37
6. Conclusions
There are several exotic species on the artificial reefs near the Oesterdam. Some more invasive than other. For now the ratio of exotic and native species seems to be close to that of the natural reef.
However the species richness on both artificial reefs is much higher at the moment compared to the natural reef. This is expected to decrease as the species find a balance by outcompeting each other.
This will give a lower species richness and a higher evenness like the natural reef. There are some exotic species which pose a threat to all reefs in the area and even to the aquacultural plots. It would be wise to see if the growth of these populations can be slowed down to give other species the change to conquer the niche hopefully keeping the invasive species at bay.
The competition between H. takanoi and juvenile C. maenas seems to be a problem for the latter species. It is likely considering the results of this study that juvenile C. maenas are outcompeted for food and shelter by H. takanoi of a similar size. This will in turn cause a problem for the population as a whole since less crabs were able to reach maturity ultimately preventing them to reproduce. Since the population of H. takanoi has already been well established on the reefs it would be impossible to get them of the reefs. If in the future more reefs were built it is recommended to look into ways to improve the design in order to help some species. Like giving the reef a more natural shape and add some open spaces in between to create patches.
7. Recommendations
This part will discuss some recommendations on the methodology in case the experiments were replicated. Also interesting ideas for further research were discussed.
7.1 recommendations on the methodology
For the shelter experiments it would advisable to use round containers. This way the crabs cannot hide I a corner and would hopefully be more inclined to use the provided shelter. Also a heavier shelter should be provided. During the experiment the mussel shell was flipped or moved to easily discouraging the crab which was trying to use it. It would be easier to use a pump to mimic the tides.
Now close attention had to be paid to the tubes to ensure all aquariums would be empty at the same time. As a final recommendation for the shelter experiment the use of a fake predator might help as well. This was the crabs were forced to seek some kind of shelter.
The only thing for the food experiment is that the crabs would sometimes run out of food. If possible it would be recommended to use more food or to be able to replace the empty shell with a new one.
During this project it has happened the crabs were out of food which may have influenced the results because they were not able to continue feeding.
38 7.2 recommendations for further research
It could be interesting to do a pilot project to see if changing the reef according to the suggestions described in the conclusion would have a positive effect.
Also further research on the effect of the location of the reefs could be helpful. For further reading on this topic (Harwell, et al., 2011) is suggested.
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Acknowledgements
I would like to thank my two supervisors Anneke van den Brink and Bram Verkruysse for their patience and guidance. Like an oyster reef a project like this can have some rough patches, but their relentless optimism and faith in me got me through. Annekes enthusiasm was addictive and I will forever have a soft spot for these brave little crabs.
I would also like to thank Cees de Jonge who helped me with the statistical analyses with a smile on his face. I have never met anyone with such a passion for statistics and it was a great to witness.
40 Annex 1 Field sheet