Generally, compared to invertebrates, fish fauna in saline lakes are limited (Brauner et al.
2012). This has been attributed to lack of direct water links to other basins, which in turn limit the dispersal ability of their fish populations relative to invertebrates, that can disperse over land, mainly facilitated by dispersal vectors such as wind and migratory birds (Hammer 1986;
Frisch et al. 2007; Brauner et al. 2012). Therefore, in the absence of human intervention, dispersal of fish relies entirely on rare chances such as during floods or by accidental transfer by fish-eating birds (Zaccara et al. 2014). Indeed, instances of floods and discharges following heavy rains may occur only in a few saline lakes (e.g. Lakes Natron and Turkana) that have temporary connections to rivers or streams. Species richness and abundance of saline lake fishes is therefore correlated to dispersal opportunities (Brauner et al. 2012), food availability and the physical complexity of individual lakes (Hammer 1986).
More often than not, less alkaline lakes (salinity 3-5 ppt e.g. Lake Turkana, Kenya) have more species, whereas hypersaline lakes (salinity ~21 ppt e.g. Lakes Magadi and Little Magadi, Kenya) have fewer species (Hammer 1986; Seegers et al. 2003). The negative correlation of salinity to fish diversity has been reported in several studies (Sosa-López et al.
2007; Cooper & Wissel 2012). Owing to the intimate physiological relationship fish have with their environment (Cossins & Crawford 2005), saline lake fishes are of great global significance as biological models to study adaptation and response to a wide variety of natural and anthropogenic environmental conditions.
As EASLs are geographically isolated and often lack inlets and outlets, the origins of the initial fish populations that seeded the lakes are not obvious. Thus, it is often difficult to state with certainty whether populations in individual lakes are natural or have even been stocked by humans in ancient times (Hammer 1986; Brauner et al. 2012). Nevertheless, owing to relatively long periods of isolation, most saline lake fish fauna are evolutionarily quite divergent from their closest freshwater relatives and have evolved unique adaptations to subsist in their individual lakes. Consequently, saline lake fish exhibit varied morphological,
Chapter 2 Fish Populations in East African Saline Lakes
behavioural, physiological and ecological adaptations. Additionally, morphological diversifications have also occurred among populations of the same species within individual lakes probably as a result of occupying diverse environments (Seegers & Tichy 1999; Tichy &
Seegers 1999). This may in turn lead to fish populations showing substantial intraspecific or interspecific morphological differentiation even in the presence of gene flow. This has been reported for Lake Natron, Tanzania where the lake’s three morphologically distinct species, Alcalicus alcalicus, A. ndalalani and A. latilabris exist in sympatry (Seegers & Tichy 1999). The most prevalent species, A. alcalicus displays considerable intraspecific morphological variation (Seegers & Tichy 1999; Tichy & Seegers 1999). Nevertheless, genetic analyses (mtDNA and microsatellites) have revealed considerable gene flow among these species and morphotypes (Zaccara et al. 2014). Owing to their characteristic restricted ranges, small population sizes and stressful habitats, fish species in EASLs are thought to be at a great risk of extinction. Model studies in other saline lakes of the world have predicted a decline in fish populations as a result of unprecedented environmental threats to their ecosystems (Jellison et al. 2004). This has been attributed to a decline in annual precipitation leading to deterioration of water quality and contraction of habitats mainly driven by anthropogenic activities (Jellison et al. 2004).
Fish species of Ethiopia’s saline lakes
Although little is known about the fish communities in the Ethiopian saline lakes, there have been suggestions that the lakes may hold a few endemic species. The cichlid Danakilia franchettii (Trewavas 1983) and cyprinodontid Lebias stiassnyae (Getahun & Lazara 2001) are endemic to Lake Alfdera (Stiassny & Getahun 1998; Golubtsov et al. 2002) (Table S2.1). Lake Chilotes (also referred to as Hora Kilole) has an indigenous Oreochromis niloticus population.
However, following recent diversions of freshwater from River Mori into Lake Chilotes, several riverine species (mainly Barbus) have colonized the lake within the last decade (Lemma 2003).
Fish species of Kenya’s saline lakes
Compared to all other EASLs, Lake Turkana has the highest number of species (Table S2.1).
