Fish finders and echo sounders discussed

Ever felt you you may not be getting the best out of your fishfinders? The little black box on your RIB’s console...

Echo sounders are the professional instruments, fish finders are for hobbiests. Neither statement is entirely true. Fish finders are not really for finding fish. They will find the kind of sea bed where the fish hang out.

The principle behind fishfinders is exactly the same as that of an echosounders. They send a short pulse of electricity to a piezo-electric crystal in a transducer, where the electrical energy is converted into a click of very high frequency sound.

The click travels down to the sea bed, and the echo bounces back to the transducer, where the same piezo electric crystal converts it back into electricity and returns it to the display unit. Sound, in sea water, travels at about 1440 metres per second, so by measuring the time interval between the initial pulse and the weaker return pulse, echo sounders can work out how far the pulse has travelled, and from that, the depth of water.

The seabed, however, is not the only thing that reflects sound energy. Some remarkably sophisticated electronic processing goes on to decide which echo has come from the seabed. That is why echo sounders often lose interest in shallow water and why others take several seconds to work out the actual depth.

For a fish finder, however, these spurious echoes are absolutely fundamental. It doesn’t have to discriminate between fish echoes and bottom echoes - or anything else. Instead, it shows everything, and gradually builds up a picture in which the seabed shows up as a continuous line, with bubbles or fish appearing as short, arch-shaped blobs. Suspended silt, stirred up by tidal eddies around obstructions often shows up clearly.

Even layers of water at a different temperature or salinity than the water above or below them can often be picked out with a well set-up fishfinder. The clever processing, which in echo sounders eliminate spurious echoes and produce a digital display of depth can be used, instead, to pick out the arch-shaped blobs, and replace them with fish-shaped graphics, while the shape of the sea bed is clarified by a feature that is variously known as white-line, grey-line or colour-line.

The sound from the transducer doesn’t spread equally in all directions. It’s focussed, at least to some extent, so that most of its energy is concentrated into a cone-shaped beam, whose sides diverge at an angle of about 20 degrees. In other words, at a depth of ten metres, most of the energy lands on a circular patch of sea bed, some three and a half metres in diameter. At a depth of twenty metres, the patch expands to seven metres, and so on.

The sea bed, however, is rarely flat, particularly not around the rocks and wrecks that are of most interest to fishermen and divers. So if a fishfinder transducer passes over a wreck lying on hard sand it would hear echoes coming back not only from bits of the wreck at various depths, but also from the sea bed around it. A basic fishfinder would show the whole lot as a thick black line making it very difficult to tell whether it was really a wreck, or just a hump in the sea bed.

Almost all fish finders nowadays have a feature known as whiteline or greyline, which adds a band of white or grey below the strongest echoes.

The effect is dramatic. Soft seabeds show up as a thin dark line with a thin lighter line below them, hard seabeds show up as a distinct black line, with a much thicker light band below them, while isolated rocks and wrecks stand out almost as clearly as if someone had run a highlighter around them.

Garmin built a reputation on GPS receivers, but over the past few years the company has dramatically extended its range to include chartplotters and fish finders.

Echo sounders

Echo sounders can do the following:

Locate where schools of fish are hiding around a structure and their depths.

Locate shipwrecks, reefs, pinnacles, sunken trees and other objects between the hull of the vessel and the bottom.

Determine bottom contours and composition to recognize soft mud, gravel, sand and rock.

Determine the water depth and locating hazards for navigation purposes.

Determine the location of areas where the temperature of the water changes dramatically.

Determine what species of fish are being shown on the echo sounder display. (An experienced operator can determine what species of fish are being displayed by their different shapes and the size of the air bladders in the fish, a trait that shows up well on high quality echo sounders. Other factors such as water temp, water depth and schooling characteristics will help determine fish species.)

Echo sounders determine the distance between its transducer and underwater objects such as fish or seabed and show the results on the display. An ultrasonic wave transmitted through water travels at a nearly constant speed of 4800 feet (1500 meters) per second. When a sound wave strikes an underwater object such as fish or sea bottom, part of the sound is reflected back toward the source.

The depth to the object can be determined by calculating the time difference between the transmission of a sound wave and the reception of the reflected sound. The sounder will then display this return as one of up to 16 different colors (color sounder) or a different level of grey scaling (monochrome sounder) depending on the strength of the returned signal.

There are two display types to choose from, CRT (Cathode Ray Tube) or LCD (Liquid Crystal Display.) Each type has different advantages that need to be considered when choosing an echo sounder.

A CRT is similar to a television screen. It has high contrast in normal to low light, allowing for bright and clear targets to be shown on the display. Bright sunlight will tend to make the CRT display fade. CRT echo sounders work best in an environment that is either covered or out of direct sunlight. With echo sounders, color units allow you to discriminate easier between small differences in signal returns.

Dual frequency echo sounders

Some echo sounders are dual frequency. The lower frequencies, (50 kHz) provide better ground discrimination and also detect fish better in deeper water. The higher frequencies, (200 kHz) will detect fish such as mackerel and squid with no swim bladders easier and usually provide better discrimination between schools of fish and individual large fish. Having a dual frequency echo sounder gives you the advantage of having a deep penetrating lower frequency and a high definition higher frequency.

All echo sounders will accept electronics navigation information from GPS or LORAN.