When the herring gull dived for a fish my reflexes were not quick enough to take turn the camera into the vertical format and get the image, so I have cropped the horizontal image to make the composition more acceptable.
Although I don't use it that often the 300mm f2.8 is my favourite lens, it just snaps into focus.
I really like the pose and the moment captured, but I'm not really settled with the crop.
Without seeing the entire frame available to work with, I wonder if you couldn't have gotten the bird more toward the upper right corner somewhat and have gone with a more rectangular crop?
I kinda like the side lighting. If the light had been more directly behind you, it would not have given the under wing detail I'm seeing.