1. Impacts of Fishing

2. 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

3. 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

4. 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

5. 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.

6. 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

10. World Landings
There are approximately million metric tons of marine animals removed from the sea (depending on China stats) (FAO 2002, Watson and Pauly 2001)
This will increase to million metric tons by 2010
Fisheries represent variable portions of national exports from <1% for Korea and Netherlands to 65% for Iceland

11. 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

12. 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

14. 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

17. Recovering Fisheries
From
Mace 2004

18. 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

19. Fishing Gear

20. 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

21. Incidental Take and Fishing Gear

22. Impacts by Gear Type

23. 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

24. Worldwide Declines of Large Predatory Fishes
From Myers and Worm 2003

25. Compensatory Responses
From Myers and
Worm 2003

26. Northwest Atlantic Longline Fisheries
From Baum et al. 2003

27. Declines in Northwest Atlantic Sharks
From Baum et al. 2003

28. 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

29. Fishing Down the Food Web
From Pauley et al. 1998

30. 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

31. 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

32. Ecosystem Changes
From Jackson et al. 2001

33. Ecosystem Changes
From Jackson et al. 2001

34. Ecosystem Changes
From Jackson et al. 2001

35. 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

36. 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

37. 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

38. 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)

39. 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