Complex sentence: Langoustines are fished by creels baited with herring or prawns, which are deployed on lines of up to 120 creels, which are left on the seabed for at least a day.
Split into simpler sentences: Langoustines are fished by creels baited with herring or prawns, which are deployed on lines of up to 120 creels. The creels are left on the seabed for at least a day.