Lake Tana's 15 large Barbus species form the only known intact endemic cyprinid species flock left in the world. The barbs contribute around one third of the total annual catch of the motorised commercial gillnet fishery which was introduced in 1986. A dramatic reduction of the adult Barbus stocks and the even lower proportion of recruits at the end of the 1990s, show the necessity for the development, implementation and control of fisheries legislation in Lake Tana. The reproductive biology of the Barbus species, essential for fishery management, is poorly known. This paper presents results on size at maturity, size at harvest and gillnet selectivity curves, which can be used to provide a scientific base for management proposals. Size at maturity varied widely among the Barbus species, ranging from 18.8 cm in Barbus brevicephalus to 44.3 cm in Barbus crassibarbis. Males matured at smaller size and reached smaller maximum length than females. Estimated selectivity curves fitted closely or were slightly larger than the observed length-frequency distribution of the commercial catch. The vast majority (85%) of barbs landed by the commercial gillnet fishery were mature. Fishing pressure on juvenile, immature fish is unlikely to be the cause of the observed decrease in Barbus stocks. Size control regulations like mesh size restrictions, intended to protect the immature part of fish populations are expected to have little positive effects on the Barbus stocks and are therefore not recommended. The drastic reduction in barbs during the 1990s is most like due to recruitment overfishing, that is, poorly regulated high fishing effort by the commercial gillnet fishery on the spawning aggregations of adult barbs during their annual breeding migration in river mouths and surrounding floodplains. Only effort control regulations limiting the gillnet fishery in the spawning season and/or areas will prevent a total collapse of the Barbus stocks as has happened to other cyprinids in African lakes. Such measures have to be implemented urgently to guarantee the conservation of Lake Tana's unique biodiversity as a sustainable source of cheap protein and as a natural laboratory to study the evolutionary processes underlying speciation in freshwater fish.

The text of this article is only available as a PDF.

