Voprosy ihtiologii

The results of long-term studies of the dynamics of population parameters of the vendace Coregonus albula in Lake Syamozero in periods with different habitat conditions are presented. The vendace is the most abundant fish species in the lake and was the main object of commercial fishing for 39 years (1932-1970) (60% of the total catch). From 1971 until 2000, the input of nutrients into the lake increased significantly due to high human economic activity in the catchment area. This led to a change in the hydrochemical and hydrobiological regimes of the lake. As a result, favorable conditions were created for vendace feeding. The biomass of zooplankton which is the main component of its diet, increased fivefold from 1950 to 2000 (from 0.4 to 2.1 g/m3). However, the conditions of reproduction of vendace significantly worsened due to siltation of spawning grounds, which led to the mortality of eggs during a long period of its embryogenesis (6-7 months). The decrease in the vendace abundance in 1971-2000 was also caused by the accidental penetration of a new species, the European smelt Osmerus eperlanus into the lake. For a long time (about 15 years) vendace was not found either in catches or in the diet of predatory fish species of the lake. In recent years, as a result of a decrease in the impact of human activity on the lake and the disappearance of the smelt in 2015 due to its infection with the parasite, the vendace population has begun to recover.

The fish species diversity, fish density, and distribution in the nearshore beds of Zostera marina and Ulva fenestrata have been assessed by SCUBA visual counts in May–September and November 2021. In total, 23 fish species have been registered. The fish species richness increases from 8–13 species in May to a maximum of 17–19 species at the end of June. Then it slightly decreases in July–September and reaches its lowest values in November (2–5 species). The same changes have been noted for the fish density. Fish diversity in the Zostera beds is higher than in the Ulva ones. The similarity of the fish species composition between Zostera and Ulva is low (the Sørensen-Czekanowski index varied from 0.32 to 0.44). In general, fish density is low (from 2.4 to 112.4 ind./50 m2 in Ulva (in November) and Zostera (in July) beds, respectively), although with an exception for Opisthocentrus spp. and Gymnogobius heptacanthus juveniles. An abnormal high bottom water temperature (above 26°C) in late July–early August has led to temporal avoidance of shallow water with plant beds by fishes (except for Gymnogobius heptacanthus) and their migration to a depth of 3.5–4.5 m, where the temperature was lower by 4.0–5.7°С.

Information has been provided on the distribution, temperature conditions of habitat and size composition of the whitecrest eelpout Petroschmidtia albonotata in the northeastern Sea of Okhotsk in July–September 2010. This species was found within the intermediate water mass at depths of 213–651 m at a water temperature at the bottom of 0–2.3°C. Accumulations of increased density were noted on the western slope of the TINRO depression and south of the Lebed trench in the bathymetric range of 213–300 and 401–500 m, respectively. The total length of the whitecrest eelpout in catches varied from 14 to 48 cm. A significant proportion of small individuals (< 20 cm in length) was recorded on the western slope of the TINRO depression at depths < 300 m, while larger individuals were caught to the south. Differences in thermal conditions of this species habitat within the study area have been revealed. North of 55°N, the whitecrest eelpout is concentrated at a temperature of 1.0–1.2°C, to the south—at 1.8–2.0°C. A comparison of the obtained data and literature data indicates that, depending on the region, species of the genus Petroschmidtia are characterized by significant differences in preferred depths and temperature conditions in their habitats.

Environmental and ecological effects of the dam of the high-pressure Bratsk Hydroelectric Power Station (HPS) on the fish population and the probability of fish downstream migration from the Bratsk Reservoir have been studied. It has been revealed that thermal stratification of the water column is a significant factor in the ecological differentiation of the fish population in the upper reaches of high-pressure hydroelectric plants. According to hydroacoustics data, most of the ichthyomass, consisting of "warm-water" representatives of the Percidae (65%) and Cyprinidae (22%) families, is concentrated in the relatively warm layer of the epilimnion. Juvenile fish from 30 to 50 mm in size (up to 70% of the total fish number in this layer) are also concentrated here. In the cold-water hypolimnion, the fish population is represented by large single individuals of whitefish (Coregoninae). The data on distribution of fish in the lower and upper reaches of the dam, fish nutrition patterns, growth rate, and the presence of injuries that occur when fish pass downstream through hydroelectric plant dams are presented. Revealed differences in the growth rate of perches from the upper and lower reaches are associated with their constant inhabiting of water masses with contrasting temperatures. Net catches in the lower reaches over the four seasons of research lack injured individuals. These facts prove the absence of mass downstream passage of fish through the Bratsk HPS dam. Juveniles of cyprinids and perches accumulate in the upper warm layer of 0–10 m, i.e. outside the zone of the intake flow formation, which makes their entrance to intake openings at a depth of more than 20 m hardly possible.

