The global marine fish catch is
approaching its upper limit. The number of overfished populations, as well as
the indirect effects of fisheries on marine ecosystems, indicate that
management has failed to achieve a principal goal, sustainability. This failure
is primarily due to continually increasing harvest rates in response to
incessant sociopolitical pressure for greater harvests and the intrinsic
uncertainty in predicting the harvest that will cause population collapse. A
more holistic approach incorporating interspecific interactions and physical
environmental influences would contribute to greater sustainability by reducing
the uncertainty in predictions. However, transforming the management process to
reduce the influence of pressure for greater harvest holds more immediate
promise.
Effects
of Fisheries on Marine Ecosystems
Fishing activities have altered and
degraded marine ecosystems through both direct and indirect effects, especially
in coastal regions where fishing and other anthropogenic perturbations are most
intense. In terms of direct effects, fisheries remove the results of about 8%
of the global primary production in the sea, but they require 24 to 35% of
upwelling and continental shelf production. Fishing reduces the abundance of
targeted stocks; the numerous examples worldwide of depletion through
overfishing are especially serious for species with high natural longevity and
low reproductive rate. Classic examples of population collapse where fishing
may have played a role include the sardine stocks off California and Japan in
the late 1940s and the anchovy off Peru and Chile in 1972. Such collapses are
of global importance because sardines, anchovies, and related species are a
dominant part of world catches (currently 7 of the top 10 species). More recent
examples of overfishing include the collapse of the Canadian cod fishery and
several New England groundfish stocks. Even where stock abundances remain high,
effects of size-selective fishing imperil future resiliency and sustainability
by markedly reducing average age, size at age, and genetic diversity. The
capture and increased mortality of less desirable, often juvenile, stages of
nontarget species are substantial, exceeding catches of targeted species in
many fisheries owing to use of efficient but nonselective fishing gears and
high prices of a few target species that subsidize that wastage .
Catch records for several Pacific
species. Catches of sardines in the California Current and off Japan increased
and decreased synchronously in the early part of this century. Off Peru and
Chile, the sardine increased after the decline of the anchovy in the mid-1970s.
The Japanese sardine increased at the same time. The sardine off California is
also increasing, but is not yet abundant enough to be harvested (not shown).
Salmon catches in Alaska have been similar to the Japanese and California
sardines.
Indirect effects of fishing can have
more important impacts on marine ecosystem structure and dynamics than do
removals of the fish themselves. Many nearshore ecosystems have been
substantially altered through destruction of benthic biogenic habitat.
Dredging, trawling, long-hauling, and igniting explosives have killed and
removed the emergent sessile organisms that provide critical structural habitat
on otherwise relatively featureless sea floors. The contributions of fishing
activities to widespread destruction of coral reefs, temperate oyster and
polychaete reefs, seagrasses, and other epibenthic organisms have repercussions
throughout the ecosystem because structural habitat plays an important role in
recruitment, prey protection, and sustaining biodiversity.
Indirect trophic (food web)
interactions induced by fishery removals represent a second class of important
indirect effects of fishing. The few documented marine examples of top-down
controls on community organization typically involve loss of a top predator
such as sea otters or lobsters from coastal benthic systems; the consequent
release from predation allows prey species to expand their cover on rock
surfaces, leading to enhanced competition and displacement of less competitive
species by a few dominants. Another example of this process is the overfishing
of herbivorous fishes on coral reefs, which together with eutrophication allows
macroalgae to overgrow and kill corals. In Chile, removal of a muricid
gastropod, loco, permits domination of its principal prey, a mussel, instead of
two local barnacles. Despite the paucity of documentation of analogous top-down
controls of community organization in the deep ocean , a few nearshore pelagic
examples combined with the selective nature of fishing preferentially on
larger, top predators in the sea imply a potential for (thus far undetected)
analogous top-down indirect trophic effects in deep oceans.
The effects of harvest on community
structure can be most easily seen in the rocky intertidal. In this example from
the Coastal Preserve of the Estacion Costera de Investigaciones Marinas at Las
Cruces, Chile, intertidal food-gathering activities were stopped in 1981. One
year before the Preserve was established, the mussel Perumytilus purpuratus
covered almost 100% of the rocky shore and the keystone carnivorous gastropodConcholepas
concholepas, “loco,” was rare due to harvest. Within a couple of years,
loco density increased and they readily consumed the mussels. Three and 12
years later, loco density was much higher, the mussels were almost completely
eliminated, and three species of barnacles and different species of macroalgae
had replaced the mussels.
