title: Effects of upland stream nutrient restoration on Atlantic salmon populations
Creators: name: Bernthal, Fionn Robert
Publisher: name: University of Glasgow
Year: 2024

description: Aquatic biodiversity has experienced severe declines over past decades, with many species requiring conservation interventions in order to preserve and protect threatened populations. However, assessing whether conservation measures work effectively with the intended outcome is important when considering their implementation. One conservation target is the Atlantic salmon (Salmo salar), populations of which have declined dramatically since the 1960s. Given that many salmon die during the spawning migration, the reduction in the number of spawning adults has also led to fewer marine-derived nutrients being deposited in upland streams in the form of salmon carcasses. These carcasses fertilise the nursery streams of the salmon, to the potential benefit of the young fish, and so the decline in the number of adult salmon carcasses may have adverse effects on the next generation.
 
In this thesis, I explore in successive chapters the potential for restoring the nutrients that are normally supplied by returning spawning salmon to upland streams, by using carcass analogue pellets. I examine the impacts of different methods of the application of these pellets, and differing doses and timing of applications, on the growth and performance of juvenile salmon populations. I also assess the effect of the application method on aquatic macroinvertebrate communities. I develop a mathematical model to predict the impact of manipulating the early freshwater growth rate of individual salmon on their life history trajectory, following a cohort of fish through to spawning and egg production; this allows exploration of the effect of nutrient additions on the viability of salmon populations. Finally, I detail an incidental study on the effects of high summer temperatures on the performance of juvenile salmon populations. 

Chapter 1 introduces the main issues and the study species and presents the ecological and conservation context for the study. Juvenile Atlantic salmon often reside in upland streams, and these streams may differ to the degree of nutrient limitation that they experience as a result of cultural practices leading to the oligotrophication of these streams. In Chapter 2, I review the sources, retention and fate of nutrients in upland streams, alongside reviewing the impacts of experimental nutrient additions on salmon populations.

In the first experimental chapter (Chapter 3), I present the results of a twoyear experiment that compared two methods of nutrient additions using carcass analogue pellets, one via bagged pellets and the second through hand-scattered pellets. I show the differing impacts of these methods on macroinvertebrates and two cohorts of Atlantic salmon populations. The results varied between treatments and between years, but mainly demonstrated increased body size of individual invertebrates in the scattered treatment. Salmon fry (fish in their first summer of growth) in the scattered treatment showed reduced growth but greater densities, whilst fry in the bagged treatment saw no change in density and a positive effect on growth in one year of the study. There was no impact of either treatment on the body size of salmon parr (fish at least one year old). 

The impacts of nutrient additions are likely to vary depending on seasonal changes to environmental variables, and the amount of nutrients added is also likely to result in different impacts based on these seasonal changes. These changes are assessed in Chapter 4, where I present the results of an experiment that tested the impact of a single dose applied in early spring against a double dose applied in early spring and early summer. The single dose resulted in increased fry density but reduced growth, whilst the double dose increased both growth and density of Atlantic salmon fry. 

No study has assessed the impact of nutrient additions over a single generation of Atlantic salmon, and nutrient additions may have unforeseen and adverse consequences. In Chapter 5, I detail an individual-based model that aims to understand the population impact of manipulating early freshwater growth of Atlantic salmon. I demonstrate that increasing early growth results in increased numbers of fish smolting, even though more precociously mature males are produced. The salmon that smolt tend to do so at a younger age but also larger size; these trends are predicted to translate into increases in offspring produced per cohort and hence increases in the population size of Atlantic salmon. 

In Chapter 6, I present an incidental study on the impacts of high summer temperatures on the density and biomass of juvenile Atlantic salmon. High temperatures can result in heat stress in Atlantic salmon, affecting their growth and behaviour. I demonstrate a negative relationship with degree hours exceeding 23ºC and the log biomass and log density of juvenile salmon. 

In the final chapter, I put into context the results of the previous chapters, and address the utility of carcass analogue application as a potential conservation tool. Though the impacts of nutrient additions may vary temporally and may be complex, the data I present in this thesis suggests that nutrient additions may be used as a conservation tool with positive impacts on the freshwater growth of salmon, which is positively related to increases in their marine survival and thus increases at the population level.