Robert, as was discussed on the other thread that was linked to in this by Colin, it comes down to sensible use of any bait that is manufactured, the exception is obviously anything thats alive.
I'll give you an example, since the product spirulina ( a form of blue green algae ) was discovered as a food source, ( which is supposed to have magical health giving properties for humans ) it quickly caught on with fish rearing enthusiasts. it contains just about everything a fish needs to survive, and flourish, and has proven major health benifits for the fish, the list is too long to go into here.
Some fish keepers decided that why mess about with all these balanced fish diets, and just feed them on Spirulina, sure enough the fish appeared to do very well on it, and as a plus the high levels of carotine in spirulina had produced some amazingly vivid colourations in the fish, which particularly suited the Koi enthusiasts.
It was some proffessor chap can't remember his name, ( couldn't pronounce it anyway ) urged caution because of the high levels of vitamin C, ( spirulina in the harvested and concentrated form has extremly high levels of vitamin C ) and it's now recognised that feeding fish exclusivley on a diet of spirulina, can have serious long term effects on fish from vitamin C poisoning.
The point is that it wasn't the spirulina that was at fault, it is an extremly usueful ADDITIVE for a fishes diet, used in correct quantities with other ingredients.
As i said in the other thread, personally i doubt in a natural flowing river enviroment, that it would be possible to get fish to exclusivley feed on one type of food source, it may become a very large part of their diet, but i do believe that fish have natural instincts which enable it to tell if a food source is making it sick, or even that as Darren George put it the reward is not worth the effort.
The same may not be true of some stillwater enviroments, - carp puddles if you like, where - and i know this is the extreme but they do exist, the natural background food has been exhausted, or possibly never existed in the first place, these places see the fish totaly reliant on anglers baits, and in such cases serious nutritional deficeiencies may occurr when even the variety of bait is reduced, and mostly a 'going' bait is used.
Shelf lifes, ready mades, whatever i doubt will do the fish any harm at all in the quantities that you are likley to use them and especially in a river.
Thats not to say you wont stuff everyones fishing up if you pile them in, but harm the fish - i doubt it, the only thing you'll do any great damage to is your pocket !!
Ian.