Characterizing the trophic position and shift in Atlantic salmon (Salmo salar) from freshwater to marine life-cycle phases using stable isotopes

Abstract

Dixon, H. J., Power, M., Dempson, J. B., Sheehan, T. F., and Chaput, G. 2012. Characterizing the trophic position shift in Atlantic salmon (Salmo salar) from freshwater to marine life-cycle phases using stable isotopes. – ICES Journal of Marine Science, 69: 1646–1655. Marine survival and recruitment of Atlantic salmon (Salmo salar) are commonly thought to be influenced by linkages between marine temperature and growth. Salmon are opportunistic feeders that are likely to be affected by the quality and quantity of prey available. During the marine phase, salmon often target larger zooplankton and fish, with a preference for fish over crustaceans. European salmon recruitment appears to be linked to forage abundance, which affects post-smolt growth critically during the first summer at sea, but similar linkages have not been shown for North American populations. Here, trophic differences within and among populations and life stages of Atlantic salmon during the freshwater and marine life-history phases are investigated. Stable isotope methods were used to characterize trophic dynamics by sampling smolts and returning adult survivors from rivers spanning a broad geographic range in Canada (southern Labrador to the Bay of Fundy), as well as non-maturing salmon sampled at West Greenland. Analyses indicate that salmon undergo a marked change in feeding as they migrate from freshwater and show the expected high reliance on pelagic foodwebs in the marine environment. Documented changes in ocean climate, pH, and the possibility of regime shifts are likely to have significant impacts on pelagic feeding fish, including Atlantic salmon. Due to the similar marine feeding ecology suggested here for adults from different age groups, these changes could have a widespread effect on multiple life-history stages of Atlantic salmon.

Lake Ontario salmon (Salmo salar) were not migratory: A long-standing historical debate solved through stable isotope analysis

“…Historical scales produced δ 13 C values consistent with modern isotopic baseline datasets for freshwater smolt and marine adult salmon scales, respectively293031. Wide separation between δ 13 C and also δ 34 S values of fish with different migratory behaviours indicates that: 1) salmon with δ 13 C and δ 34 S values below −19‰ and +12‰, respectively, completed their entire lifecycle as freshwater residents in Lake Ontario, and 2) salmon with a δ 13 C values above −17‰ and δ 34 S values above +14‰ made a round trip from their natal stream down the St. Lawrence River to the Atlantic Ocean and back over the course of their lives.…”

“…Because the organic components of both bone and scales are composed primarily of Type I collagen, these two sample types are directly comparable, and both represent long-term dietary intake49. Comparative data from modern Atlantic Salmon scales2930 as well as European505152 and North American53 archaeological bones were sourced from previously published studies and are supplemented by new analyses of a single European Atlantic Salmon individual from an Early Christian context from the site of Knowth in Ireland. Morphological analyses of scales from 5 of the 7 nineteenth-century Atlantic Salmon skin mounts were used to provide a second line of evidence for salmon origin and were conducted at the National Marine Fisheries Service (Narragansett, RI, USA) using established methods5455 (Supplementary Information).…”

Assessing the use of different marine growth zones of adult Atlantic salmon scales for studying marine trophic ecology with stable isotope analysis

“…For SIA studies of Atlantic salmon ecology scales have been used to examine aspects of their life history such as diet and migration (MacKenzie et al, 2011;Dixon et al, 2012;Vuori et al, 2012), including a long term analysis of spatial and temporal variability in marine feeding (Sinnatamby et al, 2009). However, there are differences among studies in the protocols used for sampling scale sections for use in SIA that are linked with scale architecture and growth.…”

“…marine residency (e.g. Gerdeaux, 2003, 2004;Gerdeaux and Perga, 2006;Sinnatamby et al, 2008Sinnatamby et al, , 2009Dixon et al, 2012) (Table 1). There is currently no consensus as to what portion of the scale should be used when performing SIA to study the trophic ecology of fishes.…”

A simple method of dating marine growth circuli on scales of wild one sea-winter and two sea-winter Atlantic salmon (Salmo salar)

“…Because salmon are generalist, visual predators of zooplankton and nekton at the ocean surface (Rikardsen and Dempson 2011;Dixon et al 2012), and typically are distributed at high latitudes during winter (e.g., Jacobsen et al 2012), it is intuitive that an insufficiency of light during midwinter would be a primary constraint on prey capture, and hence, somatic growth.…”