Phish feels a bit like a one-man band

Phish is not a band of equals. Not now, as the quartet returns to action after a two-year hiatus. They are all fine players, at times capable of striking passages of rock, jazz and folk, but their celebrated improvisations hinge on the creative whims of just one player: guitarist Trey Anastasio.

At least that was the case Friday at the band’s three-hour Valentine’s Day concert at the Forum. Phish’s musical energy during long instrumental sections rose or fell on the strengths of the guitarist. When Anastasio left spaces open for the others, too often the response was more noodling than focused inspiration.

In the ‘90s, Phish led the still-growing “jam band” movement, but like others from the same genre, the group was always at its best when taking off from a well-structured song. And Phish music is most charming the deeper it slips into bluegrass, as it often does on the just-released reunion album “Round Room.” At the Forum, the song “Walls of the Cave” was typically country-fried, almost sounding like an outtake from the Band, even after a long, bewildering intro dominated by keyboardist Page McConnell and drummer Jon Fishman.

Other songs, particularly “Prince Caspian,” which closed the night’s second set, were focused with energy and emotion, with explosive playing from Anastasio and lyrics that were little more than a mantra of “Oh, to be Prince Caspian afloat upon the waves.” Phish was also capable of moving re-creations of old songs, from a version early Friday of “Cover of the Rolling Stone” (originally by Dr. Hook & the Medicine Show) and an emotional, near-perfect take on the Rolling Stones’ “Loving Cup,” which closed the concert.

Two years ago, Phish made a surprise announcement that the band was going on indefinite hiatus. It was unexpected not only for longtime fans but also for new listeners finally taking notice of Phish as it crossed over into the mainstream, appearing on national magazine covers as the leaders of the jam band movement.

Its four members made no promise to reunite, and they embarked on a variety of solo projects. Anastasio seemed to be thriving last year, stretching out with Les Claypool of Primus and ex-Police drummer Stewart Copeland in something called Oysterhead, plus a big-band project inspired by the Afro-pop of Fela Kuti.

By modern industry standards, two years between albums or tours is no great stretch. But for Phish it was like an eternity, after spending most of its first 17 years on the road, with a caravan of fans permanently along for the ride, taping every show, as Phish filled a role once held by the Grateful Dead.

Those believers were in full force on Friday, dancing in the aisles, visibly lost in the music and far beyond the usual security measures that keep listeners in their seats. Many of them cheered wildly throughout the night, a wiggling, clapping throng under the colored lights.