Reconstruction of the historical distribution ranges of imperilled stream fishes

1. Understanding historical distribution patterns of freshwater fishes prior to human impacts is crucial for informing effective strategies for biodiversity conservation. However, incomplete information on species occurrence records, the existence of cryptic species and sensitivity to small sample sizes limit the application of historical records in natural history collections as well as conventional species distribution modelling algorithms to infer past distributions of species.

2. This study used molecular data as an alternative and objective approach to reconstruct the historical distribution ranges of four stream fishes from the Breede River system in the Cape Fold Ecoregion, a global hotspot of imperilled endemic freshwater biodiversity in southern Africa.

3. The study used 249 occurrence records and 208 mitochondrial cytochrome b sequences to reconstruct the potential historical ranges of four taxa: Galaxias sp. ‘zebratus nebula', Galaxias sp. ‘zebratus Riviersonderend', Pseudobarbus sp. ‘burchelli Breede' and Pseudobarbus skeltoni.

4. All four taxa historically had broader distribution ranges across the Breede River system before human impacts, but they have suffered severe attrition as the main-stem populations have been extirpated.

5. The severe decline in the historical ranges of these four taxa is a result of multiple impacts, particularly hydrological modification, habitat degradation and the introduction of non-native species, which are also global challenges for freshwater ecosystems.

6. The approach presented in this study has great potential for reconstructing historical ranges of stream-dwelling taxa from disparate regions where fragmentation has resulted from human-mediated impacts. This information is crucial for identifying appropriate conservation strategies such as river rehabilitation and eradication of non-native species, as well as guiding reintroductions and informing assisted gene flow where these are deemed necessary interventions.