Hi, great topic that is close to my heart, so just thought I would add my 2 cents worth.
I have been fishing all my life, from spear fishing, bottom fishing, shell fishing, long lining, netting, potting, trawling, I even worked on a fish farm during my summer holidays as a student.
Having seen the varying ways people dispose of fish, I think one should most definitely respect the fish and make it as humane as possible.
I have seen people beat a fish with belaying pin or other blunt object, which works sometimes, but mostly ends up in a writhing blood splattered battle with the poor fish.
A knife between the eyes, works well on small fish, but try it on a 30kg tuna!
Alcohol on the gills, to be honest I have even tried this myself but it has never seemed to calm the fish and you still have to kill the fish.
On the fish farm, we would dip net the salmon and put them directly into a bin of iced water where they would be gassed, not to be killed but to be calmed. Once calm we would use a short bladed knife to go in behind the gills and slice the heart for a very swift end. Slicing the heart drains the brain of blood in seconds. It needs to be quick because the last thing you want is damaged scales. This method also allowed for bleeding which is important for the flesh in commercial salmon. I have always considered this to be the most effective and swift way.
These days, I always have a couple of lines trailing in the wake. When I catch something, I try to get it on board as quickly as possible. Once aboard, I tie a 6mm line around the tail with a self tightening loop (already set up ready to go) then i hold the fish down and cut the heart to dispose of the fish as quickly as possible. Then he goes back over the side hanging on the line around his tail to bleed. (note, he is already dead by this stage and is not bled to death)
After 5 min or so, back on board ready to be appreciated as a culinary delight.
Our personal favorite is to change the lunch menu to a fresh plate of sushimi. Then we plan the next few meals around the fresh fish, and smoke (in the cobb) or freeze or dry the rest (fish jerky or biltong...yum!). the head and bones become fish stock.
I figure there are some simple rules.
Only catch what is legal i.e. size, bag limit etc.
Only catch what you can and want to eat
If you do catch something you don't want or is not legal, if at all possible, return it to the sea alive. If it is not legal but has died, should you throw it back anyway? Personal choice there, but it seems a waste of a life to throw it back. Just don't get caught with it, the fisheries will not see it the same way!
Dispose of what you keep as quickly and humanely as possible.
And before you put a line out, get yourself set up with all the gear you will need to get the fish aboard, kill it, clean it the boat and yourself, and cook or preserve the meat. If it is a hassle, you wont enjoy it.
Happy fishing!
...Maybe you should grow some sensitivity to the world around you. worms are nice and calm till you pick one up and they then they start squirming to escape your grasp. If that worm had no feelings it would have laid placid in your hand but it felt the dangeropus giant that had grabbed it and was frantically trying to escape. Just as you would do if you were he/she. 
