What an interesting question!
I believe that the title clearly expresses the narrator's priorities in order of importance, within the context of the story. The bass is obviously the most important since he was a keen fisherman and he states this clearly in the text. It is pertinently evident that most of his time was spent either practising his hobby or exercising what could only be deemed his first love.
... I never went anywhere that summer without a fishing rod. When I wasn’t swimming laps to impress Sheila, I was back in our driveway practicing casts, and when I wasn’t practicing casts, I was tying the line to Tosca, our springer spaniel, to test the reel’s drag, and when I wasn’t doing any of those things, I was fishing the river for bass.
Although his initial focus in the story is Sheila Mant, she was obviously not more important than fishing (for bass). This almost obsessive hobby is what later creates both his inner and external conflict. He 'automatically' stuck his fishing rod in the stern of the canoe and later also 'automatically' tied a big Rapala plug to the line and let it into the water. He then forgot about it.
When a very large bass is later attracted to the lure and sets the reel spinning, it created consternation for the narrator. He had tried to impress Sheila with his knowledge of fishing but she showed her disdain for it by calling it 'dumb'. The narrator obviously had no intention of appearing 'dumb' to Sheila and tried by all means to hide what was happening from her. He invented all sorts of excuses on Sheila's intermittent queries about the canoe and the sounds that she heard. This was the nature of his external conflict.
He was torn between continuing his secret attempt at snaring his prey and pleasing Sheila so much so that he hardly noticed her talking. The question was whether he should continue his attempts at capturing probably the biggest prize he had ever caught, or to please Sheila. This was his inner conflict.
His mind was finally made up when he looked at Sheila and saw her:
...stretching lazily up to the sky, her small breasts rising beneath the soft fabric of her dress...
The 'tug' he feels at this became too much and he took out his penknife and cut the line, setting the bass free. The memory of the bass, however has remained with him forever:
... the memory of that lost bass haunted me all summer and haunts me still.
It is clear that the narrator regrets having been overwhelmed by desire and acknowledges that he never made the same mistake again.
The river is important since it was where he practised his favourite hobby - fishing for bass and it is also where he regularly took note of Sheila, sunbathing. It is in the river where he tried to impress her with his swimming and diving. Unfortunately, his efforts were ignored. Ironically though, it is his invitation to take her to Dixford in a canoe that provides him with an opportunity to impress her. However, even this valiant effort is doomed to failure, not only for the above reasons but also because Sheila leaves Dixford that evening, not with him, but with Eric Caswell, a favourite of hers.
It is quite ironic that Sheila Mant appears to be the least significant feature in the story. The narrator was so overwhelmed by his experience with the bass during their journey, that the 'the rest of the night (became) much foggier'. He hardly remembers the occasion and only vaguely recalls dancing with her 'once or twice'.