) - 27
visserij - vismethoden - pijpleidingen - bescherming - noordzee - fisheries - fishing methods - pipelines - protection - north sea
Wageningen Marine Research was requested by ACRB B.V. to investigate the fishing activities around the BBL pipeline. This gas pipeline crosses the southern North Sea from Balgzand (near Den Helder) in the Netherlands to Bacton in the UK (230km). This pipeline is abbreviated as the BBL pipeline. Part of the activities deployed by the owner of the BBL pipeline is to secure the integrity of the pipeline, which includes checking burial status, detecting free-spans and investigating internal and external threats to the integrity of the pipeline. Fishing is considered as one of the external threats to the pipeline where a collision with fishing gear could damage the pipeline, the fishing gear, the vessel or the crew. Therefore in areas with substantial fishing activity, extra care should be taken. Such a risk inventory becomes more and more common day practice of submarine pipeline and cable owners where discussions now focus on how to best spend effort on protecting pipelines and where to relieve specific burial requirements.
Improving the knowledge basis for advice on North Sea horse mackerel : developing new methods to get insight on stock boundaries and abundance
Brunel, Thomas ; Farrell, Edward D. ; Kotterman, Michiel ; Kwadijk, Christiaan ; Verkempynck, Ruben ; Chen, Chun ; Miller, David - \ 2016
IJmuiden : Wageningen Marine Research (Wageningen Marine Research report C092/16) - 57
trachurus trachurus - fish stocks - fisheries - models - visstand - visserij - modellen
The North Sea horse mackerel stock is currently classified by ICES as a data poor stock, for which the catch advice is based on the trend in an abundance index. The development of an analytical stock assessment, necessary to give more accurate advice, is hampered by a number of limitations on the input data, among which the most important are the poor quality of catch-at-age data and the absence of a targeted survey for North Sea horse mackerel. The aim of this project was to study possibilities to improve the data quality used for an analytical stock assessment model.
Waardekaarten Markham’s Triangle & Hornsea 3
Machiels, Marcel - \ 2016
Wageningen : Wageningen Marine Research (Wageningen Marine Research rapport C085/16) - 15
visserij - waarden - tabellen - gegevensanalyse - fisheries - values - tables - data analysis
Netinnovatie Kottervisserij
Marlen, B. van; Molenaar, P. ; Bol, R.A. ; Dammers, M. ; Groeneveld, K. ; Machiels, M.A.M. ; Heijer, W.M. den - \ 2016
IMARES (Rapport / IMARES C110/15) - 130 p.
visserij - boomkorvisserij - bijvangst - vangstsamenstelling - fisheries - beam trawling - bycatch - catch composition
Als reactie op de aanlandplicht heeft de kottersector via de Coöperatieve Visserij Organisatie (CVO) het initiatief opgepakt om de selectiviteit van de vistuigen te verhogen om zo weinig mogelijk discards te vangen en aan te landen. Na een ontwerpfase met modelonderzoek in de flume tank van SINTEF te Hirtshals, Denemarken, en met ervaringen uit eerdere projecten werden op verscheidene schepen netinnovaties uitgeprobeerd. Er werd onderzoek gedaan op een schip met boomkor, verschillende schepen vissend met pulsvistuigen en op twinriggers. Hierbij werd aanvankelijk gewerkt op ‘trial-and error’ basis met in sommige gevallen zelfmonitoring. Vervolgens werden enkele uitgebreide vangst- en bijvangstvergelijkingen gedaan met medewerkers van IMARES aan boord.
Costs of seabed protection on the Frisian Front and Central Oyster Grounds for the Dutch fishing sector : addendum to LEI report 2015-145
Oostenbrugge, J.A.E. van; Turenhout, M.N.J. ; Hamon, K.G. - \ 2016
Wageningen : LEI Wageningen UR (Memorandum / LEI Wageningen UR 2016-062) - 29 p.
cost benefit analysis - fisheries - aquatic ecosystems - protection - benthos - ecosystems - oyster culture - north sea - kosten-batenanalyse - visserij - aquatische ecosystemen - bescherming - ecosystemen - oesterteelt - noordzee
This memorandum provides an estimation of the costs for four variant closures for the protection of the benthic ecosystem on the Frisian Front and the Central Oyster Grounds for the Dutch fishing sector in addition to the cost-benefit analysis carried out in Van Oostenbrugge et al. (2015). The two preferential variants lead to similar costs for the fisheries sector, whereas the costs of the two alternative combinations are either 20% higher or lower.
Omvang en overleving van schubvis bijvangst in fuikenvisserij nabij kunstwerken
Griffioen, A.B. ; Keeken, O.A. van; Chen, C. ; Blom, E. ; Schram, E. ; Graaf, M. de; Winter, Hendrik V. - \ 2016
IMARES (Rapport / IMARES C140/15) - 53 p.
visserij - bijvangst - vissen - schade - dierenwelzijn - mortaliteit - fisheries - bycatch - fishes - damage - animal welfare - mortality
Deze rapportage geeft een schatting van de omvang van de bijvangst in de beroepsvisserij nabij kunstwerken op basis van fuikvangsten en diverse interviews. Daarnaast is er een experiment uitgevoerd waarbij er gekeken is naar de overleving van schubvis nadat zij in fuiken hebben gezeten. Hierbij zijn de baars en blankvoorn gebruikt om de overleving te testen in relatie tot de volgende variabelen: aanwezigheid aal: geen (0 stuks), weinig aal (7 stuks) en veel aal (50 stuks), staduur van de fuik: 3, 6 of 9 dagen en dichtheid van vis in een fuik: 60 stuks tegenover 240 stuks.
