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Ecologically Sustainable Yield
Marine conservation requires a new ecosystem-based concept for fisheries
management that looks beyond sustainable yield for individual fish species
Richard W. Zabel, Chris J. Harvey, Stephen L. Katz, Thomas P. Good and Phillip S. Levin
I
n the 1950s, Jamaica’s coral reefs
were thriving, with branching forms
of staghorn and elkhorn corals ruling
the shallows and plate-like corals dominating deeper waters. The reefs were
popular habitats for large predatory
fish such as sharks, snappers, groupers
and jacks, and Jamaicans came to the
reefs to trap these species.
The scene looked much the same in
the 1970s—except that the large fish
had disappeared onto dinner plates.
Struggling to feed the island’s swelling
population, Jamaicans began using
motorized canoes to improve their efficiency in trapping smaller, herbivorous
fish, such as surgeonfish and parrotfish. The reefs still looked healthy,
however, and sea urchins were doing
well. As herbivorous fish were removed, the urchins no longer had to
compete with them for the mainstay of
their diet: algae.
By the 1990s, Jamaica’s reefs were
depleted of both carnivorous and herbivorous fish and smothered with
fleshy algae, such as Sargassum. Sea
urchins were uncommon, and the vast
The authors are members of the Science for Ecosystem-based Management Initiative at the Northwest Fisheries Science Center of the National Marine Fisheries Service in Seattle. Richard W. Zabel
received his Ph.D. in quantitative ecology and resource management from the University of Washington in 1994. Chris J. Harvey received his Ph.D.
in limnology and marine science from the University of Wisconsin in 2001. Steven L. Katz received
his Ph.D.in zoology from the University of British
Columbia in 1991. Thomas P. Good received his
Ph.D. in systematics and ecology from the University of Kansas in 1998. Phillip S. Levin received
his Ph.D. in zoology from the University of New
Hampshire in 1993. Address for Zabel: National
Marine Fisheries Service, Northwest Fisheries Science Center, 2725 Montlake Blvd. E, Seattle, WA
98112. Internet for Zabel: [email protected]
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American Scientist, Volume 91
diversity of the reefs was reduced. Because the algae had taken over the real
estate, there was little space for renewal by larval corals. Back in the late
1970s, surveys off the northern coastline had found corals over half the
reefs’ surface, with fleshy algae covering only 4 percent. By the early 1990s,
Terry Hughes from James Cook University demonstrated that those figures
were more than reversed: 3 percent of
the space occupied by corals, 92 percent by algae. Thus reefs that had existed for thousands of years as one of the
most diverse habitats on earth had
changed into algal mats in a few
decades.
What tipped the balance? Most scientists point to overfishing, although
pollution and natural disasters certainly also took their toll. (See “Mud, Marine Snow and Coral Reefs,” January–February.) When carnivorous fish
became scarce, populations of one of
their prey species, the sea urchin Diadema antillarum, exploded. These black,
spiny creatures flourished even more
with the demise of their competitors,
the herbivorous fish. But in 1983, a
mysterious pathogen sickened the
urchins, reducing their number by 99
percent. With few herbivorous fish and
now few urchins, the algae were free
to grow unchecked.
In the past decade, Jamaican reefs
have begun to revive—corals now cover about 10 percent of the surface. But
it is unclear whether they will regain
their dominance or whether the algae
will continue to hold sway.
This example makes an important
point: Harvesting a few types of fish
affects not only those species but disturbs whole ecosystems, which in turn
can reduce target species. We shall argue in this article that, although the
primary goal of “sustainable fisheries”
is to preserve the long-term viability of
target species, even harvest levels considered sustainable can impact marine
ecosystems. Protection of the world’s
oceans will in the future require a
broader and more integrated scientific
view than one that focuses on one or a
few species.
Marine Resources
A spectacular diversity of habitats lies
in the world’s oceans, which cover 361
million square kilometers and constitute
more than 99 percent of the biosphere’s
volume. Some habitats, such as coral
reefs, seagrass meadows and kelp
forests, are relatively well known. Others, such as deep-sea plains and vast
open waters, are alien to most people.
