Dynamics of the atlantic salmon (Salmo salar L.) population of the River Foyle, Ireland

Abstract

This study examines the dynamics of the Atlantic salmon (Salmo salar L.) population of the Foyle catchment in Ireland, through the analysis of long-term datasets and extensive field data. In chapter 1 the current conservation status of the salmon is discussed with particular reference to the Foyle. Chapter 2 considers the interaction between commercial netting and recreational sport angling and the effect of total combined exploitation on an independent measure of population size (redd counts) using a 49 year dataset. The evidence suggests that year class strength is the principal modulator of variation in commercial catches, sport angling catches and spawning escapement. Chapter 3 examines the role of density-dependence in the Foyle salmon population. A Ricker density-dependent model showed that spawning adult population size significantly predicted variation in the resultant filial generation; however, a significant amount of variation (ca. 68%) remained unexplained. Environmental factors were significant in explaining some of the remaining variance and these influences were linked to specific life stages. Chapter 4 investigates the effect of marine climatic conditions in the North Atlantic on the abundance of returning migrant Atlantic salmon, using a 126 year dataset of commercial catches and an index of climate in the northern hemisphere, the North Atlantic Oscillation (NAOI). The NAOI when below 0.151 explained a significant proportion of variation in five year running mean catches of migrant Atlantic salmon returning to the River Foyle. This indicates that a significant proportion of the variance in population size in the past was the result of variability in conditions in the marine environment. However, when the NAOI was above 0.151 this relationship uncoupled. Chapter 5 tests the capacity of local instream and broadscale catchment characteristics to predict 0+ salmon abundance within the Foyle area. This chapter also highlighted the potential detrimental impact of increasing urbanisation on the salmon stocks of the Foyle area