seagodess,
Utilizing the DFF3 (or FCV295 and FCV1150) with the R509LH and utilizing fixed frequencies is widely popular within the Southwest Alaska fishing community for salmon, cod, halibut, and herring.
Here is a picture showing the optimum frequencies of the R509LH. As you can see the R509LH has good performance using 35kHz on Low Frequency and 120kHz for the High Frequency: