You must create your entry from a single capture. While multiple images of a scene are allowed for dynamic range (HDR) and focus stacking, you can't combine elements from two different scenes into one image.
Essentially, that's what you did here: you picked one "scene" where you liked the lighting better on the sushi and a second "scene" where the fish is better. We consider them to be separate scenes because the fish position is so radically different.
We realize that is a subtle difference, but we have to remain consistent with earlier rulings: otherwise, we'd need to allow such things as swapping heads-and-bodies to combine gesture with expression in shots made with burst mode, and we have historically not allowed that.
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