PROBABLE IMPACT OF ENSO AND GLOBAL WARMING ON LAKE TANGANYIKA FISHERIES
CLIMFISH
Pierre-Denis Plisnier(1) , Jaya Naithani (2) , Eric Deleersnijder (2) , Luc André (1) , Jean-Pierre Descy (3) , Yves Cornet (4) & Christine Cocquyt (5) (1) Royal Museum for Central Africa (2) Institut d’Astronomie et de Géophysique, Université Catholique de Louvain (3) Laboratoire d’Ecologie des Eaux Douces, Facultés Universitaires Notre-Dame de la Paix (4) Unité de Géomatique, Université de Liège (5) Vakgroep Biologie, Universtiteit Gent, Belgium.
Pelagic fisheries at Lake Tanganyika 1. Introduction
Lates stappersi (Perch)
Different answers seem however presented by the fishes depending of the species.
Purse seine
Lift Net
2. ENSO and fish catches are correlated
3. Long term changes in fish catches
-1.5 -2.5
av-Anomaly CPUE month 6 to 9
SOI av.month 2+3
SOI av.month 3+4
24.0 23.5 23.0
28 20.5
27
Air T° North Tanganyika
1
av-Anomaly CPUE month 6 to 9
2.0 1969
1975
1980
0.0
Tons/boat
Wind speed
5
Meteorological conditions
20. 0 19.5
4 3 2 1 0 Feb
Jun
Aug
Oct
Dec
2002
Air T° South Tanganyika
19.0
Avr
During ENSO events, air T° increase and wind decrease although more local wind data are needed to confirm this.
Avr
Jun
Aug
Oct
Dec 2004
2003
Temperature
°C
27.3 27.0 26.7 26.4 26.1 25.8 25.5 25.2 24.9 24.6 24.3 24.0
25 Depth (m)
1989
1985
1981
1977
1973
1969
1965
1961
1957
1953
Thermic stratification
Feb
50 75
1976
1982-83
1986-87 1991
24.5 24.0 23.5 23.0 22.5 22.0
Bujumbura (1964-1992)
21.5 21.0
Air T°
Wind speed
1964
4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0
0.09 0.08 0.07 0.06 0.05 0.04 0.03 0.02 0.01 0.00
0
Nutrients
25 50 75 100
Phytoplankton biomass (µg/l Chl a)
2.5 2.0
Phytoplankton
0.0 25
6. Wind impact hypothesis The multidisciplinary CLIMFISH project is presently monitoring the lake conditions at two stations every 2 weeks. An hydrodynamic model is built. Windy years are linked to increased accumulation of epilimnion warm water in the North tied to important movement of water layers after monsoonal change (from South to North mainly) (Plisnier et al. 1999, Naithani et al. 2002, 2003).
Transparency
J /
R
0,0
0 1969
1975
1985
1990
19 94
t e mp e r a t u r e
( m / s ) s pe e d
b o a t
(m/s )
Air T° (°C)
1980
1 8. 4
Wind speed (m/s)
Large tropical lakes are sensitive to climate change because water column stability varies with small changes of temperature allowing mixing of deep nutrient-rich water toward the surface. Winds drive the hydrodynamics of the lake depending of those thermic stability conditions. Changes of climatic conditions can lead to variable primary production and fluctuating fish recruitment. In Lake Tanganyika fish catches of the main species appear correlated to ENSO.
200
Dec-03
kg/boat LS
Oct-03
Sep-03
Jul-03
May-03
Mar-03
Jan-03
Nov-02
Sep-02
Jul-02
May-02
Apr-02
CPU Clupeids
400
12-03
10-03
08-03
06-03
N
0
04-03
Wind w2
100
02-03
1600
12-02
1400
200
10-02
1200
02-02
Fish catches
800 1000
300
08-02
600
06-02
400
500 400
04-02
N
3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0
Feb-02
0
kg/boat Cl.
Upwelling
1
0,5
1 8. 8
5
w1>>w2
S0
1,0
The final objective of this project is to investigate fisheries forecasting usefull for sustainable development and fisheries optimalisation (management, gears, processing...).
15 10
200
Upwelling
Transparency (m)
20
0
2
R
1.0 0.5
S
1,5
1 9. 2
1.5
1992
Wind w1
3
2,0
The impact of increasing air and water temperature seem also linked to long term changes in fishes populations. Hypothesis linking climate anomalies to different hydrodynamic states of the lake are tested in the present CLIMFISH project.
mg/l
PO4-P
Depth (m)
1966 1969 1972
Wind (m/s)
Air temperature (°C)
100
1990
0 1994
4
7. Conclusion
6
0
25.0
1985
At the south of the lake,the increase of winds, from May to September every year, is tied to an upwelling clearly observed from temperature data allowing nutrient such as phosphorus to increase in surface waters. Plankton biomass increase attracts clupeid fishes while decrease of transparency probably explain the seasonal disapearance of Lates stapersii at the South of the Lake. CPU (catches per unit of L. Staperssi and Clupeid fish are thus inversely related to the limnological conditions. Climate-limnology partialy ENSO-related could explain the fluctuating catches. (Plisnier et al. 2005).
