STABLE-ISOTOPE ANALYSIS OF NUTRIENT PATHWAYS LEADING TO ATLANTIC SALMON

The relative contribution of terrestrial and aquatic primary energy so urces in food webs along a stream continuum in the Miramichi River sys tem, New Brunswick, was investigated through the use of stable carbon and nitrogen isotope ratios. In sites where these primary energy sourc es were isotopically distinct, quantitative mixing models were used to identify the relative importance of alIochthonous carbon in the diets of wild juvenile Atlantic salmon (Salmo salar) and other resident str eam fishes. The delta(13)C data of the stream fauna ranged from -34 pa rts per thousand to -20 parts per thousand, suggesting variable assimi lation of allochthonous and autochthonous carbon sources at the four s tudy sites. Results from the mixing model were congruent with the stre am continuum hypothesis, indicating that fishes in the headwaters of C atamaran Brook were more dependent (>85%) on allochthonous carbon sour ces than those in sites located downstream (36-52%). Stable nitrogen i sotope distributions successfully described food web structure in this study, suggesting at least 2.5-3.5 trophic levels in these lotic ecos ystems. Stable isotope ratios of carbon were particularly useful for i llustrating seasonal changes in food resources of recently emerged age 0+ salmon as maternally derived marine carbon was subsequently dilute d by freshwater carbon over the growing season.