DELTAKENNIS - carrying capacity modelling : preliminary report

Luca van Duren and Tineke Troost

Prepared for: RWS Waterdienst

DELTAKENNIS - Carrying

Capacity modelling

Workshop on shellfish modelling

Luca van Duren and Tineke Troost

Report

Title DELTAKENNIS - Carrying Capacity modelling

Abstract

Within the framework of the project “DELTAKENNIS”, DELTARES aims to develop a modelling system to assess carrying capacity for shellfish in Dutch Coastal waters. The models should not only take into account the external forcings on a system such as nutrient loading, light availability and availability of suitable substrates, but should also incorporate biological feedback mechanisms influencing carrying capacity. A prerequisite for such a model system is a shellfish module, suitable for various shellfish

species. There are a number of different types of shellfish models available that can serve as a basis. It is important to choose the right one for further development.

On the 25th of June 2008 a workshop was organised at DELTARES to discuss the various options and decide on the best way to proceed. This report reflects the discussions and the conclusions of that workshop.

References

Ver Author Date Remarks Review Approved by

Luca van Duren 04-07-2008

Project number Z4574.30

Keywords DELTAKENNIS, carrying capacity, shellfish, ecological modelling

Contents

2.3.1 Age / size / individually structured models...4

2.3.2 Model dimensions ...5

2.4 Net production vs. DEB modelling...5

2.5 Additional processes...5

2.5.1 Habitat suitability ...5

2.5.2 Motility ...5

2.5.3 Faeces – pseudofaeces ...6

2.5.4 Zooplankton...6

2.5.5 Effects of shellfish on Hydrodynamics ...6

2.6 Scenarios ...6

2.7 Additional issues ...6

3 Summary of conclusions ...7

4 Literature...8

Appendices A Presentation Luca van Duren ...9

B Presentation Tineke Troost...12

C Bespreekverslag Peter Herman (in Dutch)...15

1 Background

In shallow ecosystems, benthic filter feeders may have a large impact on the functioning of the system as a whole. The large filtration capacity of shellfish can influence the light regime in the system and thereby have an impact on carrying capacity. Moreover, these dense benthic populations can have major effects on the internal nutrient cycling of ecosystems. The impact of benthic filter feeders on the carrying capacity of a system depends 1) on characteristics of the system (residence time, nutrient loading, turbidity) and 2) on the species of the filter feeder (epibenthic or endobenthic, filtration rate, production of faeces and pseudofaeces).

Dutch coastal waters have recently seen massive invasions of exotic species of bivalves, notably the Pacific Oyster Crassostrea gigas and the American razor clam

Ensis americanus. In some areas the proliferation of the invaders appears to have

coincided with a diminished growth of native stocks, but in many cases the total shellfish biomass appears to have increased. This would lead to the hypothesis that the invading species were to some extent capable of modifying the system to such an extent that they created a niche for themselves, thereby increasing the total carrying capacity for bivalves in the system.

The central aim of the work within DELTAKENNIS is to assess the role of benthic filter feeders (shellfish) on expected developments in carrying capacity. In a simple, linear view, where carrying capacity is externally forced upon a system by nutrient loading and physical characteristics (light regime, water residence time), any share taken by

invading species will lead to a proportional decrease in resident species. However, by including biological control mechanisms as limitation for primary production, thus

considering the feedbacks between the biological community and carrying capacity, this simple view has to be extended. This makes, however, the outcome less certain. Predictive power will be enhanced by (a) cross-system comparisons yielding insight in the relative importance of physical factors, nutrient load and biological regulation (b) using recent invasions in different systems as natural experiments and (c) process-oriented studies permitting a better modelling of nutrient cycling and the possible role of invasive species on this. This project uses the large differences in residence times and nutrient loading of Dutch coastal basins to develop a generic modelling system, applicable to systems with very different driving parameters.

