Aquaculture-Mediated Invasion of the Genetically Improved Farmed Tilapia (Gift) into the Lower Volta Basin of Ghana

Abstract

The need for improved aquaculture productivity has led to widespread pressure to introduce the Genetically Improved Farmed Tilapia (GIFT) strains of Nile tilapia (Oreochromis niloticus) into Africa. However, the physical and regulatory infrastructures for preventing the escape of farmed stocks into wild populations and ecosystems are generally lacking. This study characterized the genetic background of O. niloticus being farmed in Ghana and assessed the genetic effects of aquaculture on wild populations. We characterized O. niloticus collected in 2017 using mitochondrial and microsatellite DNA markers from 140 farmed individuals sampled from five major aquaculture facilities on the Volta Lake, and from 72 individuals sampled from the wild in the Lower Volta River downstream of the lake and the Black Volta tributary upstream of the lake. Our results revealed that two farms were culturing non-native O. niloticus stocks, which were distinct from the native Akosombo strain. The non-native tilapia stocks were identical to several GIFT strains, some of which showed introgression of mitochondrial DNA from non-native Oreochromis mossambicus. We also found that the non-native cultured tilapias have escaped into the wild and interbred with local populations, and also observed potentially admixed individuals on some farms. Our results highlight aquaculture as a vector in the spread of invasive non-native species and strains, and underscore the importance of genetic baseline studies to guide conservation planning for wild populations.

Utility of DNA barcoding in native Oreochromis species

“…Despite the potential for species in the genus to interbreed naturally, few instances of shared haplotypes were detected (Figure 2). This observation illustrates the point that despite the occurrence of introgressive hybridization (Anane‐Taabeah et al ., 2019), DNA barcoding can still be used to develop informative, cost‐effective and rapid biodiversity surveys for Oreochromis within their native range.…”

“…In Africa, several Oreochromis species, in particular O. niloticus and O. mossambicus , occur naturally across multiple river basins, with genetic studies showing that some of these comprise differentiated populations, distinct lineages or cryptic species (Breman et al ., 2016; Decru et al ., 2016; Hallerman & Hilsdorf, 2014). Most genetic studies on Oreochromis have examined aquaculture stocks or introduced populations ( e.g ., Fatsi et al ., 2020; Moses et al ., 2020), and only limited studies exist on natural populations within the indigenous ranges (Anane‐Taabeah et al ., 2019; Firmat et al ., 2013; Hallerman & Hilsdorf, 2014; Syaifudin et al ., 2019; Tibihika et al ., 2020). Studies have focused on solving taxonomic issues using a phylogenetic approach (Seegers et al ., 1999; Nagl et al ., 2001; Schwarzer et al ., 2009; Dunz & Schliewen, 2013; Kavembe et al ., 2013; Matschiner et al ., 2017; Rabosky et al ., 2018; Ford et al ., 2019), improving knowledge of biodiversity (Bezault et al ., 2011; Hassanien & Gilbey, 2005; Nyingi & Agnese, 2007; Rognon & Guyomard, 2003; Syaifudin et al ., 2019; Tibihika et al ., 2020), assessing hybridization (Agnese et al ., 1997; Anane‐Taabeah et al ., 2019; Ford et al ., 2015; Lahav & Lahav, 1990; Mwanja & Kaufman, 1995; Rognon & Guyomard, 2003; Wohlfarth, 1994), understanding adaptive variations (Crandall et al ., 2000; Falk et al ., 2003; Ford et al ., 2015, 2019; Neira et al ., 2016) and elucidating phylogeographic patterns in tilapias (D'Amato et al ., 2007; Mwanja et al ., 2012; Simbine et al ., 2014).…”

“…Nonetheless, this tool might have limitations because mtDNA is maternally inherited. Hybridization and incomplete lineage sorting among Oreochromis species make assessments performed using mitochondrial markers challenging (Anane‐Taabeah et al ., 2019; Ford et al ., 2019). Threats from introgressive hybridization among species in the genus are largely due to the preference for improved strains of O. niloticus in aquaculture, which may escape into the wild and interbreed (Ford et al ., 2015; Lahav & Lahav, 1990; Syaifudin et al ., 2019; Wohlfarth, 1994; Wohlfarth & Hulata, 1981; Wu & Yang, 2012).…”

Defining Management Units for Wild Nile Tilapia Oreochromis niloticus from Nine River Basins in Ghana

“…Analysis of mitochondrial DNA sequence data from wild and farmed Oreochromis spp. collected in Ghana revealed the presence of non-native tilapia on some aquaculture facilities, including individuals showing introgression from non-native Oreochromis mossambicus, from which some stocks have escaped into the wild and interbred with native populations [22]. Given these threats, it is imperative that we assess population genetic data for wild populations of O. niloticus in a timely manner to inform conservation planning and management.…”

Biological Invasions 2020 Horizon

“…The recent exponential increase in the pet trade (Kopecky et al [15]) and the technical improvements in aquaculture to enhance productivity (Anae-Taabeah et al [16]) have significantly contributed to increasing both the invasion rate and risk. Kopecky et al focuses particularly on the importance of studying, listing, and ranking biological species traits to better prevent the transportation and subsequent introduction of species with a high risk of invasiveness.…”