Trawl Report, AMCC - Alaska Marine Conservation Council

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ALASKA MARINE CONSERVATION COUNCIL
APRIL 2002
1
Trawling the North Pacific
Understanding the Effects of Bottom Trawl Fisheries on
Alaska’s Living Seafloor
Ben Enticknap, Alaska Marine Conservation Council, Fisheries Project Coordinator
“I don’t think the problem is mobile gear per se, but too much use of it.” - Jeremy Collie
(as in Russell 1997)
“Like storms and turbidity currents, it churns and resususpends sediments. But its effects
are felt deeper than storm-generated waves, and it occurs more often.” - Elliot Norse
(1993)
Introduction
The Alaska Marine Conservation Council (AMCC) promotes habitat conservation to
sustain fisheries and a healthy ocean ecosystem. We are concerned about the impacts of
bottom trawling on seafloor life and believe that trawling is only appropriate in places
where its impact on habitat is minimal. It is intuitive to many coastal residents,
fishermen, and fisheries managers that dragging large nets across the seafloor, weighted
with chains and tires, is going to have negative impacts on benthic habitats and the
species that flourish there. This intuition is confirmed by scientists who in the past
decade have focused a great deal of research in determining the effects of trawling on
seafloor habitats.
This report updates our understanding of the effects of bottom trawling in the North
Pacific, including the U.S. Bering Sea, Aleutian Islands, and Gulf of Alaska, by
reviewing the literature of fishing effects in the Alaska region and worldwide. This
report will also characterize commercial bottom trawling in the North Pacific by
reviewing historical fishing effort, fleet composition and gear types, and the habitats that
are fished. It is important to understand the different types of trawls used and the extent
of their impact on various habitat types. It is important to understand what scientific
studies are applicable to our region, and what habitats, if any, can withstand the impacts
caused by commercial bottom trawling.
Most research on bottom trawling has been conducted off the coast of Australia, the
eastern United States and the North and Irish Seas. Relatively few studies have been
conducted in the waters of the North Pacific. However regardless of latitude, reviews of
studies worldwide have shown similar impacts caused by trawling, including the removal
or damage of benthic animals, the smoothing of the seafloor, and the reduction of habitat
complexity (Auster and Langton 1999). A recent report by the National Research
Council (2002) declares,
“Although there are still habitats, gears, and geographic regions that have
not been adequately studied and characterized, there is an extensive
ALASKA MARINE CONSERVATION COUNCIL
APRIL 2002
literature on the effects of fishing on the seafloor. It is both possible and
necessary to use this existing information to more effectively manage the
effects of fishing on habitat.”
The debate over the effects of bottom trawling is shifting away from whether or not it
reduces marine biological diversity and habitat complexity; research from across the
globe clearly indicates this is the case. The debate over bottom trawling is now focused
on where and to what degree it is appropriate. Meanwhile as this debate ensues,
commercial fishermen in the North Pacific, with some of the highest volume fisheries in
the world, are dragging bottom trawls through sensitive soft-bottom habitats, over rocky
seamounts, and through forests of corals and sponges.
What is bottom trawling?
The North Pacific (Gulf of Alaska and Aleutian Islands) and Bering Sea are home to a
diverse array of commercial fishing fleets, using many different types of fishing gear and
vessels. Commercial fishermen use pots, longlines, jigs, seines, gill nets, trolls and
trawls, in boats ranging from small skiffs to 350-foot factory trawlers. In the North
Pacific and Bering Sea, the otter trawl is the principal bottom trawl used and therefore it
will be the primary focus in this report. Bottom trawling is the fishing practice of
dragging large nets across the seafloor, weighted with chains, rock-hopper and roller
gear, or steel beams.
The National Marine Fisheries Service (NFMS 2001) reports that about 50% of the
groundfish trawl catch in the Gulf of Alaska (GOA), 27% of the Bering Sea Aleutian
Island (BSAI) trawl catch, and 30% of the overall groundfish trawl catch is caught with
bottom trawl gear. Beam trawls are permitted in State managed shrimp fisheries in
Southeast Alaska (mainly near the Petersburg and Wrangell area) but they are prohibited
in the North Pacific groundfish fisheries (ADF&G 1994). Dredges, another type of
mobile bottom gear, are used to catch weathervane scallops near Yakutat, Kodiak and in
Cook Inlet. The Bering Sea pollock fishery exclusively uses pelagic trawls since bottom
trawls were prohibited in this fishery in 1999 (BSAI Amendment 57). Also, most vessels
(90%) fishing for pollock in the Gulf of Alaska use pelagic trawls (NPFMC 2001a).
These are not considered bottom trawls because the steel doors used to spread the mouth
of the net are designed to stay off the seafloor. However mid-water or “pelagic” trawls
often contact the seafloor with their footropes, up to 85% of the duration of the tow
(Loverich 2001).
Trawlers target large aggregations of mid-water species, such as pollock and bottom fish
like cod, rock sole and yellowfin sole. Trawl vessels tow a large net bag with a wide
opening at the mouth. To keep the mouth of the trawl net open, large “doors” - weighing
up to 700 pounds and about 6 feet long - fan out the bottom of the net. Bottom trawlers
use heavy chains sometimes fixed with rock-hopper or tire gear strung along the base of
the net’s opening to hold it close to the sea floor, allowing the nets to be dragged through
rough and rocky areas that typically create complex habitats (Browning 1980). Rockhopper gear is used in the Aleutian Island Atka mackerel and rockfish fisheries and by
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ALASKA MARINE CONSERVATION COUNCIL
APRIL 2002
many vessels fishing in the GOA for cod, Pacific ocean perch (POP), rockfish, and Dover
and rex sole. Historically all slope rockfish (POP, shortraker/ rougheye rockfish, and
“other slope rockfish”) in the GOA were fished with bottom trawls. Between 1996 and
1998, 14-20% of Pacific ocean perch were caught with pelagic trawls (NPFMC 2001a).
