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1. Introduction
This report is the result of a final thesis for the Water Management bachelor program at the HZ University of Applied Sciences. The research was commissioned by the research group Building with Nature which is part of the Applied Research Centre Delta Academy at the HZ University of Applied Sciences in Vlissingen, The Netherlands.
The research group Building with Living Nature investigates possibilities to use living nature for sustainable development in and around coastal defense. This project is part of the Oesterdam project which is aiming to protect the Dutch coast and enhance the biology of the intertidal areas in the Eastern Scheldt at the same time.
The Eastern Scheldt is one of the biggest nature reserves in the Netherlands and is an important habitat for many native species due to the intertidal areas. Since the construction of the storm surge barrier the dynamics in the Eastern Scheldt changed considerably. Although there is still a free flow of water when the barrier is open, the amount of water going into the area has been reduced by 30%. (De Jong, et al., 1988). Because of this the gullies, which were already there, are now too large for the amount of water coming in. This results in the problem that the force of the water is not strong enough to deposit sand onto the sand flats which are an important part of the ecosystem. At the same time wind and outflowing water will take sand away from the sand flats. These sand flats normally fall completely dry during low tide which is an important factor in the survival of many species in the area. Since they are becoming smaller, less area is falling dry every year endangering the ecosystem as a whole. If this problem would be ignored the surface area of the tidal flats will be halved by 2050. (Van Zanten & Adriaanse , 2008)
One of the projects trying to solve this problem is the sand nourishment. In 2012, 400.000 m³ of sand was deposited next to the Oesterdam. This sand will slowly spread out over time onto the tidal flats.
The goal is to protect the Oesterdam from incoming waves and at the same time create a buffer for the tidal flats. (Linkit, 2011)
At the Oesterdam four artificial oyster reefs were built to stabilize the sediment. Over time these reefs will hopefully develop into natural reefs. The artificial reefs provide a new hard substrate into a soft substrate environment, thereby creating a new habitat. The addition of a new habitat might encourage the growth and dispersal of some of the many exotic species in the Eastern Scheldt. Of all the exotic species in the Netherlands, 55 % are found in Eastern Scheldt and 14 % of all exotic species in the Netherlands are found only in Eastern Scheldt. (Wolff, 2005)
Figure 1 Artificial oyster reef near the Oesterdam
8 The most important reason for the arrival of these exotic species is aquaculture. By accidental escapes or transfer to the ecosystem by open cage systems.
Another vector for the introduction of exotic species is hull fouling and ballast water associated with the high volume of shipping in these waters. This is the reason why shipping regulations have been tightened over the years. Natural processes such as global warming also play a role into the invasion of exotic species. Species who normally could not live in this climate are now able to survive. (Wolff, 2005)
There are many risks involved with exotic species. 10 % of total invaders in an aquatic system have a high impact (Ricciardi & Kipp 2008). They can change ecosystem functions like hydrology and nutrient cycles and might pose health risks by acting as vector for diseases. Clavero & Garcia-berthou (2005) state that exotic species are major cause of extinction because of competition, niche displacement, hybridization and predation.
Sometimes these effects can be used as an advantage by using the species an ecosystem engineer.
Ecosystem engineers are species that can create, modify and maintain habitats. Ecosystem engineers can influence the distribution and abundance of large numbers of plants and animals, by causing physical changes in biotic and abiotic materials that, directly or indirectly, modulate the availability of resources to other species. (Jones, et al., 1994). In the case of the artificial oyster reefs the exotic C.
Gigas is used to build the reefs which protect the coastline by reducing the waves.
The aim of this research project is to investigate the biodiversity on the artificial oyster reefs to get an indication of the ratio between the native and exotic species on this new habitat. As an indicator for the effect of the oyster reefs on the community composition, two crab species were chosen.
Crabs are a motile and competitive species, making them a good indicator of short term changes in an environment as they can easily enter or exit an area according to its suitability. For this reason the populations of both Hemigrapsus takanoi (exotic species) and Carcinus maenas (native species) were studied. To get a better insight on the effect both species have on each other a behavioral
experiment was set up to be able to see the competition for food and shelter between juvenile C.
maenas and adult H. takanoi according to the hypothesis suggested in van den Brink (2012) (see ‘1.2 previous experiments’ ).
1.1 Research species
Two species of crabs were investigated during this study. Hemigrapsus takanoi which is native to Japan and Carcinus maenas which is native to the Netherlands. Both being successful invaders in multiple places around the world they have proven to be resilient species (Klassen & Locke , 2007) (Dauvin, et al., 2009) (Noël, et al., 1997). In the Eastern Scheldt, among other places, they occur together.