We are only beginning to appreciate
the depth and breadth of marine diversity, but it surely rivals that on land.
Coral reefs, for instance, occupy only
0.1 percent of the earth’s surface, yet
they may support as many as 9 million
species. There also could be millions of
species in the deep sea, once thought to
be a vast desert. At higher taxonomic
levels, marine systems are much more
diverse than those on land. Of the 34
recognized animal phyla, 33 can be
found in the ocean, and 15—including
comb jellies, echinoderms and lampshells—are exclusively marine. Because certain marine groups, especially
bottom-dwelling invertebrates, are
poorly known, enormous diversity remains to be discovered. For instance,
20 new families, 100 new genera and
200 new species were recently found in
or near hydrothermal vents.
Humans have exploited marine
ecosystems for at least 10,000 years, but
overharvesting was not a significant issue when human populations were
© 2004 Sigma Xi, The Scientific Research Society. Reproduction
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Nik Wheeler/Corbis
Figure 1. Marine fish are a major food source for humans—as well as for gulls. Moreover, fisheries, especially small operations, provide employment for more than 200 million people worldwide. This continuing need for protein and income, however, will likely be impossible to meet
in the future without the development of new ecosystem-based fisheries management practices. The authors have used data from one of best
understood marine ecosystems, the Baltic Sea, to model how various approaches to managing catch might influence the future health of the
world’s fisheries. They conclude that a model based on ecologically sustainable yield will be most effective in balancing the needs of current
and future generations. This herring boat was photographed near Ahvenanmaa (the Åland Islands), Finland.
small and fishing gear was mostly lines
and hooks. Nonetheless, the first
known anthropogenically caused collapse of a marine stock occurred about
3,000 years ago, when peoples on the
Peruvian coast continued to harvest
shellfish that had been depleted by a
natural disturbance. With the technological advances of the Industrial Revolution, such as the internal combustion engine and refrigeration, the
ability of humans to extract fish from
the sea, particularly in offshore regions,
increased dramatically. The resulting
increased frequency of stock collapses
led to the realization that the sea is not
infinitely bountiful. More recent devel-
opments include factory trawlers,
which harvest a disproportionate share
of the world’s fish.
Commercial fishing now directly kills
enormous numbers of fish: More than
90 million metric tons are captured annually for consumption, and about 30
million metric tons are discarded as bycatch. Worldwide, 25 to 30 percent of exploited stocks experience overfishing,
despite numerous incidents of stock collapse. Moreover, fishing trawls dragged
across the seafloor have devastating effects on benthic communities, with impacts similar to clear-cutting in forests.
By killing benthic invertebrates as well
as key predators, fishing affects food
© 2004 Sigma Xi, The Scientific Research Society. Reproduction
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webs and fundamentally alters community composition.
Why are many marine fisheries
overexploited? Fisheries provide 19
percent of the animal protein consumed by human beings worldwide
and generate jobs for more than 200
million people, mostly in small-scale
fisheries. Understandably, this creates
enormous social, economic and political pressure to maintain current fishing levels. As fisheries managers juggle diverse and conflicting objectives,
political agendas and scientific uncertainties, they often err on the side of
overexploitation, despite calls for caution from organizations such as the
2003
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151
population size
time
annual population growth rate
maximum
sustainable
yield
population size
Figure 2. Based on the logistic population growth model, populations have a modest capacity
for growth (top) when they are small and attain the greatest capacity when they reach an intermediate size. When the number of organisms in a population becomes so large that the level of resources available to each member becomes limiting, the capacity for growth is reduced
once more. In turn, because modeling suggests that the sustainable yield of a fish species is
maximal when that species’ population is intermediate in size (bottom), optimal population
size can be maintained by harvesting the species at a rate equal to the annual growth rate. The
same general result holds for more complex models. These models, which have been widely
applied for more than half a century, do not take into account the ecologically important effects
that impacts on nontarget populations can have on target populations.
