Fisheries Management and Ecology, 1999, 6, 401±420
Fishing impacts and the degradation or loss of
habitat structure
S. J. TURNER,* S. F. THRUSH, J. E. HEWITT, V. J. CUMMINGS
& G. FUNNELL
National Institute of Water and Atmospheric Research, Hamilton, New Zealand
Abstract The wider effects of fishing on marine ecosystems have become the focus of
growing concern among scientists, fisheries managers and the fishing industry. The present
review examines the role of habitat structure and habitat heterogeneity in marine ecosystems,
and the effects of fishing (i.e. trawling and dredging) on these two components of habitat
complexity. Three examples from New Zealand and Australia are considered, where available
evidence suggests that fishing has been associated with the degradation or loss of habitat
structure through the removal of large epibenthic organisms, with concomitant effects on fish
species which occupy these habitats. With ever-increasing demands on fish-stocks and the need
for sustainable use of fisheries resources, new approaches to fisheries management are needed.
Fisheries management needs to address the sustainability of fish-stocks while minimizing the
direct and indirect impacts of fishing on other components of the ecosystem. Two long-term
management tools for mitigating degradation or loss of habitat structure while maintaining
healthy sustainable fisheries which are increasingly considered by fisheries scientists and
managers are: (1) protective habitat management, which involves the designation of protected
marine and coastal areas which are afforded some level of protection from fishing; and (2)
habitat restoration, whereby important habitat and ecological functions are restored following
the loss of habitat and/or resources. Nevertheless, the protection of marine and coastal areas,
and habitat restoration should not be seen as solutions replacing conventional management
approaches, but need to be components of an integrated programme of coastal zone and
fisheries management. A number of recent international fisheries agreements have specifically
identified the need to provide for habitat protection and restoration to ensure long-term
sustainability of fisheries. The protection and restoration of habitat are also common
components of fisheries management programs under national fisheries law and policy.
k e y w o r d s : fisheries management, fishing, habitat degradation, habitat loss, habitat
restoration, habitat structure, protected marine and coastal areas.
Introduction
The wider effects of fishing on marine ecosystems have become the focus of growing concern
*Correspondence and present address: S. J. Turner, Department of Conservation, Northern Regional Office, PO Box
112, Hamilton, New Zealand (e-mail: sturner@doc.govt.nz).
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among scientists, fisheries managers and the fishing industry over the past decade (e.g. Fowler
1989; Hutchings 1990; Northridge 1991; ICES 1992; Jones 1992; Dayton, Thrush, Agardy &
Hofman 1995). Fishing activities, such as trawling and dredging for fish and shellfish, where
mobile fishing gear is towed across the sea bed, have the capability of altering, removing or
destroying the complex, three-dimensional physical structure of benthic habitats by the direct
removal of biological (e.g. sponges, hydroids, bryozoans, amphipod tubes, shell aggregates and
seagrass) and topographic (e.g. sand depressions and boulders) features. These impacts are in
addition to documented effects of fishing on sediment dynamics (e.g. sediment suspension and
deposition), sediment chemistry (e.g. alteration of the sediment chemistry and changes in the
availability of toxic contaminants) and benthic/pelagic nutrient fluxes (e.g. de Groot 1984;
Messieh, Rowell, Peer & Cranford 1991; Black & Parry 1994).
The present review examines the role of habitat structure and habitat heterogeneity (see
Sebens 1991) in marine ecosystems, as well as the effects of fishing on these two components
of habitat complexity. It considers three examples from New Zealand and Australia, where
available evidence suggests that fishing has been associated with the degradation or loss of
habitat structure through the removal of large epibenthic organisms, with concomitant effects
on fish species which occupy these habitats. The different management actions, if any, which
have been adopted in each case are reviewed, and where documented, the outcomes in terms of
the recovery of habitat structure and fisheries resources are discussed. With ever-increasing
demands on fish-stocks and the need for sustainable use of fisheries resources, new approaches
to fisheries management are needed. Fisheries management needs to address the sustainability
of fish-stocks while minimizing the direct and indirect impacts of fishing on other components
of the ecosystem. To date, the extent and effects of fishing on habitat structure have been
largely neglected in fisheries management. The development of protective habitat management
and habitat restoration are reviewed as means of mitigating for degradation or loss of habitat
structure while maintaining healthy sustainable fisheries, and the provisions for these
management tools in recent international fisheries agreements and fisheries law and policy at a
national level are discussed.
