I have kept Koi carp for years, and one of the hard lessons I learned very quickly was not to feed the fish protein in the winter months. The specialist vet who looks after my Koi has explained that their digestive system slows right down when the water temperature falls to about 40f or below. At that point, although the fish will still eat protein they find it very hard to digest, and the food can lie in the digestive tract and rot.
When the digestive system again kicks in as the water warms quickly, this can lead to blood poisoning and death.
For my first two years of keeping Koi, through ignorance, I fed protein through the winter, and lost a high proportion of my fish each Spring. Since I have fed nothing through the cold months, I have lost none.
A natural factor leading to the fish being very vulnerable in Spring is the sudden explosion of parasites and flukes in rising water temperature. Healthy fish can generally cope with this, but the added burden of internal poisoning is lethal.
Lastly, at no other time of the year are the fish so susceptible to death through stress induced diseases. So, if we add the factors of year round angling, where carp are pursued relentlessly in the Spring months, which never occurred until a few years ago, plus more and more anglers winter carping and introducing protein foods, we have a potentially explosive cocktail.
It would be an interesting experiment if any carp syndicate, where Spring deaths have been a problem, agreed to ban any protein baits during the winter months, and just fish with carbohydrates. Those Koi keepers that do feed in the cold months use carbohydrate sticks without problem.
Any thoughts on this?

Here are my initial thoughts, Tony.
a) You have been fishing for carp for I suspect as long as me. Twenty or even ten years ago, there was no such thing as Spring Mortality Syndrome of carp. I believe the main cause is the importation of foreign carp or other cyprinids introduced into waters having contact with foreign carp.
b) I don't think you can compare Koi carp with carp kept in a more natural environment. In the latter the carp have access to and utilise a far greater range of natural food in the winter months to provide a more balanced diet. Therefore, as a user of high protein milk baits in winter for the past 25 years, none of the waters I have fished using these baits have had a fish-kill problem.

It seems to me that carp deaths happen in the spring after a stocking with new carp or coarse fish. Pollution is another reason. I can't see how bait can be responsible - the buggers don't eat enough in the winter - not on my lakes anyway!

How could you enforce a ban on protein baits in winter? You'd have to come up with some hard scientific evidence to establish there is a problem. How come all the Darenth fish survived for all those years on protein winter after winter then Tony?

Perhaps we should just stop fishing for them at all.
A load of old ******** Tony!

Let me respond to Paul first of all.
Actually, my first carp, a streaky 19lb fish taken on a whole floating bread roll, was taken in 1967, some 33 years ago!
The point about importation of foreign fish is a good one, and one thing I omitted when I posted the topic. Going back to the Koi, keepers never stock with new fish in Spring without a lengthy quarantine in holding tanks where the potential new stock can be treated for parasitic problems. Gill flukes in particular spread like wildfire when the water is warming in Spring and are very difficult to eradicate once they have taken hold.
Your comments on 25 years of winter fishing with milk proteins without a problem are very interesting and a powerful argument. However, the bulk of those years we had an enforced close season, with stress levels on the carp at minimum from mid March to mid June.
I have never said the use of high protein baits in winter is the cause of Spring Mortality Syndrome; I just wonder, with the strong evidence from Koi keeping, whether it just might be one other contributory factor.
I agree that keeping Koi is a more unnatural state that open carp lakes, but many Koi keepers hold their fish in open, unfiltered, natural pools, where they have access to ample natural food as well as introduced protein pellets. There is plenty of evidence that those Koi suffer high mortality in Spring if fed protein through the cold months. What we don't know, of course, is at what temperature the digestive system of our carp starts to shut down. With Koi, it is around 40f, but it could be much lower with other carp. This would render high protein baits only potentially dangerous during the coldest snaps.

Fred, I have not said that bait is responsible for Spring carp deaths, only that it may be a contributory factor. We must always keep an open mind on such possibilities, as was the case with peanuts for example.
Steve, your comments on Darenth are significant, and I am not advocating a ban on protein baits in winter. I merely said that it would be interesting if a group of anglers in a syndicate imposed their own controls to see the outcome.
Paddy, you may think my point a load of old *******, and in this context with Spring Carp Mortality you may well be right. What is a fact, however, is that Koi carp die in Spring in significant numbers if they have been fed protein through the winter.