Food and Agriculture Organization of
the United Nations. Such conflicts in
fisheries management are typically
characterized simply as science versus
politics, but we suggest that there may
be a deeper problem: The fundamental
goals of modern fisheries management
may be incompatible with sustaining
marine ecosystems. Currently, a primary goal of “sustainable” fishing is to
preserve the long-term viability of target species. But sustaining those
species may depend also on protecting
the vast biodiversity of the world’s
oceans, which by definition will require a more holistic approach to resource management.
Sustainable Fishing
The goal of long-term sustainable harvest has been a mainstay of fisheries
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American Scientist, Volume 91
science for the past half century. This
concept was crystallized with the development of a model in 1954 by Milner Schaefer of the Scripps Institution
of Oceanography that incorporates
both fish-population dynamics and
harvest. An important feature of this
and other early models was the recognition that a population’s size determines its growth rate. Thus the growth
rate (overall numbers of new organisms produced per year) is low when a
population is small. It is also low when
a population nears its carrying capacity, because of density-dependent
processes such as food availability (Figure 2). Intermediate-sized populations
have the greatest growth capacity and
ability to produce the most harvestable
fish per year. The key realization of
these early models was that fisheries
could optimize harvest of a particular
species by keeping the population at
an intermediate level and harvesting
the species at a rate equal to the annual
growth rate. This strategy was called
the maximum sustainable yield.
Although models are now able to
capture more of the complexity of the
dynamics of fish populations, two concepts remain integral parts of most
management plans. The first, as noted
above, is that average harvest rates
should equal growth rates. The second
is that harvests are sustainable even
when fish populations fall well below
unfished levels. The widely used term
“surplus production” implies that populations produce biomass beyond that
required to sustain them—and therefore that this surplus can be harvested
without impacts. There is a growing
sentiment, however, that we need to go
beyond considering only target species
in fisheries management. For example,
the U.S. Congress, as part of the Sustainable Fisheries Act of 1996, directed
the National Marine Fisheries Service
to establish an Ecosystem Principles
Advisory Board. An emerging question is: Do levels of exploitation consistent with sustaining marine fish populations have long-term, detrimental
effects on ecosystems?
Fishing and Ecosystems
Ecosystems are characterized by their
communities of plants, animals and
microorganisms, and by the local physical, chemical and structural environments in which those communities reside. Fishing alters marine ecosystems
both by modifying community composition and by altering local environments, as when trawls drag across the
ocean floor. Because ecosystem components interact, the effects of fishing are
more complex than the simple removal
of a few species. Ecosystems can suddenly collapse when some of their components are damaged, just as houses
can fall if their foundations rot away.
Marine community members interact in many ways: They feed on each
other, compete for key resources and,
as with kelps and corals, provide habitat structure. A species’ importance in
a community depends on its relative
abundance and the strength of its interactions with other components.
Some species have little impact on others, whereas other species greatly influence their fellow community members. A species’ position in the food
© 2004 Sigma Xi, The Scientific Research Society. Reproduction
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seals
fishing
trophic level 3
cod
adults
juveniles
herring
sprat
adults
juveniles
adults
juveniles
pelagic
invertebrates
trophic level 2
Mysid shrimps
benthic macrofauna
mesozooplankton
amphipods
isopods
bivalves
copepods
cladocerans
microzooplankton
benthic meiofauna
ciliates
ostracods
bacteria
nematodes
copepods
phytoplankton
trophic level 1
detritus
Summer/fall
blooms
D.W. Miller
Spring
blooms
Figure 3. Marine organisms interact in numerous ways, transferring energy through various trophic levels in the marine food web. Thus fishing can affect entire ecosystems as well as target species, often disturbing several trophic levels. By removing cod, sprat and herring, for example, fishing decreases the amount of food available to seals. Conversely, it favors organisms that normally serve as prey for cod, sprat and herring. When populations of organisms at this second trophic level increase, they consume more organisms at the first trophic level, reducing the
number of primary producers, such as phytoplankton, that bring energy into the ecosystem.