Role of habitat structure in marine ecosystems
Emergent structures, such as rocky outcrops, boulder shoals, epibenthic reef formations (e.g.
coral, sabellid, vermetid and oyster reefs), vegetation (e.g. seagrasses, salt marsh, mangroves,
kelp and other macroalgae) as well as other topographic features (e.g. shell, burrows, biogenic
structures and depressions), provide heterogeneity and structural complexity in marine benthic
environments. The structural framework provided by emergent features constitutes an
important organizing feature of many ecological systems and is critical to the functioning of
the ecosystem as a whole (Ryder & Kerr 1989; Peters & Cross 1992). Emergent structures
represent important habitat for a variety of marine organisms, including a number of
commercially and recreationally valuable fisheries species. These structures may provide
refuge from predation and competition, as well as physical and chemical stresses, or may
represent important food resources and critical nursery or spawning habitat (e.g. Thayer,
Stuart, Kenworthy, Ustach & Hall 1978; Robertson & Duke 1987; Rozas & Odum 1988;
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Thayer, Colby & Hettler 1988; Heck & Crowder 1991; Safriel & Ben-Eliahu 1991; Sebens
1991; Fonseca, Kenworthy & Thayer 1992; Gotceitas & Brown 1993; Posey & Ambrose 1994;
Auster, Malatesta & LaRosa 1995; Langton, Auster & Schneider 1995; Tupper & Boutilier
1995; Perkins-Visser, Wolcott & Wolcott 1996). In addition, these structures modify the
hydrodynamic flow regime near the sea floor, with potentially significant ecological effects on
food availability, growth, larval and/or juvenile recruitment and sedimentation (e.g. Fonseca,
Fisher, Zieman & Thayer 1982; Fonseca, Zieman, Thayer & Fisher 1983; Eckman 1987;
Irlandi & Peterson 1991). Although not always essential for the completion of specific lifehistory stages in terms of representing a `demographic bottle-neck', these structures may have
a significant positive influence on survival.
Habitats do not exist independently of each other, and a marine benthic system may consist
of several habitat types which are all integrally linked through biological, chemical and
physical processes to form a complex landscape. Species may utilize this entire landscape
during their life-cycle (e.g. Kenworthy, Thayer & Fonseca 1988; Parrish 1989; Baelde 1990;
Hoss & Thayer 1993; Szedlmayer & Able 1996). Thus, the integrity of the habitat landscape as
a whole may be just as important for fisheries resources as the integrity of a particular habitat
structure (Sebens 1991).
There is abundant evidence in the ecological literature which demonstrates the importance
of habitat structure and heterogeneity in influencing faunal abundance, species richness and
species composition of invertebrate and fish communities (e.g. Risk 1972; Heck & Wetstone
1977; Luckhurst & Luckhurst 1978; Carpenter, Miclat, Albaladejo & Corpuz 1981; Lewis
1984; Pollard 1984; Pihl 1986; Choat & Ayling 1987; Roberts & Ormond 1987; Rozas &
Odum 1988; Bell & Pollard 1989; Bohnsack 1989, 1991; Sale 1991; Sogard & Able 1991;
Walters & Juanes 1993; Levin, Talley & Hewitt 1998; Turner & Kendall in press). It is also
widely acknowledged that the size, vitality and spatial distribution of populations of many
fisheries species are dependent on the quantity and quality of the habitat, although the
ecological relationships which constitute this dependence have not been quantified in most
cases (e.g. Nixon 1980; Lindall & Thayer 1982; Boesch & Turner 1984; Fonseca, Kenworthy
& Thayer 1988; Karr 1991; Peters & Cross 1992; Hoss & Thayer 1993; Langton, Steneck,
Gotceitas, Juanes & Lawton 1996; Auster, Watling & Rieser 1997). If fisheries are to be wisely
managed, it is essential that fisheries managers understand the extent to which the harvested
species are dependent on habitat, together with the long-term implications of degradation or
loss of habitat structure on these species, non-target species and the ecosystem as a whole.
Degradation or loss of habitat structure
Extensive areas of benthic habitat have been lost or their physical integrity compromised as a
result of fishing (e.g. sabellid reefs in the Wadden Sea: Reise 1982; Riesen & Reise 1982;
seagrass beds in North Carolina: Fonseca, Thayer, Chester & Foltz 1984; Hsiao, Easley &
Johnson 1987; Peterson, Summerson & Fegley 1987; oyster bars in Chesapeake Bay:
Rothschild, Ault, Goulletquer & HeÂral 1994; sponge communities in the Gulf of Maine:
Auster, Malatesta, Langton, Watling, Valentine, Donaldson, Langton, Shepard & Babb 1996).
In many areas, the spatial extent (both individually and in aggregate) and severity of physical
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disturbance, the potentially slow recovery of impacted ecosystems, as well as the frequency of
occurrence over time (heavily fished areas may be impacted many times in a year), directly
attributable to fishing far exceed the effects of other disturbance agents (e.g. waves, tidal
currents, bioturbation processes, and anthropogenic processes such as dredging and extractive
activities). Nevertheless, few studies have considered the effects of fishing on the physical
modification of habitat structure and the potential implications for associated faunal
communities. Likewise, there is little information concerning how, and to what extent,
changes in habitat structure affect fisheries resources and contribute to fisheries declines.
