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– 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

From Myers and Worm 2003

Worldwide Declines of Large Predatory Fishes

From Myers and Worm 2003

Compensatory Responses

From Baum et al. 2003

Northwest Atlantic Longline Fisheries

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

Humpback Whale

Gray Whale

Northern Fur Seal

Harbor Seal

Stellar’s Sea Lion

Orca

Sea Otter

Sea Urchin

Kelp Forest Urchin Barren

Great Whale Fishery

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– 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