Length at maturity and gillnet selectivity of Lake Tana's Barbus species (Ethiopia): Implications for management and conservation

Abstract

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.