Description: The impetus for this study was the mass bleaching event of giant clams in 1997¯1998 at several reefs in the Great Barrier Reef (GBR,Australia). From September until December 1999, the study investigated the effects of high light intensities and increased temperatureaccompanied by a nutrient limitation on the metabolism of Tridacna gigas, to test, if these environmental changes can induce bleaching. In a50-day trial similar conditions of the mass bleaching event were imitated stressing clams, collected from Orpheus Island and Nelly Bay, by atwo- to threefold light intensity and an increased temperature of 4¯6 °C. The objectives of the experiments were to determine whether highlight intensities can induce changes in chlorophyll content or alter zooxanthella cell sizes and populations in the tissue. After 50 days ofexposure to high light intensity the mantle tissue of stressed clams exhibited a decreased number of zooxanthellae per unit area from19.8 ± 0.8 (× 107·cm¯2) to 0.2 ± 0.2 (× 107·cm¯2) (mean ± CL). Additionally, the average cell size of zooxanthellae were downsized from7.4 ± 0.1 m to 5.3 ± 0.1 m (mean ± CL). Subsequently, the chlorophyll content of both, chl a and chl c1, declined as well, chl a from192 ± 4 to 0.1 ± 0.1 g·ml¯1 and chl c1 from 145 ± 6 to 0 g·ml¯1 (mean ± CL). This study shows that increased light intensity andtemperature are the main causes for bleaching in giant clams. Thus, the study confirmed the four major aspects involved in bleaching: (1)loss of symbiotic algae, (2) decrease of chl a/c1 in the remaining symbiotic algae, (3) retention of small zooxanthellae in the tissue and (4)release of ammonium (NH4+) into the water column while nutrient uptake of ammonium was largely blocked.

Description: This chapter focuses on the current situation of aquaculture development in the North Sea region in the light of world-wide decline of wild fish stocks and increasing consumer demand for seafood. With 1.4 Mill. metric tons of annual production the culture of fish, especially salmon and trout, is the most commonly intensively employed aquaculture sector within the North Sea region. This has resulted in several environmental problems, such as eutrophication of the adjacent water column of the aquaculture facility due to food pellets and faeces, as well as the spread of diseases and parasites. The major shellfish species, which is extensively cultured in many countries along the North Sea, is the blue mussel, followed by the pacific oyster. Several shellfish culture techniques have emerged, which are described and discussed in respect to impacts on the environment as well as their future prospective. The chapter closes with a call for integrative action among the North Sea region states for the further promotion of ecological and socio-economic sound aquaculture development by endorsing the idea of ICZM in order to sustain its economic potential as alternative livelihood for coastal communities.