TAMPA — Adam Erne’s tenure with the Lightning ended Wednesday.Tampa Bay traded the rights to Erne, a restricted free agent, to the Red Wings for a fourth-round draft pick next year, and Erne then signed a one-year, one-way contract worth $1.05 million, the salary website CapFriendly said.Now Brayden Point is the Lightning’s only remaining unsigned restricted free agent.Detroit knows exactly what it’s getting with Erne. New general manager Steve Yzerman drafted Erne in 2013 during his tenure as Lightning GM.Erne, 24, had something of a breakout season in 2018-19, his first spent entirely in the NHL. He had 20 points (seven goals) and 40 penalty minutes (10 minors) in 65 games.The 6-foot-1, 214-pound wing added physicality to the fourth line without drawing many penalties, earning a consistent spot in the lineup.Erne was arbitration eligible this year but did not file. He was on a one-year, $800,000 contract last year, and his expected raise was to $1 million.Why did the Lightning trade Erne? Most of what it has done this offseason comes down to the salary cap.It’s possible the move is to free up cap space for Point. The Lightning is now a projected $9.4 million under the $81.5 million cap for next season, CapFriendly says.It’s unlikely that general manager Julien BriseBois, however, would have signed other free agents who are good to have but not crucial if the sides were close on a Point contract.The Lightning and Point’s agent, Gerry Johannson, may have agreed on a salary range; if so, the contract’s length could be a point of contention. BriseBois wouldn’t jeopardize his team’s biggest free agent unless it was absolutely necessary.Also, the trade might be along the lines of BriseBois’ offseason goalie moves. He could be looking down the line to the salary cap being even tighter next year and foresaw being unable to keep Erne at that point.This will make the Lightning’s training camp more interesting with another spot in the lineup up for grabs. Players such as Alex Barre-Boulet, Carter Verhaeghe, Cory Conacher and Mitchell Stephens could have a path to the roster.Contact Diana C. Nearhos at email@example.com . Follow @dianacnearhos .