Monitoring survey oyster reefs

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3. Method

The method for this project is divided into two parts. The first part is a monitoring survey on the artificial oyster reefs near the Oesterdam (fig. 5) to assess the development of the biodiversity on these reefs.

Figure 5. Location of the Oesterdam compared to the HZ University of Applied Sciences.

As an indicator for the effect of the oyster reefs on the community composition, two crab species were chosen; Hemigrapsus takanoi (exotic species) and Carcinus maenas (native species). All

collected crabs were identified to species level, counted and their size and gender were noted during the survey. This gives an overview of the population density and distribution of both species.

To gain a better insight on the effect both species have on each other a behavioral experiment were set up to document the competition for food and shelter between Hemigrapsus takanoi and Carcinus maenas.

3.1 Monitoring survey oyster reefs

Both artificial and natural oyster reefs were monitored. The natural reefs are found close to the artificial oyster reefs located near the Oesterdam (fig. 1).

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Figure 6 location natural artificial oyster reefs, gabions and natural oyster reefs.

There are two sets of two reefs located near the Oesterdam. Because of the limited time due to the tides, two out of the four reefs were chosen to be monitored. One reef out of each set was chosen to

be monitored. The first one (number 1 in fig. 6) was chosen because some of the shells have been washed out of the reefs and are have loosely collected next to the reef outside of the wire mesh (fig. 7). This makes it easier to collect crabs from between the shells. The other reef (number 2 in fig. 6) was selected because it is the largest reef and because it is the most outside and thereby exposed reef out of the upper pair.

Since that reef has no loose shells a single gabion filled with empty oyster shells was attached to that reef two months before the first sampling event. This gabion was opened during sampling by removing the cable-ties. The gabion were inspected and the organisms present were recorded and all crabs were collected.

Figure 7 Schematic drawing of the monitored reefs.

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16 To be able to compare the results obtained from the artificial oyster reefs, a natural oyster reef (fig.

4) were also monitored using the same method except instead of dividing it into section like the artificial reefs, the natural reef it is treated as one section due to its smaller size.

Figure 9 natural oyster reefs during low tide

All reefs were checked three times with one month in between (table 1).

Table 1 sample dates and times with corresponding low tide levels in NAP.

Date Time at low tide Low tide lvl NAP

30-09-2014 13.35 -134

28-10-2014 11.45 -138

25-11-2014 10.55 -144

Figure 8 loose shells next to one of the monitored reefs.

17 The artificial oyster reefs consist of empty oyster shells caged by mesh and attached to the sediment (fig. 4). These cages make quantitative sampling impractical as access to the oysters is very limited.

Therefore a systematic qualitative sampling method were applied to determine species richness along with subjective estimates on quantity.

Sampling area

Along the length of the oyster reefs three, 3 m wide sections were marked on both the more exposed and more sheltered sides. Each section was labeled so that sampling can be repeated per section as illustrated in Figure 5.

Figure 11 Example of division of sampling sections on an artificial oyster reef. The numbers represent the three meter sections on the reef. The letters A and B represent the more and less sheltered sides of the reef.

Each section was visually inspected for organisms with particular focus on periwinkles, crabs and algae.

Using a 0.25 m² quadrate haphazardly placed in each section of the reef, all species were recorded and given an abundancy class (table 2). For algae and colonial ascidians such as Didemnum sp. the percent coverage were estimated. The natural oyster reef was treated as one section.

All organisms within a quadrate were identified with the help of a field guide. This information was written on pre-made field sheets (appendix A). If a species could not be identified on site, a sample was placed in a 500 ml plastic container and taken to the HZ University of Applied Sciences for further identification. All crab species observed within the quadrate, were collected by hand and put into a bucket. For all crab species the carapace width (CW) is measured with calipers to the closest mm. Their gender were noted as well. Besides the crabs within the quadrates, more crabs were

3m

Figure 10 one of the artificial oyster reefs near the Oesterdam

18 collected by hand on other parts of the reefs. This was done to get a larger sample size for the

population comparison. One person would continuously collect crabs as long as it took to gather all the results from the quadrates.

Table 2 abundance codes for motile species

Code Estimate of

To gain a better insight on the effect exotic and native species can have on each other a behavioral experiment were set up to be able to see the competition for food and shelter between the two crab species Hemigrapsus takanoi (exotic species) and Carcinus maenas (native species).

All treatments will done in triplicate and the entire experiment was repeated 5 times for a total of 15 replicates. New crabs were used for each experiment. Before the crabs were used in the experiment they were put into holding tanks in the same room as where the experiment will take place. The holding tanks were filled with a layer of water from the Eastern Scheldt, deep enough for the crabs to be completely emerged. An air pump and some rocks will also be provided to maintain the oxygen level in the water and provide shelter. The crabs were fed every other day with some dead mussels.

Each species was held in a separate holding tank.

For each treatment a control with a single individual crab was used as a reference for the behavior without any competition.

3.2.1 Collection of H. takanoi and C. maenas Location

Both species of crabs were collected from the Eastern Scheldt. H. takanoi were collected from a dike near the village of Tholen. Carcinus maenas were collected from a dike at the Goese Sas (fig 8). They were collected at two different locations because C. maenas was more abundant at the Goese Sas and therefore easier to collect. Two people were needed to collect them so one person can turn over the rocks and the other can collect as many crabs as possible.