Fishman Transducers, Inc. has executed a 32,978 s/f lease at 6 Riverside Dr. for its headquarters. The tenant will be relocating from 340 Fordham Rd., in Wilmington in the fall.   Colliers Meredith & Grew senior vice president Matthew Daniels and assistant vice president Christopher Lawrence represented Fishman Transducers. Partner Brian McKenzie of Richard Barry Joyce & Partners represented the landlord CrossHarbor Capital Partners LLC.    Built in 1989, 6 Riverside Dr. is a two-story, 77,048 s/f R&D building located within the Andover Research Park.