1.1 Focus for 2008

We have decided to concentrate on the Oosterschelde as a first step on the way to a more generic ecosystem tool. The reasons for choosing this system were:

• Much is already known about the functioning of this system • The system is heavily controlled by benthic filter feeders

• This system is not as light limited as many other systems in the Delta area, therefore significant effects can be suspected of changes in nutrient recycling rates

• There is a large influence in this system of exotic, invasive species • There are several specific and generic management questions from

Rijkswaterstaat regarding this system, in particular related to a possible increased nutrient loading after the Volkerak-Zoommeer has been turned saline again.

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1.2 This workshop

There are different ways to incorporate filter feeders into an ecosystem model. An inventory has been made of the different modelling approaches and their pros and cons. The aim of this workshop is to decide on the best modelling approach that allows sufficient process knowledge to be incorporated, while still being computationally manageable.

The modelling tools to be developed should work for specific systems, such as this year’s case study the Oosterschelde, but should also be suitable for a generic approach.

1.3 Participants

The number of participants was deliberately kept restricted to a relatively small number of modelling specialists, some from DELTARES and some from other institutes.

Invited participants:

• Paul Boers (RWS-Waterdienst) • Peter Herman (NIOO-CEME)

• Jeroen Wijsman (IMARES / DELTARES) • Reinier Hille Ris Lambers (IMARES) • Ies de Vries (DELTARES)

• Hans Los (DELTARES) • Yenory Morales (DELTARES) • Ellis Penning (DELTARES) • Tineke Troost) (DELTARES) • Luca van Duren (DELTARES)

Unfortunately Peter Herman, Reinier Hille Ris Lambers and Jeroen Wijsman had to cancel at the last minute, leaving predominantly DELTARES participants. Tineke Troost will discuss the modelling effort at a later time with Peter Herman, Jeroen Wijsman and possibly some others working at the IMARES site in Yerseke, to assess if there are any other important issues or modelling techniques that we missed and to discuss the availability of datasets useful for calibration and / or validation.

2 Workshop

2.1 Introductory presentations

Luca van Duren kicked off with a presentation illustrating the background and the framework of this project. The PowerPoint slides of this presentation can be found in appendix 1.

Subsequently, Tineke Troost presented the results of the inventory she made of the different modelling techniques available with the characteristics and properties. The shellfish modules considered are:

• Depletion module • CONSBL • ERSEM-zooplankton module • ZOODYN • STORG • COCO&EMMY • DEB models • ShellSIM

• Size struct. model • Ind. based model

This already eliminated the first few schematisations as unsuitable for this purpose, since they are not dynamic and do not allow the inclusion of parameters such as reproduction. This PowerPoint presentation is attached in appendix 2.

In this presentation 6 questions were raised explicitly that should be answered by this workshop:

1 Are there any other models that have not been considered and are there any other model types under construction?

2 Should we opt for age structuring, size structuring or individual based modelling? 3 Should we opt for net production or DEB modelling?

4 Which extra ingredients should be incorporated, such as • The effect of silt on filtration efficiency

• The effect of mussels on hydrodynamics • Pseudofaeces production

5 How to incorporate larval production? • larval transport and predation • maximum recruitment density • habitat suitability,

• etc…

6 Which scenario studies can be identified as both useful and feasible, e.g. • Impact of the opening of the Krammer sluice gates

• Climate change

During the discussion session after the presentations all these topics were addressed, although not exactly in this order. The minutes below are therefore not a chronological reflection of the discussion, but the remarks and conclusions of the group are ordered according to the questions defined above.

2.2 Available models

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Veluwemeer. Yenory is currently looking into this model and it is considering whether to use it. There may be intellectual property issues with the use of this model.