9 Hemigrapsus takanoi
Figure 2 Male Hemigrapsus takanoi. Source: (Van Bragt, 2015)
Closely related to Hemigrapsus penicillatus, this species was only recently recognized as a separate species (Asakura & Watanabe, 2005). They are usually found under hard structures in intertidal areas. They are generally in more wave-sheltered areas than H. penicillatus, although they are also often found living in the same areas. Their native distribution ranges in the north/west Pacific Ocean around Taiwan, Korea, Japan and China. (Asakura & Watanabe, 2005)
It is believed that these crabs were introduced into Europe during the early 1990’s by the transport of Asian oyster or hull fouling or ballast water. Since then they have spread out along the European Coast at a high rate (100 km per year). They are able to spread that fast because their young reproduction age starting at 10 months old (7 mm CW) and because of their planktonic larvae (Noël et. al. 1997) which they may have up to six times a year (Dumoulin , 2004).
Their carapace is square shaped and adults can get up to 28 mm carapace width (Noël, et al., 1997).
Males have larger chelae compared to females with a patch setae on them. They vary on color from grayish to greenish or brownish (Asakura & Watanabe, 2005). Their breeding season ranges from spring until autumn. Females are mature at a size of 7mm CW which can be reached in 10 months depending on the temperature. They can have up to 6 broods per year (Dumoulin , 2004)
10 Carcinus maenas
Figure 3 Carcinus maenas. Source: A.M. Arias
Carcinus maenas or the green crab is native to the Atlantic, Baltic, and North Sea coasts of Europe from Norway to Mauritania (Groholz & Ruiz, 1996). But ranked by (Lowe, et al., 2000) as one of the world’s 100 worst invasive alien species they have spread out across the world. (Klassen & Locke , 2007)
They occur in a variety of habitats including both hard and soft substrates (Groholz & Ruiz, 1996).
They are known to migrate into deeper water when the water gets colder during autumn. (Audet , et al., 2008)
Reaching up to 10 cm carapace width they can live up to 4-7 years (Klassen & Locke , 2007). Females get mature at a minimum CW of 36 mm. Mating finds its peak in July and they can bear eggs from March and April, but also during November and December. Usually they will release their larvae during July and August. (Broekhuysen, 1936)
1.2 Previous experiments
The following papers were used as the base for both the reason and methods for this project. The first two research project took place at the same general area providing more information about it.
The first report is about natural reefs in the Eastern Scheldt and the second one about the
populations of C. maenas and H. takanoi. The second paper is the lead paper for the reason to start this research providing a hypothesis which can be investigated further. The last paper is about some behavioral experiments between C. maenas and Hemigrapsus sanguineus which is closely related to H. takanoi. This paper was used as the most important source for the methodology used in this project.
11 Fritz (2013)
This study focused on the ecological difference between dense oyster reefs, patchy oyster reefs and tidal flats at Viane and Sint-Annaland to give recommendations for the design of artificial oyster reefs. The research focused mainly on the organic material of the sediment, amount of biomass, population densities. It was found that the dense oyster reefs showed the best ecological values although the results were not significant. The method in this study was limited to only 3 species:
Arenicola marina, Littorina littirea and Pygospio elegans. This research was taken into account to get some insight on the natural oyster reefs studied around the same area, but the method for the current research project were more elaborate.
Van den Brink et al. (2012)
This research investigated the populations of the three crab species: C. maenas, H. takanoi and H.
sanguineus. This was done by monitoring the benthic communities in four Dutch delta waters and doing a snapshot survey in the Eastern Scheldt tidal bay. These waters were dominated by C.
maenas but their numbers declined in the past two decades. In 1999 the two exotic Hemigrapsus species were introduced to these waters and started to invade the habitat of C. maenas. Although they were not the reason for the initial decline for C. maenas they have taken advantage of it. H.
takanoi now dominates the hard substrates and both exotic species are equally abundant on soft substrates compared to C. maenas. On hard substrate where juvenile C. maenas normally would find shelter they now appeared to be excluded from it by H. takanoi. This increases the chance of
desiccation during low tide or in general the chance of getting preyed upon and thereby increasing their mortality.However on soft substrates the C. maenas population, were there are fewer H.
takanoi, the populations seems to be maintained by crabs that survive and reproduce.
Jensen et al. 2002
This research compares the competitive behavior of Carcinus maenas against Hemigrapsus sanguineus on the east coast and against Hemigrapsus oregonensis on the west coast of North America. By both field sampling and laboratory experiments, competition for space and food were tested. In competition for food C. maenas was dominant over H. oregonensis, while H. sanguineus were very dominant over C. maenas. By sampling rocks and sand in the habitat of C. maenas large differences in habitat use were recorded between when Hemigrapsus were present or not. If Hemigrapsus species were present only 20 % of juvenile C. maenas were found under rocks while more than 97 % of C. maenas were found under rocks in a habitat without H. sanguineus. Both Hemigrapsus species were also found to be dominant over C. maenas in competition for shelter in laboratory trials. It is likely the habitat use and by that the distribution of C. maenas is affected.
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