Habitat degradation or loss as a result of fishing remains the least understood of the
environmental impacts of fishing (Committee on Fisheries, Ocean Studies Board, National
Research Council 1994, cited in Auster et al. 1996). Because of the inadequate data which are
available to address the specifics of the complex ecological interactions involved, it is rarely
possible to predict or quantify the potential loss to fisheries production caused by the
degradation or loss of habitat structure. An understanding of the extent of this impact, and its
effect on populations of marine organisms (both target and non-target species), is essential if
fisheries are to be strategically managed by setting appropriate levels of fishing effort which
will maximize fisheries production, and therefore, yield.
The extent of fishing activities is potentially extremely large, often leaving few, if any,
undisturbed habitats within impacted areas. As well as effects on habitat structure per se, there
are important implications on a larger scale with regard to effects on habitat heterogeneity, or
the patchiness of habitats, across a landscape. Reductions in heterogeneity over large spatial
and temporal scales have implications for the maintenance of diversity and stability at the
population, community and ecosystem level (Thrush, Hewitt, Cummings & Dayton 1995). As
a landscape becomes fragmented, with reduced physical structure occurring between habitat
units, movement of adults, recruits and larvae between habitats becomes impeded.
Furthermore, as more and more of the habitat structure and the associated communities are
destroyed, sources of larvae recruiting into disturbed areas from adjacent habitats will decline,
and the recovery-time for biological communities (both habitat-forming species as well as
associated species) in these impacted areas will progressively increase.
Fishing impacts and the degradation or loss of habitat: examples
Three examples from New Zealand and Australia were considered, where available evidence
suggested that fishing has been associated with the degradation or loss of habitat structure
through the removal of large epibenthic organisms, with concomitant effects on fish species
which occupy these habitats. The management strategies (if any) which were adopted in each
example are also reviewed.
1. Benthic epifauna and finfish stocks: Tasman Bay, New Zealand
Around the northern coast of New Zealand's South Island (Tasman Bay), fishermen have long
reported an association between juveniles of a number of commercially important species of
fish [including snapper, Chrysophrys auratus (Bloch & Schneider), tarakihi, Cheilodactylus
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macropterus (Bloch & Schneider), and John Dory, Zeus faber (L.)] with clumps and mounds of
`coral-like' bryozoans. Bryozoans [predominantly the endemic species Celleporaria
agglutinans (Hutton) and Hippomenella vellicata (Hutton)], along with other calcareous
frame-building species (e.g. encrusting bryozoans, serpulids and foraminiferans) and
associated epifauna (e.g. hydroids, sponges, solitary and colonial ascidians, bivalves,
polychaetes, crustaceans, and ophuiroids), form abundant and extensive `coral-like' growths
(attaining spans of 0.3 m and heights of 0.15±0.5 m) which provide an important source of
shelter and food for juveniles of several fish species (Bradstock & Gordon 1983).
With the advent of commercial trawling in Tasman Bay in the 1940s, there was extensive
destruction of the bryozoan growths. The size and extent of these important nursery grounds
have been progressively reduced, especially since the introduction of synthetic trawl nets
(which are less prone to damage by the bryozoan growths) in the 1960s and 1970s (Saxton
1980; Bradstock & Gordon 1983). In some areas, these grounds were virtually destroyed by the
late 1970s. Observations in 1980 revealed that, where the growths persisted, these were
markedly reduced in size (not exceeding 0.15 m in height above the sea floor) and density
(clumps were often several metres apart) (Bradstock & Gordon 1983). There was a marked
reduction in the numbers of juvenile fish associated with this decline in habitat structure.
Management action
In 1980, a region delimiting an area of bryozoan growths was closed to power fishing methods
(i.e. trawling, Danish seining and dredging) (Mace 1981). The expectation was that the closure
of the fishing grounds would allow the bryozoan growths to regenerate, thereby restoring the
fish habitat to its original state and allowing recovery of the fish-stocks. To date, the area
remains closed, and according to the Ministry of Agriculture and Fisheries, the closure is
permanent. Extraordinarily, habitat recovery has not been monitored (Gordon 1994) and the
closure of the fishing grounds has not been accompanied by any scientific research. There is no
information with which to assess whether closure of the fishing grounds has led to recovery of
the bryozoan growths, and whether fish-stocks are further declining, being maintained or are
increasing.
2. Benthic epifauna and finfish stocks: north-western Australia
The continental shelf of north-western Australia supports a diverse and productive demersal
fish community which has been exploited commercially since 1959 (Sainsbury 1987, 1988;
Sainsbury, Campbell & Whitelaw 1993). Over the period that the fishery has been operating, a
marked change in the species composition of the fish catch has been observed, although the
total catch rate of the fishery has remained relatively constant. During the early years of the
fishery, the genera Lethrinus (emperor) and Lutjanus (snapper) accounted for 40±60% of the
catch, while two other genera, Nemipterus (thread-fin bream) and Saurida (grinner), together
comprised about 10% (Sainsbury 1987, 1988; Sainsbury et al. 1993). However, this structure
had reversed by the mid-1980s, with Lethrinus and Lutjanus comprising about 10% of the
catch, and Nemipterus and Saurida around 25%.