© 2004 Sigma Xi, The Scientific Research Society. Reproduction
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2003
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153
percent of community biomass
35
sprat
30
herring
cod
25
20
15
10
5
0
a
phytoplankton
bacteria
zooplankton
pelagic invertebrates
benthic meiofauna
benthic macrofauna
percent of community biomass
100
b
80
60
40
20
0
seal biomass (103 metric tons)
12
10
8
6
4
2
0
2000
unfished
20
c
40
60
80
100
PFM
percent of status quo
Figure 4. Computer modeling with Ecopath/Ecosim predicts how populations of sprat, herring
and cod (a) might respond if fishing in the Baltic Sea were prohibited between the years 2000
and 2100 (unfished) or permitted at 20 percent, 40 percent, 60 percent, 80 percent or 100 percent
of the year-2000 level (the status quo level, or SQ). One current recommendation, precautionary
fishery management (PFM), would reduce cod fishing by 35 percent and herring fishing by 65
percent. Various scenarios for fishing levels also influence the prevalence of species at lower
trophic levels (b), particularly benthic macrofauna, which profit from reduced numbers of
predators at higher fishing levels. These models suggest that changes in predator populations
affect the rest of the ecosystem. They also predict that changes in fishing pressure affect seal
biomass (c). Because humans compete directly with seals for fish, even a low level of fishing
(20 percent of SQ) reduces seal populations far below the predicted unfished level.
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American Scientist, Volume 91
web determines its average trophic level. Primary producers are said to occupy the first trophic level because they
derive energy from the sun or chemicals in seawater. Phytoplankton, seaweeds and sulfur bacteria are primary
producers. Herbivores occupy the second trophic level because they eat primary producers. Carnivores occupy
the third because they eat herbivores.
The top species is called the apex
predator.
Fishing can directly affect communities, as when it changes key life-history
traits. Fisheries typically seek larger,
more valuable fish, decreasing the average size of fish in target populations.
Because reproductive ability relates to
size in many species, this decrease has
the potential to reduce the number of
fecund fish, altering population dynamics. Moreover, large species often
have long life cycles, and long-lived
populations cannot rebound as quickly
as can shorter-lived ones. Yet fishing
has shifted many marine communities
toward shorter-lived species.
Community structure is also altered
by the practice of targeting organisms
at lower and lower trophic levels as
higher levels become depleted. Such a
progression, called “fishing down food
webs,” is exemplified by the shift from
harvesting large groundfish, such as
Atlantic cod (Gadus morhua), in the Gulf
of Maine, to the current reliance on herring, lobsters, sea urchins and shrimp.
Work by Daniel Pauly from the University of British Columbia and others has
demonstrated that the trophic level of
fish landings worldwide has declined
in recent decades, particularly in the
Northern Hemisphere. (See “Fishing
Down Aquatic Food Webs,” January–
February 2000.)
Many of fishing’s effects on ecosystems are indirect. Trophic cascades, for
example, are changes in biomass propagated across three or more trophic
levels, as when depletion of a predator
allows populations of prey to increase,
which in turn suppresses the prey’s
own prey. If the cascade reaches primary producers, the ecosystem may become less productive. For example, the
massive overfishing of coastal Atlantic
cod and other predatory groundfish increased the abundance of the sea
urchin Strongylocentrotus droebachiensis,
which transformed forests of the kelp
Laminaria, a primary producer and important habitat for young cod, to
ecosystems dominated by coralline al-
© 2004 Sigma Xi, The Scientific Research Society. Reproduction
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The Baltic Sea
We have tried to predict the effects of
fishing on marine communities by deriving a model of an ecosystem and
varying the levels at which different
species are harvested. For this task, we
used the computer software Ecopath
(available at www.ecopath.org), which
simulates food-web structure by modeling the production and transfer of
biomass among species. Ecopath can be
tailored to any food web and has been
applied to more than 130 marine communities. It handles data on abundance, production and feeding rates,
diets and harvests of key food-web
components (primary producers, invertebrates, fish, marine fishing pressure on a particular species). In addition, we used Ecosim, a dynamic model
that simulates how a community
would likely respond to changes in
fishing intensity and other forces. Ideally, field studies are used to validate
3
average community trophic level
gae. Similarly, overhunting of sea otters in the Aleutian Islands allowed sea
urchin populations to expand; the
urchins then severely overgrazed kelp.