Trawl Gear
Otter Trawl: a net towed behind a vessel that is held open by two boards (or doors) attached to
warps (cable or rope) between the net and the vessel.
Bottom Trawl: In the North Pacific, these are primarily otter trawls whose trawl doors are designed
to drag along the seafloor. The footropes are fixed with rolling discs, metal or rubber bobbins that
bounce over obstructions.
Rock-hopper gear: Bottom trawl gear designed for fishing over rocky ground. The gear may use 24inch diameter airplane tires or rubber disks that are greater than 21 inches in diameter, fitted to the
footrope of the trawl net. Bobbins are also used that can weigh up to 22 lbs each.
Pelagic Trawls: These otter trawls are designed so that the doors do not contact the seabed. The
footrope, made of metal chain, often drags over the seafloor. Performance standards for the BSAI
pollock fishery mandate that it is unlawful to have onboard 20 or more crab of any species.
Beam Trawl: A trawl with a fixed net opening, utilizing a wood or metal beam
Footrope: The weighted rope that runs along the bottom of the mouth of the net.
Codend: The rear portion of the net in which the fish collect.
Bridles: The cables that attach the doors to the trawl net.
Scallop Dredge: A dredge-like device designed specifically for taking scallops by dragging and
digging into the seabed.
(Garner 1988, Jennings et al. 2001)
Factory trawlers are equipped with on-board processing plants and can measure up to 350
feet in length. Trawl catcher vessel lengths range between 58’ to 190’. Catches can be as
enormous as 100 tons or more per tow, depending on the fishery, size and horsepower of
the vessel (Gunstrom 1994).
Technological advances are allowing fishermen to work with improved efficiency.
Electronics have become more sophisticated, nets are larger and fishing boats stay out at
sea for prolonged periods of time. Advancements in trawl technology and efficiency
have also allowed fishermen to trawl areas of the seabed that were once considered too
deep or rough to fish with mobile gear (NRC 2002).
The Effects of Bottom Trawling on Marine Habitats
Direct physical effects of bottom trawls on living seafloor communities include damage
and mortality of target and nontarget species (species with no commercial value such as
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ALASKA MARINE CONSERVATION COUNCIL
APRIL 2002
clams, corals and urchins) (Norse 1993). Indirectly, bottom trawls affect marine animals
and their associated habitat by resuspending sediments, toxins, and nutrients into the
water column by scraping and plowing 1cm – 30cm into the seafloor (Vining, Witherell,
Heifetz 1997: Dayton et al. 1995). Bottom trawls can alter the physical structures of the
seafloor by scraping, plowing, smoothing sand ripples, removing stones and turning over
boulders (Auster and Langton 1999, Schwinghamer 1998). Delayed effects on the
seafloor include post-fishing mortality of the benthic sea life and long-term changes of
the community structure (Collie et al, 1997, Jones 1992).
When trawl gear is dragged along the seafloor, sediments are kicked up behind the net.
Sediment suspension can reduce the light available for photosynthetic species and bury
benthic organisms. Repetitive trawling reduces the abundance of organic matter at the
surface of the sediments and flattens sand waves (Schwinghamer 1998). “Like storms and
turbidity currents, it (bottom trawling) churns
Infauna and epifauna:
and resususpends sediments. But its effects are
Infauna are animals living entirely
felt deeper than storm-generated waves, and it
within the sediment and epifauna
occurs more often” (Norse 1993).
are animals living on, protruding
from, anchored in, or attached to, the
substratum. Both groups are known
as benthic fauna, bottom-living fauna
or benthos. Pelagic fauna are
animals that live in the water column
(Jennings et al. 2001a).
While sediment suspension may create
anaerobic conditions for some benthic
communities, it may also stimulate primary
production by increasing nutrient levels in the
water column (Pilskaln 1998). In areas where
natural upwelling occurs, it is difficult to
determine if trawling or natural upwelling is responsible for increasing nutrient. The
effects of sediment suspension are site-specific and are dependent on sediment grain size
and hydrologic conditions. Areas that experience high amounts of natural disturbances
are less affected than more stable seafloor communities that experience few natural
disturbances from storms or strong tides (Hall 1994, Kaiser 1998, Collie 2000).
Although Hall (1999) stresses that, “while it is important to appreciate a range of natural
variation in disturbance from wind, currents and waves to put fishing in context, the fact
that the natural range is large in itself, gives no basis for arguing that the additional
perturbation imposed by fishing is inconsequential.” Kaiser (1998) states, “Presumably,
the scale and frequency of physical disturbance events can increase to a point where
lasting ecological effects are observed even against the background of natural
disturbance.”
In a review of scientific literature on fishing effects, Collie (2000) found that fauna in
stable gravel, mud and biogenic sediments were more adversely affected than in less
consolidated coarse sediments (e.g. sand). Studies in the North Pacific clearly
corroborate this finding (Freese 1999, McConnaughey 2000). When ranking the impacts
of mobile gear on habitat, Collie found that inter-tidal dredging and scallop dredging had
a greater adverse effect on benthic animals than otter trawling. Prena (1999) found that
otter trawling on a sandy bottom ecosystem of the Grand Banks of Newfoundland (120 –
146m deep) produced a detectable change on benthic habitat and communities, with a
significant reduction in the biomass of large epifauna.
