Over many decades harbor porpoises had disappeared from large parts of the North Sea coasts and their estuaries due to the massive negative anthropogenic impacts around the mid-20 th century. However, things have changed in the past several years.
Historical records show that harbor porpoises were quite common along our coasts about 100 years ago before they were decimated through hunting, overfishing, and by-catch, as well as river pollution and a decrease in fish species due to industrialization, and that they ventured far into the rivers, following schools of fish according to some descriptions:
- into the Weser river up south of the city of Bremen
- into the Ems river to Weener
- into the Elbe river as far as the city of Magdeburg
And this also seems to be the reason for their present-day journeys into the rivers.
In 2005 an increase in the number of harbor porpoises occurring along the North Sea coasts was detected.
A detailed North Sea survey (SCANS II) revealed differences in harbor porpoise densities compared with a previous survey in 1994: the distribution "centers" of these marine mammals have significantly shifted from the northern parts to the southern parts of the North Sea.
This goes hand in hand with an increase in harbor porpoises along the Lower Saxony coasts. Moreover, there are seasonal fluctuations, the harbor porpoise density being very high in the spring. Simultaneously we can find them in rivers again since then.