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One explanation for the observed changes in fish community structure is that it reflects a
change in habitat structure as a result of fishing activity on the shelf (Sainsbury 1987;
Sainsbury et al. 1993). There has been a significant reduction in the quantity of epibenthic
fauna (primarily sponges, alcyonarians and gorgonians) caught as by-catch in trawls, compared
with that recorded prior to and during the early development of the fishery. The catch rate of
sponges has decreased markedly (from around 500 kg h±1, but up to 2600 kg h±1, in 1963, to
< 300 kg h±1, and frequently, 0 kg h±1, in 1979; Sainsbury 1987). Sainsbury et al. (1993)
found that, where the fate of epibenthic organisms after impact with the trawl gear was known,
epibenthic organisms were damaged (i.e. detached from the sea-bed) in 90% of observations.
These changes in habitat structure were reflected in fish community structure, with Nemipterus
and Saurida being recorded predominantly in open sand habitats with low densities of
epibenthic fauna, while Lethrinus and Lutjanus were most common in areas with dense
epibenthic fauna (Sainsbury 1987; Sainsbury et al. 1993). Campbell (cited in Coutin 1993)
suggested that the direct impact of fishing mortality on the composition of fish communities is
less than the indirect impact of habitat degradation or loss.
Management action
An `actively adaptive' approach was adopted for the management of the North-West Shelf
fishery, including management experiments undertaken on a spatial scale of relevance to the
fishery (Sainsbury 1987, 1988, 1991; Sainsbury et al. 1993). One of the management
objectives was to assess the feasibility of maximizing the more valuable catch of Lethrinus and
Lutjanus by allowing the recovery of the preferred habitat within an area of the North-West
Shelf closed to commercial trawling for a 5-year management experiment. This resulted in an
increase in the abundance of epibenthic fauna, as well as increases in the combined populations
of Lethrinus and Lutjanus in the area closed to commercial trawling (Sainsbury et al. 1993).
However, the recovery of epibenthic habitat structure after trawling ceased was slow and full
recovery of the habitat following heavy trawling is predicted to take up to 20 years (Campbell,
cited in Coutin 1993; Sainsbury et al. 1993). Conversely, in an area where commercial
trawling continued throughout the experimental period, there was a decrease in epibenthic
fauna and a decline in the catch rates of Lethrinus and Lutjanus (Sainsbury et al. 1993).
3. Deep-sea seamounts and orange roughy stocks: Chatham Rise, New Zealand
With the continuing depletion of inshore fish-stocks, improvements in fishing technology (e.g.
the development of fishing techniques to enable trawling of deep-sea seamounts) and the
development of a new international regime for the oceans under which many coastal states
have claimed extensive Exclusive Economic Zones (United Nations Convention on the Law of
the Sea 1982), deep-water fishing grounds are being increasingly exploited (Probert 1996;
Probert, McKnight & Grove 1997). One of New Zealand's major deep-water fishing areas is
the Chatham Rise, a relatively broad submarine ridge extending for some 1000 km off central
New Zealand to the east of the Chatham Islands, and rising to within 200±400 m of the surface
from depths of more than 2000 m (e.g. Probert 1996; Probert et al. 1997). These deep-water
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areas support abundant populations of orange roughy, Hoplostethus atlanticus Collett, hoki,
Macruronus novaezelandiae Hector, and other important deep-water species, and have been
fished commercially since 1978. Seamounts in the Chatham Rise region support benthic
communities dominated by attached epifauna, in which large sessile species, in particular the
scleractinian Goniocorella dumosa (Alcock), the stylasterid Errina chathamensis Cairns and an
antipatharian, ? Bathyplates platycaulus Totton, form a distinctive and major component
(Probert & McKnight 1993; Probert 1996; Probert et al. 1997). These deep-water coral
communities support many associated species. While there is no quantitative evidence
available, commercial fishermen noted a marked decline in the abundance, biomass and
diversity of hard corals, soft corals and sponges, as well as other invertebrates, brought up in
the trawls as fishing progressed in these deep-water grounds (Jones 1992; Probert 1996).
Available evidence suggests that, once destroyed, these deep-water coral formations may
require 200±400 years to recover (Ministry for the Environment 1997). By providing habitat
structure, these deep-water communities may play a key role in the life-cycle of commercially
important fish, potentially representing important areas for aggregation, courtship and/or
mating, spawning grounds, or nurseries for juvenile fish. While it is unclear whether significant
habitat±fisheries interactions exist in such communities, damage to deep-water sessile epifauna
could have long-term impacts on the sustainability of commercial fisheries.
Management action
Because of the damage that mobile fishing gear is capable of causing to these deep-sea
habitats, and given the likelihood that these habitat structures are potentially important in the
life-history of commercial fish species, it would be prudent to adopt a precautionary approach
to the management of these deep-water fisheries and protect at least some of these areas. To
date, the principle legislation responsible for providing for the protection and preservation of
the marine environment of New Zealand's Exclusive Economic Zone has not been used to
protect any deep-water habitats from the impacts of fishing.
