A statement of theme arises from the heart of the meaning of the story. In this Wetherell story, the heart of the meaning of the story is regret. The narrator regrets all his life losing that bass because he was trying to land the bigger fish--he thought--of Sheila Mant. A statement of theme reflecting this concept might be written this way: The wisdom and passion of youth confuses things of passing importance with things of enduring importance, leading the way to lifelong regret.
The narrator had the small fish in the boat--with her lazy stretches "toward the sky" and her figure like a model--but had mistaken her for the big fish, while taking a "penknife" to release what turned out to be his life's enduring big fish. He lost interest in Sheila's spell "before the month was over," but all his life, it was the bass's "secret, hidden tuggings in the night that claimed" him.
Sheila, on the other hand, said "she would be going home in Eric Caswell’s Corvette" without him, "funny kid" that he was. Because his youthful passions and (limited) wisdom confused the enduringly important with the passingly important, regret tugs at him in his secret thoughts in the night all his life long.
I could feel the strain of the bass, steadier now, growing weaker, and this was another tug on my heart, not just the bass but the beat of the river and the slant of the stars and the smell of the night, ... I pulled a penknife from my pocket and cut the line in half. ... I never made the same mistake again.