The paper analyzes the population structure of parasitic flatworms of the Himasthla larina species (Trematoda: Echinostomatidae). Abundance and size structure of their population directly relate to the size and weight of their first intermediate hosts, mollusks of the genus Littorina. When the invasion intensity is insignificant, growth of the size of parasitic organisms occurs simultaneously with the increase in their abundance regardless of the weight and size of the mollusk. If the weight and size of the host increase, the size of flatworms can grow only simultaneously with an increase of the host's weight. Further on inverse relationship between the size and abundance of parasites is observed. As a result of this study we may conclude that the formation of populations of trematode parthenites is subject to the same laws as the formation of populations of free-living organisms.

Gray seals are widely used in biotechnical systems for various purposes. In the study of the effect of an artificial electromagnetic field (EMF) with 8 Hz frequency on the performance of gray seals, the method of conditioned reflexes has been used (one of the most informative for experimentally studying the functional state of the higher divisions of the Central nervous system). Two adult female grey seal have been used in the experiment; two series of experiments have been conducted for each individual gray seal. The first series of experiments has been conducted in the conditions of natural (background) geomagnetic field. During the second series of experiments, individuals of gray seals have been affected by an artificial electromagnetic field throughout the training. The criterion for assessing the effect of EMF on the operability of animals is the implementation of skills due to their behavior. During the study, the following parameters of instrumental reflexes have been monitored: the average retention time of the target pointer (the Target command); average target search time (the Search command); total time spent on training; the number of mistakes made by the seal per workout. It has been shown that experimental animals are characterized by similar properties of higher nervous activity. Experiments without exposure to an electromagnetic field have shown comparable values of the target index hold time, target search, and the number of errors made. When conducting training with the effects of artificial low-frequency electromagnetic field, the animals were more excited, made more errors when executing commands; the target pointer hold time was significantly reduced, and the target search time increased by one and a half times. Thus, when exposing gray seals in the artificial electromagnetic field with the frequency of 8 Hz, an annoying, alarming effect of this field has been observed manifesting itself in a decrease in the effectiveness of instrumental conditioned reflexes that means reducing the operability of gray seals.

In August–September 2015 in the southwest part of the Kara Sea, visual censuses of ringed seal (Phoca hispida) and bearded seal (Erignatus barbatus) from research vessels were carried out in the water area of 49 814 km2. For the first time, for calculation of abundance and density on ice-free area the SEAL-CALCULATION (TYULEN-RASCHET) software was used with application of separate extrapolation method by the selected sectors. Calculation for the whole census polygon yielded abundance of ringed seal of 4163 ± 16.9% and abundance of bearded seal of 1026 ± 28.4%. On the basis of the census results, a preliminary estimation of distribution of pinnipeds by various depths is presented.

The results of studying the influence of optical characteristics of sea water on the ability of earless seals to differentiate tablets of blue and red colours have been presented. The experiment involved four specimens of gray seals (Halichoerus grypus, Fabricius, 1791), five species of harp seals (Pagophilus groenlandicus, Erxleben, 1777), and two ringed seals (Pusa hispida, Schreber, 1775). The seals have been trained to choose from a pair of plates the plate of blue colour and to mark it by touching. The experiments have been conducted on air (control study) to produce the sensitivity curve in the air without affecting transparency and colour depth and on 2 m underwater. It has been found that at the illumination above 50 lux in all model animals the proportion of correct answers corresponds to the control one and is 89 ± 3.6 % for gray seals, 74.8 ± 6.2 % for harp seals, 71 ± 5.7 % for ringed seals. When light level is reduced the proportion of correct answers decreases and becomes equal to the random selection at 15 lux for gray seals, 20 lux for harp seals and 25 lux for ringed seals. It has been shown that at the depth of 2 m with illumination below 50 lux the average number of errors for the experience slightly increases from 1.2 to gray seals, harp seals at – 0.6, in ringed seals – 1.8. It has been found that the transparency and colour of water have little impact on the ability of model animals to differentiate blue and red tablets at the given depths. Only the light level has the main influence on colour perception of model animals under water.

The distribution of hydrochemical parameters observed in the Pechora Sea in July 2002 has been considered in the paper. The data collected during the sea expedition on board R/V "Dalnie Zelensy" have been used in this study. Qualitative and quantitative distributions of the main hydrochemical parameters – oxygen, silicates and nutrients

This article presents a method of assessment of the amount of polar bears on the basis of data on vessel counts. The results of 32 vessel transect counts in the southern part of the Kara Sea are used as the primary data. They are processed in an original program using separate extrapolation by sectors and summarizing of data of several different counts. In recent years, the calculated amount of bears in the studied region has varied from 3300 to 3100 individuals (3200, on average). The relative statistical error of estimations smoothly decreases from 8 to 7%. The confidence interval of the estimation at statistical error of 7% and probability level of 95% is 2700–3500 individuals.

Based on a series of specific examples, the paper addresses causes and consequences of invasion of alien fauna species into ecosystems of the Azov Sea-Black Sea Basin and the Arctic Basin and their connection with anthropogenic and climatic factors.