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The Effects of Fishing on Non-target species and Ecosystem Structure and Function

The Effects of Fishing on Non-target species and Ecosystem Structure and Function. Henrik Gislason University of Copenhagen, Denmark. Has fishing affected the structure of marine ecosystems on a global scale ?.

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The Effects of Fishing on Non-target species and Ecosystem Structure and Function

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  1. The Effects of Fishing on Non-target species and Ecosystem Structure and Function Henrik Gislason University of Copenhagen, Denmark

  2. Has fishing affected the structure of marine ecosystems on a global scale ? • On the continental shelf between 25 and 35% of the energy fixed by the primary producers is needed to sustain the current fisheries • The composition of the fish fauna has changed from large fish-eating fishes towards smaller plankton-eating fishes Pauly & Christensen(1995), Pauly et al.(1998)

  3. How can fishing change the structure of marine ecosystems ? • Changes in species interactions • Changes in habitat • Provision of discards to scavengers (e.g. seabirds) • Reduction of the abundance of slow growing, late maturing species (e.g. sharks and rays)

  4. Examples Barents Sea North Sea Caribbean North western shelf

  5. Harp seal Cod Herring Capelin Plankton Changes in species interactions A simplified Barents Sea food web

  6. Barents Sea ICES(2000) and Gjøsæter & Bogstad(1998)

  7. Barents Sea Capelin biomass Weight at age of a 6 year old cod Cod Biomass ICES (2000)

  8. The harp seal invasions Haug & Nilssen(1995), Haug et al.(1998)

  9. The harp seal invasions No of harp seals caught Capelin biomass Biomass of young herring ICES(2000), Gjøsæter & Bogstad(1998), Haug & Nilssen(1991), Nilssen et al. 1998)

  10. Habitat changeNorth western Australia • When trawling began the species composition changed from high value to low value species • To resolve why this happened a part of the area was closed to trawling area remaining open to trawling area closed to trawling 1987 area closed to trawling 1985 Sainsbury et al(1997)

  11. North western Australia Untrawled area Heavily trawled area Photos: K. Sainsbury

  12. Habitat change Coral reefs in the Caribbean In the 1970’s sea urchins controlled algal biomass After a disease hit the sea urchins in 1983 the algae increased and overgrew the corals Photos TP Hughes; Source: Reefbase

  13. Coral reefs in the Caribbean • Prior to the 1980’s fishing removed the competitors and predators of the sea urchins. • In 1983 a disease killed the sea urchins. • In the absence of herbivorous fish the algae overgrew the corals. • Fishing, hurricanes, eutrophication and coral bleaching have all been implied in the reduction in coral cover. Change in coral cover on Jamaican reefs (Hughes, 1994)

  14. Provision of discardsScavenging seabirds in the North Sea Furness(1992)

  15. Summary The ecosystems differ in their response to fishing. The most easily affected systems seem to be: • Systems where a major part of the energy has to pass a few species at an intermediate level in the food web. • Systems where biota providing a strutural habitat such as corals or macroalgae is affected directly or indirectly.

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