Following a study of the seamount fauna off southern Tasmania and an assessment of the
impact of deep-sea trawling, the Australian Government recently put forward a proposal to
include the seamounts in what will be Australia's first deep-sea marine protected area (CSIRO
1998; Hill 1998). This follows a voluntary interim closure in mid±1995 of a 370-km2 region of
the seamounts 100 km south of Tasmania to fishing and other activities which could disturb
the communities associated with the seamounts. This area includes 15 out of the 70 seamounts
in the region.
Fisheries management and mitigation of habitat degradation or loss
Fisheries management world-wide has focused (and largely continues to do so) on overfishing,
with management responses directed towards restricting catch or reducing fishing effort (e.g.
imposition of catch quotas, bag limits, minimum size regulations, limited entry, temporary area
closures and closed fishing seasons). Such management tools were developed mainly for
single-species fisheries, and little effort has traditionally been directed towards maintaining the
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ecological integrity and function of habitats and ecosystems. Fisheries management can no
longer attempt simply to maximize yield, while ignoring the impacts of fishing on habitat
structure and heterogeneity (Cooke & Earle 1993; Bohnsack & Ault 1996; Roberts 1997).
Even low fishing mortalities will have a profound effect on habitat when caused by gear such
as bottom trawls (Pauly 1997).
The conservation, protection and enhancement of benthic habitat structure is increasingly
regarded by the scientific community as fundamental to any attempt to rationally and
sustainably manage fisheries resources in the long term (Mager & Thayer 1986; Dayton et al.
1995; Bohnsack & Ault 1996; Langton et al. 1996). The need to move from a highly focused
consideration of single-species or single-habitat management strategies to integrated multispecies and ecosystem management perspectives is central to this change in management
approach (Morgan 1987; Sherman 1991; Hoss & Thayer 1993; Roberts & Polunin 1993;
Agardy 1994; Wilcove & Blair 1995; Roberts 1997). Habitats do not exist in isolation, nor is
any one habitat used exclusively during the life-cycle of any single organism. Therefore,
habitats cannot effectively be managed as separate, discrete entities. If an ecosystem
management approach is to be effective, it will require a better understanding of the
functioning, dynamics and interrelationships between organisms and habitats, and will need to
include an appreciation of the various spatial and temporal scales over which linkages between
species and habitats occur (e.g. Langton et al. 1995, 1996; Auster et al. 1997). Fisheries
management needs to address habitats on a landscape scale and consider issues such as: What
is the temporal and spatial sequence of habitats which are used by different life-history stages?
Is there an optimum mix (i.e. combination and distribution) of habitat structure to produce
optimum growth and survival of species? How does the form and extent of habitat structure
influence the use by and availability to different life-history stages and species? What are the
ecological relationships between the quality and quantity of habitat structure, and both fisheries
production and the abundance of non-target species? (Peters & Cross 1992; Hoss & Thayer
1993). Given the numerous species, their various life-history stages, the diversity of habitats,
the multitude of factors influencing the utilization of habitat structure by any one species and
the complex ecological interactions among species and habitats, there is an urgent need to
conserve a wide range of habitat structures and to move away from targeting single habitats for
single species (Peters & Cross 1992; Auster et al. 1997).
With ever-increasing demands on fish-stocks and the need for sustainable use of resources,
new approaches to fisheries management are needed. Management tools for mitigating
degradation or loss of habitat structure while maintaining healthy sustainable fisheries have
gained widespread interest from fisheries scientists and managers. These include: (1)
protective habitat management; and (2) habitat restoration.
1. Protective habitat management
Protective habitat management involves the designation of `protected' marine and coastal areas
(variously referred to as `marine reserves', `marine parks', `marine and coastal protected
areas', `sanctuaries', `replenishment reserves' and `harvest refugia'). Protected marine and
coastal areas have become a highly advocated form of marine management, providing types of
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protection not offered by other management strategies (Roberts & Polunin 1993; Agardy 1994;
Bohnsack & Ault 1996; Eichbaum, Crosby, Agardy & Laskin 1996; Allison, Lubchenco &
Carr 1998). These areas are generally afforded some level of protection from fishing (ranging
from being totally non-extractive areas to areas of restricted harvesting), and have as their
ultimate goals (among other considerations) the sustainable management and use of fisheries
resources, the conservation of benthic habitat structure and biodiversity, and the safeguarding
of ecosystem function and integrity (Jones 1994; Ticco 1995; Bohnsack & Ault 1996).