Fishing is presumed to release
competing species from competition with the targeted species, but this indirect
response is difficult to confirm. Evidence suggests that the removal of baleen
whales from high-latitude oceans has provided now unutilized zooplankton prey
to fuel alternative energetic pathways .The increase of the anchovy population
in the California Current after the decline of the sardine off the west coast
of the United States in the late 1940s suggests an analogous competitive
release, but similar covariability does not appear in longer time records.
Note, however, that the sardine population off Peru and Chile increased after
the decline of the anchoveta in 1972. Fishing intensively on sardines,
anchovies, and other forage species also harms populations of natural consumers
of that prey, including seabirds and marine mammals. Critical bottlenecks in
the life histories of many seabirds and marine mammals occur during the
energetically demanding raising of young; this rearing is typically tied to a
relatively circumscribed nearshore location, so the temporally and spatially
localized depletions of forage fishes can imperil reproduction and drive
population declines. Provision of discarded fish to scavengers, typically
larger, more aggressive seabirds, also has pronounced effects on coastal marine
ecosystems, in part because those aggressive seabirds disrupt and alter the
broader seabird community through nest predation and aggression.
Physical
Influences on Marine Ecosystems
Understanding the widespread, often
dramatic, effects of fishing on marine ecosystem structure and dynamics
requires assessing the confounding influence of the varying physical
environment. Fisheries scientists have long been concerned with the effects of
annual changes in weather and physical oceanographic conditions to enable them
to make year-to-year adjustments in management. Their traditional focus has
been on variability on yearly time scales and spatial scales encompassing the
range of the population of interest. For example, coupled changes in the
atmosphere and the ocean occur irregularly every few years to create ENSO (El
Niño–Southern Oscillation) conditions in the Pacific. These conditions involve
warmer waters over a range of latitudes in the eastern Pacific, which are
accompanied by changes in coastal circulation. Ecological consequences of ENSO
events along the coasts of South and North America include a decline in primary
productivity near the equator, a decline in zooplankton productivity in the
California Current and diminished survival and growth of some fishes such as
salmon and mackerel.
Differences of ±0.5°C from seasonal
mean 0 to 200 km from shore. ENSO (El Niño–Southern Oscillation) events cause occasional
warming and cooling at various latitudes on annual time scales. A shift from a
cool to a warm regime occurred in 1976 accompanied with the intensification of
the Aleutian low-pressure zone [redrawn from Cole and McLain].
A significant recent advance in the
understanding of how the physical environment alters ocean ecosystems is the
realization that large-scale changes called regime shifts occur across entire
ocean basins every few decades. The best documented regime shift took place
during the mid-1970s in the north Pacific Ocean, when intensification of the
Aleutian low-pressure system was accompanied by shifts in many biologically
significant physical variables, including the change in 1976 from cooler to
warmer conditions. In the subarctic Pacific, some of these changes, such as an
increase in the depth of the mixed layer, may have been responsible for
important biological changes, namely, increases in chlorophyl concentrations and
Alaskan salmon catches, and a shift from shrimp to fish (gadoids and flatfish)
dominance in the northern Gulf of Alaska. Awareness of broad-scale regime
shifts has led to increased understanding of the congruence of major changes in
populations of sardines and anchovy stocks in coastal ecosystems around the
world, and support is growing for the hypothesis that these population shifts
are the result of long-term, wide-scale changes in physical conditions, rather
than just fishing.
Recent research efforts on the biological
effects of physical oceanographic conditions are also being directed toward
finer spatial and temporal scales than traditionally treated. Analysis of the
effects of events at the spatial scale of individual fish during critical
larval and juvenile stages has the potential to illuminate how biological
productivity varies over larger spatial scales. Examples include the way in
which weekly fluctuations in upwelling winds affect primary productivity, and
the importance of occasional calm periods that allow feeding of larval fishes. These
weekly fluctuations in upwelling winds also drive mesoscale (that is, 10 to 100
km) circulation, which determines the transport of planktonic larvae and hence
recruitment to harvested populations. This short-term variability in
recruitment to fish populations appears to be responsible for yearly
differences in the spatial pattern of recruitment, which drives the spatial
dynamics of marine metapopulations (groups of populations connected by larval
dispersal) .
