never fished the kelvin but it is well talked about on here anyway ,
im a season ticket holder for the carron , It really depends what your after .
There were loads of salmon catches last year and this year... but mostly on the flying c

I very seldom see someone fly fishing for them in the carron .
Sea trout, are there but iv never seen 1 being caught , but im onto them at the moment !!
brownies , stocked 4 times a year with fish at 9 inches long probably most at half a lb to 3/4lb , My biggest has been a 2lb stockie which was put in big lastyear by coincidence so its rare to get 1 that size as those fish are more expensive for them to buy .
wild brownies , loads of wild brownies at certain stretches of the river up to about half a lb again , with loads of parr and smolts mixed in .....
some fantastic stretches that are rarely fished as people are to lazy to walk far from the car these days .
Im not sure about the Kelvin but you do get the odd thing floating past , lastweek I saw everything from a pair of seals chasing sea trout i beleive right up the river to carron works 4/5 miles from the forth

I see loads of minks , otter , and even 1 of those white plastic patio chairs !!!

but this all in the low stretch thats tidal .
18 miles of river for 40 odd quid a year and only 20% of it is actually fished .
I personally love it .