The sturgeon is our largest native freshwater fish. They can live to be 150 years old, and over five meters long. Their nurseries are found in large rivers, which they leave when they migrate to the sea. They only swim the long way back from the sea to their birthplace in the river to reproduce. In Germany, the sturgeon has been classified as ‘missing’ or ‘extinct’ for the past 40 years.
Who is the Sturgeon?
Sturgeons are ancient beings: they have lived on the earth for around 200 million years and have survived huge changes over time, such as continental drift, meteorite impacts, and the switch between ice ages and dry seasons. The sturgeon even existed at the same time that dinosaurs inhabited the earth. In the long time-span of their existence, their appearance has barely changed. Their skeleton is not made from bone, but from cartilage, like a shark. In comparison to other fish, the sturgeon do not have scales, but rather a smooth, thick layer of skin which is covered with five rows of bony plates. This armor helps to protect the sturgeon against predators.
The European sturgeon can live to the age of 150 years, almost twice as old as most people. Even compared to other native fish such as the common barbel or brown trout, which live to be around 20 years of age, this is an unusually long lifespan. The largest examples of our native sturgeons were over 5 meters long and weighed up to 800 kg – as much as a small car.
Most of the 27 sturgeon species migrate between the sea and the river. This is also the case in the two native species of northern Europe– the European sturgeon and the Baltic sturgeon. Female sturgeons spawn in gravel-sections of larger rivers in early summer. The larvae hatch only a few days after being fertilized. They find shelter between the gravel, while utilizing the nutrients of their yolk supplies. As soon as this supply is used up, the juvenile fish move into the water column and drift downstream in search of food. After two to four years, the sturgeon is able to regulate the salt content of its body not only in freshwater, but also in saltwater, which is an essential prerequisite to survive at sea. During the years it spends in marine waters, the sturgeon feeds on worms, small fish, and snails on the sea bed. Sturgeons do not have teeth, but to compensate for this, they have a protractile, extendable mouth. They can ‘pre-taste’ their prey before eating them with the long barbels on their snouts. When reaching sexual maturity at the age of 15, it returns to its natal river over up to 2500 km due to its great sense of smell and orientation. Here it migrates upstream to its place of birth to spawn. Sturgeons repeat this journey many times in the course of their long lives, spawning up to 5 Mio eggs per female.
Why is the sturgeon under threat?
Sturgeons have been living and migrating in our rivers and seas for millions of years and, for a long time, no danger was able to do them any real harm. Even as early back as 150 years ago, rivers such as the Elbe and the Oder were filled with sturgeons. It has only been since humans have begun increasingly encroaching on nature that these robust fish have come under threat – as a matter of fact, the sturgeon has been considered extinct in our waters for the past 40 years.
With the beginning of industrialization, the pollution of our waterways began to increase due to wastewater from agriculture, industry, and the increasing settlements. Simultaneously, sections of the river or entire rivers were canalized, still to this day, to make them navigable for large freight ships. These operations have led to the loss of the sturgeon’s natural habitat and spawning grounds. Additionally, the building of dikes and weirs in rivers and on the coast blocks the sturgeons’ migration path. For example, the building of a dam on the Eider River in Schleswig-Holstein near Nordfeld led to the destruction of the last sturgeon spawning grounds in the river Eider, after which it took only 33 years for the last sturgeon to disappear from the river. It is not only the sturgeon that is affected by human intervention, but other migratory fish, such as salmon, shad and smelt. However, protective measures such as fish ladders are often only built with smaller fish in mind. A fully-grown sturgeon would only be able to make the climb if the fish ladder was adapted to its size.