The behavioural ecology of migratory salmonids in the River Tweed, UK

Abstract

This study investigated various life history stages of salmonids within the River Tweed, UK with a focus on migratory movements. The River Tweed is a large upland river situated on the border between Scotland and England and is home to some of the healthiest stocks of Atlantic salmon (Salmo salar) and sea trout (Salmo trutta) in the UK. The research undertaken as part of this thesis aims to assess how management can be improved to aid the migration of salmonids moving within freshwater. This is of particular importance due to increased demand for renewable energy including small scale hydropower as well as legislation that demands improved fish passage within rivers such as the Water Framework Directive.
Sea trout smolts were captured and acoustic tagged to assess the roles that in river obstructions such as weirs play on their migration between years with varying river flow. The two study years varied radically in flow levels due to the incidences of hydrological drought in 2010 significant differences were observed in the degree of delay smolts experienced at weirs as well as differing responses to flow during years.
Sea trout and salmon were acoustic tagged and tracked during their freshwater spawning migration. The aim of the study was to examine the interspecific differences in spawning migration such as spawning location and movement rate during migration. By looking at migration rate of sea trout and salmon it was observed that both species decreased their migratory rate the further into the river system they moved. It was also observed that sea trout and salmon spawned in different locations, with sea trout using tributaries and salmon using lower stretches of the Tweed.
The small scale movements of freshwater resident trout was studied. Freshwater resident trout tend to have relatively small home ranges and often hold a territory within their home range. As a result they also tend to rapidly home back to their territory after being displaced from it. As such, the study aimed to assess the degree to which brown trout home after being displaced, particularly whether being offered a choice of empty territories at their site of displacement would affect their homing behaviour. The study found that there was no apparent difference in homing behaviour observed between treatment groups offered empty territories at their site of displacement compared to controls that were displaced into fully populated sites.

Continued research into the behaviour of salmonid species is important due to increasing demand on water resources, future conflict between man and fishes water needs is inevitable.