1. We explored how range expansion of freshwater fishes coincident with climate warming is affected by, and then in turn affects, responses to a second environmental gradient – dissolved oxygen.

2. Traits related to hypoxia tolerance, specifically various metrics of gill size and geometric morphometric proxies of gill size were quantified for a range-expanding cyprinid fish (Enteromius apleurogramma) in both its historical and novel ranges in the Mpanga River drainage of Uganda, East Africa.

3. We found that E. apleurogramma followed patterns previously established in the congener E. neumayeri. Gill filament length and some other metrics were strongly divergent in long-established populations of both E. apleurogramma and E. neumayeri, with larger gills in hypoxic populations compared to normoxic ones. Range-expanding populations were intermediate to the two long-established populations, but divergent between themselves. Other gill traits such as filament number were weakly or not divergent.

4. Furthermore, we show that grosser morphological traits such as opercular area can be successfully used as a proxy for gill size, both by direct measurement as well as using geometric morphometric techniques.

5. Finally, we show that both parapatric conspecific populations and sympatric heterospecific populations can be used as reference points to approximate the “target” of adaptation to hypoxic conditions.

Some data are presented as .csv files which should be usable in any spreadsheet or data analysis software. Other data for geometric morphometrics are presented as .tps files, and can be opened as a flat text file, or read by the geomorph package in R.