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ALASKA MARINE CONSERVATION COUNCIL
APRIL 2002
A Description of the Eastern Bering Sea
The Bering Sea ecosystem is one of the most biologically productive areas of the world
(NMFS 2001). Some of its unique physical features include a broad continental shelf,
comprising 44% of the Bering Sea, and pack ice that covers most of the eastern and
northern areas of the continental shelf during winter and spring (NMFS 2001).
Phytoplankton blooms along the ice edge account for 10 to 65% of the total annual
primary production (NMFS 2001).
Benthic substrates on the continental shelf show considerable variability on a local scale.
However, a sediment pattern is evident as one moves from the nearshore to the
continental slope. Sediments of the inner shelf (0-50m deep) in the east and southeast are
often sandy gravel that then turns to plain sand farther offshore (NMFS 2001). The
middle shelf (50-100m deep) and outer shelf (100-200m deep) seafloor is predominately
a more stable muddy sand.
But the Bering Sea seafloor is not just made up of sand and gravel. The seafloor is rich
with life, including communities of soft corals, hydroids, sea pens, tubeworms, tunicates,
and sponges. These sedentary benthic organisms help create diverse communities by
providing structural habitat (McConnaughey 2000). For example, juvenile red king crabs
have been found in close association with mats of tubeworms, north of the Alaska
Peninsula (NPFMC 2000a). Living substrates are an important component of the habitat
essential to crab species in the Bering Sea.
A Description of the Gulf of Alaska and the Aleutian Islands
The Gulf of Alaska (GOA) and Aleutian Island (AI) substrates are distinctly different
from the Bering Sea. The GOA is a much more open marine system than the eastern
Bering Sea which is partially enclosed by the Aleutians, Russia, and Alaska. The GOA
continental shelf is less than 25% the size of the eastern Bering Sea shelf (NMFS 2001).
The GOA also has relatively weaker currents and tidal actions near the seafloor. The
result of this is an array of benthic substrates such as gravely sand, silty-mud, and areas
of hard bedrock (NMFS 2001). The western GOA shelf consists of many banks and reefs
with coarse, rocky bottoms and patchy bottom sediments. In the area around Kodiak
Island, the shelf is made up of flat, shallow banks cut by deep gullies. The substrate in the
western Aleutians is predominately bedrock outcrops and coarse sediment with some
sandy bottoms (NMFS 2001). The AI shelf is extremely narrow and on the Gulf side, the
shelf edge drops off into some of the deepest waters of the world, the Aleutian Trench.
Bottom Trawl Effort in the Gulf of Alaska and Bering Sea/ Aleutian Islands
An important component in understanding the effects of trawling on marine habitats is
documenting the spatial extent and intensity of fishing effort. The NMFS observer
program has been documenting fishing effort since about 1973, allowing NMFS to make
a fairly comprehensive determination of historical effort. However most of the data is
limited to the number of trawl tows within large, 25km2 blocks. New developments in
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ALASKA MARINE CONSERVATION COUNCIL
APRIL 2002
technology, like Vessel Monitoring Systems (VMS) will allow NMFS to more precisely
track where fishing effort occurs.
From the period of 1973-1997, NMFS
observers documented
Trawl Intensity
412,040 bottom trawl tows
Very High
in the Bering Sea (NMFS
High
2001a). Throughout this
Moderate
Low
time period, bottom
Very Low
trawling has impacted most
No data
all of the Bering Sea, but
the intensity of the impact varies throughout
the region (NMFS 2001).
Historic Trawl Intensity in the Bering Sea (1973-1997) NMFS 2001. (This map does not show trawling in
the Aleutian Islands management district or Gulf of Alaska)
The highest concentrations of fishing effort in the Bering Sea are located along the shelf
edge, north of the Alaska Peninsula near Unimak Island, and Togiak Bay. Bottom trawls
in this region are targeting primarily Pacific cod and flatfish.
In the Gulf of Alaska, the greatest bottom trawl effort has occurred in the Kodiak Island
region. Trawl fisheries in this area target primarily Pacific ocean perch, Pacific cod and
flatfish. Based on observed bottom trawl effort and estimates of unobserved effort,
NMFS estimates that from 1990 to 1998 there were approximately 116,288 tows in the
Gulf of Alaska and 41,015 tows in the Aleutian Islands. In the Aleutian Islands, the
greatest bottom trawl effort has occurred in the Atka mackerel and Pacific ocean perch
fisheries (NMFS 2001).
Estimates of the total area swept by bottom trawl gear in 1998 – 2000 include 53,931 km2
in the Bering Sea, 10,201 km2 in the Aleutians, and 17,562 km2 in the Gulf of Alaska
(NRC 2002). The sum of these trawled sites (1998 – 2000) equates to an area greater
than the state of South Carolina, yet these disturbances are not occurring in a
consolidated block but spread throughout the Bering Sea, Aleutians and the Gulf of
Alaska, year after year.
The NRC reports a significant decline in the number of observed bottom trawl tows from
the early 1990’s to 2000, due to management closures to bottom trawls and the increased
use of pelagic trawls. However, it is unclear if there would be an observed decline in
trawl tows if pelagic trawls were included in these effort estimates. In addition, bottom
trawl vessels less than 60 feet in length are unobserved. The use of bottom trawls is
prohibited in a number of areas in the North Pacific such as the Gulf of Alaska (E. of
1400 ), the Nearshore Bristol Bay Closure Area, the Pribilof Islands Conservation Area,
Type I Kodiak King Crab Protection Zones, and inside state waters throughout most of
the Gulf of Alaska (Figures 2-3). The total area closed to trawling in federal waters off
Alaska is approximately 310,500 km2 (NRC 2002).
