Fishes of Lake Tumba (Democratic Republic of Congo): Evaluation of present status and comparisons with previous studies

Abstract

Background. Lake Tumba with a surface area of 830 km2 is very humic (pH = 3.6–4.6). The fishery is important for the riparian population but there are concerns about overfishing. The objective of this study was to assess changes in the fish species composition by comparing the presently reported survey (2005–2010) with earlier studies carried out in 1938–1939, 1955–1956, and 1959 Our hypothesis was that Lake Tumba had been overfished. Materials and methods. Fishing was done in the pelagial with gillnet (25 and 30 mm) and line with hooks at three sites within 166 days in 2005–2006 and 2009–2010. Results. A total of 40 species were identified. Five species were caught all years and at all sites, namely Chrysichthys cranchii (Leach, 1818), Chrysichthys ornatus Boulenger, 1902, Chrysichthys punctatus Boulenger, 1899, Mormyrops anguilloides (Linnaeus, 1758), and Tylochromis lateralis (Boulenger, 1898). Twelve species were only caught once. Weight Per Unit Effort (WPUE) differed with site and season: ranging from 0 to 4200 g · day–1 and the Number of fish Per Unit Effort (NPUE) varied from 0 to 27 fishes · day–1. Conclusion. Out of 49 pelagic species caught by Matthes in 1959, we considered five as diminished in numbers or even missing. This may be attributed to high fishing pressure. The protected site Mabali hosted the majority of previously reported species of which two were noted in Matthes’s list but not caught by him. Many parameters demonstrated a “positive” difference between Mabali (which is protected from fishing) and the other sites. This site should be a reference area for future monitoring if its status can be maintained. A trend in the fishery based on interviews with fishers indicated a change from larger species to smaller. This was deemed a result of fishing pressure and nine species were identified as diminishing. The presently reported survey introduced quantitative aspects while earlier studies were qualitative. This impairs the ability to draw strong conclusions about overfishing and other anthropogenic activities. The threat of overfishing calls for the need for consistent quantitative monitoring of the fish population, including monitoring of waters in the drainage area, to enable assessment of impact of anthropogenic pressure.

Keywords

• historical comparisons
• FishBase comparisons
• leng