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

Abstract

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. Electronic supplementary material The online version of this article (10.1007/s10750-018-3597-9) contains supplementary material, which is available to authorized users.

A survey of fishes of Hombolo Lake, Dodoma, Tanzania, with evidence for local extinction of a native tilapia as a consequence of stocking

“…Our surveys since 2011 have indicated its continued occurrence in these lakes (Shechonge et al 2018a).…”

“…The Nile Tilapia has been collected from Lake Victoria for production in hatcheries and distribution to fish farmers throughout Tanzania since the 1990s. It has also been widely stocked in natural lakes and impoundments of natural river systems (Shechonge et al 2018a).…”

“…Oreochromis esculentus is endemic to the catchment of Lake Victoria (Trewavas 1983), where it has largely been replaced by O. niloticus (Shechonge et al 2018a). It was widely stocked in Tanzania during the 1950s and 60s, most notably in Lakes Singida, Nyumba ya Mungu (Trewavas 1983) and Rukwa (Seegers 1996).…”

“…Oreochromis urolepis is known to have a higher salinity tolerance than O. niloticus (Trewavas 1983), so increased salinization would not be expected to favour the latter. If O. niloticus can competitively exclude O. urolepis, even in a relatively saline lake, this does not bode well for the long-term survival of the latter species, given that O. niloticus is established in all major catchments where O. urolepis is native (Shechonge et al 2018a) and that the two species are known to hybridise (Shechonge et al 2018b).…”

Relative growth of invasive and indigenous tilapiine cichlid fishes in Tanzania

“…Oreochromis niloticus (Nile tilapia (Linnaeus 1758)) is native to much of northern Africa, including the Nile and Niger systems (Trewavas 1983). In Tanzania, the species is naturally distributed only in the Lake Tanganyika catchment (Shechonge et al 2019a), but due to favourable characteristics, such as potential for rapid growth and broad habitat tolerances, it has been widely distributed to non-native habitats across the country (Shechonge et al 2019b). Such introductions have largely been deliberate, to promote capture fisheries, but it is also possible accidental escapes have occurred from aquaculture facilities in the country.…”