Fisheries
Management
For as long as fluctuations in
fishery landings have been a collective human concern, various concepts of
marine ecosystems have been proposed as a basis for management. Thomas Huxley's
1884 view that “probably all great sea-fisheries are inexhaustible” was
countered at the same symposium by Ray Lankester's concerns for the removal of
spawning stock and call for consideration of nontarget species. He maintained
that the fish removed were not superfluous, as claimed, but rather had “a
perfectly definite place in the complex interactions of the living beings
within their area”. However, despite appeals for ecosystem management of ocean
fisheries, development of multispecies stock assessment methods, and new
concepts of large marine ecosystems, few fisheries are actually managed on a
multispecies basis .
One goal of ecosystem management,
sustainability, has a long tradition in fisheries; because fish growth rates,
survival rates, and reproductive rates increase when fishing reduces population
density, they produce a surplus of biomass that can be harvested. This
rationale implicitly accounts for some nontarget species in that fishing was
considered to “thin” the fish population, making more prey available.
Maximizing sustained yield on this basis was a goal of fisheries management
through the middle of this century. The goal of maximum sustained yield (MSY)
was challenged 20 years ago on several grounds: It put populations at too much
risk; it did not account for spatial variability in productivity; it did not
account for species other than the focus of the fishery; it considered only the
benefits, not the costs, of fishing; and it was sensitive to political
pressure. In fact, none of these criticisms was aimed at sustainability as a
goal. The first one noted that seeking the absolute MSY with uncertain
parameters was risky. The rest point out that the goal of MSY was not holistic;
it left out too many relevant features.
Current fisheries management depends
on stock assessments to estimate population parameters of the focal species
from the age or length structure of past catches, biomass of past catches, past
fishing effort, and fishery-independent surveys . In the most common
institutional format for fisheries management, fisheries scientists formulate
potential management actions based on these estimates, then provide them to
fishery managers, who weigh their sociopolitical consequences in deciding which
to implement. This structure leads managers to constantly increase fishing
pressure to excessive levels because of the “ratchet effect”: Managers, under
constant political pressure for greater harvests because of their short-term
benefits to society (jobs and profits), allow harvests to increase when fishery
scientists cannot specify with certainty that the next increase will lead to
overfishing and collapse. This is a one-way ratchet effect for two reasons:
There is rarely political pressure for lower harvest rates (fewer jobs and
lower profits in the immediate future), and the burden of proving whether
higher harvests are harmful falls on the fishery managers, not the fishing
industry. The result is a continuous, unidirectional increase in fishing
effort, and in some cases fishery collapse. In a few instances, mistakes in
stock assessment also may have been made [for example, the Canadian cod stocks].
However, for the most part, overfishing is due to the ratchet effect. Proposed
solutions to the lack of sustain- ability of fisheries must change the two
elements of this root cause of overfishing, either by reducing uncertainty in
predicting the effects of management or by reducing the pressure on managers
for increased harvest. However, because of the limited understanding of the
complexity of marine ecosystems, the difficulty and expense involved in
sampling them, and their susceptibility to environmental variability, there
will always be great uncertainty in predictions of the effects of harvest. Thus,
reducing harvest rates will require a reduction in the pressure for greater
harvest on the management process. This could be achieved, for example, by
reductions in overcapitalization of fisheries and government subsidies of
fishing, and will require controlling the open-access nature of fisheries. In
addition, a better understanding of, and changes in, the way that management responds
to uncertainty could also reduce overfishing.
Because of recent failures to
sustain catches, fishery agencies have developed specific frameworks for
avoiding low abundance. In addition to targets that allow them to obtain the
best harvest, they also now operate with thresholds below which emergency
actions are taken to rebuild populations. For example, in the United States,
about 100 federal management plans now contain a definition of overfishing and
stipulate remedial actions once a population is overfished. This shift in focus
has increased awareness of another source of uncertainty, the behavior of
populations at low abundance (especially when considered in the context of
other induced ecosystem changes). The stock level at which recruitment to a
population will decline rapidly is not known until it happens, nor is the
subsequent behavior of competitors and predators. These threshold levels are
therefore based on empirical comparisons with similar species that have been
overfished .This aspect of fisheries management presents an opportunity for
fishery biologists and their colleagues concerned with endangered species to
collaborate on the development of methods to their mutual benefit.