Scientific monitoring of pelagic fish stocks in 2004–2005 and 2014–2017 provided information on characteristics of sexual maturation of Atlantic chub mackerel Scomber colias in the coastal waters of the Central-Eastern Atlantic. Based on these results we describe ecological and geographical variability of some biological characteristics of the species. Features of sexual maturation of females and males of the mackerel can be described by a logistic equation obtained in the R programming environment. Fork length, at which 50% of individuals aged 1+ are mature, is 18.4 cm for females and 20.0 cm for males; 100% of individuals are mature at age 3+ with a fork length of 32.3 and 30.8 cm for females and males, respectively. A high proportion of spawning chub mackerel individuals in the Central-Eastern Atlantic have been recorded from February to March and from June to July. Chub mackerel in the coastal waters of this region are similar in life cycle characteristics to the populations of the waters off the Canary Islands and the Madeira Archipelago, but differ from the populations of the waters off mainland Portugal, the Azores, South Africa, and the Bay of Biscay in faster sexual maturation and growth rates.

The effectiveness of the food chemosensory attractants for attraction and angling of wild common carp, Cyprinus carpio by the bottom tackle of two types (carp “boil” device and “makushanka” device) was evaluated in the experiments, done in 2018 in the natural water reserve—the Akhtuba River (polygon of 80x30 m). There were 6 types of the bait used: sunflower meal, and boils with flavor of “Strawberry,” “Plum,” “Tutti-frutti,” “Sweet corn” and “Mussel.” The wild common carp was attracted to the polygon by inserting into the river the attracting groundbait, combined from sunflower meal cake, corn and wheat kernels and red clay (ratio 1:1:1:5). In 2–3 days after beginning of the attraction, the number of carp and its daily catch increased sharply and in a week reached the level which stay stable up to the end of experiment, that lasted for 79 days, from June, 19 through September, 5. Total catch was 1048 fish of body weight from 2 to 14 kg, average daily catch was 7.23±2.13 specimens. The catch on the makushanka was stable during the whole time of the experiment and was about 13–15% of the total catch, while the catch on the boils of the different flavors vary. At the beginning of the experiment the most of carps were catched on the boils having “Mussel” flavor. Then the catch on this boil type decreased to the middle of July, but the catch on the boil having “Sweet corn” flavor tended to increase up to 70–75% of the total catch on the boils of all types. The maximal catch on the boils with “Sweet corn” flavor correlated with the appearance of many anglers on the river, who used kernel for fish attraction. The catch on the boils of fruit and berry flavor vary, but stay relatively low, the minor catch was on the boils of “Strawberry” flavor. There was no plausible difference in the fish body weight, landed on different baits. The obtained data support the results of other studies showing important role of chemoreception in common carp behavior. We suppose that attraction of the wild common carp to the study polygon was due to the substances released by groundbait and accepted through the olfactory system, while the catch on the particular bait was a function of its smell, taste and textural features. It was found out that effectiveness of the food attractants can change during relatively short timeline (summer season), thereafter indicates the plasticity of the fish preferences mediated by chemosensory systems.

The first recorded case of leucism is reported in the Greenland halibut Reinhardtius hippoglossoides from the waters off the northeastern tip of Sakhalin Island. The caught individual, a sexually mature female with a fork length of 83 cm, had depigmentation over almost entire body. The only pigmented parts on the eyed side of the body were pupils, a small spot on the caudal fin, a spot around the pectoral fin, and a border of the gill slit.