There are two approach options: 1) start with a complex model that includes all process knowledge and eliminate the bits you don’t need or make a coarser approximation regarding parameters where there is process knowledge lacking, or 2) start with a simple model approach and extend it as more detail is required and/or more process knowledge becomes available to a more spatially explicit version. This should not only regard habitat suitability and refuges, but also include the clustered distribution of suspension feeding shellfish species, occupying 10% or less of the total surface area, related to the dependence on advective transport of food particles. The general feeling is that option 1 may be ideal, provided that we are absolutely sure this is the right set-up but option 2 is much more practical and elegant. There have been several model attempts in the past, some more successful then others. Many of these attempts have been abandoned, because the models did not yield the desired results straightaway, and/or because time ran out. Most of these modelling efforts have been made with the aim of modelling water quality. For such a purpose a mass balance approach is all that is required. Is secondary production is the aim of the model this approach is not sufficient.

For the time being it is best to concentrate first on developing a suitable shellfish module while putting less effort into the 3D hydrodynamics.

The best option is probably a stepwise approach:

1 A box model with nutrients and algal and shellfish biomass

2 A more spatially explicit version with habitat suitability (see section 2.5.1.) and including algae refuges in the vertical and horizontal sense depending on the advective transport of food particles

3 Inclusion of different shellfish species

4 Inclusion of various intra-specific differences through size- or age classes Different opinions existed about the order of step 3 and 4.

2.3 Structuring

2.3.1 Age / size / individually structured models

Some kind of age or size structuring may be needed. A difficulty exists in the mismatch between the modelling time frame and the life cycles of shellfish. The models are characteristically run for 1 year, while the shellfish life cycle is in the order of 4 years. Complete individually based modelling may often be computationally too demanding and may not always be useful for the questions asked. Individual based modelling may be useful when population sizes are small and stochasticity plays a large role.

For mussels there is a strong link between size and age. Also, in the Oosterschelde the age structuring of the population is artificially managed. Particularly for this species age structuring may therefore be the most appropriate approach. However, for Pacific oysters the link between age and size is very tenuous, and completely absent in adults. Moreover, for wild shellfish populations, only the population size distribution are

available, and not the age distribution. For these species only size structuring is a tenable option.

If zooplankton is included as an additional grazer it may be an option to include these in an “agent based model”.

2.3.2 Model dimensions

Fairly simple 1D models, analogous to the exercise by Peter Herman, can teach us a lot about the functioning of a system. It may be advisable to continue some of this work parallel to the larger 3D models. Peter Herman will be contacted to ascertain if there have been any additional developments in the 1D models he developed in MaBenE. As Peter Herman’s models show, stratification and “algal refuges” at the surface may be important for the population dynamics of the shellfish. Also, algal modelling studies at WL have shown large differences between 2D and 3D modelling. Therefore, for the full ecosystem simulations, the only option is to use a full 3D approach. For some applications it may be possible to aggregate in the horizontal direction while retaining high resolution in the vertical. There is a 3D hydrodynamics + SPM fine scale ZUNO model available; however, this model focuses on the North Sea, and is not considered fit for modelling coastal areas in general. Also its horizontal resolution for the

Oosterschelde is not large enough. It is therefore advisable to redo the hydrodynamical calculations for a finer schematisation of the Oosterschelde. Also, it is recommended to take time for optimising the model (by horizontal aggregations, and some other specific tuneable parameters within Delft 3D, etc.) as to speed up the calculations.

2.4 Net production vs. DEB modelling

Most model systems available at DELTARES are “net production models”. DEB models have nice biological properties (size scaling, indirect dependency on environment because of energy buffer) as well as nice mathematical properties (no “switches”). DEB modelling has a special way of dealing with faeces (which is not considered to actually enter the organism), which may have consequences for the modelling of pseudofaeces production.

DEB models are being used at NIOZ and IFREMER, and are currently in development at IMARES. However, due to the absence of Jeroen Wijsman the IMARES experience with these models and their recommendations need to be explored further at a later occasion. For the time being, however, the advantages of DEB over net production models are considered to be not of large importance for the current application.

