In oceanographic terms, a region of sharp gradient in temperature or salinity, indicating a transition between two current systems or water masses. Intersection between the thermocline (temperature discontinuity) or halocline (salinity discontinuity) and the surface. Fronts are usually associated with high biological activity, high abundance of highly migratory resources (e.g. tunas), and are actively sought as fishing areas. Fronts can be monitored by satellite remote-sensing.