Lakes in Europe are subject to multiple anthropogenic pressures, such as eutrophication, habitat degradation and introduction of alien species, which are frequently inter-related. Therefore, effective assessment methods addressing multiple pressures are needed. In addition, these systems have to be harmonised (i.e. intercalibrated) to achieve common management objectives across Europe. Assessments of fish communities inform environmental policies on ecological conditions integrating the impacts of multiple pressures. However, the challenge is to ensure consistency in ecological assessments through time, across ecosystem types and across jurisdictional boundaries. To overcome the serious comparability issues between national assessment systems in Europe, a total anthropogenic pressure intensity (TAPI) index was developed as a weighted combination of the most common pressures in European lakes that is validated against 10 national fish-based water quality assessment systems using data from 556 lakes. Multi-pressure indices showed significantly higher correlations with fish indices than single-pressure indices. The best-performing index combines eutrophication, hydromorphological alterations and human use intensity of lakes. For specific lake types also biological pressures may constitute an important additional pressure. The best-performing index showed a strong correlation with eight national fish-based assessment systems. This index can be used in lake management for assessing total anthropogenic pressure on lake ecosystems and creates a benchmark for comparison of fish assessments independent of fish community composition, size structure and fishing-gear. We argue that fish-based multiple-pressure assessment tools should be seen as complementary to single-pressure tools offering the major advantage of integrating direct and indirect effects of multiple pressures over large scales of space and time.

The meso-scale habitat simulation model MesoHABSIM was applied in three Lithuanian lowland rivers to study the effect of low-head hydropower plants (HPPs) on the fish habitats. Stream flow time series on a daily scale for the period 1970–2015 were used to describe flow regime downstream of HPPs for periods before and after their installation. Conditional habitat suitability criteria were developed for 4 species of cyprinid fish, schneider (Alburnoides bipunctatus), dace (Leuciscus leuciscus), roach (Rutilus rutilus) and vimba (Vimba vimba) to simulate their available habitat at different water discharges. Modelling results showed that HPPs have a significant impact on habitat availability in the low flow period in dry years below HPPs due to insufficient released flow. The environmental flow, as prescribed by the Lithuanian national law, is estimated between 80 and 95% exceedance probability of the mean minimum discharge of 30 days. This flow leads to a significant reduction in frequency and duration of available suitable habitats for vimba and schneider during low flow period. The roach habitat is the least affected. The results of habitat modelling are in line with the actual data on the occurrence and relative abundance of considered fish species in the studied river stretches. A general comparison of the relative abundance of modelled fish species in 42 natural river stretches and 20 stretches below the HPPs also showed that the relative abundance of roach is significantly higher, and that of schneider is significantly lower in river sections below the HPPs than the abundance in natural river sections. All results indicate that the current environmental flow does not secure survival of certain fish species. The applicability of the average low flow release during summer could be a plausible alternative to the current environmental flow in order to maintain ecosystem health and services.

Following their first appearance, the invasive fishes Pseudorasbora parva and Perccottus glenii have been in Lithuania for several decades. However, until recently, information relating to their distribution and secondary spread was limited. For this reason, suitable habitats for these fish species were surveyed for their presence across the entire country. Additionally, all previously reported records on the presence of these species were summarized. Results revealed P. glenii to be widely distributed within the country with abundant populations in habitats suitable for the species. The recent distribution of P. parva is restricted to only a few water bodies. It was shown that both species are associated with human mediated transfer, while no natural dispersal of these invasive species was observed. The results of this study suggest that the invasion of Lithuanian inland waters by P. parva and P. glenii is still ongoing, and their occurrence in numerous water bodies, which are still devoid of these species, now seems probable. Demonstrated vectors of P. parva and P. glenii introductions in Lithuania highlight the importance of controlling and screening human activities related to aquaculture, recreational angling and the ornamental fish trade in order to restrict further P. glenii and P. parva expansion in this region.