Introduction

Freshwaters constitute ~3% of the Earth’s water (WWF 2020) and occupy <1% of the Earth’s surface area (Garcia-Moreno et al. 2014), but is habitat to about 40% of all described fish species (Lundberg et al. 2000). The benefits of these freshwater resources to society are immense. For example, 21 largest lakes in the world provide about 1.3 million tons of fish annually, 62.5 GW of hydropower, 5 billion m3 of potable drinking water, and 815 million m3 of water for irrigation (Sterner et al. 2020). Also, freshwater systems replenish estuarine, oceans, and seas with nutrients and water (Matthews 2016); for example, Nile River deposits 1318 and 212 kg km−2yr−1 of nitrogen and phosphorus, respectively, into the Mediterranean Sea (Yasin et al. 2010).
Freshwater species are more susceptible to human-induced threats such as climate change, pollution, habitat alteration, overexploitation, and alien species introductions compared with marine and terrestrial species (Darwall et al. 2018; WWF 2020). For example, while marine and terrestrial species have declined by 39% in the last 50 years, freshwater counterparts have reduced by 76% higher than the global average of 52% (WWF 2014). Freshwater fish species might be the most threatened vertebrates assessed by IUCN (Reid et al. 2013). The species are affected by the high levels of exploitation to support about 158 million people worldwide who derive animal protein from freshwater fish species (McIntyre et al. 2016).
Approximately 500 fish species are reported in all water bodies in Uganda (NEMA 2007; Natugonza & Musinguzi, 2020). However, several non-native fishes, including Nile perch (Lates niloticus), Nile tilapia (Oreochromis niloticus ), redbelly tilapia (Coptodon zillii) , blue-spotted tilapia (Oreochromis leucostictus ), and redbreast tilapia (Coptodon rendalii ) were introduced into various lakes and rivers within the Victoria and Kyoga lake basins (Kishe-Machumu et al. 2018). These introductions especially for Nile perch coincided with the collapse of most native fish species (Ogutu-Ohwayo 1990). In Lake Victoria, ~300+ haplochromine cichlids were extirpated (Kaufman 1992; Ogutu-Ochwayo 1990), and similar destructive ecological changes were observed in lakes Kyoga and Nabugabo (Ogutu-Ohwayo 1990; Chapman et al. 1996). This loss in haplochromine cichlids is believed to be the worst vertebrate species extinction observed in recent times (Kaufman 1992), placing Nile perch among the 100 top worst alien invasive species in the world (Lowe et al. 2000). Other native species such as Singidia tilapia (Oreochromis esculentus ) and Ningu (Labeo victorianus ), which previously dominated in fish catches from Lake Victoria and its affluent rivers (Kudhongania et al. 1992) are currently classified as critically endangered (IUCN 2020). The catfishes, including Semutundu (Bagrus docmak Forsskål, 1775), Lake Victoria deepwater catfish (Xenoclaris eupogon ), Clarias spp. ,Synodontis spp., and Silver catfish (Schilbe intermedius ) were also affected by Nile perch establishment in Lake Victoria (Goudswaard & Witte 1997; Balirwa 1998; Balirwa et al. 2003). Riverine species were mostly affected by overexploitation and habitat degradation, while the native tilapines declined mostly through interspecific competition and hybridization (Cadwalladr 1965; Kudhongania et al. 1992).
The reduction in fish species diversity in Uganda has led to numerous studies focussing on the species diversity, abundances, distribution, taxonomy, and biology of the remnant species to facilitate their recovery and reduce further extinctions (Witte & Van Oijen 1990; Kaufman & Ochumba 1993; Ogutu-Ohwayo 1993; Ogutu-Ohwayo et al. 1999; Mbabazi et al. 2004). In particular, small lakes, swamps, rivers, streams, and wetlands were documented as the main structural refugia for these fishes (Ogutu-Ohwayo et al. 1999; Mwanja et al. 2001; Chapman et al. 2002; Mbabazi et al. 2004; Balirwa et al., 2003; Wakwabi et al. 2006; Olwa et al., 2020). However, these studies were waterbody-specific with limited information to rank the waterbodies for site-based conservation given the limited available resources. Analysis of the distribution of fishes at a broader scale has been limited in the past due to the paucity of data, which have been scattered in many research institutions in unusable formats. Recently, substantial amounts of data on the occurrence of fishes for Uganda have been made available through the Global Biodiversity Information Facility portal (GBIF) (GBIF, 2020). This study aims to develop a Conservation Priority Index (CPIw) for inland water bodies to prioritize their selection for site-based fish conservation, especially when resources are limiting. We use data on the distribution, diversity, and conservation status of the fish species in Uganda, which are freely accessible through GBIF and IUCN databases.