Protected areas are especially important in providing protection for habitats which are critical
to the viability of a species or population, such as spawning grounds, nursery grounds and
those which are centres of high species diversity. They range in size from small, closed areas
or harvest refugia, designated to protect a specific resource or habitat type, to extensive areas
which are designed to integrate the management of many species, habitats and uses in a single
comprehensive ecosystem-based management plan (Agardy 1994; Eichbaum et al. 1996). As
fisheries management tools, protected marine and coastal areas are primarily intended to
enhance or replenish fisheries yields through a reservoir of broodstock. These areas also
provide for preservation of habitat diversity and integrity by protecting habitat structures from
damage, providing sources of recruits or sinks for enhanced settlement of those species
important in contributing to habitat structure, preserving genetic diversity within species, and
maintaining more natural population and community structures (cf. Carr & Reed 1993; Dugan
& Davis 1993; Roberts & Polunin 1993; Bohnsack & Ault 1996; Auster et al. 1997). Work on
the North-West Shelf fishery of Australia has demonstrated the effectiveness of large area
closures to fishing, and has shown that the impacts of fishing on epibenthic habitats and
associated fish assemblages do not occur in areas where commercial fishing is stopped
(Sainsbury 1987, 1988, 1991; Sainsbury et al. 1993).
Central to arguments advocating the development of protective habitat management is the
idea that, unlike traditional fisheries management strategies which require large amounts of
information about the life-histories of the species being fished, their implementation should not
be dependent on the acquisition of large amounts of information about the life-histories of each
species, and their interactions with habitat structures and other species (e.g. Roberts & Polunin
1993; Auster et al. 1997; Allison et al. 1998). Rather, by protecting some habitat structure
until a full understanding of the effects of fishing impacts can be determined, the risks of total
loss of habitat and of associated resources are minimized. Furthermore, areas selected for
protection need not be permanent or immutable, but can be adjusted as data improve (Roberts
1997). Management for habitat structure and heterogeneity by establishing areas closed to
mobile fishing gear is consistent with the precautionary approach and `risk aversive'
management strategies (e.g. Cooke & Earle 1993; Agardy 1994; Jones 1994; Dayton et al.
1995; Bohnsack & Ault 1996; Eichbaum et al. 1996; FAO 1996; Auster et al. 1997; Roberts
1997). Additionally, protective habitat management strategies provide a buffer against
scientific uncertainty and/or difficulties in executing management measures (e.g. Dugan &
Davis 1993; Bohnsack & Ault 1996; Roberts 1997; Allison et al. 1998; Lauck, Clark, Mangel
& Munro 1998).
Fundamental research still needs to be undertaken to determine the ideal location of
protected areas, as well as the size, number and total area required to achieve specific
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management goals (Tisdell & Broadus 1989; Salm & Price 1995; Bohnsack & Ault 1996).
This will require integrating knowledge from many different disciplines. In addition, the
proportion of habitat which should be protected is an issue that needs to be resolved. The US
South Atlantic Fishery Management Council recommended that 20% of the total shelf area
should be set aside as marine fisheries reserves, based on the goal of maintaining stock
reproductive output at levels of 30% or more of that of an unexploited population (Bohnsack
1990, cited in Roberts 1997). Ballantine (1991) recommended that 10% of all marine habitats
in New Zealand be designated as non-extractive marine reserves. While Cooke & Earle (1993)
suggested: `Whether or not there is evidence of adverse effects, any fishing method used in an
area that involves substantial disturbance to habitat shall be excluded from representative
closed sub-areas covering at least 50% of the fishing ground, to conserve part of the habitat in
its undisturbed state.' Furthermore, the above authors suggested that, if the entire fishing
ground had been subject to major disturbance, closed areas should be established to allow
habitat recovery. Sladek Nowlis & Roberts (cited in Roberts 1997) considered that larger
protected areas (up to 50% or more) may continue to provide additional benefits, especially in
heavily exploited fisheries.
Even the most well-designed and managed protected marine and coastal areas cannot alone
protect habitats, and thus, fisheries against anthropogenic impacts outside their boundaries
(ranging from local pollution events to widespread or global phenomena such as persistent
widespread pollutants, episodic climatic events, disease epidemics and the spread of exotic
species). Without adequate protection of species, habitats and ecosystems outside the
boundaries of protected areas, the effectiveness of protected marine and coastal areas will
potentially be severely compromised (Carr & Reed 1993; Agardy 1994; Jones 1994; Allison
et al. 1998). Protected areas should not be seen as a solution replacing conventional
management approaches, but need to be one component of an integrated programme of coastal
zone and fisheries management (Roberts 1997; Done & Reichelt 1998).
2. Habitat restoration
Habitat restoration seeks to re-establish important habitat structures and their ecological
function to a level, based on feasibility and historical information, which ensures the greatest
long-term continued productivity of the system. Habitat restoration is usually embarked upon
after all the alternatives to prevent or mitigate adverse impacts have been unsuccessful. While
the restoration of natural communities is an established method in land and freshwater
management, restoration of marine systems has only recently received attention. In the past,
the focus of marine ecosystem restoration has been the curtailment of the source(s) of
degradation or loss, and typically, the impacted communities have been left to recover
naturally. Nevertheless, natural recovery of impacted areas can be very slow since many
important habitat-forming species are slow colonizers, and may take a long time to attain
natural population densities, size and age structures. Ecosystems often do not recover from
anthropogenic disturbance without additional management or manipulation (Pratt 1994).
