Spatial and seasonal variation in reproductive indices of the clupeids Limnothrissa miodon and Stolothrissa tanganicae in the Congolese waters of northern Lake Tanganyika

Joost A.M Raeymaekers,Pierre-Denis Plisnier,Béatrice Marwa,Jouko Sarvala,Jean-Claude Micha,Mulimbwa N'Sibula,Leona J.M Milec

doi:10.26496/bjz.2022.96

Abstract

Knowledge on the reproductive biology of the endemic clupeids Limnothrissa miodon and Stolothrissa tanganicae, two main target species of the pelagic fisheries of Lake Tanganyika, is constrained by fragmented monitoring activities. Here, we investigate the nursing areas of L. miodon, the timing of reproductive activities of littoral and pelagic L. miodon, and the timing of reproductive activities of pelagic S. tanganicae in the Congolese waters of the northern end of Lake Tanganyika (Bujumbura sub-basin). Nursing areas were determined year-round (2009–2010) based on the presence of clupeid larvae at two sandy and two stony beaches. The gonadosomatic index (GSI) and the proportion of fish having ripe gonads were used to study variation in reproductive indices in space (littoral vs. pelagic zones) during one year (2013–2014), as well as in time (dry vs. rainy season) during three years (2013–2016). Larvae of L. miodon were more frequently encountered on sandy than on stony beaches. Mature L. miodon females were more abundant in the littoral than in the pelagic zone, while the proportion of mature males in both habitats was similar. Irregular, low amplitude peaks could be distinguished in the GSI and proportion of mature males and females, but averages only differed between the dry and the rainy season in males. In contrast, GSI and proportions of mature males and females in S. tanganicae were higher in the dry season than in the rainy season. The reproductive effort of males and females of S. tanganicae and littoral L. miodon, but not pelagic L. miodon, was strongly synchronized. Interestingly, reproductive investment was also synchronised between pelagic male L. miodon, and pelagic S. tanganicae. Our time series strongly supports the view that L. miodon reproduces year-round in the littoral zone, while reproduction in S. tanganicae is seasonal. For fisheries management, we recommend year-round protection of sandy beaches, which are the main breeding grounds for L. miodon.

Highlights

Lake Tanganyika is the oldest of the East African Great Lakes (Coulter 1991) and is the second largest and deepest freshwater body in the world
Larvae of L. miodon were frequently encountered in the sandy habitat of site 1 and 2, while they were rarely observed in the rockier habitats of site 3 and 4 (Table 1)
Direct comparison based on paired t-tests on monthly averages revealed that L. miodon of pelagic origin were generally and significantly smaller than L. miodon of littoral origin

Summary

Introduction

Lake Tanganyika is the oldest of the East African Great Lakes (Coulter 1991) and is the second largest and deepest freshwater body in the world. Around 1200 species have been identified in this lake, classifying it as the second most diverse lake in the world (Cohen et al 1993). Lake Tanganyika is bordered by four countries (DR Congo, Burundi, Tanzania and Zambia; Figure 1). Bathymetric maps of the lake show seven subbasins with different depths (Bujumbura, Kalemie, East Marungu, Mpulungu, Moba, Kigoma and Rumonge) (Tiercelin & Mondeguer 1991). Lake Tanganyika fisheries are crucial for human nutrition in the riparian countries. A major part of the catch derives from the open waters of the lake, where two endemic freshwater clupeids Limnothrissa

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