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ALASKA MARINE CONSERVATION COUNCIL
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Trawl Research in the Bering Sea
Seafloor communities of the Bering Sea are dynamic environments that provide habitat
for a wide range of both commercial and non-commercially valuable species. To better
understand the effects of trawling on these complex habitats, R.A. McConnaughey of the
National Marine Fisheries Service led an experiment to examine biological diversity in
heavily trawled versus unfished sites in the eastern Bering Sea. In “An examination of
chronic trawling effects on soft-bottom benthos of the eastern Bering Sea,”
McConnaughey (2000) systematically sampled seafloor organisms in the Crab and
Halibut Protection Zone 1 (also known as area 512) - a site north of the Alaska Peninsula
that has had a series of trawl prohibitions since 1959. Similarly, adjacent areas that have
experienced chronic trawling for yellowfin sole were also sampled. The area studied is
relatively shallow (44-52m) with a sandy substrate and rich communities of invertebrates.
The results of his analysis showed that long-lived attached or non-mobile organisms were
significantly patchier in the heavily fished areas (sponges, anemones, soft corals, stalked
tunicates). Biomass of stalked, encrusting and attached organisms was higher in the
unfished areas. Also, structural complexity of the habitat was enhanced in the unfished
areas by an abundance of “biogenic substrate” (such as empty snail and clam shells).
McConnaughey summarized his conclusions by stating, “Overall, structural complexity
and diversity are reduced by trawling. Finally, bottom trawls directly affect physical
properties of the seafloor, by increasing turbidity and by altering grain-size distributions,
sediment porosity, and chemical exchange processes.”
Eloise Brown of the University of Alaska, Fairbanks has been conducting research on
experimental flatfish trawling in the Bering Sea and the potential for long-term changes
in sediment composition and structure (Brown 2001). Brown’s study was conducted in
and around the Round Island 12-mile closure in Bristol Bay. Trawling has been
prohibited in the 12 miles around Round Island since 1992, in an effort to increase
protection for walrus habitat. This area ranges from 16 – 30 meters in depth and is a
structurally featureless area comprised of mostly fine sand with a little mud. The
research team conducted experimental trawl tows using a 132-foot commercial trawler
inside the closed area and in adjacent areas open to trawling (1 April – 15 June). This
study looked for changes in the distribution of chemical properties in the seafloor after
trawling. Brown was also trying to determine if trawling effects were measurable over
natural disturbances such as tides and storms.
Although the results of this research have not yet been published, Brown reported her
initial findings to the National Research Council symposium on the effects of bottom
trawling, in Anchorage, Alaska June 2001. Initial tests showed no significant difference
in sediment composition in the experimental and control sites. She also looked at more
sensitive indices of change, such as concentrations of chlorophyll in the top layers of
sand. Chlorophyll is an important component for the production of phytoplankton, the
base of the biological food chain (Castro and Huber 1997). Brown was interested in
whether chlorophyll concentrations in the top layers of sand were different in the trawled
and untrawled sites. She found that chlorophyll concentrations in the top layers of sand
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ALASKA MARINE CONSERVATION COUNCIL
APRIL 2002
at the trawled sites were patchier and more variable relative to the control areas. There
appeared to be some patchy disturbance and some form of mixing of the sediments but
the differences were not statistically significant.
There are important differences in the study sites used by McConnaughey and Brown that
affect the outcome of their experiments. Although tidal currents would be felt at the
Alaska Peninsula site, the area studied by McConnaughey is deeper and potentially less
affected by tides or storms. The shallow waters of the Round Island experiment are
routinely disturbed by storm-generated waves and strong tides. The organisms living in
this habitat are likely to be more adapted to disturbances than at McConnaughey’s deeper
study site. Also, the substrate near Round Island is predominantly fine sand with little
habitat complexity. The site north of the Alaska Peninsula was reported to be rich with
invertebrates and organisms like sponges and anemones that create a living seafloor
habitat, and thus are more susceptible to direct damage from trawling.
Trawl Research in the Gulf of Alaska
While Brown noticed little change after trawling at her study site, NMFS researchers
found evidence of habitat damage seven years after a single trawl passage off of Dixon
Entrance. In 1990, a single pass of a research trawl at a depth of 365m removed over
2,000 pounds of Primnoa red tree coral (Krieger 2001). The trawl used was a
Nor’eastern otter trawl fixed with rubber bobbin roller gear, with a spread of 13 to 18
meters from wingtip to wingtip.
Using a manned submersible, researchers observed the trawled area in 1997. They found
colonies of corals that were still damaged, missing up to 99% of their branches. The
NMFS researchers estimated that 27% of the corals in the net path were detached. The
corals that were detached by the passing trawl but remained on the seafloor were missing
50-90% of their polyps. The corals were most likely dying because they were no longer
oriented correctly to the current and could not feed effectively (Krieger 2001).
In 1996 Lincoln Freese and his colleagues began a study of bottom trawling on a hardbottom (pebble, cobble, and boulder) seafloor habitat east of Sitka (Freese et al. 1999).
This study area had been exposed to either no or little trawling since the 1970s. Freese
confirmed that this was a relatively pristine habitat by diving on the site with a
submersible before experimental trawling began.
The researchers used a 42.5m commercial trawl vessel, equipped with a Nor’eastern
bottom trawl fitted with .45m rock hopper discs and bobbins and 0.6m tires. Freese
reported that this bottom trawl is similar to those used in the commercial rockfish fishery
in the Aleutian Islands.
Eight sites at depths between 206 to 274 meters were trawled one time for a short
duration, and then compared to eight control sites. After trawling, researchers viewed the
study area from a submersible to document the fishing effects. This study found that a
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ALASKA MARINE CONSERVATION COUNCIL
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significant number of boulders were displaced and epifauna were removed or damaged.
