Why is Pisces

fishes

From armored car to coral fish

Whether in the ice-cold Arctic Ocean, in the warm waters of the tropics, in the pitch-dark deep sea or in shallow pools - fish have adapted to even the most adverse conditions.

Their oldest relatives, the placodermi or tank fish, still looked rather clumsy. The head and front body of the giants, up to ten meters long, were protected by thick bone plates.

The prehistoric fish, which became extinct in the Devonian (about 410 to 360 million years ago), were the first vertebrates with jaws. They originated in fresh water and later also conquered the sea.

The cartilaginous fish (sharks, rays, chimeras) and the bony fish developed from the tank fish. In contrast to cartilaginous fish, the inner skeleton of bony fish is ossified by calcium deposits.

Another achievement of the bony fish is the swim bladder, which regulates the buoyancy in the water. Cartilaginous fish like the shark, on the other hand, have to swim constantly in order not to sink to the bottom. More than 96 percent of the fish species living today belong to the bony fish.

What makes a fish a fish?

Fish are cold-blooded vertebrates that live in water and have gills and scales. Unlike most terrestrial vertebrates, fish move by winding their spine sideways. Bony fish have a swim bladder. The organ originally split off from the intestine and probably developed into the lungs in the other vertebrates.

The swim bladder of the fish regulates the buoyancy in the water. The fish can selectively pump gas into the bladder to ascend, or let it out to descend.

Most fish lay eggs for reproduction. The females often release several million eggs, the roe, into the water, which are then fertilized by the male sperm, the fish milk.

Larvae hatch from the eggs, and in the first stages of life they feed on a surrounding yolk sac. But there are also fish such as sharks, rays or guppies that give birth to their young alive.

Salt and fresh water fish

Marine fish have special mechanisms to adapt to the salinity of the water. The salt concentration in seawater is much higher than that in the tissue and blood of the fish.

Because of this difference in concentration, a sea fish would be sucked out if it were not able to constantly replace the liquid. It would dry up in the middle of the water.

Using special devices in the gills, he can desalinate the seawater so much that it corresponds to the concentration of his body fluid. So, unlike us humans, marine fish can drink salt water.

Freshwater fish have exactly the opposite problem: their body fluids contain more salts than the surrounding water. In order to compensate for the difference in concentration, water continuously flows into it over the surface of the skin. In order not to burst, a freshwater fish excretes the water through its kidneys.

Dwindling habitat

From the point of view of a fish, a river is attractive if it offers plenty of food, sufficient oxygen to breathe, and good spawning and hiding places. In addition, it should not completely freeze over in winter. To meet these demands, many fish migrate up and down the rivers in the course of their lives.

Eels, for example, spawn in the Atlantic Sargasso Sea off the Bahamas and then swim in large schools thousands of kilometers up the European rivers. They spend the years up to sexual maturity eating their fill in fresh water.

Salmon do it the other way around. They hatch in the source region of the rivers and migrate into the sea in the course of their life. They only start their way back shortly before spawning.

Even fish that are not migratory fish like salmon, eel or lamprey stay in different water zones in the course of their lives. Pike, for example, like to spawn in flooded meadows, but are otherwise more likely to be found in deep water.

Many fish need different flow patterns, food, light and temperature situations in order to be able to develop. A natural river gives them these opportunities. But more and more rivers are being blocked by dams, straightening and power plants in such a way that many fish species are deprived of their livelihood.

Water quality in German waters

Although the water in German rivers has become significantly cleaner over the past few decades, most of the more than 60 German fish species are doing badly. Weirs, hydropower plants or sluices are insurmountable obstacles, especially for non-local species.

While the salmon jumps over dams from half a meter high without any problems, the bullhead, one of our most common domestic fish, is blocked by a 15 cm high dam.

Elaborately built fish ladders are intended to solve the problem in many German rivers. In older structures, however, fish such as grayling or barbel did not make it up the "bypass". Newer fish ladders, on the other hand, try to take into account the needs of all species - if necessary with different sized steps.

In addition, industrial wastewater and fertilizers from agriculture still endanger the habitat of the fish. Predatory fish like salmon also have problems coping with the general warming of the waters.

Fish farming and its problems

We have been breeding carp successfully for centuries. They are mass fish that lay millions of spawning eggs and thus multiply at breakneck speed. They are also relatively undemanding in terms of nutrition.

The breeding of salmonids (salmonids) is difficult in our latitudes on a large scale. A lot of protein has to be fed in, which is given in the form of sea fish waste or soy. The fish meal comes from, among other things, the bycatch that occurs in sea fishing. The only way to counteract overfishing is to breed herbivorous fish.

In tropical regions and in the Mediterranean, fish farming is an important source of income. Despite the many advantages over free commercial fishing, there are also ecological problems. The intensive keeping in a confined space requires the use of medication - often only for prevention in order to avoid epidemics in the cultures.

Breeding animals that are released can also displace native species or change the genetic makeup of their wild relatives.