Management Implications of the Response of Two Tilapiine Cichlids to Long-Term Changes in Lake Level, Allodiversity and Exploitation in an Equatorial Lake

Abstract

The tilapiine cichlids Oreochromis leucostictus and Tilapia zillii were introduced into Lake Naivasha, Kenya, in 1956. Previous studies on data collected to 1987 revealed they were persistent following establishment, despite environmental variability and exploitation. Recent data, however, suggest this persistence is under threat as data indicate some significant declines in aspects of their abundance since 1999. The influence of changes in lake level, allodiversity and fishing effort on this decline was tested and showed that a decline in lake level was a significant causal factor. The recent change in allodiversity, with the establishment and dominance of Cyprinus carpio in the fishery, was not significant on the catch per unit effort of O. leucostictus but was on T. zillii. Since 1999, catches of tilapiines in the fishery have been independent of fishing effort, contrary to between 1975 and 1987, suggesting their management through application of fishery models may no longer be applicable. As it was anthropogenic-mediated lake level changes that were mainly responsible for their decline, then lake management should focus on sustainable water utilization that maximizes lake levels in accordance with the basin-wide water balance.

Life history traits of an equatorial common carp Cyprinus carpio population in relation to thermal influences on invasive populations

“…Lake Naivasha, its commercial fishery and fish sample collection Lake Naivasha is a shallow, freshwater lake in Kenya's Rift Valley located 190 km south of the equator (0 • 45 S, 36 • 21 E) at an elevation of 1890 m above sea level. Its area fluctuates according to water levels, but is approximately 100-150 km 2 and up to 6 m deep (further details available in Oyugi et al, 2011). To identify the population trends in the C. carpio since their introduction and through to the conclusion of this study, data were used from the lake's commercial gillnet fishery that has a legal fish landing size of 180 mm.…”

First record of dwarf Victorian mouthbrooder Pseudocrenilabrus multicolor victoriae (Schöller, 1903) in Lake Naivasha, Kenya, including data on its molecular identification: Management implications

“…Lake Naivasha can be described as a “hotspot” of allodiversity (taxa imported into a given area by humans) and its associated impacts (Gherardi et al., ; Harper, Morrison, Macharia, Mavuti, & Upton, ; Litterick, Gaudet, Kalff, & Melack, ; Muchiri & Hickley, ; Oyugi, Harper, Ntiba, Kisia, & Britton, ). In addition to being the only freshwater lake without surficial outlets, one of the most unique features of Lake Naivasha is its changing ecology and fish species composition over the years (Aloo, Oyugi, Morara, & Owuor, ; Britton & Harper, ; Gherardi et al., ; Hickley, Muchiri, Britton, & Boar, ; Hickley et al., ; Kundu, Aura, Muchiri, Njiru, & Ojuok, ).…”

Linking the economy to the ecosystems: land use change and ecosystem services valuation at basin level

“…Harper and Mavuti (2004) suggest that the protection and recovery of the lake's natural fringe of Cyperus-papyrus could reduce eutrophication, facilitate the transition to a clear water state and promote macrophyte growth. Oyugi et al (2011)proposed an improved water management scheme with the aim to maximize the proportion of water reaching to the lake and to maintain the lake level in order to provide a considerable ecological benefit for the Tilapiine fishes. Gherardi et al (2011) proposed the implementation of conservation management frameworks in order to prevent the high risk of species introduction in Lake Naivasha.…”

“…As well, the fish breeding grounds are exposed for illegal fishing practices and the fish stock is also endangered (Yongo et al 2013). A decline in fish population could also affect the bird community and their diversity because fish resources are the source of food for numerous piscivorous birds around the lake , Oyugi et al 2011. Therefore, as opposed to approaches that address only parts of the ecosystem or individual species, the EBFM approach would play a fundamental role to maintain the Lake Naivasha ecosystem health and functions.…”

“…Oyugi et al (2011) indicated that improvement on water management schemes would contribute to maintain lake level and provide considerable ecological benefits. A co-management approach also suggested as an opportunity for increased participation and empowerment of the fishing communities (Kundu et al 2010).…”