Chapter 11. Mortality of Seaward-Migrating Post-Smolts of Atlantic Salmon Due to Salmon Lice Infection in Norwegian Salmon Stocks

  1. Derek Mills MSc, PhD, FIFM, FLS
  1. J.C. Hoist1,
  2. P. Jakobsen2,
  3. F. Nilsen1,
  4. M. Holm1,
  5. L. Asplm1 and
  6. J. Aure1

Published Online: 20 NOV 2007

DOI: 10.1002/9780470995495.ch11

Salmon at the Edge

Salmon at the Edge

How to Cite

Hoist, J.C., Jakobsen, P., Nilsen, F., Holm, M., Asplm, L. and Aure, J. (2003) Mortality of Seaward-Migrating Post-Smolts of Atlantic Salmon Due to Salmon Lice Infection in Norwegian Salmon Stocks, in Salmon at the Edge (ed D. Mills), Blackwell Science Ltd., Oxford, UK. doi: 10.1002/9780470995495.ch11

Editor Information

  1. Atlantic Salmon Trust

Author Information

  1. 1

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

  2. 2

    University of Bergen, Norway

Publication History

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

ISBN Information

Print ISBN: 9780632064571

Online ISBN: 9780470995495



  • postsmolts;
  • migration;
  • salmon mortality;
  • lice infestation;
  • Norwegian salmon stocks



This chapter contains sections titled:

  • Since the early 1990s premature returns due to heavy salmon lice infections have been observed in Norwegian sea trout stocks. Following these observations it was hypothesized that the salmon lice could cause serious problems and mortality also in seaward-migrating postsmolts of salmon. However, due to the direct migration into the high seas of this species, evidence was hard to secure. In 1998 the Institute of Marine Research, Bergen, Norway, in cooperation with the University of Bergen initiated fjordic trawl surveys aiming at estimating salmon lice infection and mortality in seaward-migrating postsmolts of western Norwegian salmon stocks. Through the development of a live catching trawl device (Hoist & McDonald, 2000), the Ocean-Fish-Lift, it has been possible to secure live samples of postsmolt salmon with their scales and salmon lice infection intact. Fjordic trawling has been carried out yearly during the period 1998-2002, the numbers of fjords sampled varying somewhat between the years. Sampling of postsmolts has also taken place in the open ocean later in the season when the salmon lice have grown to their most aggressive stages and mortality due to salmon lice infection has more or less ceased. It has also been possible to catch live postsmolts infected with sea lice in the fjords and to run a controlled experiment to estimate mortal level of salmon lice infection on wild salmon postsmolts.

  • The mean infection levels of copepodites and chalimus stages on seaward-migrating postsmolts in western Norway have been observed to vary from 0 to 104 per fish between years and fjords. The hydrographic features of the specific fjord and year appear to be a major factor governing the infection level, with much fresh water being unfavourable for the sea lice. There have also been signs of a general decreasing trend in infection level, possibly related to the efforts by the fish farming industry to lower its mean level of salmon lice on its fish. It is, however, premature to conclude on this matter and a longer time series is necessary for a conclusion. It is also too early to evaluate the limit set on numbers of salmon lice allowed per farmed fish set by the veterinary authorities. At present there are signs that the limit at 0.5 adult females is too high in densely farmed and narrow fjords.

  • The controlled experiment suggested a mortal level at 11 adult salmon lice on wild postsmolts. This number is in close accordance with the oceanic observations, where none of several thousand postsmolts taken in the Norwegian Sea during a period of 10 years (Holm et al., 2000) has been observed to carry more than 10 adult salmon lice in July, after the fish mortality has ceased. Oceanic-caught fishes with close to 10 adult salmon lice are observed to be in a bad condition with almost no growth since sea entrance and typical haemoglobin levels below 20. Many of these fishes appear to be in a state where their chances of survival are small.

  • Based on the observed infection levels in the fjords and a conservative mortal limit at 15 adult salmon lice, estimates varying from 0% up to 95% mortality between years and fjords of western Norwegian postsmolts of salmon due to salmon lice infection have been presented during the period 1998-2002. In general, mortality due to salmon lice infection has been observed to be a major factor regulating stock size in many western Norwegian salmon stocks in all these years except 2002 when conditions improved.

  • Although the reported mean number of adult females in fish farms in the studied area is down towards the allowed level at 0.5 per fish in the spring, salmonid populations still appear to be negatively affected by salmon lice in many stocks in western Norway. At some stage the present veterinary regulation on salmon lice in fish farms should be evaluated as to whether it is acceptable for securing a sustainable development in local salmon and sea trout stocks. As it seems unrealistic that the salmon lice levels in the fish farms will be further lowered in the near future, additional measures for critically affected rivers could be to treat the running wild smolts with a protective chemical against salmon lice infection or to tow wild-caught smolts out through the salmon lice belt into open waters. Such experiments are currently carried out on the Norwegian west coast.