67% of erect sponges (morel, vase, and finger sponges) were damaged. They
documented that 55% of the sea whips, Stylea sp., were broken or pulled out of the
seafloor and there was no observed damage to anemones.
A year after bottom trawling
in the study site, researchers
returned to the area to
document recovery. They
noted that trawl tracks from
the doors were still
identifiable and little filling
of the tracks had occurred.
When examining the
sponges they found that,
“None of the 115 damaged
sponges in the trawl path
showed signs of repair or regrowth.” Freese concluded
that, “Unlike sponge
communities in warm
shallow waters, sponge
communities in the Gulf of
(NMFS – Auke Bay Lab)
Alaska do not appear to have the ability to quickly return to pre-trawl population levels,
nor do individual sponges appear to have the ability to quickly recover from wounds
suffered from trawl gear” (NPFMC 2000b).
Robert Stone (NFMS) researched the seafloor habitat in trawled areas near Kodiak Island
and compared these to nearby areas that have been closed to bottom trawling (NPFMC
2000b). The year round closures (Type I areas) to bottom trawling and scallop dredging
were implemented in April of 1987 to protect concentrations of king crab. This study
examined differences in epifauna (species living on top of the seafloor) and infauna
(species living in the seafloor) in terms of composition, diversity and abundance, along
with substrate characteristics such as total organic carbon levels and grain-size
composition (NPFMC 2000b).
Stone studied three sites that bisected the boundaries of the closed areas. However, these
closed areas do not function as true controls since state and federal researchers conduct
trawl surveys inside the Type I areas (Stone pers comm). Although all the sites are near
Kodiak Island, each site has markedly different characteristics in terms of current flow
and sediment composition. Stone noted that in Chiniak Gully, where trawl intensity is
relatively high, impacts by trawl doors leave little trace in the sandy-mud sediments.
However just south of Kodiak near the Trinity Islands where there is both less current
and trawl effort, bottom trawls leave noticeable scours in the sediments (Stone pers
comm).
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ALASKA MARINE CONSERVATION COUNCIL
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10
Initial examinations found that infauna composition; abundance and diversity were
similar in the trawled and closed areas, while organic carbon levels were higher in the
trawled zone of one of the three study sites. At one of the sites there was a clear
difference in the substrate type between the trawled and untrawled zones. Although
Stone has not finalized research on epifauna composition, his initial analyses of trawling
in the Kodiak region are that:
1) Trawling intensity, although high for the GOA, is relatively low
compared to other areas worldwide, and 2) effects on the sedimentary and
biogeochemical features of the seafloor and infauna community structure
from present levels of bottom trawling were minor and no clear patterns
were detectible (NMFS 2002 web-page).
Stone is presently continuing research of trawl effects
on the soft-bottom benthos near Kodiak, examining
the effects of trawling on sea whip habitat. He
reports that a clear relationship exists between total
epifaunal biomass and sea whip abundance,
indicating that sea whip habitat may increase
productivity (NFMS 2002 web-page). Simon Thrush
corroborates this statement when describing the
removal of epifauna and disturbance of softsediments by trawling and dredging in New Zealand.
His research indicates that the removal of habitat structure in relatively low-structure
soft-sediment systems significantly decreases the areas’ biodiversity, and ultimately that
of the greater marine ecosystem (Thrush et al. 2001).
The sea whip is commonly
found on soft bottoms below 75
feet where they often form large
fields. They are a type of
feathery gorgonian coral and
considered to be long lived with
a slow recolonization rate. They
provide vertical structure to an
otherwise low relief habitat.
The Link Between Bycatch and Habitat
Marine life hauled on board fishing vessels and then
tossed back to sea as bycatch, dead or dying, are
members of a diverse ecological community. When
parts of the seafloor habitat are crushed, upended, or
removed by fishing gear, the habitat is changed and
can no longer support all the species that once lived
there in their former abundance. It is clear that
bottom trawling impacts sensitive seafloor habitats
and the associated ecological communities. This
impact is also evident when reviewing NMFS
bycatch data. For example in 1997 – 1999 the
observed bycatch of corals in the BSAI Pacific cod
bottom trawl fishery was 8,444 lbs. This fishery also
Bycatch in the North Pacific:
Every year hundreds of millions
of pounds of marine life are
caught and discarded as bycatch.
For example, in 1999 trawl
(including pelagic trawl) vessels
accounted for 89% (~245 million
pounds) of the bycatch in the
Bering Sea and Aleutian Island
groundfish fisheries and 82% (~
43.6 million pounds) in the GOA
groundfish fisheries (FIS 2000).
ALASKA MARINE CONSERVATION COUNCIL
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took 487 lbs of sea whips and 159,352 lbs of sponges. In the Aleutian Island Pacific
ocean perch bottom trawl fishery, 40,823 lbs of corals and 149,211 lbs of sponges were
caught from 1997 to 1999 (NMFS 2001).
Bycatch is not always visible from the deck of a fishing boat. Many fish and other ocean
species may not actually be caught in fishing gear, but may be crushed by the heavy
doors of an otter trawl or killed or stunned after passing through the mesh of a trawl net.
Unobserved mortality of target and non-target species needs to be better accounted for.
An interesting story of the disappearance of corals has been unfolding in the Aleutians.
Spurred by the sensitivity of corals to disturbance, NMFS is currently examining the
distribution of Gorgonian corals in the Seguam Pass area of the Aleutians. The Atka
mackerel fishery in Seguam Pass had historically high rates of coral bycatch, but now
corals are caught less frequently (NMFS 2001).
