Population changes within a group of fish species in Lake Tanganyika following their exploitation

Abstract

Pelagic fish populations in Lake Tanganyika consist mainly of two small clupeid species and four centroponiid species which prey on them. Exploitation with purse‐seines began in the southeast arm in 1962 and catches were sampled up to early 1969. Inshore clupeid populations were also sampled with a scoop‐net (lusenga of the kind used in the traditional inshore fishery.Clupeid biomass reached annual maxima around September. One clupeid species, Stolothrissa tanganicae Regan, dominated pelagic catches except in 2 years when about equal quantities of the other clupeid, Limnothrissa miodon Blgr. were caught. Numbers of each predator (three species of Lates and Luciolates stappersii Blgr.) decreased from 1963 to 1966 and remained low thereafter. The clupeid catch rose from 1964 to 1967 and remained high in 1968. The average nightly catch weight per year of all species together altered relatively little. Changes in population size distributions occurred.Sampling methods were concluded to be valid for clupeid populations, but probably indi‐cated only general changes in predator populations. Clupeid life‐cycles are mostly accomplished within a year, and appear closely related to the periodic and spatial variations in plankton production. The two species are competitive and can replace one another in the pelagic zone. Certain distribution patterns of the clupeids and of the predator young are believed to be adaptations to severe predation. Low replacement rates under fishing pressure account for the decline of predator species, and the clupeid increase resulted from reduction in predation. A fairly stable exploited phase has apparently been reached in which clupeid biomass is much greater and predator biomass much less than in the natural state. The data encourage certain predictions. Similar major trends occurred in the much larger fishery in the Burundi sector of the lake.