Widespread colonisation of Tanzanian catchments by introduced Oreochromis tilapia fishes: the legacy from decades of deliberate introduction

From the 1950s onwards, programmes to promote aquaculture and improve capture fisheries in East Africa have relied heavily on the promise held by introduced species. In Tanzania these introductions have been poorly documented. Here we report the findings of surveys of inland water bodies across Tanzania between 2011 and 2017 that clarify distributions of tilapiine cichlids of the genus Oreochromis. We identified Oreochromis from 123 sampling locations, including 14 taxa restricted to their native range and three species that have established populations beyond their native range. Of these three species, the only exotic species found was blue-spotted tilapia (Oreochromis leucostictus), while Nile tilapia (Oreochromis niloticus) and Singida tilapia (Oreochromis esculentus), which are both naturally found within the country of Tanzania, have been translocated beyond their native range. Using our records, we developed models of suitable habitat for the introduced species based on recent (1960–1990) and projected (2050, 2070) East African climate. These models indicated that presence of suitable habitat for these introduced species will persist and potentially expand across the region. The clarification of distributions provided here can help inform the monitoring and management of biodiversity, and inform policy related to the future role of introduced species in fisheries and aquaculture

The status and distribution of freshwater fishes in the Lake Victoria basin

In terms of hydrology, the Lake Victoria Basin is part of the Nile system, most of which is within the Nilo-Sudan ichthyofaunal province. However, the ichthyofauna of the Lake Victoria Basin differs substantially from most of the downstream Nile basin. It has traditionally been assigned to the East Coast ichthyofaunal province together with the faunas of the systems of Lakes Kyoga, Edward and Kivu, and the coastal rivers of Eastern Africa (Greenwood, 1983; Roberts, 1975; Snoeks et al., 1997). This viewpoint has been challenged and inclusion of the Lake Victoria ichthyofauna into the Nilo-Sudan province has been suggested (Lévêque, 1997; Witte et al., 2009). More recent biogeographic and genetic studies have revealed that Lake Victoria and the smaller lakes in the region harbour a mosaic ichthyofauna composed predominantly of Nilotic and Congolese elements with quite balanced contributions from the two (Meier et al., 2017; Seehausen, 2002). Most of the non-endemic species of Lake Victoria are shared either with the Nile or with the Malagarasi (Congo) system, and very few are shared with coastal rivers. Most of the endemic species have their closest relatives either in the Nile or Congo systems. The large endemic cichlid species radiation evolved from a population of hybrid origins between cichlids from the Nile and the Congo (Meier et al., 2017). The endemic Nothobranchius killifish too derive from two lineages that have their nearest relatives in the Sahel and the Congo respectively (Dorn et al., 2014). Cichlids form the major component of the fish fauna of the Lake Victoria Basin. Other impor tant families are the Cyprinidae, Mormyridae, Clariidae and Poeciliidae. Prior to major anthropogenic disturbances, including the introduction of the Nile Perch (Lates niloticus) and severe habitat deterioration, the system harboured between 600 and 1,000 species of cichl ids, al l but four of them endemic haplochromines (Kaufman et al., 1997; Seehausen, 2002, 2015; Witte et al., 2007). Since the 1980s, an estimated 200 haplochromine species have, however, likely gone extinct and other species have become severely threatened (Seehausen et al., 1997b; Witte et al., 1992, 2007). Unfortunately, several hundred of the haplochromine species remain undescribed, and this includes both extant species and those likely to now be extinct. The numbers of non-cichlids reported in the past appear to have been underestimations, with 69 non-cichlid species listed from the area (excluding the Lake Kyoga region) by this project compared to Snoeks (2000) reporting 45 for Lake Victoria, and Witte et al. (2009) reporting 46 for Lakes Victoria and Kyoga together

Successive invasion-mediated interspecific hybridizations and population structure in the endangered cichlid Oreochromis mossambicus.

Hybridization between invasive and native species accounts among the major and pernicious threats to biodiversity. The Mozambique tilapia Oreochromis mossambicus, a widely used freshwater aquaculture species, is especially imperiled by this phenomenon since it is recognized by the IUCN as an endangered taxon due to genetic admixture with O. niloticus an invasive congeneric species. The Lower Limpopo and the intermittent Changane River (Mozambique) drain large wetlands of potentially great importance for conservation of O. mossambicus, but their populations have remained unstudied until today. Therefore we aimed (1) to estimate the autochthonous diversity and population structure among genetically pure O. mossambicus populations to provide a baseline for the conservation genetics of this endangered species, (2) to quantify and describe genetic variation of the invasive populations and investigate the most likely factors influencing their spread, (3) to identify O. mossambicus populations unaffected by hybridization. Bayesian assignment tests based on 423 AFLP loci and the distribution of 36 species-specific mitochondrial haplotypes both indicate a low frequency of invasive and hybrid genotypes throughout the system, but nevertheless reveal evidence for limited expansion of two alien species (O. niloticus and O. andersonii) and their hybrids in the Lower Limpopo. O. mossambicus populations with no traces of hybridization are identified. They exhibit a significant genetic structure. This contrasts with previously published estimates and provides rather promising auspices for the conservation of O. mossambicus. Especially, parts of the Upper Changane drainage and surrounding wetlands are identified as refugial zones for O. mossambicus populations. They should therefore receive high conservation priority and could represent valuable candidates for the development of aquaculture strains based on local genetic resources