The spatial scale of competition from recruits on an older cohort in Atlantic salmon
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Abstract Competitive effects of younger cohorts on older ones are frequently assumed to be negligible in species where older, larger individuals dominate in pairwise behavioural interactions. Here, we provide field estimates of such competition by recruits on an older age class in Atlantic salmon (Salmo salar), a species where observational studies have documented strong body size advantages which should favour older individuals in direct interactions. By creating realistic levels of spatial variation in the density of underyearling (YOY) recruits over a 1-km stretch of a stream, and obtaining accurate measurements of individual growth rates of overyearlings (parr) from capture–mark–recapture data on a fine spatial scale, we demonstrate that high YOY density can substantially decrease parr growth. Models integrating multiple spatial scales indicated that parr were influenced by YOY density within 16 m. The preferred model suggested parr daily mass increase to be reduced by 39% when increasing YOY density from 0.0 to 1.0 m-2, which is well within the range of naturally occurring densities. Reduced juvenile growth rates will in general be expected to reduce juvenile survival (via increased length of exposure to freshwater mortality) and increase generation times (via increased age at seaward migrations). Thus, increased recruitment can significantly affect the performance of older cohorts, with important implications for population dynamics. Our results highlight that, even for the wide range of organisms that rely on defendable resources, the direction of competition among age classes cannot be assumed a priori or be inferred from behavioural observations alone.

  • Content Type Journal Article
  • Pages 1-9
  • DOI 10.1007/s00442-011-2055-4
  • Authors
    • Sigurd Einum, Department of Biology, Centre for Conservation Biology, Norwegian University of Science and Technology, 7491 Trondheim, Norway
    • Keith H. Nislow, USDA Forest Service Northern Research Station, 201 Holdsworth NRC, 160 Holdsworth Way, Amherst, MA 01003, USA
    • Simon McKelvey, Conon District Salmon Fisheries Board, c/o C.K.D. Galbraith, 17 Old Edinburgh Road, Inverness, IV2 3HF UK
    • John D. Armstrong, Marine Scotland Science Freshwater Laboratory, Faskally, Pitlochry, Perthshire PH16 5LB, UK

    • Journal Oecologia
    • Online ISSN 1432-1939
    • Print ISSN 0029-8549