2.5 Additional processes

2.5.1 Habitat suitability

Different species have different requirements regarding sediment composition, ranges of flow velocities etc. These habitat suitability rules should be incorporated in a model set up. Parameters such as food availability are often implicitly included in models, but other parameters such as sediment composition have to be included explicitly. The best option is to include a template with a habitat suitability map before running the model, but this does require a good deal of knowledge regarding the different species. Not all this information may be available. An important factor is probably 'predictable advective flow conditions'. It may be hypothesized that an optimal location for a mussel bed is determined by advective flow directed towards an upward bottom slope.

2.5.2 Motility

For a model that includes larval settlement motility is important to include. For adult commercial species, particularly mussels, this may also be an important issue as fishermen tend to move mussel populations around from outside the Oosterschelde system to seedling patches to other areas.

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2.5.3 Faeces – pseudofaeces

It is only useful to differentiate between faeces and pseudofaeces when the “behaviour” of these particles is known to be very different. Pseudofaeces have a different

consistency and therefore may have a different rate of disintegration and a different rate of burial than faeces. A student of Mindert de Vries has done some work on the

production of faecal material by different shellfish species. The big question is whether the processes of (pseudo)faeces production and behaviour are known in sufficient detail and are sufficiently different to incorporate these in a model.

2.5.4 Zooplankton

It may be important to look into the possibility of including zooplankton as an additional grazer (see section 2.3.1.).

2.5.5 Effects of shellfish on Hydrodynamics

There are some data available on the effect of mussels on hydrodynamics, both model results and measurements from the MaBenE project. Probably in the scale of things this issue is not very high in the hierarchy of importance.

2.6 Scenarios

Apart from the scenario in which the effect of opening the Krammer sluice gates on the shellfish production in the Oosterschelde is studied, also another scenario on the Volkerak-Zoommeer was discussed:

The tidally driven currents in the Oosterschelde are relatively predictable. In a future saline Volkerak-Zoommeer with wind-driven currents, which are less predictable, areas should be identified that are suitable for shellfish cultivation, either with bed cultures or with long line cultures. For this system the management questions are: 1) optimising water quality by using shellfish and 2) optimising shellfish yield. These two aims really require different strategies. A very clear system, where grazing rates exceed

phytoplankton production rates may not yield the highest production of shellfish.

According to the simple 1D model of Peter Herman, The latter requires a relatively large standing stock of algae, i.e. a turbid system. The big question is to find the optimal solution for both management aims, but the optimal solution will depend on the

hierarchy of the aims. What is more important, a clear system with low algal densities or a high shellfish yield?

We have to bear in mind that running such applied scenarios may not be feasible for the work within 2008. The scenario study described above is a longer term aim.

2.7 Additional issues

We need to make sensible choices in the detail of modelling we go into, with respect to the available data for validation and calibration. If no data are available on e.g.

pseudofaeces production rates for various shellfish species, it is not worth spending a great deal of time and effort into building these parameters into a model. On the other hand the models that do contain these parameters may give us some indication on the ecological relevance of certain processes. When we have to make choices on devoting research efforts into quantifying certain parameters these models can give an indication which particular parameters to concentrate on and which are of marginal interest.

3

Summary

of

conclusions

Based on the discussion in the workshop, it was decided that a stepwise and bottom-up approach was to be taken in the modelling of grazers. This means that the initial grazer module should be simple, and (only if necessary) it should be step by step increased in complexity. Also, the focus should lie on inter-specific variation, less on intra-specific variation.

Furthremore, it was agreed on that STORG is the most appropriate module to start from. STORG has a general lay-out that can be used for suspension and deposit feeders, and is not size-structured nor individually based. Moreover, it has already been incorporated in DELWAQ. It

As a first step, the module could be used for a habitat suitability analysis. Such an analysis may be more workable than a dynamic simulation, as it consists of a static description of the system. Also, changes in habitat suitability are of major interest, so predictions on this issue are very welcome. Moreover, to implement a grazer module into a GEM model, a habitat suitability analysis can be used for determining the initial grazer distribution.

