V. V. Kostin

Behavioral Differentiation of Underyearlings of the Black Sea Salmon Salmo trutta labrax: Rheoreaction in the Year Preceding Smoltification

Hatchery rearing underyearlings of the Black Sea salmon Salmo trutta labrax at the age of five-six months (nine-ten months before the beginning of smoltification) divide into two spatial groups—bottom group and pelagic group. This is caused by lack of territory (bottom area): the minority of fish are bottom-dwelling—such fish are territorial, they extrude the majority of individuals to the water column. It is determined experimentally that the bottom-dwelling fish belonging to the bottom group, in comparison with the fish of pelagic group, are characterized by higher critical flow velocities and lower locomotor activity in the rheogradient. Part of individuals with the static type of rheoreaction among them is higher. The fish from the pelagic group do not have such resource as individual territory. This fact modifies the probability of manifestation of resident and migrant behavior after starvation (during ten days). The individuals from the pelagic group demonstrate the behavior characteristic of future migrants (downstream movement) to a higher degree. The individuals from the bottom group, in comparison with pelagic fish, demonstrate, to a great degree, behavior typical for future residents (upstream movement).

On methods of determination of the rheoreaction type in fish

Type of rheoreaction is investigated in some species of salmonids, cyprinids, and percids. Three types of rheoreaction are discerned: positive (the fish prefer to move against the current), static (the fish retain their position in the current), and negative (the fish prefer to move downstream). It is shown that the type of rheoreaction is probabilistic and is determined by the motivational state of fish. Procedures used in investigation of the rheoreaction type are discussed.

Hormonal status in different phenotypic forms of Black Sea trout Salmo trutta labrax

In three phenotypic forms (parrs, trout form, and kumzha form) of fish-farm Black Sea trout (age 1+) and wild Black Sea trout (age 1+ and 2+), Salmo trutta labrax, inhabiting the basin of the Mzymta River, the content of monoamines (dopamine, noradrenalin, and serotonin) and of their metabolites (3,4-dihydroxyindolacetic acid, 3-methoxy-4-hydroxyphenyl glycol, and 5-hydroxyindolacetic acid) was investigated in brain regions (forebrain, optic tectum, brainstem, hypothalamus, and hypophysis) and the level of cortisol, thyroxin, triiodothyronine, growth hormone, and adrenocorticotropic hormone was investigated in blood. Data on length and weight of fish body and maturation stage of gonads are indicated. The hormonal level differs in the fish of different phenotypic forms. It is the highest in specimens of the kumzha form which completed parr-smolt transformation. Among wild fish, numerous mature females at age 1+ were found which indicated the presence of a dwarf form of the Black Sea trout in the Caucasus in the Mzymta River.

Dynamics of cortisol and the development of the glucocorticoid function in the early ontogenesis of Atlantic salmon Salmo salar

The dynamics of cortisol were studied during the embryonic and early postembryonic development of Atlantic salmon Salmo salar (in brood obtained from the same pair of spawners, but with differing hatching dates). The initial level of maternal cortisol in the eggs after fertilization comprised of 25.6 ± 2.2 ng/g, and decreased continuously in the development process to 1.1 ± 0.2 ng/g by the 95th day after fertilization (t1) or the 18th day after hatching (t2). The first increase in the concentration of endogenous cortisol to 6.1 ± 1.2 ng/g was recorded at t2 = 24 days. Then the level of cortisol increased constantly and reached 32.8 ng/g by t1 = 149 (t2 = 72). A comparison of the hormone dynamics in two offspring groups from the same pare of spawners, differing in the longevity of embryogenesis, demonstrated that the level of cortisol increased after hatching more intensely in fishes with shorter embryogenesis. An injection of DL-aminoglutethimide, a compound preventing the synthesis of cortisol, significantly decreased the concentration of cortisol for the first time at the day of t1 = 101 (t2 = 24), i.e. at the first moment of the increase of endogenous cortisol. The effect from the adrenaline injection as a stress hormone was observed only at the day of t1 = 149 (t2 = 72), i.e., during the transition to the fry stages of development. Problems concerning when steroidogenesis begins, of the formation of the hypothalamic-hypophyseal-interrenal axis as a single hormone-competent ensemble, and of the development the stressed response in salmon juveniles are discussed.

Rheoreaction as a mechanism of formation of phenotypic groups of underyearlings of the Atlantic salmon Salmo salar

Experimental investigation on rheoreaction as a behavioral mechanism in the formation of phenotypic groups of underyearlings of the Atlantic salmon Salmo salar is performed. Juveniles of the Atlantic salmon are investigated at the age 0+ from the coastal group and from the tributary group in the period of its primary distribution from redds. The underyearlings from the tributary group differ from the coastal fish in increased critical current velocity, the fact that among them the most portion of individuals have a positive type of rheoreacrtion, and the fact that they prefer currents more often. The behavioral mechanism of spatial separation of the spreading juveniles at confluence of two currents is revealed: stronger individuals with the positive type of rheoreaction and the expressed rheopreferendum mainly move against the flow at a higher speed and manage to get into a tributary. The weaker fish with static and negative dynamic types of rheoreaction, having no expressed rheopreferendum, prefer the flow with lower current rate and turn out to be in the coastal zone of the mainstream.