5 years of Industry survey: Does the industry survey improve current stock assessments for plaice and sole?
Reijden, K.J. van der; Poos, J.J. ; Trapman, B.K. ; Verkempynck, R. - \ 2016
IMARES (Rapport / IMARES C039/16) - 31 p.
plaice - dover soles - fish stocks - fishing gear - fisheries - fish industry - schol - tong (vis) - visstand - vistuig - visserij - visverwerkende industrie
This report describes the results of the industry survey; an annual survey targeting sole and plaice on-board of the commercial fishing vessels UK45 and OD1. The survey was set up following the wish of the fishing industry to deliver data for stock assessments themselves, which are collected using commercially representative fishing gears. This survey was expected to make stock assessment results more accurate and to increase trust of the fisheries industry in stock assessments.
Wettelijke Onderzoek Taken WOT-05 Visserijonderzoek : jaarverslag 2015
Verver, S.W. - \ 2016
IJmuiden : Centrum voor Visserijonderzoek (CVO rapport 16.003) - 97 p.
visserij - onderzoek - wetgeving - jaarverslagen - nederland - fisheries - research - legislation - annual reports - netherlands
Wettelijke Onderzoek Taken WOT-05 Visserijonderzoek : beknopte jaarrapportage 2015
Verver, S.W. - \ 2016
IJmuiden : Centrum voor Visserijonderzoek (CVO rapport 16.004) - 34 p.
visserij - onderzoek - wetgeving - jaarverslagen - nederland - fisheries - research - legislation - annual reports - netherlands
Vermindering discards door netinnovatie in de Noorse kreeft visserij
Molenaar, P. ; Steenbergen, J. ; Glorius, S.T. ; Dammers, M. - \ 2016
IJmuiden : IMARES (Rapport / IMARES C027/16) - 119 p.
discards - visserij - nephrops norvegicus - innovaties - vistuig - fisheries - innovations - fishing gear
Dit rapport beschrijft de testen die zijn uitgevoerd binnen het project ‘Sectorale en Ketenintegrale aanpak Langoustines’ in de periode april 2014 – december 2015 dat was gericht op het verminderen van discards binnen de visserij op Noorse kreeft (Nephrops norvegicus). Het project is een initiatief van de Coöperatieve Visserij Organisatie (CVO) en maakte onderdeel uit van breder programma waarin de sector zich voorbereid op de aanlandplicht. De aanlandplicht, als onderdeel van het nieuwe gemeenschappelijke visserijbeleid, is sinds 2015 van kracht en houdt in dat maatse en ondermaatse vissen en schaaldieren van soorten waarvoor vangstbeperkingen gelden niet meer in zee terug mogen worden gezet. Discards in de visserij op Noorse kreeft bestaan o.a. uit kleine Noorse kreeft, schar (Limanda limanda) en ondermaatse schol (Pleuronectes platessa).
VIP Praktijknetwerk Netinnovatie Zuid
Marlen, B. van; Rink, G.J. ; Vandenberghe, C. - \ 2016
IJmuiden : IMARES (Rapport / IMARES C147/14) - 77 p.
visserij - duurzaamheid (sustainability) - pleuronectiformes - netten - noordzee - fisheries - sustainability - nets - north sea
Het VIP project Praktijknetwerk Netinnovatie Zuid beoogde een bijdrage te leveren aan een meer duurzame kottervisserij op platvis op zuidelijke bestekken in de Noordzee door het ontwikkelen van nieuwe netten met een lager brandstofverbruik, en minder netslijtage, bijvangsten en discards.
Verkenning doorvaren passieve vistuigen
Jak, R.G. - \ 2016
IMARES (Rapport / IMARES C014/16) - 20 p.
vistuig - visserij - vuilnis - vismethoden - verontreiniging - nederland - fishing gear - fisheries - refuse - fishing methods - pollution - netherlands
In deze studie is op basis van gesprekken met Nederlandse staandwantvissers en sleepnetvissers verkend in hoeverre het doorvaren van staandwantnetten kan leiden tot het ontstaan van zwerfvuil. Volgens de Nederlandse staandwantvissers zijn de problemen uit voorgaande decennia voorbij door de goede communicatie tussen staandwantvisssers enerzijds en sleepnetvissers anderzijds. Netten worden tegenwoordig nog doorvaren indien niet goed bekend is dat bepaalde activiteiten, zoals baggeren, gaan plaatsvinden. Doorvaren netten worden door de staandwantvissers zoveel mogelijk weer verzameld en eindigen dan niet als zwerfvuil. Door sleepnetvissers wordt aangegeven dat netten van Deense staandwantvissers regelmatig doorvaren worden, vooral gedurende de zomerperiode en binnen de 12-mijlzone. Deze netten worden losgesneden en dan òf als afval meegenomen naar wal, òf achtergelaten indien de netten zijn verankerd. Het is dan onduidelijk of de stukgesneden netten achterblijven als zwerfvuil. Op basis van de bevindingen wordt aanbevolen een betere communicatie te faciliteren tussen staandwantvissers en andere gebruikers van de kustzone.
Economic essays on marine invasive species and international fisheries agreements
Walker, A.N. - \ 2016
Wageningen University. Promotor(en): Ekko van Ierland, co-promotor(en): Rolf Groeneveld; Hans-Peter Weikard. - Wageningen : Wageningen University - ISBN 9789462576308 - 165 p.
fisheries - fisheries ecology - agreements - biodiversity - invasive species - europe - marine fisheries - marine fishes - visserij - visserij-ecologie - overeenkomsten - biodiversiteit - invasieve soorten - europa - zeevisserij - zeevissen

