Not sure what we would use as a BDFI, but you could do as Chuck suggested and adjust the Fish ID Sensitivity menu down so that only the biggest fish would show with the large sized Fish ID symbol. With this menu set at the default setting I think that we use the sonar return from a 1 pound LMBass as the criteria of a small Fish ID Symbol, 3 pound for medium and 5 for large. I know that this changes when you use one of the salt water settings in the Water Type menu but cannot recall what the possible equivalents are.