Similarly, on the northwest Australian shelf, trawl data showed that bycatch of sponges
and corals fell during the development of a trawl fishery (Sainsbury 1987 as in Auster
1996). Over time, fauna normally associated with sparse sandy habitats increased in
catch while fauna associated with dense habitats decreased. Species that show the
greatest decline in areas routinely trawled tend to be those that are slow growing, low in
fecundity, and physically vulnerable to damage by fishing gear (Kaiser 1998b).
This information leads us to the conclusion that trawling is changing the biological
community structure by damaging sensitive habitats. Bottom trawling is devastating
sensitive habitats such as slow growing deep-water corals and sponges. These species are
considered bycatch, an ecological cost, justified by the economic profit gained by the
catch of the target species. But as the bycatch of sensitive species continues, we will lose
the biological diversity supported by sensitive habitats, and create a seafloor community
dominated by species that thrive off periodic disturbance, resulting in human-induced
alteration of the ocean’s ecological communities.
Conclusion
After reviewing the scientific literature of bottom trawling effects in both the North
Pacific, and around the world, it is apparent that trawling negatively affects seafloor
habitats and alters the composition of species that can live there. However, the effects on
habitat and the ability of the habitat to recover is dependent on a number of variables:
“1) Type of gear employed, 2) depth of penetration of the gear into the
sediment, 3) water depth, 4) nature of the substrate (rocky, mud, sand,
pebble, etc.), 5) kind of benthic fauna being impacted (epibenthic, infauna,
emergent fauna), 6) frequency of the area being fished, 7) weight of the
gear on the seabed, 8) towing speed, 9) strength of the tides and currents,
and 10) time of year. Intensive and repeated trawling in the same area
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ALASKA MARINE CONSERVATION COUNCIL
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may lead to long-term changes in both benthic habitat and communities”
(DeAlterus, J., et al., 1999).
Shallow water habitats that are exposed to natural disturbances such as strong tides and
storm generated waves, with little sensitive epifauna, may be able to sustain some levels
of additional anthropogenic disturbance. It is important that the fishing effort does not
create a disturbance level that exceeds that of natural disturbances. Collie (2000)
speculates that shallow water sandy habitats (<60m) can recover from a disturbance in
approximately 100 days, suggesting that this habitat type could sustain 2 to 3 incidences
of physical disturbance per year. Bering Sea trawl effort data shows that some areas were
trawled up to 78 to 176 times in 1999-2000. More commonly, sites in the Bering Sea
shelf were trawled between 11 and 77 times in this two-year time frame (Figure 4).
Recognizing the importance of habitat to the health of fish species and the coastal
communities and commercial fisheries that rely on them, Congress amended the
Magnuson-Stevens Act to ensure the designation and protection of “essential fish habitat”
(EFH). The Magnuson-Stevens Act describes EFH as “those waters and substrate
necessary to fish for spawning, breeding, feeding, or growth to maturity.”
Here in the North Pacific, federal fisheries managers are working to find appropriate
methods to designate essential fish habitat and ways to reduce damage from fishing gear
to habitat. The North Pacific Fishery Management Council (NPFMC) has approved a
range of alternatives to be analyzed in an environmental impact statement (EIS) for the
designation of EFH and Habitat Areas of Particular Concern (HAPC). HAPCs can be
types of habitat like corals, or specific sites like the Sitka Pinnacles. These habitats may
be given elevated importance based on their significant ecological function, rarity, or
sensitivity to human disturbance.
In the past the NPFMC and state Board of Fish have taken a number of steps to protect
benthic habitat from bottom trawl gear including the Bristol Bay Crab Protection Zone,
the Pribilof Islands Habitat Conservation Area, the Southeast Alaska (E. of 140o ) Trawl
Closure, and selected state waters. These areas all provide year round protection from
bottom trawling, while allowing the use of other commercial gear types.
Many different management tools can be used to protect habitat. The appropriate tools
should be considered based on the vulnerability and sensitivity of the site to a
disturbance. Tools being considered for the protection of essential fish habitat include a
harvest prohibition for HAPC biota, gear restrictions, gear conversions, areas of limited
effort, rationalization (minimizing impacts by slowing down the pace of fishing), and
habitat conservation areas.
ô
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ALASKA MARINE CONSERVATION COUNCIL
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Literature Cited:
ADF&G, 1994. Wildlife Notebook Series. Alaska Department of Fish and Game.
www.state.ak.us/adfg/notebook/shellfish/shrimp.
Auster, P.J., Langton, R.W. 1999. The Effects of Fishing on Fish Habitat. American
Fisheries Society Symposium, 22: 150-187
Auster, P.J., et al. 1996. The Impacts of Mobile Fishing Gear on Seafloor Habitats in the
Gulf of Maine: Implications for Conservation of Fish Populations. Reviews in Fisheries
Science, 4(2): 185-202.
Brown, E. 2001. Commercial Flatfish Fishing in the Bering Sea: Impacts to sediment
structure from experimental trawling, submitted to the National Academy of Science,
Evaluating the Effects of Bottom Trawling on Seafloor Habitats. Anchorage, Alaska.
June 2001.
Browning, R., 1980. Fisheries of the North Pacific: History, Species, Gear and Process.
Alaska Northwest Publishing Company, Anchorage, Alaska.
Castro, P., Huber, M.E. 1997. Marine Biology, second edition. Wm. C. Brown
Publishers, pp 210-211.
Collie, J.S., Hall, S.J, Kaiser, M.J., and Poiner, I.R. 2000. A quantitative analysis of
fishing impacts on shelf-sea benthos. Journal of Animal Ecology, 69: 785-798.
Collie, J.S., Escanero, G.A., and Valentine, P.C. 1997. Effects of bottom fishing on the
benthic megafauna of Georges Bank. Mar Ecol Prog Ser, 155: 159-172.
