A320 EXPERIENCES LOSS OF SEPARATION WITH VFR MOONEY ON ARR TO LGA.

Narrative

WHILE AT 13000 FT ON MIP3 ARR INTO LGA; ATC ADVISED OF A MOONEY 1 O'CLOCK POS AND ABOUT 8 MI. WE DID NOT SEE THE ACFT; BUT ATC ADVISED THAT THE MOONEY RPTED US IN SIGHT. WE NOTICED AN ACFT ON TCAS AT ABOUT 1 O'CLOCK POS AND 5 MI; 200 FT ABOVE OUR ALT AND DSNDING. WE STILL HAD NOT ACQUIRED THE ACFT VISUALLY BEFORE WE GOT A TCAS 'DSND; DSND.' THE FO (PF) PERFORMED THE TCAS RA MANEUVER AND DSNDED AT ABOUT 1100 FPM TO 12300 FT. I (PNF) ADVISED ATC OF RA SIMULTANEOUSLY AND ADVISED ACFT NOT IN SIGHT. THEN WE GOT A 'CLB; CLB NOW' RA WHICH THE FO FOLLOWED AT ABOUT 400 FPM CLB TO 12600 FT. THE TCAS THEN ADVISED 'CLR OF CONFLICT' AND THE FO CONTINUED CLB BACK TO OUR ASSIGNED ALT OF 13000 FT. THE OTHER ACFT PASSED ABOUT 1.5 LATERALLY AND WITHIN 200 FT VERTICALLY. ATC IS REQUIRED TO SEPARATE IFR AND PARTICIPATING VFR TFC. HOWEVER; BECAUSE THE OTHER ACFT RPTED US IN SIGHT; THAT RELIEVED ATC OF HIS RESPONSIBILITY OF SEPARATION. THIS TYPE OF SITUATION CONTINUES TO OCCUR AND I BELIEVE IT IS IMPERATIVE THAT ATC SHOULD BE REQUIRED TO SEPARATE ACFT AT NIGHT IN THESE SITUATIONS BECAUSE THE HUMAN EYE IS NOT WELL SUITED TO MAINTAIN SEPARATION IN THESE CONDITIONS.