Vulnerability to a small-scale commercial fishery of Lake Tana's (Ethiopia) endemic Labeobarbus compared with African catfish and Nile tilapia: An example of recruitment-overfishing?
Graaf, M. de; Zwieten, P.A.M. van; Machiels, M.A.M. ; Lemma, E. ; Wudneh, T. ; Dejen, E. ; Sibbing, F.A. - \ 2006
Fisheries Research 82 (2006)1-3. - ISSN 0165-7836 - p. 304 - 318.
oreochromis-niloticus - species flock - reproductive segregation - perch upsurge - barbs barbus - cyprinidae - victoria - impact
In 1986 a motorised, commercial gillnet fishery was introduced in Lake Tana, Ethiopia's largest lake (3050 km2) in addition to the artisanal, predominantly subsistence fishery conducted from reedboats. The three main species groups targeted by this fishery are a species flock of endemic, large Labeobarbus spp., African catfish (Clarias gariepinus) and Nile tilapia (Oreochromis niloticus). The commercial gillnet fisheries was monitored during 1991-1993 and in 2001 (CPUE, effort allocation, catch composition, yield) and the development of the stocks of the three species groups was determined using an experimental trawl program during 1991-1993 and 1999-2001. In 1991-1993 the vast majority of fishing activities took place in the southern Bahar Dar Gulf (71%). Each species group contributed roughly one third to the total catch. In 2001, 41% of the effort was allocated to the north-eastern shores of Lake Tana. Both the CPUE and the contribution of O. niloticus to the total catch had doubled. However, the proportion of large specimen (>50 cm TL C. gariepinus; >20 cm FL O. niloticus) in the trawl surveys decreased significantly. A three-fold decline in abundance of the anadromous Labeobarbus species occurred. The 15 large labeobarb species are long-lived, ecologically specialised endemics. The seven riverine spawning Labeobarbus species form aggregations in the river mouths in August-September, during which period they are targeted by the commercial gillnet fishery. A sharp decrease in abundance by ca. 75% of the migratory riverine spawning Labeobarbus species in the sublittoral and pelagic zones of the lake, areas where no fishing takes place and the collapse of juvenile Labeobarbus (between 5 and 18 cm FL: by 90%) during the 1990s suggest recruitment-overfishing. To prevent the possible extinction of the unique Labeobarbus species flock all fishing effort should be severely restricted near the river mouths and on the upstream spawning areas during August-September (peak breeding period) to protect the vulnerable spawning aggregations