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Preparing for ecosystem-based fisheries management: A Namibian case study

Preparing for ecosystem-based fisheries management: A Namibian case study. Nico E. Willemse. International Symposium “Marine fisheries, ecosystems, and societies in West Africa: half a century of change.” Dakar, Senegal, 24-28 June 2002. . Introduction: Namibian Fisheries.

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Preparing for ecosystem-based fisheries management: A Namibian case study

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  1. Preparing for ecosystem-based fisheries management: A Namibian case study Nico E. Willemse International Symposium “Marine fisheries, ecosystems, and societies in West Africa: half a century of change.” Dakar, Senegal, 24-28 June 2002.

  2. Introduction: Namibian Fisheries • Location: West coast of Southern Africa, bathed by the Benguela upwelling, which supports rich fishery resources; • Fisheries heavily exploited prior to Independence in 1990; • No effective management in place before 1990; • >300 distant water fleets (DWFs) roamed offshore from 1960s to 1980s.

  3. Data and Methods 1 • Landings: not recorded continuously – there was no effective monitoring agency; • Hence landing time series had to be reconstructed; this was done for 1950-2000; • Mean trophic level of landings were then estimated: • First, a TL estimate was obtained (mainly from FishBase (www.fishbase.org) for each species; • Then mean TL values we obtained by weighing species’ TL by their landings.

  4. Data and Methods 2 • P/Z ratio of landings was obtained by: • Identifying species as either piscivorous (P) or zooplanktivorous (Z); • Deriving annual P/Z ratio by weighing each species by its landings. • Plot mean TL vs landings, and mean P/Z vs. landings over 50 year period; • Interpret trends based on biology of species, and behaviour of the DWFs.

  5. Results 1: 1950-2000 • Three developmental stages observed: • Undeveloped (1950-64): not much change in indicators; • Developing/ mature (1965-69): rapidly increasing indicators; • Senescent (1970-2000): declining trends of indicators. • Post Independence: Most stocks are depressed: shadow of past overfishing and unfavourable environmental conditions.

  6. Results 2: 1970-2000 • Catch composition analysis: shift from hakes (TL=4.5) to horse mackerel (TL=3.5); • Landings, mean TL and P/Z ratio of landings declined, suggesting ’fishing down marine food webs’; • Backward-bending plot of mean TL vs. landings plot implies: • ’Fishing down’ does not lead to catch increases; • Hence the bottom-up tranfer of biolgical is impared by ecosystem impact of fishing.

  7. Trends 1: Total landings Senescent Undeveloped Developing/ mature

  8. Trends 2: Mean TL • Mean TL declined from 4.0 between 1970-74 to 3.7 between 1990-94.

  9. Trends 3: Mean TL vs P/Z Ratio • Similar trends observed with difference in amplitude. • Short-term fluctucations in P/Z ratio – periodic Benguela Niño.

  10. Trends 4: Mean TL vs. Landings

  11. Ecosystem Impact 1 • Impaired bottom-up production transfer. • Sardine collapse: • Replacement by other small pelagics; • Plankton community change; • Reduced competition for phytoplankton; • Food availability changed, which affected the piscivores; • ’wasp-waist’ control impaired.

  12. Ecosystem Impact 2 • Decline of hake: • increase in other pelagic predators; • Increase in adult horse mackerel – opportunistic feeder. • 16% Decline in ecosystem biomass in 1990.

  13. Fishery Resources - 1990s • Cape hake, horse mackerel and sardine: • Increased after 1990; decreased thereafter; • Similar for most species caught off Namibia. • Current state of stock: severely depressed, due mostly to the Benguela Niñoevent of 1994/5.

  14. Fisheries Management - 1990s • National issues: • Stock rebuilding policy; • 90% reduction of DWFs; • Biological vs. political imperatives; • Benguela Niño impacts. • Regional issues (Namibia, South Africa and Angola): • Management of shared and migratory stocks; • Improved research capacity; • Ecosystem modelling and transition to ecosystem-based management.

  15. Preparing for ecosystem-based management: • Reconstructed time series of landings for 50 years, i.e., a long period, to allow for contrasts; • Interpreting the resulting trends in terms of fisheries impact on ecosystem, mediated by biological interactions, and environmental change, and thus: • Considering more than one species, one fleet at a time; • Which his should contribute to better management, and more long-term benefits gained from the resource.

  16. Acknowledgements • Thanks are due to my professors who helped me extensively in finishing my thesis in time: Prof. Jorge Santos of the University of Tromso and Prof. Daniel Pauly of the University of British Columbia; • The Sea Around Us project, which funded my participation in this symposium, is a UBC Fisheries Centre partnership with the Pew Charitable Trusts of Philadelphia, USA.

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