Sorry, man! That is one of the worst decisions to have to make at any time, no matter if the dog is old, sick or injured... I cried like a baby when I had to put my Sheltie down (quite a while ago). The cried even more when an envelope came from the vet containing a printout of that sappy "Rainbow Bridge" poem and an ink impression of the dog's footprint on the bottom.
Maybe you'll have the chance to find another fishing buddy. They are never a replacement, but they find their way into filling that quiet void in our lives.
Sorry for your loss! He was a great looking Dobie!