A TRAWL net scrapes the seabed, funnelling a precious catch into its trap.
Cod, haddock and flounder are the intended prey but this device is
indiscriminate. The hard wire of the net鈥檚 leading edge cuts a swath through the
jungle of bottom-dwelling creatures. Sponges, bryozoans and many others are
smashed to bits. Some come up in the net, others will lie rotting on the bottom.
In the aftermath, the dominant life forms are scavenging amphipods鈥攖he
marine equivalent of ants鈥攁ttracted to the carcasses left behind.
鈥淚t鈥檚 like looking at a recent clearcut,鈥� says Barbara Sullivan of the
University of Rhode Island Graduate School of Oceanography. Photographs of
trawled areas show bare gravel on the seabed鈥攁 marine desert. But
undisturbed parts of the northwest Atlantic seafloor are covered in miniature
forests of sponges and delicate tubes, built by thousands of polychaete worms.
鈥淏efore we had remote photography,鈥� says Jeremy Collie, also at URIGSO, 鈥減eople
were largely unaware of what the seabed was like.鈥� Even now many marine habitats
remain as poorly understood as tropical rainforests鈥攚hich makes the
analogy between clearcutting and trawling a good one. But there is a crucial
difference.
Conservationists concerned about the destruction of rainforest often
highlight its economic potential as a reason to preserve it. The value of marine
ecosystems, on the other hand, has long been recognised. This gives added
impetus to the work of ecologists like Collie, who are pioneering a new wave of
research exploring the complex biology of the seabed. Using a variety of modern
technologies, including remote photography, sonar and computer modelling, these
researchers are beginning to reveal the effects of habitat destruction, not just
on species that live on the seabed but on commercial fish stocks throughout the
oceans. What they are finding adds new urgency to recent calls for better
regulation of heavily fished areas, such as Georges Bank and the Gulf of Maine
in the northwest Atlantic.
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Undisturbed areas of the seabed are complex. Gravel bottoms and boulder beds
are often crowded with sponges, hydroids, bryozoans and a variety of other
invertebrates that live on or are embedded in the sediment and rocks. These
creatures, called benthic epifauna, provide millions of nooks and crannies that
serve as cover for shrimps, amphipods and small crabs. The epifauna also shelter
juvenile fish including cod and other commercial species, which feed on the
shrimps and amphipods. Even a sandy seabed can provide shelter to many animals
on the downcurrent side of sand waves formed by storms or tidal currents. All
these habitats can be destroyed by fishing trawls and dredges designed to catch
scallops and bottom-dwelling fish.
Disappearing world
One of the difficulties in studying seafloor habitats is that it can be
almost impossible to find undisturbed areas to compare with the heavily fished
ones. 鈥淚n the last century we had ad hoc reserves because there weren鈥檛 many
fishermen harvesting far offshore,鈥� says Peter Auster, science director for the
National Undersea Research Center at the University of Connecticut. 鈥淚n
addition, there were certain areas that just couldn鈥檛 be fished because of
topography or because they were difficult to locate. As navigation improved, as
strong artificial fibres were used in the fishing industry, as there was an
increase in the power of boats, those ad hoc reserves disappeared.鈥�
This is a real problem for researchers doing research in the northwest
Atlantic. Georges Bank, a submerged mountain 80 kilometres off the New England
coast, has been steadily fished since the 1720s. By the 1980s, advances in
fishing technology and continual increases in fishing effort had taken a heavy
toll. Halibut and redfish had been fished out years before. Populations of cod,
haddock and yellowtail flounder鈥攖he principal commercial fish species
still being harvested in the area鈥攈ad reached all-time lows. The US and
Canada share jurisdiction over Georges Bank. In 1994, large areas under US
control were closed to fishing to allow the stocks to recover. Canada reduced
the numbers of fish trawlers were allowed to catch.
Putting some areas off limits is providing a rare opportunity to study the
recovery of a seafloor, or benthic, ecosystem. Before the restrictions were put
in place, Collie began a study comparing relatively undisturbed areas of Georges
Bank with heavily fished ones. He used fishermen鈥檚 records to work out which
areas had been fished most often. He also used sidescan sonar, which can detect
the tracks that trawls and dredges leave on the seabed to confirm the
whereabouts of heavily fished areas. Using a small research dredge, he took
samples from the seafloor and counted all the animals he鈥檇 caught. He found that
the abundance, biomass and diversity of benthic creatures were all lower at the
more disturbed sites.
