Catch and release varies and IMHO at the end of the day is really down to your own morality ... any water that is specifically requesting catch and release probably has a good reason to do so as illustrated above. In particular in Scotland numbers of sea trout and salmon have been (dangerously) low in recent years. the beautiful silver are coming back though, it seems due to catch and release policy on a number of rivers and anglers' changed attitudes (ie don't kill them just cos you caught them) .
My catch and release morals are roughly --
-Never take more than two salmon (a lot of people would say that's greedy)
-Put back big salmon (big fish beget big fish)
-Put back hen salmon (the spawners, look online for how to tell difference between cock and hen - not easy)
-Put back big sea trout
-Never take more than one small (half pound) sea trout
-Put back big brown trout (anything over 2 lb)
-Keep any damaged, injured or exhausted fish (these can introduce disease into a water and will probably die anyway)
-I don't have morals that apply to coarse fish (pike, carp etc) for me these taste crap so i put them back
-For sea fish i keep middle size fish that i can and will eat.
For general on trips any fish more than 2lb is a lot to eat at one sitting and fish goes off quickly if you're thinking of your freezer. Slaughter for the freezer can be wasteful unless you know you can get them home in time. Also fishing, camping, hiking and setting up an earth smoker can take more energy than you (or i anyway) actually have. Point being not to kill unless you know, for sure, you will eat the fish.
Highland Scotland is covered in burns, lochs and rivers full of fairly small, naturally occurring, brown trout, the problem in many waters is too many small fish and stunted populations, away from the roads few gamekeepers will bother if you take a couple. In stocked, or easily accessible waters it can be a different game; I suggest checking the web for info on the rivers or lochs in your chosen area and you'll get an idea of what the setup is.
For an outdoor taste experience you're not gonna get at a restaurant take some vinegar (or lemon) with you, catch your salmon, kill on the bank and skin the back of the head/neck above the gills (the otters' choice) , add vinegar and bite ...
@munkiboi
those environmental agency regs look like they only apply to England.