Noteworthy is hardiness of rotan. According to the data of many authors (Yakovlev, 1925; Kirpichnikov, 1945; Sinelnikov, 1976; Kudersky, 1980, and others). It inhabits water bodies of different types endures well low oxygen content and water chemical composition. Rotan occurs mostly in water bodies that either have weak current or are stagnant with well developed higher water vegetation, in river flood plains with well developed vegetation, in littoral of lakes in swampy water bodies and even in swamps. It is particularly abundant in small water bodies with ground feeding, where in most cases rotan is the only representatives of their ichthyofauna.
Its single specimens usually after high water occur also in rivers. Adult individuals prefer deeper areas and juveniles stay in overgrown shallow water areas.
Rotan definitely belongs to eurytherm fishes (Kirpichnikov, 1945; Konstantinov et al., 1987; and many others). It demonstrates normal vital activity within a very wide temperature range from 1-2 to 20 and even 37 degrees C.
Its ability to freeze into ice and then to "melt" is well known (Yakovlev, 1925; Zaloznykh, 1982, and others). According to the information of sportfishermen from Belogorsk city of Amur Region (Sokolov, 2001) cavities in the ice where rotan winter have the appearance of a hemisphere of different diameter (from 20-30 cm to 1.5-2 m) the upper point of it is situated 30-60 cm of ice surface. It is filled with air mixed with water and pieces of ice, i.e. air ice wet mass.
Such hemisphere is very frequently situated above a hillock or a bunch of aquatic vegetation. Temperature in the place of wintering of rotans. Temperature in the locality of wintering of rotan is close to 0, they are in state of torpor and when taken out of ices slightly move. When they are put in a container with water their torpor passes quickly and fishes begin swimming actively (Reshetnikov, 2001). In the opinion of Bolonev and others (2002) by autumn salts accumulate in cavity liquids of rotan and they become a kind of a natural "antifreeze". As a result, no internal freezing occurs and ice crystals do not destroy tissues of fishes. According to the information from the book by Bolonev and others (2002) O.N. Yunchis, an expert in fish diseases from St. Petersburg demonstrates in his lectures a piece of ice with rotan. By the end of the lecture when the ice is melted the students can see a live rotan.
Such freezing into ice is described for rotan in introduced range (Vechkanov, 2000; Naumenko, 2002).
Sometimes rotan winter in a different way, like Carassius species they are buried in silt (mud) and stay immobile for several months. According to the data of Kirpichnikov 1945) rotan was found in small water bodies in the flood plain of the Sintuhe River in the Lake Khanka basin frozen to the bottom. In one such water body with an area of less than 2 square km more than 150 specimens of rotan were obtained at the age of 1-4 years immersed in a thick layer of silt constituting the bottom of the water body. No signs of suppression of vital activity of fishes were observed, moreover, evidence of cannibalism was found.
Better than other species of Amur fauna rotan survives also warming and drying of water bodies. According to the evidence of Kirpichnikov (1945) (Khanka Lake basin) rotan was discovered in the drying pond. As the water bodies dry the amount of bicarbonates increases considerably, and growth of total water hardness and carbonate hardness and also shift of environment response toward alkaline reaction, increase of chlorine concentration and decline of the amount of dissolved oxygen (to 14.2%) were observed. A day before complete drying, 224 specimens of rotan were found, none being sick or dead. There is evidence that in the summer when the sun dries out some overgrown small water bodies to such an extent that hard silt crust is formed on their bottom, rotan are covered by mucus and in a dense capsule fall in dormancy (summer "hibernation"?), like frogs (Bolonev et al., 2002).
The physiological study (Pilipenko, Tishchenko, 1986) shows a certain physiological specificity of rotan. Structural characteristics of hypotalamo-hypophisial neural system, low values of G6PhDH activity and at the same time rapid growth rate in all probability are related to the highly eurybiont state and ecololgical plasticity of this species. High abundance of rotan population in water bodies of the European part of Russia is probably determined not only by ecological factors, but also by some physiological characteristics of functioning of several systems of its organism. An abrupt acceleration of growth of juveniles in alternating temperature regime may be regarded as an index of extreme adaptation of this fish to abrupt temperature fluctuations in natural habitats (Pilipenko, Tishchenko, 1986; Konstantinov, et al., 1987).
Many authors (Seleznev, 1973; Lysenko, Vorobyova, 1975; Zaloznykh, 1982) emphasize that within the limits of the natural distribution range and in new areas rotan attains high abundance in small water bodies with poor ichthyofauna. In large water bodies with multicomponent ichthyocenoses density of rotan population is low. Probably in complex ecosystems increase of abundance of rotan is restricted by the pressure of predatory fish species.
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