Etho-biochemical mechanisms of early differentiation in juveniles of the Atlantic salmon Salmo salar

The social status of hatchery-reared juveniles of Atlantic salmon Salmo salar at the age of 11–19 months in conditions of lack of shelters was investigated experimentally. It was demonstrated that biochemical differentiation in such juveniles begins at the age of 13 months—one month before the first differences in the social status of fish and four months prior to the beginning of smoltification. The dominants occupy bottom areas with shelters and extrude the subordinates to the water column. The subordinates undergo smoltification or die from aggression of the dominants. Concentrations of dopamine, noradrenalin, and of their metabolites in the brain of fish in the beginning of smoltification were higher in the dominants and, by the end of smoltification, were higher in smolts. The social behavior of fish is considered to be one of the ethological mechanisms of differentiation of salmon juveniles into smolts and parr.

Influence of shelters and food resources on smoltification of juveniles of the Atlantic salmon Salmo salar

Influence of feeding conditions and of the presence of a various number of shelters in basins on development of differentiation of juveniles of Salmo salar at the age 1+ and formation of smolts were studied under experimental conditions. The experiments had been carried out on groups of ten fish. The influence of investigated factors turned out to be significant. Under normal feeding four times a day, the quantity of formed smolts depends on the number of shelters: the more shelters that are available, the fewer smolts appear in groups. The presence of one shelter is a strong stimulant enhancing aggressive activity towards conspecific individuals, resulting in destruction of some fish in the group. Under feeding once a day, i.e., under food shortage, more fish perished, and the number of smolts decreased as the fish did not accumulate a sufficient energy potential for the morphophysiological transformation. One of the potential mechanisms of development of differentiation of fish juveniles into parrs and future smolts is discussed, based on domination of some fish possessing the individual territory and exclusion of subdominant specimens into open water as a result of social interaction.

Patterns of movements of fish with negative type of rheoreaction

Experiments carried out on underyearlings of four fish species—roach Rutilus rutilus, carp Cyprinus carpio, bream Abramis brama, and perch Perca fluviatilis—at illumination of 200–300 lx showed that the specimens with negative type of rheoreaction move downstream mainly (47–93%) in the active form, with their heads downstream, and rarer (3–37%) in the active-passive form, with their tails downstream. Increase of motivation for downstream movement caused by starvation leads to a higher part of fish with an active form of movement and to greater speed of movement of underyearlings. Active downstream movement is a manifestation of rheoreaction.

On the mechanism of orientation and navigation of sockeye salmon underyearlings (Oncorhynchus nerka Walb.) during feeding migration in the inlet–lake–outlet system

Mechanisms that define the choice of migration direction of sockeye underyearlings in the water system of Lake Kurilskoe have been experimentally explored. The choice of direction of feeding migration and maintenance of lake habitats by sockeye are determined by the combination of two factors: water current and lake odor. In the presence of lake odor, early juveniles select migration against the water current; without lake odor they select downstream migration. Such a combination of factors is universal for the orientation of the early juveniles of sockeye that have emerged in the outlet, inlets, and right in the lake.

Comparison of behavioral and physiological–biochemical indices in fingerlings of roach Rutilus rutilus (L.) from the Ild River and Ild Bay in the Rybinsk Reservoir

It is found that the size–mass, physiological–biochemical, and behavioral parameters in roach fingerlings from the upper and lower (mouth) reaches of the Ild River differed during the autumn season. Compared with the roach from the upper reach of the river, the fingerlings of the same age from the mouth part have larger lengths and weights and a higher concentration of cations in the body. In experiments, the fish from the mouth part selected a lower (~4°C) temperature and showed a lower resistance to the water flow. The reason for differences in behavioral responses in two groups of juveniles of roach is probably, on the one hand, their physiological and biochemical characteristics and, on the other hand, the motivational component of fish behavior in different environments—hydrological, thermal, and hydrochemical conditions and different food items in geographically remote parts of the same river. It is assumed that in autumn roach fingerlings migrate from the littoral of the mouth part of the river into deeper parts under a decrease in water temperature before the downstream migration of the fingerlings in the rivers. This is probably due to the earlier achievement of the migration status of fingerlings from the mouth part than that in fish from the upper reach of the river and due to the peculiarities of their physiological state. Negative rheoreaction, manifested in the experiment in ~80% of the fish from the bay of the reservoir and ~55% of the fish from river, confirms the fish readiness to migrate and is likely due to the behavioral mechanism of fish migration from the littoral and downstream the river.