This thesis is divided into two parts, as explained in Chapter 1, which focus on different aspects of marine ecological change. Part A considers marine Invasive Alien Species (IAS), which are taxa introduced outside of their native range. The detrimental consequences of invasions for human welfare necessitate management of IAS. There are two types of IAS management. These are (i) management of the risks that an invasion will become established, termed “prevention”, and (ii) management of already established invasions, termed “control”. Chapter 2 considers prevention of invasive species with Ballast Water Management (BWM). Vessels transport invasive species in their ballast water. BWM involves treating ballast water to reduce the risk of successful invasion establishment. Chapter 2 studies the determinants of optimal ballast water treatment standards from a theoretical perspective. Chapter 3 considers control of already established invasions from a spatial and dynamic perspective. We model a non-native habitat divided into patches, where each patch may contain a population of the invasive species, and where spread of the invasion between patches is a stochastic process. In this context, we derive optimal management policies.

The second part of this thesis: Part B, considers International Fisheries Agreements (IFAs). IFAs facilitate cooperation in the management of fish stocks. Cooperation is necessary to ensure sustainable management. Part B focuses on two issues which may affect the stability of cooperation within IFAs. These are; in Chapter 4, changes in stock location, which may occur due to climate change, and in Chapter 5, the risk of stock collapse, which may exist due to overfishing. Part B uses game theory to analyse the effects of these two issues on the stability of the Grand Coalition, which is the state of affairs where all parties cooperate to maximize their joint benefit from the fish stock.

The methods and findings of the thesis are summarized as follows: in Chapter 2 (Part A), we construct a model to study optimal BWM standards. The model is built around the assumption that invasions arriving via ballast water are irreversible, i.e. once an invasion has arrived, it is not possible to reduce the size of the invasive population to zero. The hazard rate of invasion establishment can be reduced by setting a BWM standard. The hazard rate is also affected by the Minimum Viable Population (MVPs) of the species and the possibility of an Allee effect. An MVP exists if there is some population size below which there is an insufficient number of invasive individuals to sustain a population. An Allee effect exists if the probability that a population survives increases at an increasing rate in the size of the population. Our analysis focuses on the conditions under which a BWM standard which aims to reduce invasive populations in ballast water to below their MVPs (as is aimed for by the BWM convention) can be optimal. We find that the current aim of the BWM convention can only be optimal in the case that the hazard function (which determines the hazard rate) is not continuously differentiable around the MVP. We find that Allee effects are a requirement for a continuously differentiable hazard function. Therefore, we find that whether or not an Allee effect exists fundamentally affects whether it is optimal to aim to reduce an invasive population in ballast water to marginally below its MVP.

