Darwininan fisheries: the collapse of the
Newfoundland cod populations
An outline of this talk
• Part 1: General background on fishing as a selective agent
Esben Moland Olsen1, Mikko Heino2, George R. Lilly3,
M. Joanne Morgan3, John Brattey3 Bruno Ernande1
& Ulf Dieckmann1
Adaptive Dynamics Network, International Institute for Applied Systems
Analysis (IIASA), A-2361 Laxenburg, Austria
1
Institute of Marine Research,
P.O. Box 1870 Nordnes, N-5817
Bergen, Norway
2
Department of Fisheries and Oceans,
P.O. Box 5667, St. Johns,
Newfoundland, Canada, A1C 5X1
3
Fishing as a major source of mortality
– Fishing as a major source of mortality
– Ecological and evolutionary consequences: predictions from
theory
– How to disentangle phenotypic plasticity and evolutionary
change: the probabilistic maturation reaction norm approach
• Part 2: An example with the Newfoundland cod, which
collapsed to the state of commercial extinction in the early
1990s
– Trends in maturation reaction norms suggest an evolutionary
change of life history in response to heavy fising pressure
What can happen if you fish too hard?
• Reduction of population size
1995
UN Food and Agriculture Organisation (FAO):
70 % of the world fish stocks are either ”fully exploited,
overfished, depleted or rebuilding from previous overfishing”
2005
UN Millennium Ecosystem Assessment:
”Current patterns of use of capture fisheries are
unsustainable...
Marine fish landings are now declining as a result of the overexploitation of this resource”
• Change in distribution and migration patterns: the
Norwegian spring-spawning herring
• Truncation of age-structure: decline in the abundance of
older age-classes
• Life history changes
1
Changes in age at maturity
The Northeast Arctic “Barents Sea” cod
Age at maturity, years
Age at maturity tends to decrease in exploited populations:
the Atlantic cod
Evolutionary changes or phenotypic
plasticity?
Scenario 1: Compensatory growth
leads to earlier maturation
Maturity ogives
The traditional measure of maturation, the age at which
50%
of the fish are mature (A50), does not separate the
two alternative hypotheses
Scenario 2: Fisheries-induced
evoluiton of maturation patterns
2
Maturation reaction norms
Phenotype
Reaction norms
Environment
Part 2: Case study on Newfoundland cod
Newfoundland cod fisheries
The fishery is ancient: harvested by European nations
since early 1500s
2J3KL: ”Northern cod”
3NO: ”Grand Bank cod”
Inshore fisheries: hook and line, gill nets, cod traps
3Ps: ”St. Pierre Bank cod”
3
Newfoundland cod fisheries
• 1950s: Trawler fishery develops, offshore overwintering areas
no longer refugia
•
Record year
– 1968: 800.000 t harvested
Newfoundland cod fisheries
Late 1980s – early 1990s: collapse
July 1992: Moratorium, Canadian government
close down the cod fishery
The northern cod has
not recovered
• Early 1970s: Major decline
• 1977: 200 mile Canadian Exclusive Economic Zone
• Late 1970s and early 1980s: signs of improvement
DFO survey
Newfoundland cod, recent development
Newfoundland cod biomass
Catch data
4
Newfoundland cod distribution
Newfoundland cod survival
Newfoundland cod distribution
Newfoundland cod growth
Moratorium
5-6 year old cod
5
Temporal trends in Newfoundland cod
maturation reaction norm midpoints
Newfoundland cod maturation reaction norms
6 year
Length, cm
5 year
An example showing the 1980 cohort from area 3Ps,
south of Newfoundland
Maturation reaction norm midpoints
An example with female cod from the 2J area, off southern
Labrador and northern Newfoundland
Geographical trends in maturation reaction
norm midpoints
S
N
6
Some conclusions
Support a “selective episode” hypothesis: early maturation
favored during the collapse
An updated analysis of St. Pierre Bank (3Ps)
cod
Intense fishing may lead to undesirable evolutionary changes in
harvested populations
Trends in plasticity not related to growth may cause bias
Social effects?
Signs of rebuilding of maturation patterns
Suggests shift in selection regime, however….
Publications
Olsen et al. 2004. Maturation trends indicative of rapid evolution preceded
the collapse of northern cod. Nature 428: 932-935.
Olsen et al. 2005. Assessing changes in age and size at maturation in
collapsing populations of Atlantic cod (Gadus morhua). Canadian Journal of
Fisheries and Aquatic Sciences. In press, April issue.
Olsen et al. 2005. Case study on Atlantic cod off southern Labrador and
eastern Newfoundland: the northern cod. In U. Dieckmann, O. R. Godø, M.
Heino and J. Mork, eds. Fisheries-induced adaptive change. Cambridge
studies in adaptive dynamics, Cambridge University Press, Cambridge, U. K.
In press.
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