The Mediterranean Sea is oligotrophic: it is rich in oxygen and poor in nutrients. Oligotrophy increases from west to east. The fauna and flora is one of the species richest of the world and there is a high rate of endemism. Introduced alien species are increasing in the eastern basin. Compared with the Atlantic, the Mediterranean marine communities have many different species with generally smaller individuals (Mediterranean nanism). Eutrophication in coastal areas has almost certainly resulted in an increase in fish catches of some pelagic fish species in the formerly low-nutrient waters of the Mediterranean Sea. The immunosuppressive effects of contaminants arising from agriculture, industrial activity and population growth may have contributed to the severity of mass mortalities among marine mammals in the Mediterranean Sea and the additional chronic effects of organochlorines could hinder, or even prevent, recovery of individuals from pathogenic disease. Introduction of alien species through ballast waters, fouling, import and invasion has resulted in the establishment of dense natural populations of species. However, the impact of some intruders like the tropical alga Caulerpa taxifolia has had catastrophic effects on the natural environment. Fishing has resulted in overexploitation of several fish stocks in the Mediterranean. Mortality of the monk seal is mostly associated with fishing. Overexploitation by intensive collection has led to a serious decline in some corals and many shellfish.