Overharvesting played a key role in
major shifts of entire communities in
both these examples, as in the shift
from corals to algae on Jamaica’s reefs.
In most cases, we do not know how
alterations in community composition
affect ecosystem function, although
they are generally believed to decrease
stability. Because marine systems are
immense, variable and affected by
many abiotic and biological forces,
they are inherently difficult to study.
Qualities such as species diversity are
easily measured but not explicitly
linked to community function. More
relevant qualities, such as stability (the
ability of a community to maintain a
state) and resilience (the ability of a
community to return to a state after
perturbation), are nearly impossible to
assess in natural communities. Furthermore, communities are dynamic—
some species are seasonal because they
migrate, many populations naturally
fluctuate in size, and all communities
are at the mercy of natural disturbances, such as hurricanes. To compound these problems, we lack the
comparisons in time or space that
would allow us to quantify the effects
of fishing or other disturbances. This
lack of perspective severely limits our
ability to predict how communities
will respond to human pressures.
secondary consumers
(predators on primary
consumers)
primary
consumers
(grazers)
2
primary
producers
1
0
0.2
0.4
0.6
proportion of current fishing levels
0.8
1
Figure 5. Models suggest that fishing decreases an ecosystem’s average trophic level by removing predatory secondary consumers, such as cod, and enabling populations of primary
consumers, such as herbivorous fish, to increase. This shift occurs at low fishing pressures, resulting in a curve shaped like a decay function.
model parameters, thus improving
model performance and our understanding of community dynamics.
We applied Ecopath/Ecosim to the
Baltic Sea, a northern European arm of
the Atlantic Ocean. This marine system
is ideal for this type of application because it is relatively simple and wellstudied. Because fisheries are a mainstay of the region’s commerce, there is
a rich body of data on local fish species
and the prey, predators and fisheries
that interact with them.
The most visible components of the
Baltic Sea food web (Figure 3)—fish
and seals—are currently in undesirable
states: Sprat (Sprattus sprattus), a fish of
low commercial value, dominates the
fish biomass; herring (Clupea harengus)
and cod (Gadus morhua) are near alltime lows; and seals are slowly recovering from near-total collapse during
the past century. These conditions have
resulted largely from overharvesting.
But how has overfishing affected the
rest of the community?
We do not know how the Baltic Sea
community was structured before fisheries developed, so we simulated an
unfished state by eliminating all fishing from the model for 100 years.
Compared with the year 2000, the 2100
community featured much larger biomasses of herring and cod at the expense of sprat (Figure 4a). These
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changes cascaded through the rest of
the community. For example, herring’s
increased abundance reduced the level
of one of its main prey, pelagic invertebrates such as small shrimp (Figure 4b),
to half the 2000 level. Similarly, the recovery of cod cut down the biomass of
benthic macrofauna, allowing one of
their major prey, benthic meiofauna, to
double in biomass by 2100. The overall
growth in total fish biomass in the unfished scenario tremendously increased seal biomass (Figure 4c).
We also determined what the system
might look like if fishing pressure were
maintained at the status quo. After 100
years of status quo fishing, sprat continued to dominate the fish community, cod biomass was low, and herring
were totally extirpated. The rest of the
community was profoundly different
from the unfished state: The abundance of benthic macrofauna was the
same as at present, pelagic invertebrates were more abundant because
they were not being eaten by herring,
but seal recovery was only modest.