In Chapter 3 (Part A), we combine aspects of previous modelling approaches to provide new generalized management insights for controlling established invasions. We employ a metapopulation network consisting of patches which are arranged one-dimensionally (i.e. in a line), which is relevant, among other cases, for invasive species spreading along coastlines. We allow for the population size of the invasion within patches to be reduced, which we term “removal”, and we allow for the probability of spread between patches to be reduced without affecting the population sizes directly, which we term “containment”. We employ numerical stochastic dynamic programming to explore how these two interventions (removal and containment) can be optimally applied to minimize the sum of damages from the invasion and the costs of removing and containing the invasion. We find that allowing for varying stock sizes within patches facilitates optimal timing of the application of containment. We also identify two novel optimal policies: the combination of containment and removal to stop spread between patches and the application of up to four distinct policies for a single patch depending on the size of the invasion in that patch.

Chapter 4 (Part B) considers how Grand Coalitions can be stabilized in the face of changing stock location. To do so, we employ the Gordon-Schaefer fisheries model. We consider farsightedness as a mechanism by which stability of the Grand Coalition can be increased in the face of changing stock location. Farsightedness allows players to respond to deviations of other players by deviating themselves. This reduces the incentives to leave the Grand Coalition. This is in contrast to shortsightedness, whereby players cannot decide to leave the Grand Coalition in response to such a choice by another player. We begin by modifying the farsightedness concept such that it can be used in games with asymmetric players and transfer payments. We proceed to analyse the modified farsightedness concept in the case where players are symmetric (stock location does not change) in order to identify the properties of the concept in the base case. We find that farsightedness increases Grand Coalition stability with respect to shortsightedness. We proceed to analyse the extent to which farsightedness increases Grand Coalition stability, relative to shortsightedness, as fish stock location changes, using sensitivity analysis. We find that farsightedness increases the stability of the Grand Coalition, but also increases the sensitivity of stability to changes in fish stock location. Thus, for any fish stock location, a Grand Coalition is more likely to be stable if players are farsighted, but shifts between a stable and an unstable Grand Coalition will occur more frequently if players are farsighted.

In Chapter 5 (Part B), we analyse how the stability of Grand Coalitions is affected by an endogenously determined risk of stock collapse. We do so using the Levhari and Mirman (LM) fisheries model, which is adapted such that there is a risk of stock collapse which increases as the fish stock size decreases. We numerically solve the model and calculate the stability of the Grand Coalition. We find that the effect of an endogenously determined risk of stock collapse depends heavily on the assumptions made regarding how payoffs are determined. A common assumption in the literature is that payoffs are determined at the steady state fish stock. Under this assumption, endogenous risk means that for specific discount and growth rates, a Grand Coalition is stable for any number of players. This is a very different result from the original LM model whereby Grand Coalitions can never be sustained. This is because players can essentially follow two strategies in response to the risk. Firstly, they can attempt to maintain the fish stock by fishing less. In doing so they are running the risk of collapse. Secondly, they can avoid the risk by pre-emptively depleting the fish stock, i.e. harvesting the stock to zero immediately to avoid the risk. Grand Coalitions of any number of players are stable for parameterizations for which a Grand Coalition attempts to maintain a non-zero fish stock and if a deviation from the Grand Coalition would result in pre-emptive depletion. We proceed by relaxing the assumption that payoffs are determined in the steady state by allowing for deviators to obtain payoffs in the transition between steady states. In this case, only Grand Coalitions of two players are stable, and then only for certain parameterizations. The reason is that players can now gain payoffs in the process of pre-emptively depleting the stock, i.e. payoffs are received from the process of fishing the stock down to zero. This increases the benefit of deviating from the Grand Coalition. In this case, Grand Coalitions are only stable for two players for specific parameterizations.

Chapter 6 summarises the research questions formulated in Chapter 1 and evaluates the work of the thesis. Regarding Chapter 2, we justify our theoretical approach with the following two points. Firstly, BWM management is a global and complex problem, which means that the information required to formally calculate an optimal standard is prohibitively burdensome. Secondly, we argue that the complexity of BWM necessitates a sound theoretical understanding of the problem in order to evaluate the current BWM standard, and also to aid in future policy formulation. Similarly, in Chapter 3, we focus on deriving generalized management insights which are applicable to a variety of real-world cases, as opposed to deriving an optimal management strategy for a specific case. In addition to the data requirements necessary to derive such a management strategy, the complexity of such applied cases leads to potentially excessive computational burden. Chapter 3 analyses systems of two and three patches, which are likely to be too simple to analyse specific real world cases, but are sufficient to derive generalized management insights.

