Sea level rise threatens ecosystem function in coastal lakes by increasing salinity and altering community composition. As mobile generalists, fish have considerable influence on the stability of a system by impacting trophic network structure. Understanding their diet across abiotic gradients, including salinity, can show key characteristics of their ecological role, and predator diet shifts may serve as warning signs of food web restructuring. Stomach contents analysis was used to look at the diet of Retropinna retropinna (common smelt) across shallow coastal lakes on Rēkohu (Chatham Island). Smelt stomachs from 10 lakes (seven freshwater; three salt-influenced) were analysed. Comparisons of salt-influenced and freshwater lakes showed shifts in composition, with reduced beta diversity with salinity. Smelt in freshwater lakes predominantly consumed herbivorous, omnivorous, or predatory insect larvae while diet in salt-influenced lakes was dominanted by crustacean shredders. The between lake taxonomic variation amongst salt-influenced lakes was lower than between freshwater lakes, suggesting homogenisation of smelt diet with salinity. Differences in composition and heterogeneity did not translate to consistent differences in total niche width or individual specialisation between freshwater and salt-influenced lakes.  Further development of knowledge on biological structure across salinity gradients is essential to understand ecosystem function under future climate scenarios.