The cold Benguela current flows along the southwest coast of Africa, extending from the very tip of the continent up into the waters of Angola. The current is one of the world's four main upwelling systems which transport nutrient rich waters from the depth to the upper layers of the oceans, thus providing a basis for abundant fish populations. The most important species sustained by the current are pilchard, anchovy, hake, horse mackerel and orange roughy, but also rock lobster, deep sea crab, sole, monk and, seasonally, tuna. However, these fish stocks are subject to pronounced natural fluctuations due the general instability of the oceanographic and biological environment. Some fish stocks can even break down completely for a period of time and changes in species composition can occur. Understandably, such variations have severe consequences for the fisheries sector and are an enormous challenge for fisheries management.