Transcript From Jackson et al. 2001

Impacts of Fishing
Historical Impacts of Harvesting
• Overfishing and overharvesting is not a new
phenomenon
• In the modern era, our rate of fishing and
harvesting is much greater than historical levels
• However, even low levels fishing can have
significant impacts over long periods of time
• Fishing and harvesting by aboriginal people
hundreds or thousands of years ago affected
marine systems
Historical Impacts of Harvesting
• Historical records, paleontological and
archaelogical records can be used to assess
past
• Stellar’s Sea Cows were distributed throughout
the northern Pacific Rim through the late
Pleistocene
• Aboriginal hunting reduced numbers, they only
survived in areas of Aleutians without humans
(Jackson et al. 2001)
• When Europeans arrived in 1741, sea cows
restricted to Commander Islands in western
Aleutians, died 27 years later
Historical Impacts of Harvesting
• Aleutian peoples also harvested sea otters and
had significant impacts 2500 y.a.
• Without otters eating urchins, the urchins
responded by getting larger, more numerous
• Then Europeans hunted otters to near extinction
and kelp forests, which are controlled by urchins,
crashed
• This was helped along but harvesting of spiny
lobsters, sheepshead and other urchin predators
Historical Impacts of Harvesting
• Historical impacts include coral reef areas
• Large reductions of most fishes had already
occurred before the 20th century (Jackson et al.
2001)
• Species like cucumbers were harvested in
Australia in massive numbers in 18th and 19th
century
• Pearl oysters, dugongs, turtles have all been
harvested prior to this century
• Overharvesting of fishes have contributed to
declines in coral reefs due to algal overgrowth,
crown-of-thorns outbreaks, etc.
Historical Impacts of Harvesting
• Influence of European fishing predated
Columbus with Basque fisherman
harvesting cod on George’s Bank
• Founding of Newfoundland and much of
the impetus for finding the New World was
based on finding new fishing grounds
World Landings
• There are approximately 70-80 million metric
tons of marine animals removed from the sea
(depending on China stats) (FAO 2002, Watson
and Pauly 2001)
• This will increase to 80-105 million metric tons
by 2010
• Fisheries represent variable portions of national
exports from <1% for Korea and Netherlands to
65% for Iceland
World Fishery Condition
• The world’s capture fisheries are in
various conditions:
– 10% are depleted
– 18% are overexploited
– 47% are fully exploited
– 25% are underexploited
State of the World’s Fisheries
• Trends are difficult to assess
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Trends in catch
Stock by stock assessment
Trends in trophic level
Trends in catch for individual stocks
• All methods have limitations
• Trends in total yield suggest stability, but these
may not represent depletion of individual stocks
• Stability may persist until the last stocks are
depleted
State of the World’s Fisheries
• Stock by stock assessment
– 33% of US stocks are overfished or depleted
(NMFS 2002) yet many are still producing
large yields
• Of the 495 largest fisheries in the world,
many are near their peak production
Recovering Fisheries
From
Mace 2004
Incidental Catch
• Some species are threatened by incidental
catch
• 90% of white marlin mortality, a species
petitioned for federal protection occurs via
incidental catch on tuna and swordfish
longlines
• Marine birds including several species of
albatrosses are killed on long lines
Fishing Gear
Trawling Impacts
• Trawling in Australia resulted density and
biomass of soft-bodied immobile fauna by 80%
(Koslow et al. 2001)
• In the Grand Banks (eastern North America)
trawling has had big impacts on crabs and sea
urchins
• 26% of the worlds fishery catch is bycatch and
discarded (Alverson et al. 1994)
• Shrimp and prawn trawl fisheries catch 5.2 kg of
bycatch for every kg of landed catch
Incidental Take and Fishing Gear
Impacts by Gear Type
Impacts on Top Predators
• Fishing impacts in offshore ocean habitats
have disproportionately affected higher
trophic levels
• Populations of top predators like billfishes
(tuna, marlin, sailfish, swordfish) and
sharks have been depleted much more
than lower trophic levels
• Many implications of this including
increases (compensatory) in fishes at
lower trophic levels
Worldwide Declines of
Large Predatory Fishes
From Myers and Worm 2003
Compensatory Responses
From Myers and
Worm 2003
Northwest Atlantic Longline Fisheries
From Baum et al. 2003
Declines in Northwest Atlantic Sharks
From Baum et al. 2003
Fishing Down the Food Web
• Studies have shown that the mean trophic
position of fishes has declined (Pauley et al.
1998)
• Shifted from larger piscivorous fishes to smaller
fishes and invertebrates
• This was suggested to preceed system collapse
• Areas like the Mediterranean which have been
fished down for years still has high productivity
• Suggestion is that fishing lower trophic levels
may be maintained at high levels without system
collapse
Fishing Down the Food Web
From Pauley et al. 1998
Other Impacts of Overharvesting
• Overfishing of sea grass consumers like
dugongs and manatees has altered seagrass
systems
• Moderate herds of dugongs could remove 96%
of above ground biomass of seagrass areas
(Preen 1995)
• Without manatee and dugong grazing, seagrass
detritus builds up in many areas and anoxic
events are common in places like Moreton Bay
Australia and Florida Bay
Other Impacts of Overharvesting
• Loss of oyster reefs in eastern U.S. and
elsewhere is a good example of fishery impacts
on entire system
• Oysters used to filter the entire volume of
Chesapeake Bay every three days
• Overfishing coupled with disease resulted in
oyster collapse in the 1960s
• Since then there have large changes in turbidity,
anoxia and hypoxia, complete loss of
seagrasses, and shift from benthic to planktonic
primary production and eutrophication
Ecosystem Changes
From Jackson et al. 2001
Ecosystem Changes
From Jackson et al. 2001
Ecosystem Changes
From Jackson et al. 2001
Trophic Cascades
• Populations of seals, sea lions and sea
otters have collapsed in the North Pacific
Ocean and the Bering Sea
• Initially (in 1990s) nutritional limitations
and overhavesting of fin fish or changes in
oceanographic processes implicated
• Declines in pollock and other ocean fishes
were initially implicated
Trophic Cascades
• Recent evidence has clearly shown that
predation by killer whales (Orca) is the
cause of these declines (Estes et al. 1998)
• Most likely responsible for declines in
Stellar’s sea lions, Northern fur seals and
habor seals over past decades
• Clearly responsible for recent declines of
sea otters over in last decade
Great Whale Fishery
Northern Fur Seal
Orca
Gray Whale
Harbor Seal
Humpback Whale
Sea Otter
Stellar’s Sea Lion
Kelp Forest
Urchin Barren
Sea Urchin
Trophic Cascades
• What caused shift in Orca’s diet?
• Increased whaling following WWII drastically
reduced large whale populations
• This reduction in whales resulted in a shift in the
diets of Orcas over long periods of time Fewer
than 40 Orcas could produce the declines of
Stellar sea lions in the Aleutian Islands
• A pod of 5 individuals could reduce and maintain
low abundances of sea otters in Aleutians
• Whaling decades ago can have current
ecosystem wide affects on mammals (Spring et
al. 2003)
Trophic Cascades
• Impacts of sea otters on kelp forests have been
demostrated for several decades
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Otters eat sea urchins
Urchins eat kelp
Decline of otters resulted in abundant urchins
Kelp forests turned into urchin barrens
• Return of otters and fishing of urchins had both helped
kelp forests recover in recent decades
• Now decline of otters is shifting systems back to urchin
dominated barrens
• So whaling decades ago is affecting present day kelp
forests hundreds of miles away