Switch from density dependent larval mortality in an accumulated cod stock caused by exploitation

1
Switch from density dependent larval mortality
in an accumulated cod stock caused by exploitation
ICES WKIZC Copenhagen 7-9 June 2005

• Stig Skreslet
• Faculty of Fisheries and Natural Sciences
• Bodø University College, Norway

2
Spatio-temporal dimension of relations between
Calanus finmarchicus and Arcto-Norwegian
year-class formation
(Based on Skreslet 1976, 1986)
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Positive correlations between vernal freshwater
discharge and recruitment in Arcto-Norwegian cod
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Arcto-Norwegian cod stock size 1946-92
Milllion tons
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Ricker relation between spawning stock biomass
and recruitment in Arcto-Norwegian cod
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Recruitment of one year old Georges Bank cod as
function of abundance of 6 mm cod larvae one
year in advance (MARMAP time series)
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0-group strength of Arcto-Norwegian cod as
function of average UVR in Lofoten in April-May,
calculated from measured ozone and cloudiness
observed by satellite
(Skreslet et al., in press)
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Population growth with density dependent and
density independent cod larval mortality
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Hypotheses

• Ontogenetic survival and hatching success of cod
  eggs is controlled by effects of UVR on the
  microbial environment.
• Diel light abundance (PAR) controls the
  production of a phytoplankton diet that define
  the time window available for cod larvae to
  locate patches of first prey (nauplii).
• In unexploited cod stocks, yearclass strength is
  regulated by the abundance of reproducing Calanus
  finmarchicus which causes density dependent
  mortality in years with low copepod abundance
  (low carrying capacity).
• In heavily exploited cod stocks, being smaller
  than the carrying capacity defined by the
  abundance of reproducing Calanus finmarchicus,
  0-group strength is a function of several factors
  that regulate larval encounters with nauplii (egg
  mortality, behaviour. microturbulence, Langmuir
  circulation) and juvenile growth factors
  (temperature, food abundance, cannibalism).

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Variation in number of three-year old recruits to
the exploited stock of Arcto-Norwegian cod