Salmonids, such as Arctic charr (Salvelinus alpinus), are of high socio-economic and ecological value, but are in widespread decline because of anthropogenic disturbance, especially near their southern range edge in Wales. It is therefore essential to understand the threats these species face and identify appropriate management actions to secure persistence of populations across space and time.

To investigate deep-water thermal-oxygen habitat, I collated disparate data sets for lakes across the British Isles. Presence-absence models found the temperature at the 7 mg/l dissolved oxygen isocline (TDO7) was a strong predictor of Arctic charr occurrence. TDO7 values of 10, 12.5, and 15 oC are suggested as boundaries for classifying lakes as offering excellent, good, moderate, or poor-quality habitat. In stratified lakes where most populations reside, spatial and temporal analysis identified that elevated nutrients (total phosphorus and dissolved organic carbon) are associated with a lower suitability of thermal-oxygen habitat. At Welsh lakes there has been a significant trend toward stronger thermal stratification driven by warmer summers between 2007 and 2020. Significant declines in hypolimnetic oxygen were detected at two out of four Welsh lakes, whereas hypolimnetic temperatures have been stable. Llyn Padarn had the lowest oxygen concentrations of 31 lakes assessed.

The survival of eggs is critical to understanding the population dynamics of salmonids. However, studies of Arctic charr spawning habitat and reproductive success have been rare. Chapter 3 adds important new information to the sparse literature, including the timing and duration of fry emergence, the characteristics of spawning habitats, and data showing significant temporal variation in fry production at Llyn Padarn. The suitability of swim-up traps for sampling emergent fry was demonstrated and a new and apparently productive spawning site was discovered, indicating greater resilience to ongoing disturbances at the second spawning site.

In cases where an intractable environmental disturbance is causing population decline, supplementation may be the only action available to prevent imminent extinction. However, the efficacy of supplementation has been questioned by many; in particular, there are concerns regarding the impacts on population genetics. Chapter 4 involved analysis of population dynamics and genetics between before and approximately two generations after supplementation at Llyn Padarn. The results showed the census and effective population size has increased in recent years, and that this increase has been largely driven by supplementation, rather than improvements in natural recruitment. Microsatellite markers did not detect a significant difference in genetic diversity between before and after supplementation or between wild and stocked adults. Likewise, both stocked and wild adults obtained a similar body condition and length, indicating an equal capability to exploit the wild environment. Thus, the supplementation programme has been successful in preventing seemingly imminent extirpation whilst maintaining favourable genetics. However, the reproductive contribution of stocked fish (i.e., fitness) remains unknown.

An important demographic parameter for conservation is the relative contribution of life-stage vital rates to population growth rate (λ). All other things being equal, management interventions should focus on life-stages with a large effect on λ. Analysis of a demographic model for Padarn charr indicated fry and juvenile survival are most important for λ and that larger adults (> 280 mm) are relatively unimportant (Chapter 5). However, comparisons of models constructed for 41 populations (Chapter 6) showed the importance of life-stages can vary substantially according to population life-history traits associated with climate and ecological niche. Accordingly, consideration needs to be given to the phenotypic life-history state of the population being managed.

The study represents a comprehensive investigation of topics pertinent to the conservation biology of Arctic charr and provides important information for managers at Llyn Padarn, and lakes with cold-adapted fish populations more generally.