Dayton, P.K., Thrush, S.F., Agaardy, M.T., and Hofman, R.J. 1995. Viewpoint –
Environmental effects of marine fishing. Aquatic Conservation: Marine and Freshwater
Ecosystems, 5: 205-232.
DeAlterus, J., Skrobe, L., Lipky, C. 1999. The significance of seabed disturbance by
mobile fishing gear relative to natural processes: a case study in Narragansett Bay, Rhode
Island. In Fish Habitat; Essential Fish Habitat and Rehabilitation, American Fisheries
Society Symposium 22.
FIS 2000. Discards in the Groundfish Fisheries of the Bering Sea/ Aleutian Islands and
the Gulf of Alaska 1998 & 1999. Fisheries Information Services. Juneau, Alaska.
Freese, L., Auster, P.J., Heifetz, J., and Wing, B.L. 1999. Effects of trawling on seafloor
habitat and associated invertebrate taxa in the Gulf of Alaska. Mar Ecol Prog Ser., 182:
119-126.
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ALASKA MARINE CONSERVATION COUNCIL
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Garner, J. 1988. Modern Deep Sea Trawling Gear. Fishing News Books Ltd. Farnham,
Surrey England.
Gunstrom, G. 1994. “What kind of fishing boat is that?” Alaska Department of Fish and
Game 1994.
Hall, S.J. 1999. The effects of fishing on marine ecosystems and communities, Blackwell
Science, Oxford. 274 pp.
Hall, S.J. 1994. Physical Disturbance and Marine Benthic Communities: Life in
Unconsolidated Sediments. Oceanography and Marine Biology: an Annual Review, 32:
179-239.
Jennings, S., Kaiser, M.J., Reynolds, J.D. 2001. Marine Fisheries Ecology. Blackwell
Science, pp 93-102, 277.
Jones, J.B. 1992. Environmental impact of trawling on the seabed: a review. New
Zealand Journal of Marine and Freshwater Research, 26: 59-67.
Kaiser, M.J. et al, 1998. Changes in megafaunal benthic communities in different
habitats after trawling disturbance. ICES Journal of Marine Science, 55: 353-361
Kaiser, M.J. 1998b. Significance of Bottom-Fishing Disturbance. Conservation Biology,
12(6): 1230-1235.
Krieger, K., 2001. Coral (Primnoa) Impacted by Fishing Gear in the Gulf of Alaska.
(Unpublished manuscript). Auke Bay Laboratory, Alaska Fisheries Science Center,
NMFS.
Loverich, G. 2001. NET-Systems. Trawl dynamics and its potential impact on habitat.
Report submitted to the National Academy of Science, Evaluating the Effects of Bottom
Trawling on Seafloor Habitats. Anchorage, Alaska. June 2001.
McConnaughey, R.A., Mier, K.L., Dew, C.B. 2000. An examination of chronic trawling
effects on soft-bottom benthos of the eastern Bering Sea. ICES Journal of Marine
Science, 57: 1377-1388.
NMFS. 2001. Alaska Groundfish Fisheries Draft Programmatic Supplemental
Environmental Impact Statement. National Marine Fisheries Service. Alaska Region.
January 2001: pp. 3.1-1 – 3.2-15, 4.7-1 – 4.7-40.
Norse, E.A. 1993. Global Marine Biological Diversity. Island Press, Washington D.C.
(USA). Pp.110-112.
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NPFMC. 2001a. Appendix B, Stock Assessment and Fishery Evaluation Report for the
Groundfish Resources of the Gulf of Alaska. North Pacific Fishery Management
Council. Anchorage, AK. November 2001: pp 1-2, 6-5.
NPFMC. 2001b. Appendix D, Economic Status of the Groundfish Fisheries off Alaska,
Stock Assessment and Fishery Evaluation Report for the Groundfish Resources of the
Gulf of Alaska and Bering Sea/ Aleutian Island Area. North Pacific Fishery Management
Council. Anchorage, AK. November 2001: pp 1-2, 6-5.
NPFMC. 2000a. Draft Environmental Assessment/ Regulatory Impact Review, Habitat
Areas of Particular Concern. North Pacific Fishery Management Council. Anchorage,
AK. January 12, 2000: pp 115.
NPFMC. 2000b. Appendix D, Stock Assessment and Fishery Evaluation Report,
Ecosystem Considerations for 2001. North Pacific Fishery Management Council.
Anchorage, AK. November 2000: pp 24-30.
NRC. 2002. Effects of Trawling and Dredging on Seafloor Habitat. National Research
Council. National Academy Press, Washington D.C. March 2002.
Prena, J., et al. 1999. Experimental otter trawling on a sandy bottom ecosystem of the
Grand Banks of Newfoundland: analysis of trawl bycatch and effects on epifauna. Mar
Ecol Prog Ser., 181: 107-124.
Pilskaln, C.H., Churchill, J.H., Mayer, L.M. 1998. Resuspension of Sediment by Bottom
Trawling in the Gulf of Maine and Potential Geochemical Consequences. Conservation
Biology, 12(6): 1223-1229.
Russell, D. 1997. Hitting Bottom. Amicus Journal, Winter 1997, 21-25.
Sainsbury, K.J. 1987. Assessment and management of the demersal fishery on the
continental shelf of northwestern Australia. Pp 465-503. In: Tropical Snappers and
Groupers: Biology and Fisheries Management (Polovina, J.J. and Ralston, S., Eds.)
Boulder, Colorado: Westview Press (1987)
Schwinghamer, P., et al. 1998. Effects of Experimental Otter Trawling on Surficial
Sediment Properties of a Sandy-Bottom Ecosystem on the Grand Banks of
Newfoundland. Conservation Biology, 12(6): 1215-1222.