Greater
Holism
Greater holism in fisheries
management can be achieved by consideration of multiple species interactions,
broad-scale physical forcing, and the response of management to pressure for
greater harvests under uncertainty. To the extent that lack of sustainability
of fisheries is due to the ratchet effect, whether such an expanded focus
improved sustainability would depend on whether the first two of these reduced
the uncertainty in prediction, and whether the third reduced the effects of
political pressure for short-term gain in the management process.
Multiple species approaches. Virtually all fisheries in the world target more than one
species or affect secondary species. Yet fisheries science has diverged from
traditional oceanography and limnology, as well as community ecology, in
maintaining a focus on single-species descriptions. Presumably, the rationale
for this was to simplify the system by omitting the details of ecosystem
complexity. That trend has been questioned in recent years; new assessment
methods and management approaches account for both biological and technological
(for example, through nets harvesting several species) interactions among
species.However, ecosystem management of marine systems requires a
sophisticated understanding of ecosystem dynamics and the organization of
component communities. The development of marine ecosystem management lags
significantly behind management of terrestrial and freshwater systems due to
undersampling of the oceans, their three-dimensional nature, and the difficulty
in replicating and controlling experiments. Thus far, the value of multispecies
approaches in marine fisheries has been in terms of post hoc explanations of
long-term changes, rather than year-to-year predictions. Even in cases of
replacement of collapsed species by competitors, it is not clear whether
knowing the dynamics of the competitive interaction would have prevented the
collapse.
At present, the ability of marine
ecology to incorporate multispecies and ecosystem information into a model that
would reduce uncertainty in forecasting the effects of alternative management
choices is limited. Food web descriptions and even energy-flow models represent
static descriptions of the past and do not predict dynamics arising from future
perturbations such as alternative exploitation scenarios. Dynamic models of
interacting species are uncertain in their predictions, and factoring in
physical forcing, such as the effects of local turbulence on feeding success,
of mesoscale circulation on metapopulation structure, and of global regime
shifts on entire communities, will add further complications. One promising,
but challenging, protocol for the development of ecosystem models for
management involves use of adaptive management to identify strong interactors
and erect interaction webs that include physical as well as biological
components.
Only in coastal regions, where
habitat alteration, water pollution, and other serious anthropogenic influences
are pervasive, are the costs of such a holistic, multispecies approach likely
to be compensated by short-term benefits to the fisheries industry.
Nevertheless, if sustainability over the long term depends on retention of the
integrity of ecosystem structure, then there may be long-term payoffs, even to
pelagic fisheries, of adoption of an ecosystem approach. Furthermore, because
fishing represents such a significant disruptor of ocean ecosystems, wildlife
conservation objectives on behalf of seabirds, marine mammals, and sea turtles
also require and justify an immediate commitment to progress in multispecies management.
Physical forcing. Recent identification of the dramatic effects of
basin-scale, decadal variability on marine ecosystems and component species,
such as small pelagics and salmon in the north Pacific, have reduced the
uncertainty surrounding some fluctuations in fish stocks. However, in most
instances the mechanisms of physical-biological coupling have not been
identified, a necessary step to greater utilization of this understanding for
prediction. Because optimal management and expected catch will vary with
climatic regime, such knowledge should improve management. However, knowledge
of the potential effects of regime shifts can also introduce ambiguity. For
example, the regime shift in the north Pacific in the mid-1970s has been
proposed as an alternative to the completion of the last several dams on the
upper reaches of the Columbia River as an explanation for the dramatic decline
in chinook salmon stocks. An understanding of the mechanisms underlying regime
shifts is needed to differentiate between causes. Such information will also
provide clues as to the possible effects on marine ecosystems of changes in
climate due to global warming. Physical effects on weekly time scales and
mesoscale spatial scales have the potential to provide better explanations of
annual variability in the abundance and distribution of fish and invertebrates
than currently used monthly averages. Better understanding of the effects of
mesoscale circulation on dispersal within coastal metapopulations will provide
information for rational management of populations distributed along
coastlines, especially important for those crossing jurisdictional boundaries
Pressure for greater harvests. The influence of political pressure for short-term gain on
the fishery management process needs to be reduced. Greater holism in this case
involves expanding our view of fisheries management to include aspects of
economics and political science. One approach to combating the common property,
open access nature of fisheries has been to provide a sense of ownership to
fishermen, either through individual transferable quotas or greater involvement
in management through comanagement schemes. Both of these still require
estimates of the effects of different levels of harvest, but they are designed
to reduce pressure for short-term gain by increasing vested interest in the
long term. In practice, this is effective only under certain conditions. Basing
management on a degree of ownership by fishermen works best in small-scale,
artisanal fisheries in coastal zones, where overcapitalization is not present
and short-term economic interests can be overcome by appeals for cooperation
based on clear scientific demonstration of the utility of such an approach .However,
it will be more difficult to change large, overcapitalized fisheries.
