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Cabin display of a commercial or oceanographic fathometer sonar

A fishfinder is a type of fathometer, both being specialized types of echo sounding systems, a type of Active SONAR. ('Sounding' is the measurement of water depth, a historical nautical term of very long usage.) The fishfinder uses active sonar to detect fish and 'the bottom' and displays them on a graphical display device, generally a LCD or CRT screen. In contrast, the modern fathometer (from fathom plus meter, as in 'to measure') is designed specifically to show depth, so may use only a digital display (useless for fish finding) instead of a graphical display, and frequently will have some means of making a permanent recording of soundings (which are merely shown and subsequently electronically discarded in common sporting fishfinder technology) and are always principally instruments of navigation and safety. The distinction is in their main purpose and hence in the features given the system. Both work the same way, and use similar frequencies, and, display type permitting, both can show fish and the bottom. Thus today, both have merged, especially with the advent of computer interfaced multipurpose fishfinders combining GPS technology, digital chart-plotting, perhaps radar and electronic compass displays in the same affordable sporting unit.

Contents

Operating theory

Display of a consumer type fishfinder

In a generalized sense, an electrical impulse from a transmitter is converted into a sound wave by the transducer, called a hydrophone, and sent into the water. When the wave strikes something such as a fish, it is reflected back and displays size, composition, and shape of the object. The exact extent of what can be discerned depends on the frequency and power of the pulse transmitted. The signal is quickly amplified and sent to the display. Knowing that the speed of the wave in the water is 4921 ft/s (1500 m/s) in seawater, 4800 ft/s (1463 m/s) in freshwater (typical values used by commercial fish finders), the distance to the object that reflected the wave can be determined. The process can be repeated up to 40 times per second and eventually results in the bottom of the ocean being displayed versus time (the fathometer function that eventually spawned the sporting use of fishfinding.) Note: This discussion of the propagation of sound in water is simplified, speed of sound in water depends on the temperature, salinity and ambient pressure (depth). This follows approximately this formula (del Grosso, 1974):
c = 1448.6 + 4.618T − 0.0523T2 + 1.25 * (S − 35) + 0.017D
where
c = sound speed (m/s)
T = temperature (degrees Celsius)
S = salinity (pro mille)
D = depth
TS = TeamSamick This will give variations in speed through the water column

General interpretation

The image above, at right, clearly shows the bottom structure -- plants, sediments and hard bottom are descernible on sonar plots of sufficiently high power and appropriate frequency. Slightly more than halfway up from the bottom to the left of the screen center and about a third away from the left side, this image is also displaying a fish -- a light spot just to the right of a 'glare' splash from the camera's flashbulb. The X-axis of the image represents time, oldest (and behind the soundhead) to the left, most recent bottom (and current location) on the right; thus the fish is now well behind the transducer, and the vessel is now passing over a dip in the ocean floor or has just left it behind. The resulting distortion depends on both the speed of the vessel and how often the image is updated by the echo sounder.

Fish symbols in the screen of the fishfinder

Fishfinders usually have a "Fish Symbol" feature, which will display a FISH symbol for almost any echo not connected to the bottom or the surface. More specialized units use a detection of a special fingerprint in echosignals. With those you can find living fish and fish that have recently died. With these units also in some cases fish spawn will be shown as a large fish. Some kinds of seafish will be not marked as fish, but you will see an echo.

Fish Arches

With the Fish Symbol feature disabled, an angler can learn to distinguish between fish, vegetation, schools of baitfish or forage fish, debris, etc. Fish will usually appear on the screen as an arch. This is because the distance between the fish and the transducer changes as the boat passes over the fish (or the fish swims under the boat). When the fish enters the leading edge of the sonar beam, a display pixel is turned on. As the fish swims toward the center of the beam, the distance to the fish decreases, turning on pixels at shallower depths. When the fish swims directly under the transducer, it is closer to the boat so the stronger signal shows a thicker line. As the fish swims away from the transducer, the distance increases, which shows as progressively deeper pixels.

Sonar image of a white bass feeding frenzy

The image to the right shows a school of white bass aggressively feeding on a school of threadfin shad. Note the school of baitfish near the bottom. When threatened, baitfish form a tightly packed school, as the individuals seek safety in the center of the school. This typically looks like an irregularly shaped ball or thumbprint on the fishfinder screen. When no predators are nearby, a school of baitfish frequently appears as a thin horizontal line across the screen, at the depth where the temperature and oxygen levels are optimal. The nearly-vertical lines near the right edge of the screen show the path of fishing lures falling to the bottom.

General history in sporting and fishing

Early sporting fathometers for recreational boating used a rotating light at the edge of a circle which flashed in sync with the received echo, which in turn corresponded to depth. These also gave a small flickering flash for echos off of fish. Like today's low-end digital fathometers, they kept no record of the depth over time and provided no information about bottom structure. They had poor accuracy, especially in rough water, and were hard to read in bright light. Despite the limitations, they were still usable for rough estimates of depth, such as for verifying that the boat had not drifted into an unsafe area.

Commercial and naval units

Commercial and naval fathometers of yesteryear used a Strip Chart Recorder where an advancing roll of paper was marked by a stylus to make a permanent copy of the depth, usually with some means of also recording time (Each mark or time 'tic' is proportional to distance traveled) so that the strip charts could be readily compared to navigation charts and maneuvering logs (speed changes). Much of the world's ocean depths have been mapped using such recording strips. Fathometers of this type usually offered multiple (chart advance) speed settings, and sometimes, multiple frequencies as well. (Deep Ocean -- Low Frequency carries better, Shallows -- high frequency shows smaller structures (like fish, submerged reefs, wrecks, or other bottom composition features of interest.) At high frequency settings, high chart speeds, such fathometers give a picture of the bottom (and any intervening large or schooling fish) relatable to navigation position data. Fathometers of this constant recording type are still mandated for all large vessels (100+ tons displacement) in restricted waters (i.e. generally, within 15 miles (24 km) of land).

Birth of the fishfinder

Eventually, CRTs were married with a fathometer for commercial fishing and the fishfinder was born. With the advent of large LCD arrays, the high power requirements of a CRT gave way to the LCD in the early 1990s and fishfinding fathometers reached the sporting markets at prices nearly anyone of modest means could afford. Today, sporting fishfinders lack only the permanent record of the big ship navigational fathometer, and that is available in high end units that can use the ubiquitous computer to store that record as well.

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References

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