Since parts of the bank have been closed to fishermen, Collie has returned to
his study sites each year to monitor their recovery. By good fortune, his most
heavily disturbed site turned out to be within the closed area. 鈥淲e saw fairly
immediate changes in the site that was closed,鈥� he says. 鈥淎fter only one year,
the abundance of animals increased, biomass and diversity increased.鈥� The bare
gravel is now being colonised by sponges, bryozoans and other epifauna. But
Collie points out that the recovery is far from complete. 鈥淎lthough we see a
rapid recovery, there鈥檚 still a long way to go,鈥� he says. Growth so far leads
him to believe that full recovery may take about ten years. 鈥淏ut nobody really
knows,鈥� he admits.
Fish stocks are also bouncing back. The US National Marine Fisheries Service
(NMFS) has been conducting fishery surveys in the Georges Bank area for 35
years. NMFS researchers take samples at a series of randomly selected stations,
and count and weigh all the fish they catch. 鈥淚t鈥檚 a Dow Jones Index of what鈥檚
going on with the fish,鈥� says Steven Murawski, a biologist with NMFS. Since the
closure of the bank, they鈥檝e supplemented this research with additional trawls
inside and outside the protected areas. 鈥淲e鈥檙e seeing a very good growth of the
yellowtail flounder population, and declines in the number of haddock and cod
have stabilised,鈥� says Murawski. 鈥淲e haven鈥檛 seen any big years for baby fish
yet, but that will come.鈥�
Auster is focusing his research on how changes in seabed habitat structure
affect the population dynamics of fish. It鈥檚 a difficult question to answer. How
much of the recovery of Georges Bank fish populations is due to the restored
benthic habitat and how much is due simply to a reduced harvest of fish? James
Lindholm, a graduate student at Boston University who is working with Auster,
has set up a series of lab experiments to see how changes in habitat complexity
affect the survival of juvenile cod. He has just finished a series of
experiments that show greater habitat complexity correlates with increased
survival rates in the young fish.
Nowhere to hide
Now Lindholm and Auster are combining this laboratory data with information
on existing habitat conditions in the northwest Atlantic. 鈥淭he idea is to build
a computer model where we can look at what the changes in habitat are at large
regional scales and how they might be affecting the survival of different year
classes of fish,鈥� says Auster.
Preliminary results from this computer model suggest that when cod
populations are at a high level, habitat complexity does not greatly affect the
survival of young fish. But at low population levels鈥攖he kind of situation
that now exists on Georges Bank鈥攈abitat complexity becomes crucial. 鈥淭he
more complex the habitat, the harder it is for predators to get at the juvenile
fish,鈥� says Auster. That makes very little difference when the habitat is
saturated with fry, but it is important when the population is low.
Auster sees computer modelling as a useful research tool rather than a
reliable predictor of what will happen in a real ecosystem. 鈥淲e use the model to
generate questions about how the world is working,鈥� he says. 鈥淭hen we can go out
and test whether our model actually is predicting what goes on in the real
world, and we can adjust our model based on empirical evidence.鈥�
This kind of modelling is part of adaptive management鈥攁 process that
Auster believes is the key to sustainable fisheries, and which he hopes to see
adopted over the long term in the northwest Atlantic. The idea is that fisheries
managers make changes based on their best current information, then monitor the
effects of those changes over time and make adjustments based on the
results.
An excellent example of this kind of adaptive management in action comes from
the work of Keith Sainsbury and his colleagues at the CSIRO Division of
Fisheries in Australia, half a world away from Georges Bank. Intensive trawl
fishing began in the North West Shelf off Western Australia in 1971.
Photographic surveys of the seabed during the early 1980s showed that most large
benthic organisms鈥攍ike the soft corals that once dominated the
area鈥攈ad been decimated by trawling. And further research showed that
economically valued fish such as emperor bream and snappers declined with the
development of trawl fishing, while less valuable species of bream and Bombay
ducks became more abundant.
Sainsbury and his co-workers developed several models of what might be
happening in the North West Shelf ecosystem. The observed shifts in fish
populations might be due to interactions between fish species. The more valuable
bream and snappers might normally win out over their less valued counterparts,
for instance, so that as fishing reduced the abundance of the former, the
latter鈥檚 populations would be able to expand. Or, the changes might be
determined by trawl-induced changes in habitat, with the cheaper fish the more
successful species in more barren conditions.
Off the shelf
Based on the researchers鈥� recommendations, part of the North West Shelf was
closed to trawl fishing in 1985. Results from five years of monitoring using
research trawls showed that populations of small benthic organisms and of
emperor bream and snappers rose steadily in the closed area, while they
continued to decline in the trawled area. This is strong evidence that the
abundance of the most highly valued fish in the area depends on the health of
the benthic community.
