Small cetaceans are no longer deliberately hunted in the ASCOBANS area. However, many species of whales, dolphins and porpoises migrate seasonally between their breeding, feeding and over-wintering ranges, or follow their prey over long distances. En route they encounter a variety of man-made threats:

• Bycatch, the accidental entanglement in fishing gear, is considered the most serious threat to cetacean populations in the ASCOBANS area. Every year, several thousand animals are ensnared in fishing nets. Since cetaceans are mammals, they have to surface at regular intervals to breathe. Once caught in fishing gear they can no longer come up for air and eventually drown. Due to their coastal and inshore distribution, harbour porpoise populations are especially vulnerable to bycatch in bottom-set gillnets. According to estimates by the International Council for the exploration of the Sea (ICES), more than 4400 porpoises are killed annually in fishing operations throughout the North Sea. In the Skagerrak, annual bycatch probably exceeds 4% of the total population and coincides with a decline in stock size. This high mortality, coupled with relatively low reproduction rates means that harbour porpoise populations will continue to decline and not be able to recover until bycatch rates are significantly reduced.
  
  
• Marine pollution is another serious threat that calls for an international, co-ordinated approach. Hazardous substances like heavy metals and organic compounds accumulate in marine organisms, pass up the food chain and reach highest levels in the body tissues of marine mammals, adversely affecting their health.
  
  
• Acoustic disturbance - due among other things to heavy shipping traffic in the Baltic and North Seas, seismic surveys and military activities - is a further cause for concern.
  
  
• Competititon with fisheries can be a problem. Most whales and dolphins feed opportunistically on a wide variety of fish species, squid an sometimes bottom-living invertebrates. However, schooling fish, such as herring, mackerel and sand eel, are often preferred prey. Unfortunately, commercial fisheries often target the same species. Thus, cetaceans and fisheries may compete for the same (waning) resources.

The extent of the threat to small cetaceans in the Baltic and North Seas is dramatically illustrated by the decline in the populations of harbour porpoises. The harbour porpoise (Phocoena phocoena) is the most common cetacean species in the North Sea and the only native cetacean inhabiting the Baltic. Harbour porpoises are small dolphin-like animals with a low triangular dorsal fin. They generally occur singly or hunt in small groups. Until the 1930s the range of the Baltic population extended north-eastwards as far as the Åland Islands north of Stockholm, and included the Gulf of Riga and the entrance to the Gulf of Finland. Since then a dramatic decline in numbers has been accompanied by a steady retreat from large areas of their former range in the eastern and central Baltic. Today, Baltic harbour porpoises are generally restricted to the Kattegat and Belt Sea in the very west of their original habitat, with only occasional sightings along the German, Polish and Swedish Baltic coasts. In the North Sea cetaceans face similar threats.


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