Migration

Who travels where?

There is a huge number of migratory fish in our local waters: eels, shad, salmon, sturgeon or smelt. They all travel between the sea and the river, but the direction in which the different species travel varies. But almost all fish species and even insects make compensational migrations between spawning and nursery habitats in the rivers. Migration is a way of life that is closely linked to living in a dynamic system.

The European eel is a catadromous fish, it spawns south of Bermuda in the western Atlantic Ocean. It takes many years for the developing eels to arrive at the coast of Europe as glass eels. Here, they swim up the rivers, feeding on insects, worms and smaller fish and growing to a length of up to 1.2 metres. Then, these snake-shaped fish travel downstream back into the sea on their 5,000 km journey across the Atlantic to breed and produce offspring themselves in the Sargasso Sea.

Salmon and sturgeon as well as a lot of anadromous migrating fish do things differently. They spend their lives as adult fish in the sea and swim upriver to spawn at an age of 3-4 years. Salmon have their feeding grounds near Greenland, and the adult fish swim upriver to lay their eggs in the upper reaches of the rivers. Most of them carry out this difficult journey only once in their lives. In contrast, sturgeon take longer to grow and build up reserves, the return to their natal river after 15 years and span in the mid-section of the larger rivers. They invest less energy in this process and come back to spawn in the rivers time and time again over a period of more than 100 years.

Why do migratory fish need so many different habitats?

There are many reasons for migration. Many beings are forced to leave their habitats due to a change in environmental conditions. For example, flooding events can frequently cause migration among water dwellers who try to avoid suboptimal conditions.

In the case of migratory species, the reason for migration is the fact that changing habitats allows a larger number of offspring to survive and grow since spreading out allows to utilize more and different feed sources. The larger the fish get, the more drastic the change in environment can be.  While adult fish mostly feed on larger prey items such as small fish or shrimp, fish larvae feed off their egg yolk sac reserves and of tiny crustaceans and worms afterwards. To protect them from predators, the smaller juvenile fish also need more sheltered areas than their parents, who often do not fall victim to predators due to their size.

In order to meet these various challenges, fish make use of various habitats. The most extreme form of such migrations can be found in those fish that travel between the river and the sea. This strategy has a number of benefits, as the habitats mainly serve one purpose each: the spawning grounds offer the optimal conditions for the safe development of the eggs, but the juveniles which hatch from these eggs have to migrate to find enough food. Furthermore, the division of the habitats among different ages reduces the amount of competition between the generations. Migration thus represents an alternative survival strategy for these fish, and occurs in a variety of species in numerous places.

What challenges do migratory fish face?

On their journey between river and sea, migratory fish have to face many natural challenges. Predators with differing hunting strategies lie in wait in the areas of water which they travel through. This means that fish have to learn to adapt to their predators.

Furthermore, the change between environments also requires being able to adapt between the saltwater of the sea and the freshwater of the rivers. As salt removes water from the body, the fish would dry out when in saltwater, if they would not be able to regulate their water contents. Some fish, which drink the salty seawater or absorb it through their skin, emit the excess salt through their gills. Other sea dwellers avoid taking in too much salt or produce highly-concentrated urine, whereby as little water as possible is excreted. On the other hand, the low salt concentrations of the rivers also present migratory fish with the challenge of preventing to loose valuable minerals from their body.  For this, they use their gills and excrete a very thin urine.