Spatial distributions and productivity in salmonid populations

Abstract

This thesis presents analysis and fieldwork investigating how the effect spatial distributions and spatial relationships have on population growth in two anadromous salmonid species, Atlantic salmon and brown trout. The thesis looks at how the spatial distribution of spawning within a river can shape the distribution of juveniles within a river throughout their freshwater stage, and investigates how habitat characteristics, ranging in spatial scale from micro-habitat to catchment, affect population productivity.

Increased knowledge of how spatial relationships can affect population production is important both for theoretical understanding of population dynamics and management of these economically and culturally important species.

Salmon and trout spawn 100s to 1000s of eggs in discrete gravel nests, and the preferred sites for spawning are often are patchily distributed within rivers, often with multiple females spawning in the same area. The densities experienced by newly emerged juveniles are often high, and can be reduced either by mortality or by juveniles dispersing to areas with fewer competitors. This implies the potential for patchy spawning distributions to limit productivity via local density dependent mortality and/or by limiting the river area accessible to juveniles.

Questions regarding this were investigated in three separate studies where the spatial distribution of year classes were followed over one or more life stages ranging from eggs, to juveniles in their third year. The results showed a close spatial relationship between spawning and juveniles during the first year, and although dispersal increased with age, the initial distribution was partly retained throughout the riverine stage.

The relationships between productivity and microhabitat, river morphology and catchment properties were analysed using three large datasets. Both large and small scale properties of the environment effect productivity, and insights from this analysis will improve management when assessing production potential of rivers.