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

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.

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.

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.

Results 1: 1950-2000

Three developmental stages observed:

1. Undeveloped (1950-64): not much change in indicators;

2. Developing/ mature (1965-69): rapidly increasing indicators;

3. Senescent (1970-2000): declining trends of indicators.

Post Independence: Most stocks are depressed: shadow of past overfishing and unfavourable environmental conditions.

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:

1. ’Fishing down’ does not lead to catch increases;

2. Hence the bottom-up tranfer of biolgical is impared by ecosystem impact of fishing.

Trends 1: Total landings

R2 = 0.7324

R2 = 0.7858

R2 = 0.8285

0

500

1000

1500

2000

2500

1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000

Year

La

nd

ing

s (

x 0

00

't)

Undeveloped

Developing/ mature

Senescent

Trends 2: Mean TL

Mean TL declined from 4.0 between 1970-74 to 3.7 between 1990-94.

R2 = 0.1693

3.5

3.6

3.7

3.8

3.9

4.0

4.1

4.2

1970 1975 1980 1985 1990 1995 2000

Year

Me

an

TL

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.

0.0

0.3

0.5

0.8

1.0

1970 1973 1976 1979 1982 1985 1988 1991 1994 1997 2000

Year

P/Z

Rat

io

3.2

3.4

3.6

3.8

4

4.2

Mea

n TL

PS/ ZP Ratio Mean TL

Trends 4: Mean TL vs. Landings

3.3

3.4

3.5

3.6

3.7

3.8

3.9

4.0

4.1

4.2

10.5 11.5 12.5 13.5 14.5

Log Landings

Mean

TL

1950

19702000

1960

1980

1990

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.

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.

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 Niño event of 1994/5.

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.

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.

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.