Freshwater ecosystems in Lithuania are threatened by the introduction of invasive fish species including Neogobius fluviatilis, N. melanostomus, Perccottus glenii, and Pseudorasbora parva. Data on helminth parasites of these fishes have not been comprehensively studied, with only two reports on parasites of N. melanostomus from the Curonian Lagoon and Baltic Sea, Lithuania. We examined 278 fish individuals representing 4 invasive species from 13 waterbodies. Using morphological and molecular analyses, we identified 29 helminth taxa representing 15 digenean trematodes, 6 nematodes, 4 cestodes, 2 acanthocephalans, and 2 monogeneans. With 18 species, N. fluviatilis had the highest helminth diversity, followed by N. melanostomus (11 species) and Ps. parva (8 species). Perccottus glenii was found to be free from helminth infection. The availability of historical information on the native fish parasites in Lithuania allowed us to conclude that out of the 29 recorded species, invasive fish serve as hosts for 22 local fish helminth species, while 7 helminth species have been reported exclusively in invasive fish. Based on newly obtained and previously published data, a total of 34 helminth species parasitise invasive fish in Lithuania, of which 30 use these fish as intermediate or paratenic hosts.

Aquatic ecosystems are particularly vulnerable to anthropogenic activity and climate change. The changes in flow regimes in Lithuanian lowland rivers due to the operation of hydropower plants (HPPs) and the impact of altered flow on some fish species have already been studied. The impact of climate change on future natural river runoff and the structure of fish assemblages was also investigated. However, it is still unknown how the combined effect of climate change and flow regulation related to hydropower generation may affect fish assemblages in the downstream river reaches below the Lithuanian HPPs. In this study, the physical habitat modelling system MesoHABSIM was used to simulate spatial and temporal changes in aquatic habitats availability for different fish species under the influence of HPP at different climate change scenarios. Changes in the available habitat were assessed for common fish species in four HPP-affected rivers representing different hydrological regions of Lithuania. The modelling results showed that the operation of HPP under climate change conditions in most rivers could be beneficial for small benthic fish species such as gudgeon Gobio gobio and stone loach Barbatula barbatula. Meanwhile, for larger fish species (e.g., chub Squalius cephalus and vimba Vimba vimba) the alteration in the temporal availability of suitable habitat was relatively higher.

Trematode metacercariae are the most abundant and frequently encountered helminths in freshwater fish. Yet, accurate species identification remains challenging, potentially leading to an underestimation of trematode diversity. Using data from parasitological examinations of 1,030 fish (47 species) collected from diverse freshwater habitats in Lithuania (2022–2024), we assessed the contemporary diversity of trematode metacercariae, host associations, microhabitat preferences, and changes in diversity patterns and transmission dynamics. Through integrated morphological and molecular techniques, we identified metacercariae belonging to 51 species from eight families, more than doubling previously reported diversity (25 species). While trematode family composition remained largely unchanged – the Diplostomidae and Strigeidae remained the most diverse families – notable differences were observed at the species level. Metacercariae of the Echinochasmidae and Echinostomatidae were detected for the first time, while previously reported Clinostomidae were absent. Fish of the Leuciscidae hosted the highest trematode diversity. Host specificity of metacercariae was generally low, with most species being euryxenous. At the microhabitat level, eyes harboured the highest number of species, while muscles showed the highest metacercarial density. Notably, we detected species first genetically characterised in North America ( Echinoparyphium sp. 2 and Ichthyocotylurus sp. 2) and species potentially belonging to the genus Neogogatea , previously known only from Asia and North America, highlighting potential invasion risks and suggesting that European trematode diversity remains substantially underestimated. Future efforts should obtain molecular data from correctly identified adult specimens to resolve the identity of species currently identified only to the genus or family level, thereby enabling assessment of their geographical distributions and ecological roles.

One of the most abundant fish species, Rutilus rutilus, is widely distributed in Lithuania and its potential to adapt to environmental changes attracted our interest. Unfortunately, it is not properly understood how anthropogenic activities can affect the genetic diversity within this species. We studied three populations of roaches (samples collected in the Neris and Žeimena rivers, and Lake Drūkšiai) over a period of five years (from 2017 to 2022) to determine genetic diversity using mtDNA D-loop and ATP6 genetic markers. Genetic diversity parameters, AMOVA analysis, haplotype network, and PCoA analysis revealed a greater genetic variability in roach samples collected in 2017, and the greatest differences were noticed in the population inhabiting Lake Drūkšiai, as compared with other samples studied over a five-year period. Differences in genetic diversity detected after a five-year period led us to the assumption that roach populations may be related to the effects of natural (changing climatic conditions) and anthropogenic (operating nuclear power plant) origin.

The radiocarbon, together with several isotopes of noble gasses such as 41Ar, 85mKr, 87Kr, 88Kr, 133Xe, 135Xe, 135mXe, is one of the main radionuclides discharged to the environment by nuclear industry. Different materials of neutron moderator, composition of reactor fuel and constructions, and the concentration of target nuclei for activation in these constructions leads to variations in amounts of discharged 14C. The RBMK (Russian Acronym for ”Channelized Large Power Reactor”) is a graphite-moderated boiling water channel-type reactor with the principle of electricity generation the same as for boiling water reactors (BWRs). 14C produced in this type of reactor is released mostly in a gaseous carbon dioxide form and in much smaller quantities as liquid effluents [1].The Ignalina Nuclear Power Plant (INPP) in north-eastern Lithuania, operated two RBMK-1500 Units (design electric power 1500 MWe): Unit 1 came online in December 1983 and was shut-down on December 31, 2004 whereas Unit 2 started operation in August 1987 and was shut-down on December 31, 2009.The INPP used Lake Drūkšiai as a cooling pond by the closed cooling loop and for technological water supply, as well as for controlled industrial drainage discharges from the plant.In this study radiocarbon activity measurements were performed in two organic fractions of the lake sediment core layers: alkali- soluble and alkali insoluble. During the period of 1945-1999, the radiocarbon activity in both fractions exhibited the parallel course. During first 14 years of operation, excess of 14C activity in both fractions reached 0.34÷0.41 109 Bq. The period from 1999 to 2013 is distinctive by considerable increase of 14C activity in alkali soluble fraction (by 2.4 109 Bq) but in alkali insoluble fraction this increase did not exceeded 0.21 109 Bq. However, this radiocarbon distribution in both fractions during this period could be related with additional releases of 14C incorporated alkali soluble organic compounds used in technological process of NPP operation and/or maintenance. However, no information about increased activity levels of aquatic effluents or different chemical agents used could be found in INPP reports.[1] IAEA, Management of waste containing tritium and carbon-14, Technical Report Series No. 421, International Atomic Energy Agency, Vienna, 2004. 

The application of alizarin dye for the marking of fish is a widely adopted practice in post-stocking monitoring programmes. Nevertheless, concerns regarding the welfare implications of alizarin staining persist. The present study conclusively demonstrated that ARS dye exerts instantaneous and protracted deleterious effects on the physiological parameters (gill ventilation frequency, homeostasis in the gut microbiota, total number of erythrocytes and leukocytes) and body fitness (total length, weight and Fulton’s condition factor) of S. trutta fry. The validity of the dye-marked fish stocking effectiveness studies is called into question by these findings.