The sea trout (Salmo trutta trutta)
Photo: E. Hensel
The fish enjoy hiking: hiking is not uncommon in nature. We find species both in land creatures and in inhabitants of watery habitats that carry out such migrations with great regularity. The goal is always to improve the chances of survival. The ability to adapt to living conditions is the key to a successful evolution and thus to life on our planet. Because if you don’t adapt, you have no chance of survival in the long run. However, this does not mean leveling out; on the contrary, it has led to the immense and fascinating diversity of organisms. This enormous abundance of different characteristics is also reflected in the hikes. There are daily and annual hikes, horizontal and vertical.
The daily vertical migrations of organisms in the water column are often a compromise between the requirements of nutrition and enemy avoidance. Only at night many animals stay in the upper, bright water layers, where they find the best feeding conditions. During the day they evade optically oriented enemies by seeking in deeper, darker layers. Because many animal species that only appear in deeper water layers during the day can also endure the much lower pressure on the water surface, they can often be kept in show aquariums without any problems.
But migrations serve not only for nutrition, but also for reproduction, such as in the case of sturgeon, eels, salmon, sea trout and many other species.
Many sturgeon species migrate up the rivers from the sea to spawn and thus ensure their continued existence. The adult animals then go straight back into the sea, while the young animals need a much longer period of time before they reach the river mouth. In the young of the European sturgeon (Acipenser sturio) this takes about 3.5 years.
As the name suggests, this sturgeon species was widespread in almost all of Europe’s seas and their tributaries. In Germany, the European sturgeon was a very popular fish species because of its meat and eggs (caviar), but it was almost completely extinct by overfishing, water pollution and river construction by the end of the 1930s.
The last specimen of this sturgeon species, which was caught in the North Sea around Helgoland at the end of the 1960s, and made its rounds in the arena tank of the Helgoland Aquarium for many decades. Despite his old age – he was more than 50 years old – he was in a good shape and received vitamins only and a remedy for osteoarthritis to maintain his flexibility.Despite the global decline in fish stocks, after more than thirty years of research, French scientists have succeeded in securing the continued existence of this fish species through artificial reproduction in aquaculture. The restocking of rivers in Europe with this sturgeon species has already started.
The last European sturgeon to be caught in the North Sea around Helgoland in the late 1960s.
Photo: E. Hensel
Ocean east of Florida and south of the Bermuda Islands, with extensive populations of the floating brown alga Sargassum, starting from the European coasts he covers a distance of approx. 5000 kilometers. He swims around 30 to 40 kilometers a day for five to seven months without eating. When it reaches its destination, it spawns in the upper, warmer layers of water, and then dies. The fish larvae then embark on the same long journey – but in the opposite direction – to the coasts and rivers of Europe, where they develop the shape of the adults, grow and then migrate again to reproduce. How the migration takes place exactly, at what depths, and with what orientation aid is still largely unknown. Presumably, the adult animals swim to the Sargasso Sea at considerable depths and orient themselves on the earth’s magnetic field, while the larvae use the Gulf Stream as a “means of transport”.
The European eel (Anguilla anguilla)
Photo: E. Hensel
For orientation, fish basically use the same senses as humans, i.e. sight, hearing, smell and touch. In addition, a so-called lateral line organ can also perceive pressure differences in the water. The mechanism of stimulus perception and the structure of the sensory organs differ fundamentally from those of land dwellers due to the special properties of the surrounding medium – sea water. In addition, many fish also have a very pronounced electrical sense. On the one hand, they can perceive electrical fields in the immediate vicinity and use them to find prey, on the other hand, they also use the earth’s magnetic field during its migrations.
In order to find the local waters of sturgeon, eel, sea trout and other regularly migrating fish species, the sense of smell with which the animals perceive and recognize the river’s typical “smell” – that is, the chemical composition – is ultimately decisive.