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

Asilatu Shechonge,Jennifer J Freer,Alan M Smith,Julia J Day,Benjamin P Ngatunga,Rashid Tamatamah,Tabitha Richmond,Alexandra M Tyers,George F Turner,Stephanie J Bradbeer,Semvua Mzighani,Jonathan Kihedu,Martin J Genner,Emmanuel A Sweke,Antonia G P Ford

doi:10.1007/s10750-018-3597-9

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.

Highlights

In Africa, inland aquaculture is a rapidly growing food sector (FAO, 2016), but one of the major consequences of expansion of aquaculture can be the associated spread of cultured species into non-native ecosystems (Naylor et al, 2001), which has led to detrimental effects for many local habitats (Ehrenfeld, 2010; Gichua et al, 2014)
The spread to natural habitats from culture facilities has been both unintentional, with individuals escaping from aquaculture facilities (Canonico et al, 2005), and deliberate, with tilapia being released into natural water bodies to improve capture fisheries (Canonico et al, 2005; Genner et al, 2013)
Two further taxa are native to Tanzania, but were translocated beyond their native range, namely O. niloticus, and O. esculentus

Summary

Introduction

In Africa, inland aquaculture is a rapidly growing food sector (FAO, 2016), but one of the major consequences of expansion of aquaculture can be the associated spread of cultured species into non-native ecosystems (Naylor et al, 2001), which has led to detrimental effects for many local habitats (Ehrenfeld, 2010; Gichua et al, 2014). Among the most widely cultured groups of freshwater fish species are tilapiine cichlids They have been introduced to over 140 countries, and established feral populations in at least 114 of these (Deines et al, 2016). Spread of tilapia species into non-native habitats has resulted in negative ecological effects on native species and their habitats through competition and habitat alteration (Canonico et al, 2005). It has resulted in the loss of unique population genetic structure through hybridisation (D’Amato et al, 2007). When tilapia introductions are being considered, benefits need to be evaluated in light of potential ecological and economic costs

Methods

Results

Conclusion