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.