European sturgeon Acipenser sturio
Photo: E. Hensel
Many older residents of the North Sea island of Helgoland still remember that the island’s fishermen caught a spectacular and sensational catch at the end of the 1960s: a fish over a meter long, brownish-gray in color, with five rows of bony scales, which was one of the last specimens of the European sturgeon Acipenser sturio to be found in the North Sea.
Three more specimens were caught, but already died and found their way into a freezer. The surviving conspecific was in very good health, only vitamins and occasionally a remedy for osteoarthritis were given to him in addition to his food. A special method was developed for this targeted feeding, which ensures that only the sturgeon receives the feed additives: the feed slid down to the bottom of the tank in a red-colored, approximately one and a half meter long plastic tube (diameter approx. 5 cm). By tapping lightly on the upper end of the tube, the sturgeon was signaled acoustically that its food was about to be available at the lower end of the tube. In the meantime, not only he, but also other fish in the largest tank in the Helgoland Aquarium had learned this, so that a cloud of fish was already gathered around the end of the pipe when the sturgeon slowly approached. Only when it had reached the lower end of the pipe with its trunk-like mouth was the pipe pulled up a little, which released the food for the sturgeon. Just as the fish had learned that the knocking sound meant food, experience taught it to take in the food offered quickly, otherwise it would be devoured by the assembled competitors.
The main food of the fish in the Helgoland Aquarium consisted mainly of sprats and squids, which are obtained frozen from wholesalers. Cuttlefish were particularly suitable for feeding the vitamins and medication, and the food supplements could be “hidden” inside their bodies after defrosting. After successful feeding, the tube was removed from the tank and all the fish could now regain the pieces of sprat distributed in the tank.
The European sturgeon is characterized by four barbels standing in a row in front of the mouth. These identify him as a food specialist who prefers to feed on organisms living on the ground (benthic organisms). This also includes various lugworms that occur in the Gironde estuary on the French Atlantic coast, where the last remaining population of this fish species can be found.
As the name suggests, this species was originally distributed throughout almost all of Europe, both in its seas and its tributaries. As a migratory fish, this species migrates up to 1000 km up the rivers. There are historical sources which report that the animals have even reached the Havel near Potsdam. The adult animals spawn over a gravelly, current-rich bottom and then return immediately to the sea. The hatched fry, however, need about 3.5 years to migrate to the river mouth before they also live in the sea. But there are also sturgeon species, such as the Sterlet Acipenser ruthenusin the Danube, that live exclusively in fresh water.
Because of its special foraging behavior, in which it carefully scans the ground with its barbels, the sturgeon needs longer to find and take in food than other fish that intentionally strike on their prey. For this reason, care was taken in the aquarium to ensure that the food reached the bottom in sufficient quantities so that the sturgeon was also “fed”.In addition to its special diet, the European sturgeon has other peculiarities: From an evolutionary point of view, it is an “old” species of fish that lived on earth at the time of the first dinosaurs about 200 million years ago. This originality is also reflected in the anatomy of this “Living fossils”. The animal lacks the bones of the more highly developed “real” bony fish (Teleostei), which include, for example, the sea bass (Dicentrachus labarx) or the cod (Gadus morhua), which are also found in the arena tank.
In addition, the spine of the sturgeon consists of cartilage, and it has very original so-called ganoid scales, mainly consisting of a substance similar to tooth enamel. That is why it is classified in the group of cartilaginous ganoids (Chondrostei). However, five rows of scales made of bony substance already indicate the real bony fish. There are two rows of scales on the stomach side, another two on the side of the body and one on the back. The number of scales is species-specific, the European sturgeon has 24 to 40 bone scales per row. But not only its anatomy, but also its reproductive biology shows the sturgeon to be a very special fish: the European sturgeon does not reach sexual maturity until very late, males between the ages of 6 to 8 and females between the ages of 14 and 16 years. In addition, sexually mature males can reproduce every two years, but females only every four years. Yes – there are even individuals who, according to their age, should have been sexually mature long ago, but are still not capable of reproduction. A particular problem for sturgeon breeding in aquaculture is the fact that the sexes do not differ externally. In the Helgoland Aquarium, too, it was not known whether the European sturgeon was a male or a female. A biopsy or an ultrasound examination could have clarified this, but in view of the advanced age of the fish, this was not done.
In sturgeon aquaculture, the procedures mentioned are also too complex, so that research is being carried out into alternative methods. One possibility of gender recognition and separation would be the bioacoustic evaluation of the sounds made by the sturgeon. Because in sturgeon breeding, females are preferred because of caviar.
For this reason, too, the European sturgeon had almost completely disappeared in German waters in the 1930s after around 50 years of fishing. After the large males, the sexually mature females and young animals, which are of particular importance for the continued existence of the population, were increasingly caught. In addition, there was water pollution and the construction of the rivers, which prevented the fish from reaching their spawning grounds above weirs and dams.
The demand was so high at that time that a separate hall was built on the fish market in Hamburg for the sale of boneless sturgeon meat and caviar. And the sturgeon even found its way into the labor regulations of the Hamburg housemaids; in it the servants insisted that they should eat sturgeon meat twice a week at most.
Today the European sturgeon Acipenser sturio is considered to be extinct in Germany, and in the Washington list of species protection it ranks in the category of highly endangered animal species. Fortunately, after more than 30 years of research, French scientists near Bordeaux succeeded a few years ago in breeding a large number of juvenile fish in aquaculture. They form the basis for restocking measures both in the Gironde and other European rivers, such as the Elbe and the Oste in Germany. Perhaps sturgeon can even be fished in the North Sea again in a few decades.
The sturgeon in the Helgoland Aquarium was last around 50 years old and has been swimming in the largest tank in the Helgoland Aquarium – the arena tank – for over 40 years. It could be up to 100 years old, 6 meters long and weigh up to 600 kilograms. These enormous body dimensions are only surpassed by the beluga sturgeon Huso huso(also known as European Hausen), which can reach a length of 10 meters and a weight of 1000 kilograms, making it the largest freshwater fish in the northern hemisphere. It is now also known that animals in captivity can get even older than in nature.
Despite the high growth potential of the species, the sturgeon in the Helgoland Aquarium was now probably fully grown. Its size was proportionate to the size of the pool, so that it was kept in a species-appropriate manner. A longer transport to a larger aquarium would be associated with enormous health risks due to the animal’s old age and therefore not recommended. This also applies to a release into the wild, as it is uncertain whether the animal could feed itself in the sea. This sturgeon was of such great importance that it should not have been released from care. In the Helgoland Aquarium, he pointed out to visitors the problem of species loss and the research efforts to preserve and breed endangered species.
Author: Dr. Emanuel Hensel