Why would you motion track something that you animated? Simply child a null to the dolphin, and position the null at the spot of the dolphin you want to “track”, then use the toWorld transform expression to get the world position of that null as it moves with the dolphin. From there you can either use both the x and y (and even Z if you need it) or just one of the axis.
That way if you want to change your animation later on (’cause that never happens) you don’t have to retrack the motion. (plus you don’t get the inherent positional noise added when you do any kind of motion track)
SteveD, I think its was probably the rotation of the dolphin that didn’t allow my first suggestion to work for you. If the rotation point of the dolphin was the point you wanted to follow it would work, but if the point you wanted was somewhere else that point would be offset by the distance but also by an ever changing amount based on the how far the dolphin’s angle had changed. This could be calculated with some simple Trig, but its far easier to use the toWorld function that tells you the position of something like a null even though that null is a child of an object and the object is rotating. If you just asked for the position of the childed null with the pickwhip, all you would get is the position of that child object in relation to the parent and not in relation to the viewport.