Saulius Stakėnas

Impact of beaver dams on abundance and distribution of anadromous salmonids in two lowland streams in Lithuania.

European beaver dams impeded movements of anadromous salmonids as it was established by fishing survey, fish tagging and redd counts in two lowland streams in Lithuania. Significant differences in abundancies of other litophilic fish species and evenness of representation by species in the community were detected upstream and downstream of the beaver dams. Sea trout parr marked with RFID tags passed through several successive beaver dams in upstream direction, but no tagged fish were detected above the uppermost dam. Increase in abundances of salmonid parr in the stream between the beaver dams and decrease below the dams were recorded in November, at the time of spawning of Atlantic salmon and sea trout, but no significant changes were detected in the sections upstream of the dams. After construction of several additional beaver dams in the downstream sections of the studied streams, abundance of Atlantic salmon parr downstream of the dams decreased considerably in comparison with that estimated before construction.

Ecological Characteristics of 0+ Fish Communities in the Shore-Zones of the Middle Nemunas and the Neris Rivers

5243 individuals of 0+ juvenile fish belonging to 20 species were caught in the Middle Nemunas. The following fish species predominated in the fish communities: bleak, roach, minnow, riffle minnow, chub, bitterling, and dace. In the Neris River, 6226 individuals of 0+ fish belonging to 22 species were caught. Such species as roach, dace, bleak, chub, gudgeon, riffle minnow, and barbel predominated over others. While comparing the mean of density and the dispersion index of fish species, we can maintain that ecological conditions in the Neris River are suitable to lithophilous, lithophilous-phytophilous and psamophilous fish species. In the Middle Nemunas, there are enough biotopes suitable to lithophilousphytophilous and psamophilous fish species, but lithophilous fish species are highly influenced by the environmental factors, and more sensitive species are eliminated (barbel). Correlation of the zooplankton and 0+ fish densities (R2 = 0.667) shows, how important is the amount of forage resources for 0+ ...

Habitat Use of 0+ Fish Communities in Rivers of Lithuania

A relationship between environmental variables and 0+ fish communities was investigated in inshore river zones and offshore zones in 21 different river of Lithuania. In 278 study sites, a total of 59,864 young-of-the-year fish from eight families and 31 species was caught using a fish dredge. In warmwater rivers, species diversity and density were higher than in the same-sized coldwater ones. Based on ANOVA results, 0+ fish density was mostly influenced by water limpidity, filamentous and attached algae, river zone, and average water yield. Less impact was made by shade, channel shape, current velocity, and shallow water zone width. Quantitative parameters of 0+ fish communities were principally determined by larger scale environmental variables. Qualitative parameters of the communities were conditioned by properties of a narrow living habitat. Environmental variables that are important to the abundance of 0+ fish food decide quantitative parameters of 0+ fish communities, whereas they are not so importa...

The Growth of Brown Trout (Salmo trutta fario L.) in Different Climatic Regions of Lithuania

The growth rate of brown trout (Salmo trutta fario L.) varies across its distribution range, as well as in the rivers of three different Lithuanian climatic regions. Interspecific and intraspecific competition that is closely related even to minor climatic differences is considered to be one of the major factors determining differences in trout growth. The highest significant (p < 0.05) growth rate was found in juvenile trout at the age 0+ and 1+ in the rivers with the lowest species richness, whereas the growth rate in trout at the age 2+ and over was highest in the rivers with the highest species richness. No statistically significant correlation between trout size and density in different age groups was found in the investigated climatic regions of Lithuania, spare environmental capacity probably plays the major role here. However, significant differences in different age groups were observed in trout size and species richness, which attests to the impact of interspecific competition on trout growth.

Habitat Use by 0+ Fishes in Small Rivers of Lithuania

Studies of habitat use by the most common 0+ fish species in 16 small rivers of Lithuania were performed. The density of 0+ fish was proved to be most heavily dependent on environmental variables of the microhabitat, whereas species diversity of 0+ juveniles turned out to be almost evenly affected by environmental variables of all levels. Abiotic microhabitat variables were the ones that exerted a somewhat stronger influence on the share of 0+ juveniles in communities. The influence of mesohabitat variables on the share of 0+ fish in communities was proved to be comparatively weak, the impact of macrohabitat variables being the strongest. River temperature conditions exerted reliable influence on the greatest number of 0+ fish species. Stone loach, freshwater sculpin and three-spined stickleback were recorded as most heavily dependent on microhabitat variables, whereas roach, chub and perch were found to be most susceptible to the impact of mesohabitat variables, and minnow – to that of the macrohabitat o...

The State and Morphological Characteristics of Vimba (Vimba Vimba L.) Subpopulation in the Middle Nemunas

In 1996-1998, the isolated subpopulation of vimba (Vimba vimba L.) in the Middle Nemunas was investigated. It has been established that vimba makes approximately 0.06-0.15% of the total ichthyomass of Kaunas reservoir and of the Middle Nemunas. The same amount of vimba has been found in other reservoirs (Cimliansk water reservoir, Sengilejev water reservoir), where local subpopulations have developed (Volskis et al., 1976). In spite that the isolated subpopulation of vimba is very small, it is stable and its abundance is determined by natural factors (such as nutritive competition, abundance of predators, etc.). The isolated subpopulation of vimba of the Middle Nemunas is distinguished from other subpopulations of the Nemunas basin by its peculiar plastic features. The main differences are determined by L, l, ao and id features. (L – body length to the end of the tail (cm), l – body length (cm), ao – the length of a head, id – the length of a caudal peduncle (%) in relation to a body length l cm). The vim...