Although much knowledge can be gained from the bank, to read a big pit well, a boat is essential. Bankside observation cannot reveal the underwater structure of out-of-reach areas, and at best only shelves near the bank, depth changes and other structures can be charted. By rowing round the pit and sounding the bottom with a long pole or plumb line, however, you can assess the underwater structure fairly well.
With a boat, you can also investigate scum lanes – oily surface strips that remain calm in a ripple.
The reservoir angler uses these to find trout, the pit angler pike.
Many gravel pits were dug dry and did not fill with water for several years. Trees and shrubs grew, and later, when the pits were filled, there were large areas holding submerged or partly submerged trees. The water killed them, but they remained hard, in some cases as if petrified. These trunks, together with fallen branches, provide cover for many species of fish.
When searching for underwater structures from a boat, any indication of a stump or underwater tree should be investigated. The bottom there may be littered with snags, but it is an undeniable fact that these areas are fish larders and sanctuaries. How to fish them is a pro-blem, but such fish as rudd, roach, perch, pike – and occasionally bream and tench – can be taken just off the bottom, a little away from the snags. Never ignore these places.
Anchor the boat and watch for movements near the surface. See the terns swoop and grab small fish, and the heron perched on one of the emergent stumps. They are there because they have seen fish.
Many gravel pits have underwater banks and channels running in the same direction. Locate them and mark them well so that you can follow one or the other and not cross over. Fish tend to lie either over the banks or in the channel – temperature and food availability being two of the obvious reasons for this – although on occasion they swap from one place to the other. Drifting for pike with dead baits or lures along, say, a channel, is critical to within a few feet, and if the pike are in the channel there is no point in drift-fishing the banks.
Information about the pit bottom can be easily obtained by using a transistorized sonar unit. Many are available today, and some even chart the bottom on paper as the boat proceeds. This type is not strictly necessary for gravel pits, and simple units which give the exact depth on a rotating dial are quite adequate.
It takes a while to be able to read what the unit is saying, but once mastered, you will be able to recognize mud, weed, branches, rocks, hard gravel, ledges, drop-offs, deep holes, and the exact depth.
Locating the shoals
The permanent reading at zero represents the surface, while the movable signal, shown at various places on the dial as the boat proceeds, indicates the bottom. Little blips between zero and bottom indicate fish or fish shoals. Do not imagine, however, that you can get a very direct reading of fish location from the sonar unit. Rather it enables you, once you have learnt to translate the signals, to transcribe a picture of the underwater topography, which in turn will help you locate the shoals.
The unit gives the strongest signals on a bottom of gravel or hard clay. Such signals are very clear at a depth of around 40ft, but at greater depths, up to 80ft, you need to turn up the ‘gain’ or power.
If you then move over a bottom of mud and decayed vegetation, the clear, distinct signals will disappear at times. This does not mean that you have suddenly come to very deep water – the dial would have shown the drop-off with a wide band of signals – but that the mud and vegetation have absorbed the sound waves, making them disappear.
Over a gravelly bottom with weeds, the signal unit will transmit many thin signals, which climb towards the zero point as the boat approaches and return towards the bottom point as the boat leaves the weeds. The same signals will register as your boat approaches or leaves trees or bushes. You can use these signals to steer your boat along the edge of weedbeds.
Sudden changes in depth
The unit also registers sudden changes in depth, for example where there is an underwater cliff, or a submerged tree. In either case the unit will give two readings, at the highest point and at the bottom, but an underwater tree gives weaker signals than does a reef or an under-water cliff. It is possible to mistake a tree for a shoal of fish. If you are not sure, anchor your boat bow and stern so that it cannot move, and take further readings. Constant signals indicate a tree, while intermittent signals indicate fish.
The sonar unit itself is simple to operate. There is a transducer and a dial. The transducer is hung over the boatside into the water and the unit switched on. As the dial rotates, a red light shows at zero and at the bottom. Some adjustment is needed to ensure that the dial is recording correctly and only experience will tell you if you are using too much or too little gain.
There is no doubt that transistorized sonar units make the reading of big gravel pits much easier. They never lie and always record accurately. It is more or less certain that if an angler has the ability to read the signals, he will also be able to read the water.