I remember fishing Rainbow Lake in France where this type of phenomenon can be very common in many of the swims at times. (It can be unbelievably frustrating hearing a fish crash out literally sounding like a cow just fell in and then putting a hook bait on the spot only to never get a take there for an entire 2 weeks straight!)
Carp as I mentioned are benthic feeders obviously evolved for feeding on the bottom and in the case of carp and bream species, they are extremely well adapted to feed upon bloodworm (dipteran or mosquito or midge larvae) and tubifex (a different insect larvae group.)
White bream, barb bronze bream and carp are probably the best adapted to specialise in the most efficient sieving type particulate feeding in sediments on dipteran larvae in particular. Carp can often spend significantly long periods with their heads totally buried in silt feeding on bloodworm and other larvae, etc which may be present. The diet may be supplemented by seeds, molluscs, algae of various types, water weeds and of course anglers baits more and more.
Analysis of this larvae shows it contains high levels of many of the most essential amino acids absolutely crucial in a carp diet to maintain survival. But perhaps just a little more pertinent to us is the fact that this larvae most anglers refer to as bloodworm also contains among the most stimulatory amino acids to carp feeding which impact most intensely upon carp chemoreception and olfactory systems. It is no surprise therefore that bloodworm and its extracts have been adopted for use in fishing baits effective for carp and many other species too.
Other smaller benthic feeding fish are familiar to anglers; barb and the humble gudgeon, although barbel are highly regarded; a hard-fighting double figure barbel is a very worthy goal. Crucian carp are noted bottom feeders and are often extremely gentle biting, giving little indication of a bait being nibbled or fully taken in the mouth. These fish can be very hard to hook especially considering I discovered very many hooks we use today just do not hook deeply enough quickly enough to be of any use whatsoever.
This knowledge has been applied to carp fishing very effectively and testing baits on crucian carp is also an extremely valuable way to find out how effective your baits for the far bigger cousin Cyprinus carpio really are. Crucian carp are easily caught on earthworms, bloodworm extracts, yeasts and more bacterially active cheeses for example. (These probably ring a few bells with you carp anglers...) In fact other benthic feeders are very keen on bloodworm extracts including tench.
The impact of the introduction of large volumes of anglers baits upon a fishery including that of pellets, boilies, and ground baits of many forms including live maggots, jokers and bloodworms etc definitely change feeding behaviours.
In the case of very pressured carp it may well be this simply leaves more natural food much more easily and consistently available to eat safely and sustain the largest fish in a water. Hence one significant reason why certain very big fish are sometimes seen but only caught rarely (if at all!) Some fish are so attuned to their natural environment of natural food items and their distinct combinations of chemical signal for example that it is very possible they may be totally blind to most anglers baits not triggering them to feed, especially at times when natural food is extremely abundant.
It is no coincidence that December, January or February are times when rarely caught monsters are sometimes landed when their natural food may well be in insufficient densities to be consumed energy efficiently for example.
When it comes to carp, pump filter feeding with very slow swimming motion or a stationary holding position is often a common one and seeing very big fish feeding like this can be absolutely fascinating to the angler even though such fish may be totally absorbed in consuming zooplankton and initially not your baits. In this form of feeding they are utilising the many taste buds in the pharyngeal cavity which select the food items as the fish swallows water; through hence the pump effect.
Feeding behaviour of carp can differ at different stages in the life cycle yearly food cycles naturally available and carp can be observed picking up individual anglers baits in careful methodical fashion one minute, while having gorged on fry having darted after them and gulping them down into their stomachs live previously. I recall catching a big carp which disgorged its load of live roach fry which on the unhooking mat. (Who says carp do not turn carnivorous at time!)
Carp are the masters of particulate feeding, but often their methods of feeding cause more mobile food items to escape because of the water turbulence they can cause. But always being on the look-out for discoloured water or even the odd appearance of sticks and rotting leaving and of course unnatural bubbles especially at dawn or dusk, are very easy ways to identify carp feeding spots.
The strength of suction of carp is powerful enough to have a bearing on exactly how to make some carp rigs perform better and in many cases I have found making a rig more weighted at the hook by the addition of tungsten shot or putty or very dense homemade hook baits to be extremely effective.
I rate the ability of the homemade bait maker to change and manipulate bait density and weight especially, as well as texture, shape and buoyancy, to be massive edges over those anglers purely using machine-rolled or extruded boilies and pellets...
This fishing bait secrets ebooks author has many more fishing and bait edges - just one can impact very significantly on your big fish catches!
By Tim Richardson.