Why do you think that they could NOT summon fish? That paragraph right before the Salmon Accioing was:
"My mother," said Ron on night, as they sat in the tent on a riverbank in Wales, "can make good food appear out of thin air."
He prodded moodily at the lumps of charred gray fish on his plate. Harry glanced automatically at Ron's neck and saw, as he has expected, the golden chain of the Horcrux glinting there. He managed to fight down the impulse to swear at Ron, whose attitude would, he knew, improve slightly when the time came to take off the locket.
Clearly, they HAD fish. They were just too bad of cooks to cook it in a Molly-quality way.