We introduced fishing into the unfished system by incrementally increasing fishing pressure on cod, herring and sprat. As we moved from no
fishing to 20 percent of the status quo,
the three species and the overall community responded most strongly. Seals
also responded dramatically: This rela2003
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155
! " # Figure 6. Data demonstrate that marine fisheries landings have, at best, remained level since the
late 1980s. Further, the amount of primary productivity required to sustain these yields (as much
as 35 percent on temperate coastal shelves) is quite high. It is unlikely, therefore, that improvements in efficiency or management will increase harvests. Instead, the authors propose that future fisheries management be based on ecologically sustainable yield, a concept that will support
marine ecosystems and therefore maintain productivity for generations to come.
tively small increase in fishing allowed
human competition for fish to greatly
impede seal recovery.
We also used the model to predict
the consequences of a more precautionary fisheries management plan,
which some Baltic fisheries scientists
advocate. Their approach would cut
fishing pressure on cod and herring by
about 35 percent and 65 percent, respectively, from current levels and
would slightly increase sprat fishing. It
aims to bolster cod and herring populations by protecting them against
overfishing. We detected a desirable
response from the fish community:
Biomasses of cod and herring clearly
were higher in 2100 than in 2000 or status quo conditions . Unfortunately, the
rest of the community retained the
characteristics of a heavily fished system, falling somewhere between the 40
percent and 60 percent status quo scenarios. These results suggest that this
type of fishery rehabilitation would do
little to shift community structure toward the unfished state.
An additional simulation looked at
the effects of fishing pressure on community trophic structure. As seen
above, fishing directly and indirectly
changes the relative abundances of
species in a food web. While others
have focused on the change in average
trophic level of the harvested species,
we calculated the average trophic level
(weighted by the abundance of each
community component) of the entire
Baltic community across a range of
fishing pressures. As pressure in156
American Scientist, Volume 91
creased from zero to the status quo level, the average trophic level decreased,
following a curve shaped like a decay
function (Figure 5). Much of that decrease resulted from a shift of biomass
from relatively large fish to a community increasingly dominated by phytoplankton and benthic macrofauna. The
dominant fish species changed from a
major predator, cod, to one of its prey,
sprat, and the biomass of the overall
community shifted closer to the primary consumer level (trophic level 2). Our
model therefore supports the idea that
fishing can change an ecosystem’s
trophic structure.
Global Consequences
The Baltic Sea ecosystem is but one of
hundreds of marine ecosystems that
provide commercially useful fish and
other resources. Taken together, these
systems operate within the larger global system, which is closed and therefore
sets important limits on exploitation
rates. Because harvestable biomass depends on assimilated solar and chemical energy, as well as on individual animals capable of reproduction, the
absolute limit on the sustainable rate of
harvest is determined by the rate at
which primary producers can assimilate the energy that supports higher
trophic levels. If fisheries remove a substantial proportion of the energy acquired from primary producers, the
system becomes potentially unstable.
Daniel Pauly and Villy Christensen
estimated that 8 percent of the ocean’s
primary productivity is necessary to
sustain global fisheries, which is four
times greater than previous estimates.
However, this figure was dominated
by the open ocean, which represents
most of the ocean’s surface area but relatively little of its productivity. For
commercially important regions, the
primary production required to sustain
fisheries was much greater: 25 percent
for upwelling zones, 24 percent for
tropical coastal shelves and an extraordinary 35 percent for temperate coastal
shelves.
It is apparent that we are close to the
limit on sustainable harvests and that
simply improving fishing efficiency or
management practices will not allow us
to significantly increase global marine
exploitation. In fact, since Pauly and
Christensen published their study,
global landings have leveled off (Figure
6), suggesting that we might already
have reached some sort of threshold.
Humans are now the apex predators in
many marine communities, but, unlike
other predators, we generally do not recycle energy back into the appropriate
ecosystems. We also have little understanding of the long-term effects of removing huge amounts of energy and
biomass from the sea.
