Rog2323:
Post the make and model of the fish finder - can't see from the picture. Even better, post a link to the manual or datasheet for it that has info on what ports are available on the fish finder. Or a datasheet for the transducer if you want to connect it directly to the Arduino.
thishttp://www.google.com.ph/url?sa=t&rct=j&q=portable%20fish%20finder%20operation%20guide&source=web&cd=1&cad=rja&ved=0CCgQFjAA&url=http%3A%2F%2Fwww.phiradar.com%2Fen%2FUploadDrivers%2FFD01_MANUAL_ENGLISH.pdf&ei=gkPHUOjaNYePrgfRtgE&usg=AFQjCNGqOOCNxr8RH6GCPSks3Z0vOLVWzw