Particularly challenging are large international fisheries, where existing
institutional structures are inadequate to overcome short-term economic
interests, and where socially and culturally diverse participants have little
tradition of cooperation.
Greater management involvement of
stakeholders who do not have an actual long-term interest in the fishery may
even have negative effects on sustainability. The concept of optimum sustained
yield, allowing for economic, social, and other considerations, rather than
simply maximizing biological yield, emerged at a United Nations oceans
convention in Geneva in 1958 and was used in subsequent management. In the
United States, for example, the Magnuson Act of 1976, which created the current
federal management structure, charged regional councils with taking into
account socioeconomic consequences of management actions, and added the
possibility of industry participation in management. The record of management
since then, evidenced especially by collapses of New England groundfish stocks,
has led to charges of foxes having too great a role in guarding the henhouse.
The definition of optimal sustainable yield in the Magnuson Act was changed in
1996 to be MSY or less as determined by economic, social, and ecological
considerations. Changes such as these that counteract the ratchet effect will
occur more frequently with increasing public education and awareness of fishery
problems. Responsible public policy demands inclusion of all stakeholders in
the decision-making process, but more effective means of implementing
comanagement so that biological judgments are not compromised need to be
devised. Political forces for short-term economic gain are present in countries
at all levels of development of management capability and operate through
local, national, and international channels. For example, Third World countries
under pressure to repay their external debt may increase allowable catches to
do so.
Better
Management
Several changes in the way fisheries
are managed would improve sustainability without changes in scientific
approach. Using a precautionary approach to fishery management is one example.
The precautionary principle, as applied in other areas of environmental law,
involves taking a conservative approach to management issues until there is
compelling evidence that a less conservative approach would pose no added risk.
The burden of proof that it is safe to be less conservative is then shifted to
those favoring that option. Another policy option, spatially explicit
management, has great potential to improve sustainability of ocean ecosystems.
Harvest refugia can preserve a specified fraction of an exploited population by
shielding that fraction of the population's range instead of specifying a
certain fishing effort. That approach removes the dependence on uncertain
assumptions about the link between fishing effort and future biomass. Use of
marine reserves also reduces uncertainty regarding the effects of harvest on
ecosystems because portions of the ecosystem remain intact . Spatial
variability in management also provides the potential for more efficient
harvest, as well as the possibility of experimental harvesting and adaptive
management, which is the most direct empirical way to reduce uncertainty in
fisheries. Finally, closures and moratoria should be used more liberally to
protect and allow recovery of declining stocks or stressed marine ecosystems
well before, instead of after, collapse has occurred.
A holistic, ecosystem approach to
fishery management requires the integration of information from a wide range of
disciplines, levels of ecological organization, and temporal and spatial
scales. New, expanded mathematical models that synthesize multiple processes
are critical to the scientific basis of ecosystem management of marine
fisheries. Such modeling should integrate the many anthropogenic influences on
ocean ecosystems, now treated in isolation: eutrophication and induction of
nuisance algal blooms; habitat destruction, fragmentation, and degradation;
species introductions, extinctions, and endangerments; chemical pollution of
the sea; and effects of anthropogenic and natural global change on ocean
physics.
In conclusion, ocean ecosystems are
influenced as much by changes in the physical environment as by humans, but it
would be a fallacy to conclude that the effects of fishing can be dismissed as
unimportant. The effects of the physical environment on marine ecosystems make
it difficult to define sustainability in the context of ecosystem management,
but it is clear that the root cause of the lack of sustainability is the
sociopolitically biased response of management to intrinsic uncertainty. A more
holistic approach involving expanded consideration of other strongly interacting
species, marine habitats, and the physical environment has the potential for
incrementally improving sustainability by reducing uncertainty. However,
attention to changing the institutions and processes by which fisheries
management is implemented will have more immediate payoffs in improving fishery
sustainability. The challenge for the next century lies in crafting new local
and regional institutions, not just in filling the scientific gaps. The best
hope for greater sustainability of marine ecosystems is to insulate management
from pressure for greater harvest while attempting to reduce uncertainty
through a comprehensive ecosystem view.
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