Where human-induced impacts have resulted in the loss of habitat structure, associated
changes in the hydrodynamic, physical and/or chemical conditions make habitat restoration
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DEGRADATION OR LOSS OF HABITAT STRUCTURE
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more difficult, and in the majority of cases, the original environmental conditions will need to
be re-established before habitat restoration can be successfully achieved (e.g. Thayer, Fonseca
& Kenworthy 1986; Fonseca, Thayer & Kenworthy 1987; Kirkman 1992). Proper site
selection, species selection and the timing of restoration effort are of paramount importance to
the success of restoration projects (e.g. Fonseca et al. 1987; Fonseca 1989).
Habitat restoration should not only focus on the physical habitat type, but also its ecological
functions, and the success of any restoration project should be measured not only by the
persistence of the habitat, but also the extent to which the habitat functions and is used by
organisms which would normally be found there (e.g. Fonseca et al. 1988; Fonseca 1989;
Thayer, Fonseca, Kenworthy, Colby & Currin 1989; Thayer, Fonseca & Kenworthy 1990;
Levings 1991; Pratt 1994). Ignoring ecological linkages may result in a poorly functioning
ecosystem with low probability of persistence (Pratt 1994; Race & Fonseca 1996). As with the
establishment of protected marine and coastal areas, the restoration of habitat structure must be
addressed in the context of integrated coastal zone management policies if the goal of habitat
restoration is to be achieved.
Protected marine and coastal areas may also exist as restoration areas, allowing ecosystems
to return to a condition more closely approximating the natural state in terms of habitat quality,
species abundance and diversity, population structure, and reproductive output (Irving 1994;
Lindeboom 1995). In the future, the establishment of protected areas selected for their potential
to be restored could usefully complement networks of areas set up to conserve habitats
(Gubbay 1995; Done & Reichelt 1998).
Although techniques exist to restore salt marshes, seagrass beds, mangroves, kelp forests,
coral reefs and temperate reefs (see Thayer 1992), in many cases, the process of restoration has
not been overly successful nor have there been any long-term evaluations of restoration
success (Pease, Thayer & Lillestolen 1994). Furthermore, there is currently little evidence that
restored habitats are as capable of supporting or sustaining the ecological function and fisheries
production which their natural counterparts do. While studies clearly show, for example, that
increasing habitat structure and heterogeneity using artificial reefs can increase the local
abundance of fish, it has not been conclusively demonstrated that the total fish yield for any
body of water increases significantly as a result of structural modification, rather it may be that
the artificial habitat tends to concentrate fish that were already present (Bohnsack 1989, 1991;
Ryder & Kerr 1989; Sheehy & Vik 1992). There is also little evidence that restored or created
seagrass and salt marsh habitats function ecologically in the same way as their natural
counterparts. The time required for equivalent functioning of restored or created habitats may
be on the scale of years (e.g. Homziak, Fonseca & Kenworthy 1982; Smith, Fonseca, Rivera &
Rittmaster 1988; Fonseca, Kenworthy, Colby, Rittmaster & Thayer 1990; Meyer, Fonseca,
Colby, Kenworthy & Thayer 1993). Meyer et al. (1993) reported that these habitats were still
not fully functional in terms of living resource utilization even after 2 years of created marsh
and 3 years of created seagrass development,.
Because of a fundamental lack of understanding of the ecology and dynamics of natural
habitats and their biological resources, as well as lack of detailed understanding of the biology
of individual species in restored ecosystems, restoration of marine systems has not yet evolved
into a reliable management tool. The science of habitat restoration is still `young, imperfect
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S. J. TURNER ET AL.
and experimental', and in most cases, the extent to which restoration compensates for the loss
of natural habitat structure, the trade-offs that occur if out-of-kind restoration is undertaken or
the rate of functional replacement if restoration is successful, are unknown (Thayer, Fonseca &
Kenworthy 1985; Thayer et al. 1990; Race & Fonseca 1996). Since habitat loss still cannot
be controlled, Fonseca et al. (1988) advocated that, at least in the case of seagrass beds,
complete conservation of seagrass habitat should be undertaken as the alternative to restoration.
Fisheries law and policy
A number of recent international fisheries agreements have recognized the effects of fishing on
habitat as a key management issue, and specifically identify the need to provide for habitat
protection and restoration to ensure the long-term sustainability of fisheries resources (Garcia
& Newton 1994; Hey 1996). These instruments include the 1995 International Code of
Conduct for Responsible Fisheries (FAO 1995) which sets out principles and standards for
responsible fishing to ensure effective conservation, management and development of all
living aquatic resources with due consideration for the ecosystem and biodiversity. The Code
includes a specific requirement that `critical fisheries habitats' are protected from destruction,
degradation, pollution and other significant impacts resulting from human activities which
threaten the health and viability of the fishery resources (Article 6.8). In addition, the Code
provides that every effort should be made to ensure that fisheries resources and habitats critical
to the well-being of such resources which have been adversely affected by fishing or other
human activities are restored (Article 7.6.10). The Code also seeks to improve fishing
practices. Of particular relevance to the protection of habitat, is the requirement to ensure that
assessments of the implications of habitat disturbance are carried out prior to the introduction to an area of new fishing gear, methods and operations on a commercial scale
(Article 8.4.7).
