The houting (Coregonus oxyrhynchus) is closely related to the common whitefish (Coregonus lavaretus) which is found in larger freshwater lakes and brackish waters.
The most striking difference between a whitefish and a houting is the pointed “snout” of the houting.
The houting also grows longer and heavier than the whitefish, and can tolerate waters with higher concentrations of salt.
This is clearly shown by the different habits and distribution of the two species. The houting moves extensively around the salt waters of the Wadden Sea in search of food, whereas the whitefish mainly stays in freshwater lakes or fjords with low salinity.
The houting spawns in the watercourses
The houting lives and matures in the Wadden Sea , but in the autumn it ascends the larger watercourses to spawn.
Only those watercourses which allow free passage can be used for spawning. Even small falls and dams can effectively block the passage of the houting to the spawning grounds, and it will not use fish ladders. The only form of fish passage which the houting can certainly clear are sweeps with large flows.
The houting spawns in the lower and central parts of the watercourses where the bed is at least 5-6 metres wide. The mature houting looks for reaches with a good current, a firm bed and evergreen aquatic plants. The spawning period is relatively short. In Denmark it lasts for only 2-3 weeks around November to December.
The houting spawns its eggs freely into the water. The adhesive eggs attach themselves to aquatic plants, stones or gravel. A single female houting has 20-30,000 eggs per kilo of its body weight, whereas, by comparison, a sea trout has 2-3,000.
The male houting matures sexually faster than the female. Thus some male fish are sexually mature when two years old, while most do not spawn until they are three years old. The female fish, however, do not mature until they are four years old, with some exceptions which spawn at three years old.
The distribution of the sexes for mature fish in the spawning grounds is very uneven. Normally there are considerably more males than females. There are two reasons for this imbalance. One is the fact that the males mature a year earlier than the females, another is that the males spend longer at the spawning grounds.
Migration back to the Wadden Sea
Once they have finished spawning, from early spring until May, the fish migrate down into the lower reaches of the watercourses and return to the Wadden Sea . This migration is probably determined by a rise in the temperature of the water.
The studies conducted by the counties of Ribe and Southern Jutland have also shown that some of the immature houting over-winter in the lower sections of the water systems, as has been observed for sea trout, for example.
The adaptation of the fry to the Wadden Sea
The houting succeeds in producing fry in some watercourses, but not in others. The environment required by the fry to reach the Wadden Sea once they have hatched is not as well known as, for example, for the sea trout and salmon. It does seem, however, that it is at this stage of the life-cycle of the fish where the most important causes of the decline of the houting are to be found.
The houting eggs hatch in February – March, and the newly-hatched fry are approximately 10 mm long. Studies have shown that the newly hatched fry cannot survive a level of salinity such as that found in the Wadden Sea . As the fish reaches a length of 30-40 mm , its physiology changes so it can withstand the move from freshwater to saltwater.
It is therefore probable that the survival of the houting depends on leading the newly hatched fry to areas with static water, such as flooded meadows, smaller lakes, large river bends and similar areas which constitute the natural rearing grounds. The fry live for the first spring months on zooplankton, which exists in abundance in such areas. Whether the small houting swim deliberately out to the Wadden Sea , like sea trout and salmon, or whether they are just passively carried out on the current, is not known. It is probable that the fry reach the Wadden Sea in April to May, as by now they have achieved a length of 30-40 mm .
The genealogy of the houting
The houting is a salmonoid, and belongs to the whitefish family. Studies conducted on Danish houting and whitefish show that the genetic difference is very small, whereas whitefish from the Baltic Sea show significant differences from the Danish whitefish and houting. The small genetic differences between North Sea houting and whitefish probably mean that the houting and whitefish diverged from one another relatively recently in evolutionary terms, i.e. within the last 10,000 years. It is probable that the Danish houting and whitefish migrated into the area after the last Ice Age via the river system of the Elbe , as it then existed.
The fact that the houting lives in both freshwater and saltwater has led to some fairly important adaptations which the whitefish does not display, such as tolerance to high levels of salinity and a different diet. Thus from the point of view of population genetics and evolution, it is very important to preserve the houting.
