Population Changes of Sardines in Northern Lake Tanganyika

Abstract

Population changes of sardines, so called ndakala, in northern Lake Tnaganyika and some possible causes of their decrease in catch in the 1980s were studied using the catch statistics of Zaire and Burundi from 1974 to 1987. Neither immigration nor emigration could be proved in the northern part of the lake. Yearly changes in catch and catch per unit effort suggested that the ndakala population size had mildly but steadily decreased since 1980 and the size in 1987 was about a half of that in 1980. Density dependence was detected and the mild population fluctuation might be related to recruitment compensation among the multiple cohorts born in the same year. A stable level of fishing effort in Burundi in the 1980s supported a view that the decrease in catch was caused by the population decrease. A hypothesis that an increase in predation pressure had caused the decrease was rejected. A preliminary analysis using simplified population dynamics models implied that the population had not been overfished, though consideration of overfishing will be still indispensable.