The scope of this overall project is to conduct a practical test for an ecosystem approach to controlling crayfish based on Van Kleef et al, 2022. In a compartment of more than 2 ha (figure), the most complete system approach possible will be carried out. And in the entire Molenpolder Nature Reserve (50 ha) measures for an ecosystem approach will be implemented that are feasible and affordable on this larger scale. In this way we can compare an intensive and a less intensive approach.

The project team wants to keep control and therefore divides the project into plots. Part of the work will be carried out by Waternet, part of the work will fall under the parcel scheme and will be carried out by a privately selected market party. A detailed description of the Assignment per plot is given in the relevant Program of Requirements of a plot.

Direct award, see section II.2.14.

This plot is about the insulation of a compartment of more than 2 ha by the construction of two fish walls, so that a compartment is created within which the most complete practical test for the ecosystem approach can be carried out. A water-permeable and navigable fish and lobster protection is being built in two places.

The fish defences are intended to minimize exchange (between compartment and environment) of the somewhat larger fish and lobsters. Water must be exchanged because of. follow the maintenance of the surface water level according to the level decision in the compartment. A maximum mesh size of 10x3 mm is maintained. Experiences of ATKB (Jouke Kampen) are that it is very difficult to make a fish protection that does not have or get a 'leak'. ATKB is therefore involved in the design.

The barrier is constructed with beams with H-profile, sheet steel and wooden fitting beams to secure the sheet steels in the H-profile.

The width of the barrier is approximately 9 m in the wet profile and extends 2 m into the bank.

The height of the barrier is approximately 1.2 m.

Preconditions of the fish defences:

• Take into account a flexible level management from -0.90 to -1.15 m NAP at the inlet and outlet. Locally, the level can be even higher or lower due to suspension due to the large distance to inlet and outlet. So when considering the height of fish protection, take into account level variation between -0.80 and -1.25 m NAP to be sure.

• It is very important that the meadows are not passable by fish and lobster via the water.

• A good connection to the bank so that no short circuit can occur and fish or lobster can come behind.

• Insert the beams with H profile and sheet steel deep into the ground so that no fish can pass underneath.

• Exchange of water so that no difference in level is created. The water exchange takes place by placing a stainless steel trench panel with slots of 3 by 10 mm. The slots are placed vertically so that they can be easily wiped clean.

• A gangway above the grille for the purpose of wiping the trench panels.

• Navigable by an insert of 0.5 m high and 2.5 m wide, which can be easily removed.

• The navigability must also not allow fish to pass through. Ie.dat the boat that sails through has to push away the navigable part and close it immediately after passage.

• The work carried out on the construction of the fish defences must not damage the existing N2000 habitats and habitat species.

• The fish guards must be ready before 15 March 2023.

This plot is about the construction of 30 structures, "nesting rafts", in the water on which grebes can build a nest.

Grebes predeter on lobster but are rare in the Molenpolder. The number of breeding pairs of grebe in the Molenpolder (50 ha) has been estimated in recent years at approximately 6, or 1 pair on more than 8 ha. A territory of 1 ha or smaller is often sufficient. In the Molenpolder there is a lot of landscape structure and more grebes could breed in the Molenpolder.

The hypothesis is that the impeding factor for more breeding pairs of grebe is the nesting opportunity due to the virtual lack of preferred substrate. That is why we offer more nesting opportunities to attract grebes.

The intended net rafts will be made of natural material, (willow) branches in combination with bundles of reeds. By using willow branches, water roots are created that provide shelter for young pike, which is a beneficial side effect. The design must be able to carry a weight of 1.5 kg for a longer period of time and last several seasons.

The nesting assistance structures do not come to locations with N2000 Habitats. If a grebe nest is placed, it should not shade a significant part of the habitat. In addition, it must be ensured that the work involved in the construction of a nest does not damage the existing habitat.

In sub-compartment 2, a pilot is being carried out for the construction of emer vegetation. In this pilot, 209 m2 of emerse vegetation will be created. The emer zone with helophytes, such as reeds, is especially important for pike, predator of lobster that we want to stimulate.

In the pilot we want to build water cane in compartment 2 on approximately 268 m2 (2 plots 209 and 59 m2). In the Molenpolder there is hardly any shallow water. The hard water bottom is deeper than 1 meter almost everywhere, often around 1.2 m. I.dat it is not possible to create a reed zone without deepening.

It is not buried in the Molenpolder because it will then be too deep. Instead, soil is added to reach a depth acceptable to reeds for planting and expansion. With 350 blocks, 700 m2 can be realized instantaneously, but not all of this is necessary because if reeds catch on well, they expand quickly. So the blocks are laid out at some distance from each other

In order to arrive at the intended 2 m wide reed zone, 1 reed block of 1x2 m is laid out per 5 m bank length on a 0.5 m from the bank and surrounded with replenished soil. For example, with 2 m2 of reed layer, 10 m2 of reed zone is created. Per 5 m, 11 m3 of additional land is needed. Of which 7 m3 is needed for the one-sided embankment.