鈥淚 think Australia is out ahead of the rest of the world on this. They鈥檝e had
a habitat perspective in their management for five or ten years,鈥� says Collie.
鈥淭hey are establishing protected zones and zones where different activities can
occur.鈥� Many governments now fund studies of benthic habitats but few have taken
concrete conservation action. In the US, the Magnuson-Stevens Fishery
Conservation and Management Act became law in October 1996. It requires the
regional fishery management councils that regulate fishing in American waters to
identify 鈥渆ssential fish habitat鈥�, but at this point it is unclear what kind of
protection will be afforded to such habitats.
鈥淩ight now our fishery management is based only on variations in fish
populations,鈥� says Auster. 鈥淚t鈥檚 not based on managing habitat impacts.鈥� He
points out that current understanding of both is filled with uncertainty.
鈥淧rotected areas can play an important role as a hedge against that
uncertainty,鈥� he adds. Although the US now has several areas designated as
National Marine Sanctuaries, Auster describes them as more akin to heavily
logged National Forests than to protected National Parks. 鈥淭hey are multiple-use
areas and the management of most sites rests with the regional fishery
management councils.鈥� He is research coordinator for Stellwagen Bank National
Marine Sanctuary in the Gulf of Maine, and believes that such areas should be
protected from the impacts of fishing.
Meanwhile, as the Georges Bank scallop population recovers, pressure is
mounting to reopen closed areas to scallop dredging. This could mean the end of
a rare opportunity to study and find new ways to manage fish and their habitats.
But a continuing decline in cod numbers in the Gulf of Maine has prompted calls
for a closure there. If this is enacted, Auster plans to launch a study of
habitat changes over time in the protected area, coupled with a survey of
habitat use by juvenile fish in closed and trawled areas.
鈥淲e鈥檝e been studying this area for over a century,鈥� says Auster, 鈥渂ut we鈥檙e
still making the kind of primary ecological observations that people like Darwin
and Beebe made. And we鈥檙e doing it not as exploration for its own sake, but in a
rush to get some ecological answers to pressing management questions.鈥�
BARBARA SULLIVAN studies a diminutive drama, played out over and over again
in her laboratory aquariums. A tentacled, predatory beast with many mouths is on
the hunt. The predator is a hydroid, a cousin of anemones and jellyfish which
grows in spherical colonies about the size of a dandelion seedhead. The prey is
a larval cod, 5 millimetres long.
The hydroid鈥檚 tentacles attach so gently that it may take several seconds
before the young fish begins to struggle. Few manage to escape. 鈥淭he hydroid
keeps latching on, and actually sucks the insides of the fish right into its
body,鈥� says Sullivan, a researcher at the University of Rhode Island Graduate
School of Oceanography.
In 1994, Sullivan and her colleagues set out to study predator-prey
relationships of larval cod and haddock, which drift in the water column and
feed on smaller zooplankton. It was part of an effort to understand all of the
threats facing declining commercial fish stocks on Georges Bank. When the
researchers began sampling Georges Bank plankton, they were startled to find
that their nets came up full of hydroids, which normally live anchored to the
bottom.
鈥淲e haven鈥檛 found this phenomenon any place else in the world,鈥� says
Sullivan. 鈥淭his species lives on the bottom in other places, even in the
Canadian part of George鈥檚 Bank, which is colder and deeper than our study area.鈥�
Hydroids living on the bottom are unlikely to prey much on larval cod and
haddock, which float higher in the water column. But in the lab, Sullivan has
found that free-floating hydroids at the densities found on Georges Bank reduce
survival of larval cod by 50 per cent. Hydroids also prey on planktonic
copepods, which are a major food source for larval fish.
So how do the hydroids break free? Georges Bank is one of the most turbulent
areas in the northwest Atlantic. 鈥淲e know that the mixing induced by tidal
currents is responsible for keeping the hydroids suspended in the parts of the
Bank where they occur,鈥� says Laurence Madin of the Woods Hole Oceanographic
Institution. 鈥淲e don鈥檛 know if the same forces actually break them off and get
them into the water column in the first place.鈥�
Disruption of the seafloor by fishing trawls and dredges may be breaking
hydroids loose from the bottom. So far there is little evidence to either
confirm or disprove this idea but Madin and Sullivan hope to answer this
question in the lab. Using a flume to create turbulence in an aquarium, they鈥檒l
measure the force needed to break hydroids loose from the bottom and compare it
with the natural turbulence on Georges Bank. 鈥淚f it takes more energy to disrupt
them than we think occurs out there, that would suggest some other force is
involved,鈥� says Madin.