Furthermore, it was agreed that the Oosterschelde was the most appropriate system for testing the grazer module, as many shellfish data are available on the Oosterschelde, and shellfish plays an important role in the system’s dynamics. A case study may then analyse the impact of the opening of the Krammersluizen on the secondary production in the Oosterschelde.

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4 Literature

van Nes,E.H., Noordhuis,R., Lammens,E.H.H.R., Portielje,R., Reeze,B., and Peeters,E.T.M. (2008) Modelling the effects of diving ducks on zebra mussels Dreissena polymorpha in lakes. Ecological Modelling 211: 481-490.

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Bespreekverslag Peter Herman (in Dutch)

Mondeling onderhoud te:

Yerseke Gesproken met : Peter Herman

Bedrijf : NIOO

Datum bespreking : 4 juli 2008

Onderwerp : Draagkracht modellering Oosterschelde Opgemaakt door : T. Troost

Bijlage(n) :

Kopie(en) :

Overleg met Peter Herman over Draagkrachtmodellering Oosterschelde

Tijdens het gesprek zijn er zo’n 5 verschillende punten aan bod gekomen. Hieronder worden ze een voor een besproken. Dit verslag is dus geen chronologische weergave van het gesprek.

1. Draagkracht-gerelateerde modellen van Peter Herman

Peter Herman is bezig met een aantal principiële modellen die van belang zijn voor draagkracht modellering in de Oosterschelde. Een daarvan heeft te maken met algen refuges. Deze algen refuges kunnen een grote rol spelen bij de grazers-productie. Bij afwezigheid van refuges ligt de grazers-biomassa lager dan bij aanwezigheid van refuges.

Een ander model gaat over de optimale diepten voor grazers. Op te grote diepte zijn er voor de grazers weinig algen beschikbaar omdat die zich meer aan de oppervlakte bevinden, terwijl in te ondiepe gebieden er weinig

watervolume en dus ook weinig algenproductie is. Het model laat een optimum zien juist op de dieptes waarop schelpdierbanken voorkomen.

2. Algen refuges

De refuges kunnen zich zowel in het verticale als in het horizontale vlak

bevinden. Refuges in het verticale vlak komen voor in gestratificeerde gebieden zoals in de Scandinavische fjorden. Horizontale refuges komen juist voor in verticaal goed gemengde gebieden zoals de Oosterschelde. Een intermediate situatie komt voor in de Waddenzee.

Er zijn ook aanwijzingen dat de compactheid en structuur van de mosselbedden gerelateerd zijn aan deze horizontale en verticale menging in het gebied. Zo zijn de mosselbedden in de Waddenzee minder compact dan die in de

Oosterschelde en is er sprake van horizontale banden om het voedselaanbod te optimaliseren. In de fjorden is eerder sprake van een random of fractale distributie van de mosselen.

De keuze voor 2D of 3D simulatie is gekoppeld aan deze verschillende vormen van algen-refuges. Voor goedgemengde gebieden met horizontale refuges zoals de Oosterschelde, is geen verticale resolutie nodig, maar wel een fijne horizontale resolute. Dit ligt omgekeerd in gestratificeerde gebieden.

3. Interessante issues / mogelijke scenarios

Interessante vragen op het gebied van de draagkracht-modellering liggen er vooral op het vlak van competitie tussen verschillende soorten (kokkels, mosselen, ensis, oesters). Er is op het moment een algehele afname in mosselconditie te zien, maar ook een afname in oesterlarves. Verder is er een opmars van ensis waar te nemen. Hebben de oesters een soort van

evenwichtssituatie bereikt waarin mosselen niet meer thuis horen? Of zullen zowel de oesters als de mosselen weggeconcurreerd worden door ensis? Een interessante insteek is hierbij het verschil in strategie tussen de soorten. Mosselen komen voor in afgebakende mosselbedden, waardoor er vanzelf (horizontale) refuges ontstaan. Kokkels komen niet voor in compacte bedden. Hierdoor optimaliseren ze hun eigen voedselaanbod niet, maar omdat ze solitair zijn kunnen ze wel overleven bij lagere voedselconcentraties.

