Air Traffic Control Fully Certified
Air Traffic Control Time Certified In Pos 1 (yrs) 4.5
Communication Breakdown; Situational Awareness
ATC Issue All Types; Conflict NMAC
Flight Crew Requested ATC Assistance / Clarification; Flight Crew Took Evasive Action
Airspace Structure; Human Factors; Procedure
Aircraft X was on my frequency departing to the north squawking 1200. I restricted Aircraft X to maintain at/or below 2;000 feet for inbound traffic from the northeast; a VFR [aircraft] worked by Approach at 2;500 feet. I noticed there could be potential additional traffic from the northeast traveling southwest bound to intercept the final; Aircraft Y at 2;500 feet still talking with Approach. I issued traffic to Aircraft X regarding Aircraft Y; then I noticed Aircraft Y descending from 2;500 feet to 2;300 feet then to 2;200 feet. I told Aircraft X about the traffic and immediately suggested a right turn to avoid the traffic. Then I saw Aircraft Y changing heading from southwest to southeast right into Aircraft X. I tried to reach for Aircraft Y; but he wasn't on my frequency. At that point the 2 aircraft were about 1 1/2 miles apart and I issued a traffic alert to Aircraft X and Aircraft X reported Aircraft Y in sight. I saw the targets merging on radar at the same altitude of 2;000 feet.The recommendation would be to restrict descend on an arriving aircraft until passing any VFR aircraft below that could become a conflict.
A Local Controller reported an inbound aircraft still on Approach Control frequency descended through the altitude of a VFR departure.