Thrush, S.F., et al. 2001. Fishing disturbance and marine biodiversity: the role of habitat
structure in simple soft-sediment systems. Mar Eco Prog Ser, 223: 227-286.
Vining, I., Witherell, D., Heifetz, J. 1997. The Effects of Fishing Gear on Benthic
Communities. North Pacific Fisheries Management Council. Ecosystem Considerations
for 1998.
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Web Page Citation:
NMFS. 2002. Effects of bottom trawling on soft-bottom sea whip and other habitat in the
central Gulf of Alaska, National Marine Fisheries Service, Auke Bay Lab.
http://www.afsc.noaa.gov/abl/MarFish/effectsoftrawlonseawhips.htm
Figure 1.
Diagram of an Otter Trawl with description of roller and rock hopper gear
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ALASKA MARINE CONSERVATION COUNCIL
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Figure 2.
Trawl Closures: Bering Sea/ Aleutian Islands
Figure 3.
Trawl Closures: Bering Sea/ Gulf of Alaska
Maps from North Pacific Fishery Management Council (2001)
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ALASKA MARINE CONSERVATION COUNCIL
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Figure 4.
Bering Sea: 1999-2000: Number of Bottom Trawl Tows/ 25km2
(NPFMC 2001)
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ALASKA MARINE CONSERVATION COUNCIL
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Figure 5.
Aleutian Islands: 1999-2000: Number of Bottom Trawl Tows/ 25km2
(NPFMC 2001)
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ALASKA MARINE CONSERVATION COUNCIL
APRIL 2002
Figure 6.
Gulf of Alaska: 1999-2000: Number of Bottom Trawl Tows/ 25km2
(NPFMC 2001)
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ALASKA MARINE CONSERVATION COUNCIL
APRIL 2002
Table 1.
Number of Trawl Vessels that caught groundfish by year, area, target, and vessel
type: CV = Catcher Vessel, CP = Catcher Processor
Target/Year
Pollock
1996
1997
1998
1999
2000
Pacific cod
1996
1997
1998
1999
2000
Flatfish
1996
1997
1998
1999
2000
Rockfish
1996
1997
1998
1999
2000
Atka mack.
1996
1997
1998
1999
2000
All groundfish
1996
1997
1998
1999
2000
GOA
BSAI
All Alaska
CV
93
119
120
112
82
CP Total
4
97
6
125
2
122
0
112
0
82
CV
117
104
99
114
102
CP
41
35
38
17
16
Total
158
139
137
131
118
CV
157
171
167
160
163
CP
41
36
39
17
16
Total
198
207
206
177
179
108
133
117
106
95
16
9
13
9
6
124
142
130
115
101
107
82
85
81
84
39
41
36
26
27
146
123
121
107
111
190
182
171
170
173
41
43
36
27
27
231
225
207
197
200
51
48
37
29
29
19
15
14
11
11
70
63
51
40
50
12
12
8
2
5
38
33
30
29
29
50
45
38
31
34
62
59
45
31
44
41
34
30
29
30
103
93
75
60
74
29
23
26
28
32
17
17
14
12
11
46
40
40
40
43
0
0
0
1
0
15
10
8
13
6
15
10
8
14
6
29
23
26
29
32
21
17
17
17
12
50
40
43
46
44
0
0
0
0
0
9
0
0
0
0
9
0
0
0
0
0
0
0
0
0
17
12
14
17
12
17
12
14
17
12
0
0
0
0
0
18
12
14
17
12
18
12
14
17
12
160
173
167
154
122
38
32
24
18
18
198
205
191
172
140
129
108
115
126
117
62
59
51
40
39
191
167
166
166
156
213
201
205
202
207
63
60
51
40
40
276
261
256
242
247
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ALASKA MARINE CONSERVATION COUNCIL
APRIL 2002
Table 2.
Table 5.
Mean vessel length (feet) of factory
trawls in the Gulf of Alaska
Number of factory trawl vessels in
the Gulf of Alaska by year and
vessel size.
<124 124-164 165-234 235-259 >260
1996
1997
1998
1999
2000
109
103
114
113
111
147
149
155
155
152
204
207
213
207
205
241
238
238
238
238
271
303
287
295
<124 124-164 165-234 235-259 >260
1996
1997
1998
1999
2000
19
16
14
7
8
4
1
1
1
1
2
2
0
2
1
Number of factory trawl vessels
fishing in the Bering Sea/ Aleutian
Islands by year and vessel size
Mean vessel length (feet) factory
trawlers in the Bering Sea and
Aleutian Islands
<124 124-164 165-234 235-259 >260
111
107
115
114
116
149
149
152
152
152
204
206
211
207
204
241
241
241
245
245
302
302
302
306
308
Table 4.
<124 124-164 165-234 235-259 >260
1996
1997
1998
1999
2000
149
149
152
152
152
204
206
211
207
204
241
241
241
245
245
5
5
4
4
4
23
21
18
10
11
7
5
5
3
3
18
16
16
14
13
Number of factory trawl vessels
fishing for groundfish in All Alaska
by year and vessel size.
<124 124-164 165-234 235-259 >260
108
107
115
114
114
9
12
8
9
8
Table 7.
Mean vessel length (feet) of factory
trawlers in All Alaska
1996
1997
1998
1999
2000
4
3
2
2
4
Table 6.
Table 3.
1996
1997
1998
1999
2000
9
10
7
6
4
302
302
302
306
308
<124 124-164 165-234 235-259 >260
1996
1997
1998
1999
2000
10
13
8
9
9
5
5
4
4
4
23
21
18
10
11
7
5
5
3
3
Tables adapted from: NPFMC 2001b
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