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Andere interessante issues zijn het voorkomen van grazers in andere gebieden zoals het Veerse meer, de Grevelingen, het Volkerak Zoommeer, en de

Waddenzee. Met name in het VZM met de hoge nutrientenconcentratie is het interessant te onderzoeken op welke locatie de mosselcultuur zou moeten worden geplaatst. Met de keuze in locatie kan namelijk de mosselproductie worden geoptimaliseerd, maar ook bijvoorbeeld de doorgifte van nutrienten aan de Oosterschelde (door de grazers respectievelijk naast of op de

algenproductie-hotspots te plaatsen).

4. Praktische tips / stepwise approach

Wat betreft draagkracht in de Oosterschelde liggen de meeste vragen op het gebied van verschillende soorten en onderlinge competitie. Met het oog daarop zou in een stepwise modelling appraoch voorrang gegeven moeten worden aan interspecifieke vergelijken boven intraspecifieke (grootte- of leeftijds)

verschillen.

Een aardige eerste stap om de habitatgeschiktheid en concurrentiepositie voor verschillende soorten uit te rekenen is door te kijken of een minimale

voedselhoeveelheid wordt gehaald. Deze kan worden berekend door het voedselaanbod per locatie (gridcel) over een geheel jaar te integreren.

5. Overige opmerkingen

Mogelijk is BLOOM ongeschikt voor het modelleren van algen-grazers

interacties, doordat het niet volledig proces-based is. De mortaliteit zit namelijk verweven in de lineaire optimalisatie waarbij versnelde mortaliteit en groei van de types optreedt. Dit platslaan van tijdschalen biedt voordelen op het gebied van reken-intensiviteit en robuutsheid, maar staat wellicht een goede algen-grazer interactie in de weg.

D Bespreekverslag Jeroen Wijsman (in Dutch)

Mondeling onderhoud te:

per email Gesproken met : Jeroen Wijsman

Bedrijf : IMARES

Datum bespreking : 9 juli 2008

Onderwerp : draagkrachtmodellering Oosterschelde Opgemaakt door : T. Troost

Bijlage(n) :

Kopie(en) :

Schriftelijke enquete Jeroen Wijsman

1a. klopt het dat er op het moment aan DEB modellen wordt gewerkt binnen

IMARES?

Ja, wij doen mee met het EU netwerk AquaDeb, dat wordt getrokken door

IFREMER. Bas Kooijman en Henk van der Veer (NIOZ) participeren hier ook in.

Binnen dit netwerk worden allerlei DEB modellen en toepassingen ontwikkeld voor

schelpdieren en vissen. Ook worden er voor diverse soorten parameters geschat.

1b. wie is daar mee bezig?

Ik, samen met Aad Smaal

1.c voor welke soorten wordt dat gedaan?

Wij werken aan Deb modellen voor kokkel, mossel en oester

2a. wordt er nog wel eens gewerkt met COCO en EMMY?

Ja, voor sommige toepassingen zijn deze nog prima toepasbaar

2b. zonee, wat is de voornaamste reden dat nu met andere (DEB) modellen wordt

gewerkt

De COCO en EMMY modellen zijn scope for growth modellen. Er zijn veel

parameters nodig om de modellen te calibreren en toepassen van de modellen in

gebieden/situaties waar ze niet gecalibreerd zijn leidt soms tot problemen. Door het

groot aantal parameters is het vervolgens moelijk te doorgronden waar het misgaat.

