A fishfinder is an instrument used to locate fish underwater by detecting reflected pulses of sound energy, as in SONAR

Sonar

Sonar is a technique that uses sound propagation to navigate, communicate with or detect other vessels...

. A modern fishfinder displays measurements of reflected sound on a graphical display, allowing an operator to interpret information to locate schools of fish, underwater debris, and the bottom of body of water. Fishfinder instruments are used both by sport and commercial fishermen. Modern electronics allow a high degree of integration between the fishfinder system, marine radar

Radar

Radar is an object-detection system which uses radio waves to determine the range, altitude, direction, or speed of objects. It can be used to detect aircraft, ships, spacecraft, guided missiles, motor vehicles, weather formations, and terrain. The radar dish or antenna transmits pulses of radio...

, compass and GPS navigation systems.

Fathometer

Fish finders were derived from fathometers, active sonar instruments used for navigation and safety to determine the depth of water. The fathom

Fathom

A fathom is a unit of length in the imperial and the U.S. customary systems, used especially for measuring the depth of water.There are 2 yards in an imperial or U.S. fathom...

 is a unit of water depth, from which the instrument gets its name. The fathometer is an echo sounding

Echo sounding

Echo sounding is the technique of using sound pulses directed from the surface or from a submarine vertically down to measure the distance to the bottom by means of sound waves. This information is then typically used for navigation purposes or in order to obtain depths for charting purposes...

 system for measurement of water depth. A fathometer will display water depth and can make an automatic permanent record of measurements. Since both fathometers and fishfinders work the same way, and use similar frequencies and can detect both the bottom and fish, the instrumnets have merged.

Operating theory

In operation, an electrical impulse from a transmitter

Transmitter

In electronics and telecommunications a transmitter or radio transmitter is an electronic device which, with the aid of an antenna, produces radio waves. The transmitter itself generates a radio frequency alternating current, which is applied to the antenna. When excited by this alternating...

 is converted into a sound wave by an underwater transducer

Transducer

A transducer is a device that converts one type of energy to another. Energy types include electrical, mechanical, electromagnetic , chemical, acoustic or thermal energy. While the term transducer commonly implies the use of a sensor/detector, any device which converts energy can be considered a...

, called a hydrophone

Hydrophone

A hydrophone is a microphone designed to be used underwater for recording or listening to underwater sound. Most hydrophones are based on a piezoelectric transducer that generates electricity when subjected to a pressure change...

, 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. 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.)

The speed of sound in water depends on the temperature, salinity and pressure (depth). This follows approximately this formula (del Grosso, 1974):



where

c = sound speed (m/s)

T = temperature (degrees Celsius)

S = salinity (pro mille)

D = depth

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 discernible on sonar plots of sufficiently high power and appropriate frequency. Slightly more than halfway up from the bottom to the left of the screen centre 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

Forage fish

Forage fish, also called prey fish or bait fish, are small fish which are preyed on by larger predators for food. Predators include other larger fish, seabirds and marine mammals. Typical ocean forage fish feed near the base of the food chain on plankton, often by filter feeding...

, debris

Marine debris

Marine debris, also known as marine litter, is human created waste that has deliberately or accidentally become afloat in a lake, sea, ocean or waterway. Oceanic debris tends to accumulate at the centre of gyres and on coastlines, frequently washing aground, when it is known as beach litter or...

, 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 centre 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.

The image to the right shows a school of white bass

White bass

The white bass or sand bass The white bass or sand bass The white bass or sand bass (MoroneIt is the state fish of Oklahoma.- Range :White bass are distributed widely across the United States, particularly in the midwest. They are very abundant in Pennsylvania and the area around Lake Erie...

 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.

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. 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.

Commercial and naval units

Commercial and naval fathometers of yesteryear used a strip chart recorder

Chart recorder

A chart recorder is an electromechanical device that records an electrical or mechanical input trend onto a piece of paper . Chart recorders may record several inputs using different color pens and may record onto strip charts or circular charts...

 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 reef

Reef

In nautical terminology, a reef is a rock, sandbar, or other feature lying beneath the surface of the water ....

s, wrecks

Shipwreck

A shipwreck is what remains of a ship that has wrecked, either sunk or beached. Whatever the cause, a sunken ship or a wrecked ship is a physical example of the event: this explains why the two concepts are often overlapping in English....

, 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 that can be related to position. Fathometers of the constant recording type are still mandated for all large vessels (100+ tons displacement) in restricted waters (i.e. generally, within 15 miles (24.1 km) of land).

External links

• Post-war economics: Fisherman who caught multibillion-dollar deal Financial Times
  Financial Times
  The Financial Times is an international business newspaper. It is a morning daily newspaper published in London and printed in 24 cities around the world. Its primary rival is the Wall Street Journal, published in New York City....
  
  , 9 December 2009.
• How to choose a fish finder History on fishfinders, how a fish finder Works, black & white fish finders vs. colour fish finders, about transducers, transom mount or hull mount, ceramic transducers, transducer frequencies, major brands of fish finders.