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NEWS & COMMENT
Ecosystem effects of fishing
M
arine fisheries are in decline worldwide. Increasing problems of bycatch and habitat destruction, and now a
growing realization of the role of climate
on fished populations, are leading to widespread recognition that the single species
approach to fisheries management is not
effective. These problems are moving
management from its traditional focus on
maximizing the yield of individual resources towards broader considerations
of direct and indirect impacts of fishing
on ecosystems as a whole. Thus, the
International Council for the Exploration
of the Sea (ICES) and the Scientific Committee for Oceanic Research (SCOR) convened a meeting on the Ecosystem Effects
of Fishing this March in Montpellier,
France. The objective was to provide
a global synthesis of the impacts of
fishing on marine ecosystems, to report methods for quantifying ecosystem
effects, and to provide a forum for discussion of how objectives relating to
the conservation of nature can be integrated into fisheries management. Led
by Michael Sinclair (Bedford Institute
of Oceanography, Dartmouth, Canada)
and Henrik Gislason (University of
Copenhagen, Charlottenlund, Denmark),
the symposium attracted more than 300
participants from 54 countries*.
Impacts on ecosystems and species
groups
The meeting was highlighted by
reviews of the effects of fishing on different marine ecosystems and species
groups. These illustrated the challenges
of relating the causes of a broad range of
impacts. For estuaries, nearshore systems and enclosed seas, the effects of
fishing are hard to isolate from the results
of other human activities, especially
runoff of nutrients. John McManus [International Centre for Living Aquatic
Resource Management (ICLARM), Manila,
Philippines] noted that although it might
be difficult to separate the effects of overfishing from organic pollution on coral
reef communities, reductions in herbivory
greatly enhance the likelihood that
organic pollution will lead to a coral–algal
phase shift. Widespread coral bleaching
during the 1997–1998 El Niño on reefs
already stressed by organic pollution and
overfishing raises concerns that these
*Invited presentations and a selection of
papers from the many posters that added
to the discussion will be published in the
ICES Journal of Marine Science.
TREE vol. 14, no. 7 July 1999
interacting disturbances might often
prevent return to coral domination.
In contrast, kelp forests offer clear
demonstrations of ecosystem effects of
fishing in situations where alternate
stable states exist between kelp forests
sustained by sea otters (Enhydra lutris)
and sea urchin- (Strongylocentrotus spp.)
dominated barren grounds. Marine
reserves in New Zealand and offshore
refugia from trawling in New England
(USA) offer further evidence that fishing
of urchin predators has cascading
impacts through kelp community structure. Boreal ecosystems are characterized
by relatively few dominating species with
strong interactions in highly dynamic
environments; the critical question here
is whether excessively large fluctuations
in landings could have been avoided or
significantly damped with lower fishing
pressure or whether climate-induced
anomalies are the dominant influence.
In contrast with most shallow species,
continental slope and deep-sea fishes
exhibit extreme K-selected life history
strategies, with high longevity, slow
growth, relatively low fecundity and
apparently high recruitment variability.
Most aggregate around seamounts.
J.A. Koslow [Commonwealth Scientific
and Industrial Research Organisation
(CSIRO), Hobart, Australia] described
how the combination of low productivity
with high vulnerability has rapidly
depleted stocks such as orange roughy
(Hoplostethus atlanticus) to the point of
commercial extinction or very low (1–2%
of virgin biomass) sustainable levels. Furthermore, trawling has devastated the
highly endemic suspension-feeding communities that characterize these habitats.
These sessile animals, mostly corals, tend
to be very slow growing and the obliterated populations were almost certainly
extremely old, long-lived individuals. This
suggests a poor prognosis for sustainability. The ecological consequences of
trawling in shallow water vary with substrate and gear type; areas with high
degrees of biogenic structure (long-lived
corals, sponges, bryozoans, etc.) could
be permanently harmed, whereas some
high energy areas show minimal impact.
The effects of fishing on other species
groups depend strongly on individual life
histories, ranging from relatively mild in
the case of some tunas, to devastating for
certain K-selected elasmobranchs. Two
once abundant skates (the common skate,
Raja batis, and the barndoor skate
R. laevis) may be approaching extinction,
both victims of bycatch. Turtles, sea birds,
and marine mammals suffer direct (mortality in fishing gear) and indirect effects
(e.g. alterations of their food supply).