Ecologically Sustainable Yield
We have seen that the traditional approach to fisheries management relies
on single-species models of population
dynamics that aim to sustain harvests
of commercial species. Such an approach ignores a broad suite of interactions among exploited species and between exploited species and other
members of their communities. Widespread recognition of this shortcoming
has spawned considerable interest in
ecosystem-based fisheries management, and scientists are beginning to
identify suitable metrics for monitoring marine ecosystems. Ecosystembased management requires a longterm commitment to monitor all
trophic levels of marine organisms and
the physical forces that influence their
communities. The well-studied Baltic
Sea community and a few others, such
as the Bering Sea community, lend
themselves to such analyses, but these
studies must be extended to include all
impacts of fishing, including habitat
destruction and bycatch, which in
some cases depletes more biomass
than does harvest.
Achieving the goal of sustainable
harvest of fish, however, does not
© 2004 Sigma Xi, The Scientific Research Society. Reproduction
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mean that the impact on marine
ecosystems of fisheries is eliminated.
The Baltic Sea example shows us that
careful assessment of fish stocks in
concert with limited levels of fishing
effort allow sustainable fisheries. At
the same time, prosecution of the
Baltic Sea fishery, even at a limited level, fundamentally alters community
structure at all trophic levels. In our
hypothetical scenarios, fishing above
intermediate levels (approximately 40
percent of the status quo) elicited no
appreciable positive response from the
marine community. This inertia implies that even precautionary levels of
harvest can diminish the ecological
importance of target species by significantly weakening their linkages to
other community members.
Thus, the goal of sustainable fisheries is simply different from, and in
some cases may be incompatible with,
the goal of maintaining natural marine
communities. This should not come as
a shock. Sustainable fishing will lower
the size of fish populations well below
the size of unfished populations. Thus
we might expect community shifts in
systems where fish strongly influence
the population size of their prey or
predators. We argue, however, that the
cost of mismanaging a community
might be far greater than the cost of
mismanaging a fishery. Although overfished stocks have been known to recover, revival of communities that have
changed states can be excruciatingly
slow or even impossible. Such potential impacts are rarely considered in
cost-benefit or risk analyses.
Fisheries agencies around the globe
focus on the management of natural
“resources.” To the extent that ecosystems affect exploited resources, these
agencies have embraced ecosystem research that allows better resource management. However, a shift in the
species composition of benthic invertebrates or seaweeds has not historically
been the concern of agencies unless
such changes affected fish production.
We suggest the time has come to consider the status of marine ecosystems,
in addition to the status of fisheries.
One solution to the environmental crisis in our oceans might be to redefine
sustainability. If we can shift our way of
thinking about fisheries and ecosystems
we may prevent the shifting of the
ecosystems themselves. We propose a
change from maximum sustainable
yield to ecologically sustainable yield (ESY):
the yield an ecosystem can sustain without shifting to an undesirable state.
To determine ESY, one will have to
simultaneously consider the impacts of
all harvested species on an ecosystem,
quantifying important qualities such as
community stability or resilience. This
will be challenging because of the uncertainty and indeterminacy inherent
in ecological systems and because
ecosystems change in response to natural processes in ways that we do not
fully understand. However, we can
and should improve our understanding of the bounds of expected ecosystem behavior and define ESY within
the limits of predictability. Maximum
sustainable yield or other more conservative fisheries targets are critical tools
in achieving the goal of sustainable
fisheries. We submit that a concept
such as ESY is a necessary instrument
in achieving the objective of marine
conservation.
Widespread implementation of ESY
is obviously an issue of policy, rather
than science. For many ecosystems,
harvesting at ESY levels will require
even stricter limits than those used to
achieve maximum sustainable yields.
But we believe that until we fully incorporate the notion of ecological sustainability into our philosophy of fisheries management, we will continue to
degrade the communities that nurture
commercially valuable fish.
Acknowledgments
Rick Methot, Church Grimes, Marc Mangel, Paul Dayton, Mike Schiewe, John
Williams, Jon Drake and Doug Dey provided stimulating discussions that greatly
improved this article. The views expressed
in this article are those of the authors and
not necessarily of the National Marine
Fisheries Service.
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Links to Internet resources for further
exploration of “Ecologically Sustainable
Yield” can be found at the American
Scientist Web site:
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