Morgan Geekie, Elias Lindholm and David Pastrnak give the Boston Bruins a good No. 1 line. New coach Marco Sturm will have options when it comes to forming a smashmouth fourth line. The defense and goaltending are set.

There is no such certainty when it comes to the second and third lines, though.

The Bruins begin on-ice sessions on Thursday at Warrior Ice Arena. It will be Sturm's first opportunity to begin teaching systems and the attitude with which he wants the Bruins to play.

It also starts the process of identifying the middle-six forwards. This is the coaches' and management's No. 1 priority in terms of forming the 2025-26 roster out of training camp. The identities of these positions will go a long way toward answering the Bruins' biggest issue: How will they score?

The openings and the candidates:

Candidates: Matej Blumel, Marat Khusnutdinov, Pavel Zacha

Zacha is the runaway favorite in terms of performance and experience. The 28-year-old checks multiple boxes when it comes to being a trusted two-way forward: hockey sense, creativity, strength on the puck and defensive awareness. Zacha averaged 19:04 of ice time per game last season, second only to Pastrnak among team forwards.

If Sturm uses Casey Mittelstadt as the No. 2 center, Zacha's defensive presence could be good to have on the wing. Mittelstadt is an offense-first pivot. However, Zacha is a natural center. It's possible Sturm prefers Zacha in the middle on the second line.

Playing on the No. 2 line would be a big step for Khusnutdinov. The shifty wing was a third-liner upon arriving from the Minnesota Wild. The 23-year-old would have to bring more offense to earn such shifts.

Last season, Blumel led the Texas Stars, the Dallas Stars' AHL team, with 39 goals. The Bruins signed Blumel, 25, to a one-year, $875,000 contract. It's a low-risk bet to see if the left-shot Blumel can make NHL noise with an opportunity or be a high-end AHLer.

Candidates: Zacha, Casey Mittelstadt, Matt Poitras

Mittelstadt has top-six skill. He is good at controlling the puck and shifting speeds to find his linemates.

But the No. 8 pick from 2017 was traded twice in a 366-day span, first by the Buffalo Sabres, then by the Colorado Avalanche. He left both clubs wanting more. Perhaps Mittelstadt, who appeared in 18 games following last season's trade, will find better traction by starting the season with the Bruins.

If Mittelstadt is quiet during camp, it will give Matt Poitras a chance to take his next step. Poitras is trying to make the opening roster for the third straight season. The challenge for Poitras, 21, is to become a consistent NHL offensive presence while earning Sturm's trust away from the puck.

Zacha's consistency puts him in the mix if Sturm likes him better at center.

Candidates: Viktor Arvidsson, Fabian Lysell, Alex Steeves

Last season, the Bruins had their fingers crossed that Lysell would knock down the door. He got a 12-game look, 11 of them coming late in the season. He showed serious speed and skill. But he is running out of time to make his case away from the puck.

In 2021-22, Arvidsson scored 20 goals for the Los Angeles Kings. Sturm was a Kings assistant. Sturm knows Arvidsson can skate, get inside and be strong on the puck. The 32-year-old, who is unrestricted after this season, is eager to prove himself for this season and beyond.

Steeves led the Toronto Marlies, the Toronto Maple Leafs' AHL club, with 36 goals last season. Like Blumel, the 25-year-old Steeves will have a chance to make his NHL case.

Candidates: Johnny Beecher, Tanner Jeannot, Riley Tufte

Jeannot has the inside line because of his experience and ruggedness. There's also the matter of his contract. The Bruins would not have given Jeannot a $3.4 million average annual value if they identified him as a fourth-liner. In 2021-22, he hit a career high with 24 goals for the Nashville Predators, setting up the blockbuster trade with the Tampa Bay Lightning the following season. But he's now coming off three straight seasons with seven goals or fewer.

Beecher did not end 2024-25 well. He was a healthy scratch for the final two games. The 2019 first-rounder scored three goals after scoring seven as a rookie the previous season. He would have to make a significant jump to enter the third-line picture. He has the speed and size to play the position.

The 6-foot-6, 230-pound Tufte has the size and speed to be a third-line menace. But he went without a point in six appearances for the Bruins last season.

Candidates: Sean Kuraly, Georgii Merkulov, Fraser Minten

The Bruins acquired Minten with the projection that he could become Charlie Coyle's third-line replacement. That could happen as soon as October. The 21-year-old is smart, responsible and coachable. If he increases his offensive pace, the job is all but his.

"It's a hard thing to tell a player, because we love when a guy is on the right side of the puck," player development coordinator Adam McQuaid said. "You never want guys who cheat the game. But sometimes not defaulting to thinking so defensive-minded."

On the flip side, Merkulov trends more toward offense. He led Providence last season with 54 points. But that has not translated in 10 NHL appearances.

Kuraly is most likely a fourth-liner. But the two-time Bruin has the savvy, energy and experience to play up in the lineup.

Candidates: Riley Duran, Mikey Eyssimont, Mark Kastelic

At the start of 2024-25, Kastelic centered a good fourth line between Beecher and Cole Koepke. He also took some shifts at right wing. Kastelic's muted offensive play makes him a better fourth-line fit. But his all-out style could be helpful on the forecheck on Jeannot's opposite flank. No team would enjoy retrieving pucks against those two heavyweights.

In 2023-24, Eyssimont scored 11 goals in 81 games for the Lightning. The abrasive player has the motor and hands to create offense, even if he's better suited for the fourth line.

Duran made two appearances for the varsity last season. The rugged 23-year-old is a longshot to start this season up top.

Bruins training camp battles: 6 jobs up for grabs and who's in the running