For those that have not seen fish react to a "spod" (or bait carrying devise' being cast into the water, this has to be seen to be believed. Fish will actually feed on baits on the way down; often the bigger fish will monopolise this baiting-up "dinner bell" effect. This way, they can get all the free "safe food without a hook" they want by gobbling it up as it falls through the top and middle water layers. Few anglers I've spoken to appear to realise this.
Until you've caught big fish "on the drop" when you definitely had a "PVA stringer" or a "PVA bag" of bait still attached, it may be difficult to more fully appreciate the full impact of this behaviour in so many ways! ('Match fishermen' manipulate and exploit such fish movements often in order to win.)
Boilies can often follow a "wobbly" route to the bottom of a lake or river; even rolling upwards and sideways. Many pellet type baits can be even slower to sink. Pellets made mostly by extrusion methods and some are not as dense as a boiled or steamed "boilie" bait. Some pellets have a slightly flattened shape and are often packed with oils which can produce slower sinking being more buoyant in water. Cheaper fish and animal pellets may have more air in them too.
In fact, such properties and characteristics are all important weapons at your disposal in the correct fishing situations. For example; getting your free offerings down quickly to the larger fish, where smaller ones abound is sometimes important. Or conversely getting you free baits to stand-out on silt or weed. By having them land much more gently than denser or heavier baits which sink and become obscured, catches can sometimes be much improved. (Even fishing slow-sinking bread flake on top of weed beds as an "instant bait" can be extremely effective, despite the plethora of various baits available today!)
An angler may use a "spod" or "bait rocket" throwing stick or catapult to introduce his free ground baits of various forms into a water. The fish can very physically directly "hear" and experience the sounds of the baits and tackle etc via the lateral line and vibrations in the swim bladder etc. Think about the last time you went swimming and heard sounds under water, like people talking around the swimming pool and the characteristic echoing splash of someone jumping in! (This can be deafening in water.)
These sounds can seem magnified in strange ways to our "terrestrially designed" ears. So why not imagine the affects of a big heavy lead sinker or a full large PVA bag, or a heavy "spod" striking the water. Those highly evolved aquatic senses of a fish must be so well aware of such activity and such a bang in close proximity in shallow instead of deep water must be akin to a bomb going off! As an example a fully laden pre-wetted bait carrying "spod" hitting the water, produces a very significant set of sound waves. These are vibrations which fish receive directly through the water even from long distances away via their lateral lines specially adapted cells.
The fish may respond or react using various behaviours , in particular one being to associate this sound with a threat and retreat away from it. However, do not fall into the trap of always applying this "rule" as some big fish in particular waters will respond completely differently!
For instance, I've used small balls of mud to "bait-up" a swim, (when I've run out of bait) quite a few times and the clouding of the water and sound seemed to keep the fish interested. I notice that renowned anglers like Terry Hearn are fond of using special tactics using natural materials to alter the characteristics of the bottom of a swim, cloud up the water and so on.
I admit to pouring gravel onto spots, introducing soil, sand and even garden compost rich in natural worms, and larvae in to a spot to enhance it in various ways. Stirring up of the bottom sediment to release bloodworm and other benthic organisms can really attract fish into an area. Carp are curious creatures, always checking out objects and changes in the water for potential opportunities and threats, so take advantage of their natural behaviours.
The detection by fish of movement in the "bait," chemical changes in the localised water involving flavours etc, colouration, presence of suspended particles, bubbles rising in the water and so on, all add to potential attraction. There is yet another bait fashion, that of 'active ground baits' and these certainly aim to exploit more carp senses more effectively than conventional less active baits and ground baits. (Very many modern ground baits can be used as a base for effective boilies and paste baits and ground bait digestibility has become a far more significant factor in ground bait design than in previous decades.)
