Mollusks growth is a very important and sensitive response to environmental stresses sincethey are good indicators of the available amount of contaminants in the water; reduced growth represents adverse environmental effects and possible effects on the population.Sweden has about 3,000 Natural Reserves and 28 National Parks. Kosterhavets is the first National Marine Park and it is located on the west coast of Sweden, it is also considered one of the most vulnerable areas since 6000 marine species can be found here and about200 are found nowhere else. It is not only a touristic destination; it is also a home and a work place to many people. The type of pollutants and environmental impacts that areproduced by recreation activities on marinas on such park depend very much on the amount of boats. Mytilus edulis has been widely used to monitor the biological effects of contamination by different ways, such as chemical analysis and biological responses. Themain goal of this project was to assess the status of three different marinas with high and low boat traffic and use the shell length of the blue mussel M. edulis as a potential bioindicator to detect effects from boating activities pressures. The main findings arising from this study are that the sizes of the mussels from the three areas with high boat traffic are significantly smaller than the area with little boat traffic. / <p>www.ima.kth.se</p>