Over 50 species have been described, 30% of which are endemic to the lake (Table S2.1) (Hopson 1982; Kolding 1989; Seegers et al. 2003). The non-endemic species are derived from the Nile drainage (Kolding 1989, 1995) as a result of past connections of the Turkana Lake basin to River Nile (Johnson & Malala 2009). The high diversity of fishes in Lake Turkana is
attributed to its mild saline conditions, a well-mixed water column and well-oxygenated waters among other factors (Ferguson & Harbott 1982; Kallqvist et al. 1988). Recent studies have however suggested a decline in the lake’s fish populations owing to fishing pressures (Muška et al. 2012) and fluctuations in water levels (Kolding 1995). In contrast to Lake Turkana, the other saline lakes of Kenya have very limited fish fauna. The hyper-saline Lake Magadi contains only a single species, the endemic tilapia, Alcolapia grahami. The Magadi tilapia was introduced to Lake Nakuru between 1952-1962 to combat mosquito larvae (Vareschi 1979) and probably also in Lake Elementaita (Okeyo 2006). However, massive deaths of the introduced fish were reported in 1991 for unknown reasons (Githaiga 1997). In a recent expedition (March, 2010) to Lake Elementaita, a thriving population of tilapia resembling Alcolapia grahami but with slightly bigger body sizes was found (Geraldine D.
Kavembe, personal observation). The current population status of Alcolapia grahami species in Lake Nakuru is, however, unknown. Although Lake Bogoria does not have endemic species of its own (Harper et al. 2003), fish from affluent streams have been found to “stray” into the lake following heavy rains that make the water less saline.
Fish species of Tanzania’s saline lakes
Lake Manyara is the most important of the Tanzanian saline lakes in terms of fisheries. The lake is dominated by the endemic Lake Manyara tilapia Oreochromis amphimelas (Figure 2.3).
This species is also found in other soda lakes in Tanzania such as Lakes Kitangiri, Eyasi and Singida (Froese & Pauly 2014). A sharp decline of fish catches in Lake Manyara has been recorded, from a high of 1,800 tons in 1970s to a low of 0.5 tons in 1990 (Mugisha et al. 1993).
The decline was attributed to a drought that wiped out most of the populations of O.
amphimelas (Mugisha et al. 1993). Nevertheless, more recent surveys suggest that Lake Manyara still supports a significant population of O. amphimelas (Yanda & Madulu 2005).
However, information on the status of the species in other Tanzanian saline lakes is scanty.
Catfish (Clarias gariepinus) have been reported in Lake Manyara (Yanda & Madulu 2005) and Lake Kitangiri (Bwathondi 2002). Oreochromis esculentus and Tilapia rendalli were introduced from Lake Victoria to Lakes Kitangiri and Singinda (Thieme et al. 2005) (Table S2.1).
Bwathondi (2002) reported sightings of Protopterus aethipicus and Oreochromis niloticus, as well as some unspecified Haplochromine species, in Lakes Kitangiri and Singinda respectively.
Lake Natron, which displays extreme conditions (similar to but less extreme than those of Lake Magadi) is well known for its three endemic soda lake tilapia of the genus Alcolapia: A.
alcalicus, A. latilabris and A. ndalalani (Figure 2.3, Table S2.1) During the wet season, several
Chapter 2 Fish Populations in East African Saline Lakes
riverine fish species have been reported to stray into Lake Natron from rivers (Ewaso Nyiro, Peninj, Moinik and Engare Sero) and seasonal streams that feed the lake (Seegers & Tichy 1999).
Figure 2.2 Representative fish species from East African Saline Lakes: a) Alcolapia grahami from southern lagoons of Lake Magadi; b) Alcolapia alcalicus from Shompole swamps, north of Lake Natron;
c) Alcolapia aff. alcalica from south western lagoon of Lake Natron; d) Alcolapia ndalalani from Olomotony, southern Lake Natron; e) Alcolapia latilabris from Olomotony, Lake Natron; f) Oreochromis amphimelas from Mto wa Mbu, northern affluents of Lake Manyara; g) Oreochromis niloticus vulcani from Loyangalani springs, eastern shore of Lake Turkana; h) Haplochromis rudolfianus from south of Loyangalani, Lake Turkana, i) Hemichromis exsul from south of Loyangalani, eastern shore of Lake Turkana; j) Clarias gariepinus from Mto wa Mbu, Tanzania northern affluents of Lake Manyara.
Photographs kindly provided by Dr. Lothar Seegers.