| Summary: | Investigations on marine feeding of Atlantic salmon (Salmo salar L.) in the Northwest Atlantic are rather limited in comparison with studies carried out in the Northeast Atlantic. Climate induced changes to food webs in marine feeding areas for Atlantic salmon have been noted in recent years, along with an increase in mortality and decrease in recruitment and growth, despite a cessation in most ocean fishing activities. Since foraging may be hampering salmon survival, it is important that this knowledge gap be addressed. Accordingly, Atlantic salmon were sampled at three communities on the West Greenland coast across three years, 2009-2011, and 15 rivers spanning a broad geographic range in Atlantic Canada in 2008-2010. Conventional gut content analyses were combined with stable isotope methods to assess spatial and temporal differences in Atlantic salmon marine feeding. An investigation of differences between juvenile feeding in freshwater and adult feeding in the marine environment indicated that Atlantic salmon undergo a dramatic change in feeding as they migrate. Smolts show much larger δ13C variability, due to the benthic-pelagic coupling which occurs in the freshwater ecosystem, while adults foraging in the marine environment demonstrate much smaller δ13C variability due to a high reliance on pelagically sourced carbon, as demonstrated by a mixing model. There was an overall decrease in the variance of δ15N (and therefore degree of omnivory) between smolts and returning adults, although this was not consistent across all rivers. Capelin (Mallotus villosus) was found to dominate diet at Nuuk and Qaqortoq, while boreoatlantic armhook squid (Gonatus fabricii) was the dominant prey at Sisimiut. Hyperiid amphipods (Themisto spp.) and sand lance (Ammodytes spp.) were also important. Significant differences were found among communities for gut contents analysis (dietary overlap and diversity) and stable isotope analysis, while fewer differences were evident temporally. Dietary differences were also evident across ...
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