Ecosystem-based fisheries
management: an introduction
Villy Christensen and Daniel Pauly
Fisheries Centre
University of British Columbia
Vancouver, Canada
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Under the UN Convention on the Law of the
Sea, nations have accepted:
• A mutual obligation to consider the impact of
their policies on marine ecosystems;
• to take all appropriate actions to preserve the
marine environment; and
• to manage ecosystem resources based on the
interdependence of the system components...
“in accordance with their capabilities.”
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We cannot and should not
replace the top predators
• Many examples exist of ecosystem disasters
due to fishing down of predators;
• To maintain or improve catches we must
maintain functioning ecosystems;
• We already assert major influence over the
marine environment:
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Ecosystem effects of fishing
• Removal of large sharks in South
Africa  more small sharks  less of
their prey fish;
• Removal of grazers (such as these
surgeonfishes) led to Jamaican reefs
being overgrown by algae and more
susceptible to hurricane damage.
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Ecosystem effects of fishing
• Overfishing triggerfish,
pufferfish, hump-head wrasse,
and triton (which all feed on
juvenile crown-of-thorns) led to
crown-of-thorns outbreaks on
coral reefs
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Ecosystem effects of fishing
• Overfishing
removed
pufferfishes,
the predators of
sea urchins:
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Ecosystem effects of fishing
Overfishing removes
the predators of sea
urchins: pufferfishes.
Obviously, the sea
urchins of any
given reef will have
more than one
species of
predators.
However, the one
shown here was
dominant.
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Trophic level: the concept
4
 10%
3
2
1
 10%
 10%
.. . . .. . ..
. . *. . . .. . *. .*. ..
*.
*
*. *. *. .
.
*.
...
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Processes involved in ‘fishing down’
• Initially fisheries target the upper-trophic
level species, the top predators;
• As these decline in abundance focus is
gradually shifted toward the prey of the top
predators;
• Ultimately only the lower-trophic level
species (small fish and invertebrates) are
left to be caught.
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Catch
Fishing down the food web:
succession of target organisms
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Fishing down the food web:
succession of target organisms
Catch
Sustainability?
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Fishing effort
Fishing down the food web:
succession of fleets
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Global fishing down the food web
TL of landings
3.4
3.3
3.2
3.1
3.0
Marine
2.9
2.8
2.7
1970
1975
1980
1985
1990
Freshwater
Science March 1998,
279
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innri.unuftp.is/pauly/Iceland%202%20Dec%2016%2002.ppt
Gulf of Thailand fisheries
Trophic level of catch
3.4
3.3
3.2
3.1
66
67
68
68
70
71
72
73
74
75
76
77
78
79
80
81
Year
Fishing down the food web
Science, OctoberSUMBER:
1998
innri.unuftp.is/pauly/Iceland%202%20Dec%2016%2002.ppt
Gulf of Thailand fisheries
TL of catch
3.4
3.3
1966
3.2
1974
1981
3.1
0
200
400
600
800
Catch (thousand tonnes)
Science, OctoberSUMBER:
1998
innri.unuftp.is/pauly/Iceland%202%20Dec%2016%2002.ppt
1000
Feeding triangles: North Sea
4
Other
fish
1
2
5
50
Norway
pout
17
Krill
11
100
Copepods
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Feeding triangles: North Sea
4
Other
fish
1
2
5
50
Norway
pout
17
Krill
11
100
Copepods
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Feeding triangles: North Sea
4
Other
fish
1
2
5
50
Norway
pout
17
Krill
11
100
Copepods
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Requirements for adoption of
ecosystem-based approaches:
• A framework for integrating the wealth of
information available at the species level;
• Basic understanding of ecosystem structure
and function;
• Concepts and software integrating first two
items in a transparent, practice-oriented
context.
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Do we have tools for
ecosystem-based management?
• Yes, using Ecopath with Ecosim we can
incorporate ecological, economic and
social considerations into a rigorous
framework
• Based thereon we can use Ecosim and
Ecospace to explore policy options for
ecosystem based management
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Policy exploration
• Policy may be defined as an approach towards
reaching a broadly defined goal;
• In fisheries policies are often implemented via TACs
that are recalculated annually, and through regulation
that affects fleet and deployment;
• The task of fisheries scientists is to advise both on
policy formulation and on its implementation;
• So far much fisheries research is on implementation
only;
• Ecosystem-based policy exploration is in its infancy.
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Some policy objectives
1
2
3
4
Maximize fisheries rent;
Maximize social benefits;
Mandated rebuilding of species;
Maximize ecosystem ‘health.’
Ecopath with Ecosim offers a method for
evaluating the fleet configuration and
effort levels that optimizes each of these
objectives individually or jointly.
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Maximizing profit
1 Profits are calculated as:
the value of the landings (landings * price, by
species) less the cost of fishing (fixed +
variable costs);
2 Often optimization is achieved by:
• phasing out most fleets except the most
profitable ones; and
• wiping out ecosystems groups competing
with or preying on the more valuable target
species.
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Maximizing social benefits
1 Social benefits are expressed through the
employment supported by each fleet;
2 The benefits are calculated as
job / catch value, and is fleet specific;
3 Therefore social benefits are largely
proportional to fishing effort;
4 Optimizing efforts often leads to more
extreme scenarios than optimizing profit.
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Mandated rebuilding
1 External pressure (or legal decisions) may force
policy makers to concentrate on preserving or
rebuilding the population of a given species in a
given area;
2 In Ecosim this corresponds to setting a threshold
biomass (relative to the biomass in Ecopath) and
optimizing towards the fleet effort structure that
will ensure this objective;
3 Implications will be case-specific: consider Monk
seal/shark interaction in Hawaii, or Steller sea
lion/Alaska pollock in the Eastern Bering Sea.
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Maximizing ecosystem ‘health’
1 The ecosystem ‘health’ definition is inspired by
Odum’s description of ecosystem ‘maturity’,
wherein mature ecosystems are dominated by
large, long-lived organisms;
• The group-specific biomass/production ratio
provides the default weighting factor for the
maximization of overall biomass;
• The ‘health’ optimization often implies:
phasing out of all fisheries except those
targeting species with low weighting factors.
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Balancing economic, social and
ecological objectives
1 The starting values of the objective functions have
each been standardized relative to their base values
(from Ecopath), making them roughly comparable;
2 Two of the measures tend to pull towards increasing
fishing, and two pull towards reducing fishing. Care
should be taken to consider this balance when
giving relative weightings to the objectives;
3 Optimizations should be performed with a range of
weighting factors for each objective function.
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KESIMPULAN
1. A transition to ecosystem-based management of fisheries is
unavoidable, if we are not to lose more of our fisheries
resources
2. Such transition if actually happening in some countries, if
only in the form of ecosystem consideration informing singlespecies management;
3. In the longer term, detailed simulation of trophic interactions
within exploited ecosystems, along with explicit consideration
of environmental effects, may allow for forms of fishing that
are compatible with the continued existence of the underlying
ecosystem, something we presently do not have.
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