References

Armstrong, D. W., Ferro, R. S. T., MacLennan, D. N. and Reeves, S. A.
1990
.
Gear selectivity and the conservation of fish
.
J. Fish. Biol.
,
37
(suppl. A)
:
261
262
.
Banister, K. E.
1973
.
A revision of the large Barbus (Pisces: Cyprinidae) of East and Central Africa. Studies on African Cyprinidae, part 2
.
Bull. Brit. Mus. Nat. Hist. (Zool.)
,
26
:
167
180
.
Bini, G.
1940
.
I pesci del Lago Tana. Missione di Studio al Lago Tana richerge limnologiche
.
B. Chimica e Biologia, Reale Accademia d'ltalia
,
3
:
137
179
.
Boulenger, G. A.
1911
.
Freshwater fishes of Africa
,
Vol. II
,
1
512
.
London, , UK
:
British Museum (Natural History)
.
Chen, Y. and Mello, L. G. S.
1999
.
Growth and maturation of cod (Gadus morkua) of different year classes in the Northwest Atlantic
.
NAFO subdivision 3PS. Fish. Res.
,
42
:
87
101
.
Conover, D. O. and Munch, S. B.
2002
.
Sustaining fisheries yields over evolutionary time scales
.
Science
,
297
:
94
96
.
Craig, F. J.
1992
.
Human-induced changes in the composition of fish communities in the African Great Lakes
.
Rev. Fish. Biol. Fisher.
,
2
:
93
124
.
Crawford, S. S. and Balon, E. K.
1994
.
Alternative life histories of the genus Luciano: 3. An ecomorphological explanation of altricial (L. parva) and prococial (L. goodei) species
.
Env. Biol. Fish.
,
41
:
369
402
.
De Graaf, M.
2003
. “
Lake Tana's piscivorous Barbus (Cyprinidae, Ethiopia)
”. In
Ecology, Evolution and Exploitation
,
249
Wageningen, , The Netherlands
:
Wageningen University
.
Ph.D. thesis
De Graaf, M., Machiels, M. A. M., Wudneh, T. and Sibbing, F. A. in press.
Declining stocks of Lake Tana's endemic Barbus species flock (Pisces; Cyprinidae): natural variation or human impact?
.
Biol. Conserv
,
De Silva, S. S., Schut, J. and Kortmulder, K.
1985
.
Reproductive biology of six Barbus species indigenous to Sri Lanka
.
Env. Biol. Fish.
,
12
:
201
218
.
Dgebuadze, Y. Y., Mina, M. V., Alekseyev, S. S. and Golubtsov, A. S.
1999
.
Observations on reproduction of Lake Tana barbs
.
J. Fish Biol.
,
54
:
417
423
.
Diamond, S. L., Crowder, L. B. and Cowell, L. G.
1999
.
Catch and bycatch: The qualitative effects of fisheries on population vital rates of Atlantic croaker
.
T. Am. Fish. Soc.
,
128
(
6
):
1085
1105
.
Gabriel, W. L., Sissenwine, M. P. and Overholtz, W. J.
1989
.
Analysis of spawning stock biomass per recruit: an example for Georges Bank haddock
.
N. Am. J. Fish. Man.
,
9
:
383
391
.
Kawamura, G.
1972
.
Gillnet mesh selectivity curve developed from length-girth relationship
.
Bull. Jpn. Soc. Sci. Fish.
,
38
(
10
):
1119
1127
.
LFDP
.
1997
.
Fisheries Statistical Bulletin Number 5 Lake Tana Fisheries Development Project Working Paper no 25
,
Addis Ababa
:
Ministry of Agriculture
.
Loreau, M., Naeem, S., Inchausti, P., Bengtsson, J., Grime, J. P., Hector, A., Hooper, D. U., Huston, M. A., Raffaelli, D., Schmid, B., Tilman, D. and Wardle, D.
2001
.
Biodiversity and ecosystem functioning: current knowledge and future challenges
.
Science
,
294
:
804
808
.
Mina, M., Mironovsky, A. N. and Dgebuadze, Y. Y.
1996
.
Lake Tana barbs: phenetics, growth and diversification
.
J. Fish Biol.
,
48
:
383
404
.
Nagelkerke, L. A. J. and Sibbing, F. A.
1996
.
Reproductive segregation among the large barbs (Barbus intermedius complex) of Lake Tana, Ethiopia. An example of intralacustrine speciation?
.
J. Fish. Biol.
,
49
:
1244
1266
.
Nagelkerke, L. A. J. and Sibbing, F. A.
1997
.
The barbs of Lake Tana, Ethiopia: morphological diversity and its implications for taxonomy, trophic resource partitioning and fisheries
,
105
170
.
The Netherlands
:
Wageningen Agricultural University
.
Ph.D. thesis
Nagelkerke, L. A. J. and Sibbing, F. A.
2000
.
The large barbs (Barbus spp., Cyprinidae, Teleostei) of Lake Tana (Ethiopia), with a description of a new species, Barbus osseensis
.
Neth. J. Zool.
,
50
:
179
214
.
Nagelkerke, L. A. J., Sibbing, F. A., van den Boogaart, J. G. M., Lammens, E. H. R. R. and Osse, J. W. M.
1994
.
The barbs (Barbus spp.) of Lake Tana: a forgotten species flock?
.
Environ. Biol. Fish.
,
39
:
1
22
.
Ochumba, P. B. O. and Manyala, J. O.
1992
.
Distribution of fishes along the Sondu-Miriu River of Lake Victoria, Kenya with special reference to upstream migration, biology and yield
.
Aquacult. Fish. Manage.
,
23
:
701
719
.
Ogutu-Ohwayo, R.
1990
.
The decline of the native fishes of Lakes Victoria and Kyoga (East Africa) and the impact of introduced species, especially the Nile perch, Lates niloticus and the Nile tilapia, Oreochmmis niloticus
.
Env. Biol. Fish.
,
27
:
81
96
.
Pet, J. S., Pet-Soede, C. and van Densen, W. L. T.
1995
.
Comparison of methods for the estimation of gillnet selectivity to tilapia, cyprinids and other fish species in a Sri Lankan reservoir
.
Fish. Res.
,
24
:
141
164
.
Pet, J. S., Gevers, G. J. M., van Densen, W. L. T. and Vijverberg, J.
1996
.
Management options for a more complete utilization of the biological fish production in Sri Lankan reservoirs
.
Ecol. Fresh. Fish
,
5
:
1
14
.
Rochet, M. J.
1998
.
Short-term effects of fishing on life history traits of fishes
.
ICES J. Mar. Sci.
,
55
:
371
391
.
SAS Institute Inc.
1990
.
SAS Technical report P-243, SAS/STAT Software: The GENMOD procedure Release 6.09
,
Gary, NC
:
SAS Institute Inc.
.
Sechin, Yu. T.
1969
.
A mathematical model for the selectivity curve of a gillnet
.
Rbynoje Khozdajstwo
,
45
:
56
58
.
Sibbing, F. A. and Nagelkerke, L. A. J.
2001
.
Resource partitioning by Lake Tana barbs predicted from fish morphometrics and prey characteristics
.
Rev. Fish Biol. Fish.
,
10
:
393
437
.
Skelton, P. H., Tweddle, D. and Jackson, P.
1991
. “
Cyprinids of Africa.
”. In
Cyprinid Fishes, Systematics, Biology and Exploitation
, Edited by: Winfield, I. J. and Nelson, J. S.
211
233
.
London
:
Chapman & Hall
.
Sokal, R.R. and Rohlf, F.J.
1995
.
Biometry: The Principles and Practise of Statistics in Biological Research
, 3rd ed.,
New York
:
W.H. Freeman and Company
.
Taylor, E. B.
1999
.
Species pairs of north temperate freshwater fishes: Evolution, taxonomy and conservation
.
Rev. Fish Biol. Fisher.
,
9
:
299
324
.
ter Braak, C. J. F. and Looman, C. W. N.
1995
. “
Regression.
”. In
Data Analysis in Community and Landscape Ecology
, Edited by: Jongman, R. H. G., ter Braak, C. J. F. and Van Tongeren, O. F. R.
29
77
.
Cambridge, , UK
:
Cambridge University Press
.
Tilman, D., Reich, P. B., Knops, J., Wedin, D., Mielke, T. and Lehman, C.
2001
.
Diversity and productivity in a long-term grassland experiment
.
Science
,
294
:
843
845
.
Tomasson, T., Cambray, J. A. and Jackson, P. B. N.
1984
.
Reproductive biology of four large riverine fishes (Cyprinidae) in a man-made lake, Orange River South Africa
.
Hydrobiologia
,
112
:
179
195
.
Wudneh, T.
1998
. “
Biology and management offish stocks in Bahir Dar Gulf, Lake Tana, Ethiopia
”.
Wageningen, , The Netherlands
:
Wageningen Agricultural University
.
Ph.D. thesis