Some albatross species, for example, are
experiencing high levels of mortality in
long-line fisheries (e.g. fisheries of the
Patagonian toothfish Dissostichus eleginoides and southern bluefin tuna Thunnus
maccoyii). Because several marine birds
and mammals and all marine turtles are
endangered, the potential for fishing to
cause irreversible ecosystem changes is
very real.
Tools for assessment and
management
In 1996, Larkin1 suggested that the
essential components of ecosystem management are sustainable yield, maintenance of biodiversity and protection
from the effects of pollution and habitat
degradation. Tackling the problem of
defining overfishing from this perspective, Steve Murawski [National Marine
Fisheries Service (NMFS), Woods Hole,
MA, USA] offered a proposal. ‘Ecosystems
are considered overfished when the
cumulative effect of catches, nonharvest
mortalities and habitat modifying effects
result in one or more of the following:
biomasses of important species or
assemblages fall below minimum biologically acceptable limits, such that
(1) recruitment prospects are significantly impaired; (2) rebuilding times to
levels allowing catch near the maximum
sustainable yield are unacceptably long;
(3) prospects for stock recovery are jeopardized because of adverse species interactions; or (4) any stocks are threatened
with biological extinction.’
Other management tools presented for
their potential to assess fisheries impacts
and achieve ecosystem objectives in fishery management plans included various
modelling approaches, legal and economic
instruments, gear modifications to limit
bycatch and marine protected areas.
Ecosystem objectives usually come
from high level national policies or international agreements with broadly stated
goals; the challenge is to put the objectives into practice, deal with uncertainties
and develop evaluation methods and appropriate reference points. Keith Sainsbury
(CSIRO, Hobart) described working examples – the US Marine Mammal Protection
Act and a strategy for the Northwest
shelf of Australia. The Convention on the
Conservation of Antarctic Marine Living
Resources (CCAMLR) is probably the
leading practitioner of ecosystem management in the world today. Established
in 1982 to minimize fishing impacts on
krill, CCAMLR manages fisheries south of
the Antarctic Convergence. Its objectives
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261
NEWS & COMMENT
include maintenance of ecological relationships and populations at levels that
ensure high levels of recruitment, restoration of depleted stocks, minimum risk
of irreversible changes and sustainable
harvest. Andrew Constable (Australian
Antarctic Division, Kingston, Australia)
described CCAMLR’s development of
decision rules, its application of the precautionary principle in view of long-term
environmental change, and methods to
account for uncertainty and achieve
consensus.
Conclusion
The meeting ended with progress
reports from different regions and perspectives of various national players.
The USA Sustainable Fisheries Act of 1996,
for example, mandates development of
approaches maintaining ecosystem health
and viability, and contains provisions for
essential fish habitat and restoration of
stocks. Fishermen generally remain concerned about economic hardship and
property rights, but acknowledge increasing public concerns over the environmental impacts of fishing. The
common agenda of nongovernmental
organizations is that the goal of management should not be the maximization of
production or efficiency, but rather the
conservation of biodiversity and avoidance of irreversible genetic impacts.
Although most national representatives
reported efforts towards more holistic
approaches, managers from the European Union and Norway were conspicuous for their reluctance, notwithstanding
evidence presented at the meeting, to
accept change and continued insistence
on single species management.
Despite frustrations voiced by some
players, Keith Sainsbury noted progress;
at an ICES meeting in the late 1970s, there
wasn’t even agreement that ecosystems
are altered by fishing. Direct and indirect
effects are clear in many systems, and
the direct effects are strong. The precautionary approach offers an alternative
to insufficient data, but governments,
managers and stakeholders want better
indicators, standards and robust strategies from scientists. With better tools,
precaution and adaptive management,
an approach based on maintaining
healthy ecosystems provides realistic
prospects for sustaining fisheries in variable environments, as well as protection
of biodiversity.
Mia J. Tegner
Paul K. Dayton
Scripps Institution of Oceanography,
University of California,
San Diego, La Jolla,
CA 92093-0201, USA
(mailto:[email protected];
mailto:[email protected])
References
1 Larkin, P.A. (1996) Concepts and issues in
marine ecosystem management, Rev.
Fish Biol. Fish. 6, 139–164
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TREE vol. 14, no. 7 July 1999