New approaches to fisheries management, including the development of protected marine
and coastal areas, and habitat restoration, are also increasingly common components of
fisheries management programmes under fisheries law and policy at a national level. The
Magnuson±Stevens Fishery Conservation and Management Act is the United States' foremost
federal law governing fishing activity in the 200-nautical mile Exclusive Economic Zone. The
1996 amendment to the Act (the `Sustainable Fisheries Act') addresses a number of major
fisheries management issues, including that of habitat degradation and how to strengthen
provisions for habitat protection and restoration (Magnuson-Stevens Fishery Conservation and
Management Act 1996). The Act recognizes that some fish stocks have declined to the point
where their survival could become threatened as a consequence of direct and indirect habitat
losses which have resulted in a diminished capacity to support existing fishing levels [Section
2(a)(2)], and that one of the greatest long-term threats to the viability of commercial and
recreational fisheries is the continuing loss of habitat [Section 2(a)(9)]. The new Act mandates
guidelines be established for the description and identification of `essential fish habitat', as
well as the adverse effects of activities on such habitat (including impacts from fishing), and in
the consideration of actions to ensure the conservation and enhancement of such habitat
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DEGRADATION OR LOSS OF HABITAT STRUCTURE
413
[Section 305(b)(1)(A)]. Fishery management plans are required to include management
measures which minimize the adverse effects on habitat caused by fishing and must include an
assessment of the potential adverse effects of all fishing gear types used where there is
essential fish habitat [Section 303(a)(1)]. The Act also provides for fisheries research to be
undertaken to develop and test new gear technology and fishing techniques to minimize any
adverse effects on essential fish habitat [Section 404(c)(2)].
Similar developments in fisheries management have been seen elsewhere. The new New
Zealand Fisheries Act was enacted in 1996 to provide for the utilization of fisheries resources
while ensuring their sustainability, and at the same time, avoiding, remedying or mitigating
any adverse effects of fishing on the environment (Section 8) (New Zealand Fisheries Act
1996). Under the Act, fisheries management decisions must take into account a number of
`environmental principles', including maintenance of the long-term viability of species which
are associated with or dependent upon the harvested species, maintenance of biological
diversity, and protection of habitats of particular significance to fisheries management (Section
9). The Act provides for the setting or varying of `sustainability measures' for stocks or areas,
after taking into account any effects of fishing on the environment, among other factors
[Section 11(1)(a)]. Thus, the Act sets out to ensure that account is taken of the wider effects of
fishing in the ecosystem in managing fisheries resources. Likewise, the Queensland Fisheries
Act provides for the management, use, development and protection of fisheries resources and
`fish habitats' within the coastal waters of Queensland, Australia, with the objective of
ensuring that fisheries resources are used in an ecologically sustainable way [Section 3(1)(a)]
(Queensland Fisheries Act 1994). The Act provides for the protection and conservation of fish
habitats through the declaration of `fish habitat areas' which are managed under management
plans (Sections 120±122). There are also provisions for the rehabilitation or restoration of fish
habitat (Sections 124±125). The objectives of the Western Australian Fish Resources
Management Act similarly include the conservation of fish and protection of their
environment, and the designation and management of `fish habitat protection areas' [Section
3(2) and Sections 115±119] (Western Australian Fish Resources Management Act 1994).
Regulations relating to the protection or management of fish habitat protection areas may
prohibit or regulate fishing or any other activity which may adversely affect the designated
area [Section 120(2)]. While there is undoubtedly a consensus on the substantive issues to be
addressed in fisheries management, as reflected in international agreements and recent
developments in fisheries law and policy at a national level, the question remains whether
sufficient political determination, funding and resources exist to do justice to these important
and ambitious undertakings to prevent further habitat degradation or loss.
Conclusions
As more fisheries species are exploited and technological advances increase the range of
habitats which are likely to come under threat from fishing, understanding and effectively
managing for the effects of fishing on habitat structure becomes ever more important. There is
a growing international consensus that a `precautionary approach' should be adopted towards
fisheries management, and that this includes a duty to assess the impact of fishing on the
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environment, and to adopt management strategies to protect and restore habitats of special
concern. The continued loss of habitat structure important in the completion of the life-history
of fisheries resources is likely to have significant implications for the fishing industry, and its
management and sustainability, as well dramatic and potentially long-lasting ramifications for
the maintenance of habitat diversity, integrity and function.
Acknowledgements
This review formed part of a research programme (CO1502) funded by the New Zealand
Foundation for Research, Science and Technology for which we are grateful. We thank
two anonymous reviewers for constructive comments on an earlier version of this manuscript.
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