Nice look at this cool looking Gull. The dead fish is to decomposed to add much to the image IMO.
Just some friendly advice:S3:
If you are manually controlling the aperture and shutter speed then you might as well start learning/getting comfortable with shooting totally manual, without auto ISO.
Exactly as Gail just stated, most people that shoot manual not only shoot manual ISO, they figure out a minimum ISO that their camera performs best at and hardly ever go below that setting as it gives you more room to crank up the ss and aperture if needed. You need to assess your camera's ISO performance by looking at some images and then figuring out what you like as an acceptable ISO. On my 1DX I like to shoot at ISO640 as my base/minimum. The images look great and I don't notice any noise so I always stay there and go up if need be. Sometimes as I am sitting with a subject getting lots of close portraits I will go down to 400 if I remember but it dosn't make that much difference.
You also underexposed this image at capture. You mentioned "maybe underexposing whites is not a bad thing" I think it is! It tends to lead to gray looking whites as opposed to bright clean whites. I see many people exposing thinking only for the whites, or underexpose their whites, and wind up with whites that are muted and dingy.
Look at your finished file and you can see the gray whites and the gray V below the neck that isn't there in the original file. I would have exposed this shot with a some blinkies on the shoulder utilizing my highlight alert. This blinking area is usually not fried and easily recovered with a highlight slider or brush.
You then wind up with nice "clean" whites and more detail in the darks, although the darks look pretty good on this particular shot.
Hope this helps
Keep em coming