For (after natural expansion) 100 linear meters of reed zone, it is necessary:

- 20 rietblokken (40 m2)

- 220 m3 of land (at 30 m3 of cargo that is 7.3 cargo loads)

With the transplant, a root zone should be included. Then it is an advantage if large plots can be used: relatively little root zone is broken.

Waternet will work out the exact way in which the pilot will be carried out together with the contractor.

In summer 2021, lobster reduction started in Molenpolder – West. In the winter of 2021-2022, fish stocks have been reduced in the entire project area, in particular the bream, roach and kolblei stock has been reduced. After the fish reduction, it is undesirable for young bream, roach and kolblei to enter the area via the pumping pump. In the Molenpolder, directly behind the pumping station and valve thruster, there is a trap. . In the summers of 2021 and 2022, quite a lot of fish of 10-15 cm was frequently found in good condition in this trap, which indicates successful immigration. And we want to prevent catfish from swimming from the Molenpolder to the Nedereindse vaart.

Waternet therefore wants to build:

1. a row of slats under the bridge ;To prevent fish from swimming into the pumping bowl: to be applied (Bruijs and Vriese, 2023, chapter 4.3)

2. the active self-cleaning wedgewire drum sieve at the pumping station (Bruijs and Vriese, 2023, chapter 4.1.4

3. a hinged fine grid on either side of the valve thruster. (Bruijs and Vriese, chapter 4.2)

The aim is to have these adjustments ready before the 2023 intake season starts.

Preconditions Fish protection:

• The fish protection must prevent fish from reaching the Molenpolder nature reserve via passage of the pumping station and valve weir. Effective exclusion of cyprinids, 0+ to 1+ (5 to 15 cm): especially bream, kolblei, roach;

• The fish protection must prevent released catfish from swimming from the Molenpolder to the Nedereindse vaart.

• The application of the fish protection must guarantee the required throughput capacity of the pumping station and flow over the valve weir. Working under the worst case scenario (highest flows);

• Operates during periods without flow (fish may not actively pass through the system itself);

• Effective management of aquatic plants/floating waste, without the need for manual cleaning work;

• The integration and application of the fish protection must be acceptable from a price point of view.

This plot is about:

1. Reintroduction of submerged aquatic plants and crab shear

2. Spawning pike, eel and catfish

3. Releasing native mussels

4. Creation of emer vegetation

5. Afrasteren emerged vegetatie

6. Construction of wood habitat

7. Monitoring and modelling and interpreting Practical test

Further information in the schedule of Requirements of lot 5.

Activities for dredging problems

• Develop a strategy together with private individuals and Staatbosbeheer about the dredging problem.

• Determine the extent of the problem, monitor it: quantity and quality of dredging and solid soil.

• Soil quality for

• Develop a strategy to mitigate the problem. Thinking more broadly than just dredging.

• Develop and stimulate together with the private individuals that they take the initiative to implement the dredging strategy.

• Organize consultation moments with the private owners and Staatsbosbeheer and develop a strategy for the dredging problem together with these owners.

• Encourage private individuals to take the initiative to implement the dredging strategy jointly.

• Part of the assignment will be to test the strategy against Natural Law.

• The result is a dredging strategy, supported by the owners and elaborated in a report.

Activities Trees in riparian problems

1. Develop a strategy together with private individuals about the problem of trees in the bank.

2. Determine the extent of problems, the number of trees in the bank that hinder water quality and the development of emerse vegetation.

3. Develop a strategy to mitigate the problem.

4. Discuss moments with the private owners and organize Staatsbosbeheer and together with these owners develop a strategy for the trees in the bank problem.

5. Encourage individuals to take the initiative to implement the strategy jointly.

6. Part of the assignment will be to test the strategy of Natural Law.

7. The result is a strategy for the tree problems in the bank, supported by the owners and elaborated in a report.

8. If it is known what the pilot entails, it will be tested against the Natural Law.

9. Implementation of a pilot for the implementation of the strategy remove trees in the bank.

A sufficiently low phosphorus load is essential for this project. The external phosphorus load from water inlet is below the critical limit. Nevertheless, the Molenpolder seems to be too nutrient-rich given the enormous amounts of lobsters that are caught, especially in the extreme Southeast. The cause of the excessive nutrient richness lies in internal phosphorus sources.

The water surplus from the Southeast must flow through the rest of the Molenpolder towards the outlet at the Krom.

If the monitoring (lot 5) and quantifying the dredging problem (lot 6) show that the phosphorus load from the Southeast is too large, then it is necessary to eliminate this load. This could be done by letting out the water surplus from the extreme Southeast on the eastern side of the main waterway. This requires a navigable weir and automation eastern outlet In this way, this phosphorus-rich water no longer enters the clean part of the Molenpolder in the middle and west. And the main part of the Molenpolder will have more chance to recover.

In 2024, ATKB will be commissioned to reduce lobsters in Molenpolder Oost. In Molenpolder West there will no longer be a current assignment in 2024.

Activities

• Reduce lobsters in Molenpolder-West (35 ha) in 2024

• Reduce lobsters in Molenpolder Oost (15 ha) and Molenpolder West (35 ha) by 2025.

• Lobsters reduce at least from the beginning of May to the end of November, but preferably start earlier than May and longer than November. Find a method that takes fewer hours to perform.