The can be a number of causes for this condition.
Some of the more common causes are a poorly mounted transducer and an air stream passing over the transducer.
The transducer needs to be mounted in the correct location for your type of boat. Here is a guide
showing the do's and don'ts
If there is anything forward of the transducer It can cause an area of turbulence or bubbles:
look for water intakes or discharge openings; strakes, struts,
fittings, or hull irregularities forward of the transducer