SST Niño 4(°C)
24.5
Air T° Mbala (°C)
Air T° Buj (°C)
29
4.0
5. Two yearly cycles in Lake Tanganyika
4. Climate conditions during ENSO An ENSO signal is well observed in the time series of climate data in East Africa. Annual average air temperature increases by up to 0.8°C during warm events) (Plisnier et al, 1998, 2000,). 30 SST Niño 4
-0.8
2
3,0
1 9. 6
a ir
-0.6
3
Wind speed
2,5
p e r
-0.4
-0.5
i
6.0
4
5
3,5
Av .
1993
1991
1989
1987
1985
1983
1981
1979
1977
1975
1971 1973
1969
1967
1965
-1.5
1963
-4.0
2 0. 0
W i n d
-1.0
0.5
Wind speed
T o n ns
-3.0
-0.2
Wind speed (m/s)
-0.5
1.5
1967 1969 1971 1973 1975 1977 1979 1981 1983 1985 1987 1989 1991 1993
-2.0
8.0
0.0
1963 1965
0.0
S.O.I.
-1.0
5
s pe e d
2.5
0.2
W in d
0.5
6
Air T°
Air T°
r =-0.60
b o a t
0.0
0.4
p e r
3.5
Lates stappersi 6
T o n n s
1.0
Lates stappersi
Anomaly CPUE (T./boat)
1.0
r = 0.58
Anomaly CPUE (T./boat)
1.5
4.5
2.0
S.O.I.
Stolothrissa tanganyikae
2.0
Stolothrissa tanganyikae
( °C )
Catches per unit of fishing effort (CPUE) of the sardine Stolothrissa tanganyikae decrease in the South while CPUE of Lates stappersi increase. Limnological changes such as decreased upwelling intensity and increased lake stability are possible causes. A warming trend of the air in this area (>0.9°C) and of the lake (>0.34°C in the upper 100 m) has been shown in the recent decades while average wind speed decrease (Plisnier, 1998, 2000; O’Reilly et al 2003). Response to those changes seem different depending of the species.
Catches per unit of fishing effort (CPUE) of the sardine Stolothrissa tanganyikae in the South are positively correlated to the SOI (Southern Oscillation Index) with a lag of 4 to 7 months while CPUE of Lates stappersi are negatively correlated to the SOI. Cold and windy years influence the catches of those species in a different way. Strong upwelling favour sardines fishes while decrease transparency seem not favourable for Lates stappersi catches, a probable visual predator. The same is observed in the North (not shown) (Plisnier, 1997, 1998).
3.0
Length: 670 km Width: 50-80 km Depth: 1470 m Age: 10 to 20 106 yrs Fish Prod: 200 000 T/yr
Limnothrissa miodon (clupeid)
Stolothrissa tanganicae (clupeid)
The fisheries catches of the main pelagic species at Lake Tanganyika (Lates stappersi and the clupeids: Limnothrissa miodon and Stolothrissa tanganicae) display an ENSO (El Niño /Southern Oscillation) type of variability as well as a trend that are correlated to changes in environmental conditions.
Lake Tanganyika
CPU L. Stapersii
600 800
Acknowledgments The Federal Sciences Policy (Belgium) and the Belgian Development Cooperation are funding the CLIMFISH project. Thanks are adressed also to the Department of Fisheries (Zambia), the Tanzania Fisheries Research Institute and the Nyanza Project for their collaboration.
1000 1200 1400
BIBLIOGRAPHY
1600
Epilimnion
Metalimnion+ Hypolimnion
Southern upwelling
SE Wind
Seasonal upwelling is monsoon driven. It occurs at the southern end every year between June and August.
Naithani, J., Deleersnijder, E., Plisnier, P. D. 2002. Origin of intraseasonal variability in Lake Tanganyika. Geophysical Research Letters, 29(23). Naithani J., E. Deleersnijder, P.-D. Plisnier, 2003. Analysis of wind-induced thermocline oscillations of Lake Tanganyika. Environmental Fluid Mechanics, 3, 23-39. O'Reilly C.M., S.R. Alin, P.-D. Plisnier, A.S. Cohen and B.A. McKee, 2003. Climate change decreases aquatic ecosystem productivity of Lake Tanganyika, Africa. Nature, 424, 766-768. Plisnier, P.-D., 1997, Climate, limnology, and fisheries changes of Lake Tanganyika. FAO/FINNIDA Research for the Management of Fisheries on Lake Tanganyika.GCP/RAF/271/FIN-TD/72 (En) 38p. Plisnier P.-D. 1998. Lake Tanganyika: Recent climate changes and teleconnections with ENSO. Proceedings of the International Conference "Tropical Climatology, Meteorology and Hydrology" (Brussels, 22-24 May 1996). G. Demarée, J. Alexandre and M. De Dapper (ed.): R. Acad. of Overseas Sc. & R. Met. Inst. of Belgium: 228-250 Plisnier, P.-D. 2000. Recent climate and limnology changes in Lake Tanganyika. Verh. Internat. Verein. Limnol. 27: 2670-2673. Plisnier P.D. & J.P. Descy (Eds) 2005. Climlake : Climate variability as recorded in Lake Tanganyika. Final Report (2001-2005). FSPO - Global change, ecosystems and biodiversity: 105p.
Information: Pierre-Denis Plisnier Royal Museum for Central Africa Leuvensesteenweg,13 B-3080 Tervuren, Belgium
[email protected]
