Trophospatial ecology of Atlantic Salmon in the River Tamar

Supervisor: Clive Trueman (National Oceanographic Centre, Southampton)
Determining the migratory and foraging habits of Atlantic salmon (Salmo salar) assists stock-based management and conservation efforts.

In place of tagging, stable isotope analysis provides an economical alternative to determine the ecology of migratory fish species. Atlantic salmon are of particular concern firstly, due to their commercial importance and secondly, due to their sensitivity to anthropogenic and environmental disturbances.

In the years 2003 to 2009, isotopic analyses were conducted on aged salmon scales taken from returning Atlantic salmon from the River Tamar. The last summer of growth was used for isotopic analyses for δ15N and δ13C. Salmon were categorized into two age classes, one sea winter fish (1SW) or multi sea winter fish (MSW), according to their time spent at sea.

Nitrogen analyses showed 1SW having lower δ15N values and smaller size than MSW fish. No significant difference in δ15N was found between years, meaning that salmon did not change trophic levels between years. A positive correlation between salmon size and δ15N was found, showing that larger fish fed at higher trophic levels than smaller ones. MSW fish, on average, were found to be larger than 1SW fish. 1SW fish were found to have a stronger correlation between size and δ15N than MSW fish. The strongest positive correlation was found when analysing the population as a whole.

Carbon isotope data showed a variation between 1SW and MSW fish in the years of 2003 and 2004, leading to the conclusion that 1SW and MSW fish fed at separate locations. Years 2005 to 2009 lacked variation in δ13C between 1SW and MSW fish, leading to the conclusion that Atlantic salmon changed their behavioural foraging fidelities and fed in a common feeding area, although changes in marine isotope values cannot be ruled out.