Climate change has been identified as a global threat to Arctic char Salvelinus alpinus L. populations. Bayesian statistical models were used to identify important physical and ecological factors explaining Arctic char presence and persistence in Irish lakes. Maximum lake depth and the presence of mixed fish communities (i.e. including non‐salmonid fish species) were the most important drivers of Arctic char presence. There was a 75% probability that an Irish lake would contain Arctic char when maximum depth exceeded 40 m, if only a salmonid community was present. However, the required depth increased to 57 m when a mixed fish community (e.g. pike, perch or roach) was present. Similar variables explained char persistence, with surviving Arctic char populations being more likely in lakes with greater maximum depth and fewer non‐salmonid fish species. Tested temperature covariates were not important in explaining Arctic char presence or persistence in Irish lakes. A risk analysis was undertaken to identify which Arctic char lakes are vulnerable to colonisation by thermally plastic mixed fish communities due to flooding. Results indicated that 32 out of 45 lakes were not at any risk from the colonisation of thermally plastic mixed fish communities and 13 lakes had some level of risk.

Pollan, Coregonus autumnalis pollan Thompson, requires an appropriate standardised sampling protocol for conservation assessments. The suitability of hydroacoustics with gillnetting was evaluated, and the effects of sampling design, effort and statistical analysis on the repeatability of results were tested. Summer hydroacoustic surveys appear appropriate as pollan were not abundant in acoustically unsampled areas. However, pollan density estimates were significantly affected by the gillnet sampling design used to ground‐truth the acoustic data. Density estimates from the >12‐ m layer were more robust to gillnet design than estimates from the 3‐ to 12‐ m layer. Estimates from different transect designs yielded some statistically significant results. Comparison of transects common to both transect designs suggests that observed differences reflect temporal changes rather than transect design effects. The reduction in the systematic parallel transect effort by half significantly affected target strength distributions but not pollan density estimates. Density estimates were affected by statistical analysis method where pollan density was highest, with geostatistical analysis providing higher estimates than the arithmetic mean. Hydroacoustic assessments may provide a reliable index of pollan abundance over time and between lakes but future research should investigate the effects of temporal and abiotic factors.

An environmental study of pike Esox lucius recorded their presence in 522 Irish lakes and that they coexisted with brown trout Salmo trutta in 97 of these. Statistical models, accounting for spatial non‐independence among lakes, suggested that lakes with greater area, maximum depth and stream connectivity show a higher probability of coexistence. Introductions of E. lucius are likely to have negative effects on S. trutta stocks in small isolated lakes, but coexistence may be possible in larger systems.