This work explores the development of highly sensitive salinity sensors. The demonstrated sensors are based on optical fibres and consist on Fabry-Pérot optical cavities formed by optimized processes that include chemical etching and fusion splicing, on which microfluidic channels are milled by focused ion beam. Two configurations are presented and their performance compared, including a design that makes use of Vernier-effect for the simultaneous measurement of salinity and temperature with high sensitivity. The interrogation of the devices is carried out by spectral measurements using a broadband light source yielding sensitivities to salinity up to 82.61 nm/M, or 6830.0 nm/RIU.