The game theoretic methodologies in Part B are evaluated principally in terms of the assumptions about changes in stock location in Chapter 4 and the numerical method in Chapter 5. In Chapter 4, the fish stock is conceptualised as existing at a single point in space. The location of this point is determined in relation to fishing nations, which are also conceptualised as single points in space. Changes in stock location result from rises in ocean temperatures due to climate change. Such rises in temperature are likely to lead to other changes in the fish stock such as the size of the area where the fish stock can be found and increases in the maximum fish stock size which the ecosystem can support. These other aspects of changing stock location need to be considered in evaluating Chapter 4, as well as in formulating more applied models. In Chapter 5, a numerical method is adopted to analyse the effects of an endogenous risk of stock collapse. To do so, the utility function in the LM model is adapted such that it can be used in a numerical model. In order to isolate the effect of endogenous risk from changes in the utility function, a validation procedure is carried out by comparing analytically derived results in the deterministic case (without endogenous risk of stock collapse) to numerically derived results in the deterministic case. This reveals that changes to the utility function have a negligible effect and thus the results, in terms of the stability of Grand Coalitions can be attributed solely to endogenous risk of stock collapse.

Overall, Part A of this thesis presents new insights into the determinants of optimal BWM standards. These insights demonstrate the conditions under which the current BWM standard, which aims to eliminate the risk of invasion establishment, may or may not be optimal. Part A therefore provides a novel theoretical framework which aids in the evaluation of current, and the determination of future standards. Part A also provides new insights into the control of established invasions, by extending existing spatially explicit optimal control models. Specifically, dividing space into patches and allowing for varying invasive population sizes within patches facilitates the optimal timing of management interventions and, in general, more detailed, and thus more efficient, management strategies. Part B provides a novel analysis of the effects of changing stock location on Grand Coalitions by explicitly introducing fish stock location in the analysis, and shows how farsightedness can stabilize Grand Coalitions in the face of such changes. Part B also shows how the effects of an endogenous risk of stock collapse on the stability of Grand Coalitions depends vitally on whether transition payoffs are included. These results can form the basis for more interdisciplinary analyses, analyses of different types of marine ecological change, and analyses of these changes in different settings, such as non-European countries.

Minimum aanlandingsmaat Brasem (Abramis brama)
Hal, R. van; Miller, D.C.M. - \ 2016
IJmuiden : IMARES (Rapport / IMARES C148/15a) - 17
abramis brama - grootte - visserij - lengte - ijsselmeer - size - fisheries - length - lake ijssel
Ter ondersteuning van een besluit aangaande een minimum aanlandingsmaat voor brasem, primair voor het IJsselmeer en Markermeer, heeft het ministerie van Economische Zaken IMARES verzocht een overzicht te geven van aanlandingsmaten voor brasem in andere landen en waar mogelijk de motivatie achter deze maten te geven. Er blijken in verschillende, voornamelijk oost, Europese landen minimum aanlandingsmaten te zijn ingesteld. Het is echter niet altijd duidelijk of deze voor de commerciële en/of recreatieve visserij gelden. De gegeven maten zijn niet eenduidig, in een deel van de geval gelden de maten voor de gehele vis (puntje neus tot eind staart) voor een ander deel gelden ze voor een kleiner deel van de vis (puntje neus tot begin staart). Het was voor ons niet altijd duidelijk welke meetmethode gebruikt wordt. De gevonden minimum aanlandingsmaten variëren tussen 20 en 45 cm. Er zijn helaas geen gegevens gevonden voor de onderbouwing van deze maten. Een mogelijke methode om een minimum aanlandingsmaat vast te stellen is gebruik te maken van gegevens die per lengte aangegeven welk deel van de populatie volwassen is. Op basis van deze gegevens kan een volwassenheid ogive worden bepaald. Deze was beschikbaar gebaseerd op IMARES survey en afslag gegevens van het IJsselmeer. Met behulp van deze ogive is de lengte waarop 50, 70 en 100% van de populatie brasem volwassen is bepaald. Bij de doelstelling “het zorgdragen van voortplanting” zijn dit maten die indicatief zou kunnen zijn voor een eventuele minimum aanlandingsmaat voor het IJsselmeer en Markermeer.
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