Changes in large-scale climate conditions in the Northwest Atlantic caused a phase shift in productivity, altering trophic pathways that influence the growth, survival, and abundance of many species. Despite diverse population structures and management regimes, concurrent abundance declines in disparate North American and European Atlantic salmon populations suggest that conditions experienced at common marine areas may be causative. To understand the trophic mechanisms contributing to population declines, 1451 Atlantic salmon stomachs were collected and examined from individuals caught between 2006 and 2011 at the West Greenland feeding grounds. Standardized stomach content weights and stomach composition varied among years but not between stock complexes. Atlantic salmon consumed a variety of prey taxa, primarily capelin and Themisto sp., over a broad size spectrum. Standardized stomach content weight and proportions of taxa consumed were similar between historical (1965−1970) and contemporary samples, although lower-quality boreoatlantic armhook squid, nearly absent from historical data, was of moderate importance in contemporary samples, while higher-quality capelin decreased in importance. Furthermore, from 1968−2008 mean energy density estimates of capelin, the regional keystone forage species, decreased approximately 33.7%. This resulted in lower estimates of total energy consumption by Atlantic salmon over time. Results indicate that altered trophic dynamics caused by 40 yr of changing ocean conditions negatively influenced Atlantic salmon and likely many other commercially, culturally, and ecologically important species in the Northwest Atlantic. Determining the primary mechanisms that influence marine food-webs is necessary to fully understand and evaluate survival and productivity trends and to establish realistic management targets for commercial, recreational, and protected species.
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.
How colouration provides information about individuals in birds has been a central issue in recent decades. Although much information has been derived, little is known about the adaptive significance of egg colouration in birds. A recent idea suggests that biliverdin- and porphyrin-pigmented eggs may act as a post-mating sexual signal for males to assess female quality. In birds, it is common for males to influence prelaying female condition by courtship feeding. Using Eurasian kestrels, a species that lays protoporphyrin-pigmented eggs, we descriptively assessed the influence of male feeding on egg pigmentation by considering female phenotype, condition, breeding parameters and male body condition. We found that older females and females with greyer tails (an index of individual quality) produce highly pigmented eggs. However, male body condition was the only variable that explained egg colouration when considered together with the female-related variables. Therefore, females that mated with males in better condition laid highly pigmented eggs. With the same species, we also explored the cost of producing protoporphyrin-pigmented eggs using a food-supply experiment before the laying period. Food supplementation did not increase egg pigmentation, but hatching success and egg mass were positively related to egg colouration only in food supplied pairs. We suggest that egg colouration might be costly to produce and probably suggests egg quality. However, this cost cannot be explained by female quality, but by male condition instead. In general, our results do not support the theory that egg colouration is a post-mating sexual signal in species where males determine female condition at the time of laying.
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