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Equipment Class: Fishing technologyEquipment Type: Wheelhouse
Characteristics
The most simple hydroacoustic equipment is the echosounder which acts in a vertical sense sending an echo pulse directly downwards to the seabed and recording the returned echo. The sound pulse is generated by a transducer on the bottom of the vessel which vibrates up to 200 000 times per second. As the speed of sound is around 1 500 metres/second, the time interval, measured in milliseconds, between the pulse being transmitted and the echo being received, allows the depth to be measured. Not long after echosounders were introduced it was found that fish also reflected the transmitted pulse - sometimes with just as strong a reflection as the sea bottom. Other seafarers termed these phenomena as "false echoes", however fishers recognized the value of these false echoes to the efficiency of their operations and echosounders also became known as fishfinders.

Fish detection equipmentEchosounders are among the many modern instruments used for detecting fish
Fish detection equipment
FAO/15668/R.Cannarsa


Although earlier models were analogue instruments, modern echosounders are invariably digital allowing digital processing of the information plus display of the water column on a colour monitor. Usually, the display shows a series of colours, from blue (indicating no echo) to red (indicating a very "hard" echo from a dense fish school or the sea bottom). Digital processing allows various scales of display at different depths (ie phased ranges) as well as providing facilities such as "bottom lock" which fixes the display (or a part thereof) to the bottom, indicating where a bottom fishing gear, such as a trawl, might catch fish.

The development of the transducer, the sensor on the hull of the vessel, which transmits and receives the underwater signal, has seen similar changes with advances in electronic technology. The original large magnetostrictive transducer has given way to the modern multi-element transducer which, by means of electronic switching, provides directional capabilities similar to a sonar. In fact, this multi-element transducer was developed for sonar but is now used in advanced echosounders and netsounders.

The more advanced systems use "echo counting" and "target strength" measurements to provide more information to the skipper on the species that is returning the echo. However, these only give an indication of the species to a certain level of probablity as many species return similar types of echo. Although mackerel, for instance, which return a far weaker echo than a herring from the same level of insonification can, under certain circumstances, be readily identified.

Despite the efforts of electronics experts over a long period of time there are still some problems with the interpretation of the echoes seen on the echosounder. Demersal fish very close to the sea-bottom are masked by the so-called "dead zone", which is a function of the echosounder pulse length and the distance between the transducer and the sea bottom. The "dead zone" can be reduced but cannot be eliminated hence the detection of fish close to the bottom remains problematic.

The value of underwater acoustics to the fishing industry has led to the development of other acoustic instruments that operate in a similar fashion to echosounders but, because their function isslightly different from the initial model of the echosounder, have been given different terms. The sonar is merely an echosounder with a directional capability that can show fish or other objects around the vessel. The netsounder is mounted on the net and relays its signal back to the vessel via a cable or an accoustic signal. Bottom discrimination echosounders analyze the echoes from the bottom and classify the type of bottom as to whether it is rocky, sandy etc.

 
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