Those seagulls are rapacious bastards for sure. Waterproof crows. This one knows the eagle is at a big disadvantage at that moment and takes advantage of it. Pretty smart for a birdbrain.
Anyone who has ever eaten outside at a seaside restaurant knows just how aggressive and wily they can be.
Of course, if the seagull was really smart, he would have let the eagle catch the fish first, then nailed him.
Seagulls, particularly Herring Gulls are rats with wings. However they have much to fear from the Sea Eagle. I have seen some footage where a Sea Eagle plucks a near full grown gull from its cliff side nest. During the breeding season they take a lot of gulls this way.