3a. hoe kijk je inmiddels tegen STORG aan? (positieve en negatieve punten) 3b.

welk deel van STORG zou je als eerste kandidaat stellen voor aanpassing?

STORG is ook een scope for growth model. Het voordeel van STORG is echter dat

de basis code universeel toepasbaar is voor benthische en pelagische filterfeeders en

dat het is ontwikkeld rond de Delft 3D interface. STORG lijkt al heel erg op DEB

vanwege zijn eenvoud en universaliteit. Ik zou daarom eerst kijken of het kan

worden herschreven tot een DEB model

4a. in het keyzones-project waren er ind. mosselgroei curves van 1 locatie in de

Oosterschelde (Yerseke natuurlijk) beschikbaar waarmee de modellen "gevalideerd"

werden. Gebruiken jullie diezelfde data, en/of doen jullie dat inmiddels aan de hand

van veel meer data?

Voor de mosselen binnen Keyzones hebben wij onderandere gewerkt met gegevens

van mosselgroei bij Yerseke. Binnen Keyzones zijn er echter ook metingen gedaan

op percelen verspreid door de Oosterschelde. Ook is de groei van oesters gemeten,

zowel op percelen als bij Yerseke in onze opstelling. Binnen andere projecten zijn

wij nu ook bezig dergelijke groeigegevens te verzamelen, zowel in de Oosterschelde

als in de Waddenzee. Tenslotte hebben wij nog een heel interessante dataset over

kokkels in de Oosterschelde en de Westerschelde (groei en sterfte). Deze

gegevens willen wij onderandere gebruiken binnen draagkrachtstudies en de

ontwikkeling van de DEB modellen

5a. klopt het dat er in de afgelopen jaren intensieve oesterbank-surveys in het

sublittoraal gemaakt zijn? En zijn die data (vrij) beschikbaar?

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Er zijn wel verkenningen gedaan met side-scan sonar, maar wij hebben geen

gegevens over de ligging van sublitorale oesterbanken. Wij proberen wel

verschillende instanties (RWS, LNV en Provincie) te overtuigen dat dat wel van

belang is in verband met de draagkracht problematiek.

5b. hoe zit dat met de BIOMON data, kan ik daarvoor het beste terecht bij de

DONAR database, bij IMARES, of bij het NIOO?

Er zit wel wat in DONAR, maar dat is niet voor alle parameters en alle

jaren compleet. Het NIOO heeft deze database compleet en up-to-date. Wij

gebruiken de data ook maar hebben de afspraak deze niet verder te verspreiden.

5c. klopt het dat de Cockle-survey data ook vrij beschikbaar is, en zoja, hoe kan ik

daar het beste aan komen?

Ik weet niet waar je het vandaan hebt dat de kokkel survey gegevens vrij

beschikbaar zijn. Wij kunnen ze wel gebruiken bij de uitvoering van onze projecten.

5d. ten slotte, hoe zit het met de gegevens over mosselconditie van de gekweekte

mosselen, is daar via jullie aan te komen, of moet ik dan bij de veiling oid zelf zijn?

De gegevens over mosselconditie hebben wij van de veiling gekregen. Omdat de

basisgegevens veelal vertrouwelijke informatie bevat hebben wij de afspraak met de

veiling dat wij alleen bewerkte resultaten presenteren die niet zijn terug te leiden tot

perceelsniveau.

Overige opmerkingen:

Hebben jullie al een concrete vraag op het gebied van draagkracht onderzoek.

Wellicht is het interessant dat we hier samen in optrekken. Dat lost gelijk het

probleem van data beschikbaarheid op. Tevens kan aanvullend veldwerk vanuit

Yerseke makkelijk uitgevoerd worden. Ik weet dat ook Aad Smaal initiatieven heeft

genomen op het gebied van draagkracht onderzoek in de Oosterschelde. Wil je mij

svp op de hoogte houden van jullie plannen op dit gebied.