Chapter 4. The Ecology of Post-Smolts of Atlantic Salmon

  1. Derek Mills MSc, PhD, FIFM, FLS
  1. L.P. Hansen1,
  2. M. Holm2,
  3. J.C. Hoist2 and
  4. J.A. Jacobsen3

Published Online: 20 NOV 2007

DOI: 10.1002/9780470995495.ch4

Salmon at the Edge

Salmon at the Edge

How to Cite

Hansen, L.P., Holm, M., Hoist, J.C. and Jacobsen, J.A. (2003) The Ecology of Post-Smolts of Atlantic Salmon, in Salmon at the Edge (ed D. Mills), Blackwell Science Ltd., Oxford, UK. doi: 10.1002/9780470995495.ch4

Editor Information

  1. Atlantic Salmon Trust

Author Information

  1. 1

    Norwegian Institute for Nature Research, PO Box 736, Sentrum, N-0105 Oslo, Norway

  2. 2

    Institute of Marine Research, PO Box 1870, Nordnes, N-5817 Bergen, Norway

  3. 3

    Fiskirannsoknarstovan, Noatun, PO Box 3051, FO-110 Torshavn, Faeroe Islands

Publication History

  1. Published Online: 20 NOV 2007
  2. Published Print: 7 JUL 2003

ISBN Information

Print ISBN: 9780632064571

Online ISBN: 9780470995495



  • postsmolts;
  • atlantic salmon;
  • marine ecosystem;
  • life cycle;
  • commercial fishery



This chapter contains sections titled:

  • Anadromous Atlantic salmon move from fresh water to the ocean and gain weight. The fish move over large areas and are exposed to different environments and ecological communities during the marine phase of the life cycle. Although the biomass of salmon in the ocean is very small compared with the biomass of other pelagic marine species, Atlantic salmon are still part of the marine ecosystem and subjected to fluctuations and changes in that system.

  • The distribution of salmon in the sea is probably dependent on several environmental factors like surface temperature and surface currents. Furthermore, the oceanic distribution of salmon may probably also be determined by genetic components, for example that salmon have developed navigation systems that bring the fish to the right place at the right time, and thus maximize fitness. It is also reasonable to assume that the availability of suitable food organisms affects the distribution of salmon in the sea, as growth and survival are important fitness characters as well.

  • Salmon spend most of their time in the ocean close to the water surface, and prey on different pelagic animals such as crustaceans, fish and squid. Several authors have suggested that Atlantic salmon are opportunistic feeders, but selective feeding has also been described.

  • The abundance of Atlantic salmon in the north Atlantic has declined considerably in recent years. The decline is evident from most of the salmon distribution area, and is most pronounced for large salmon, but there appears to be no significant density-dependent mortality of salmon in the sea as there is in fresh water. The abundance of salmon is dependent on several factors such as smolt production, natural and manmade mortality. It has been shown that marine mortality accounts for a significant proportion of the decline, and is associated with a decline in the temperature to which postsmolts are exposed during the first months at sea.

  • There are many factors thought to influence marine mortality of salmon. Although it has been suggested that the highest mortality takes place at the postsmolt stage, there are also indications that heavy mortality may occur later in the life cycle. Important sources of postsmolt mortality are predation, infestations of parasites and diseases, influences from freshwater life, as well as synergistic effects between these and a number of other factors. Furthermore, salmon may respond to changes in the ecosystem by altering their life histories, which may be seen by, for example, changes in growth rates, sea age at maturity and seasonal return pattern.

  • Salmon are harvested in the ocean, but exploitation in commercial fisheries is now close to zero. On the other hand, recent information from surface trawling experiments in the Norwegian Sea has raised concern about the significance of by-catches of salmon postsmolts in pelagic fisheries for marine species.