The following are definitions or descriptions relating to fish deflection systems


Refers to Section 316(b) of the US Clean Water Act, requiring facilities that withdraw more than two millions gallons of cooling water per day to reduce the entrainment and impingement of fish and shellfish. The rule was announced on May 19 2014 and enables acoustic fish deflection technology to be used as part of control measures installed by a facility to meet the required targets. To read more please see our 316(b) page.


Acoustic Fish Deflection. A term used to describe a fish deflection system based upon acoustic technology.

Acoustic Fish Deterrents

There are a variety of acoustic deterrents available, based upon infrasound, low frequency signals and high frequency signals. High frequency based systems are useful when specific fish species need to be deflected, such as alewife and shad, as the signals are typical of the sounds from potential predators, and so elicit a defensive flight response from the fish. However, the effectiveness of the systems is limited to only a few species, and so their use is limited. Infrasound systems rely on the fish sensing the particle motion (vibration) associated with a sound signal, and therefore means the fish have to be close to the sound source to be deflected. This can compromise the effectiveness of a system, as the fish can easily be drawn into an intake if they don’t respond to the system. The most effective acoustic fish deterrent systems are based upon systems that produce sound that cover the hearing range of fish, and typically cover the frequencies between 20 and 3,000Hz. It is these low frequency systems that are supplied by FGS.


A component of a High Intensity Light based system, which regulates the power supply to the LED lights. The Accumulator is usually located on a water intake, close to the High Intensity Lights.


BioAcoustic Fish Fence. Patented technology developed by FGS where an acoustic signal is incorporated into a bubble curtain and used to guide fish away from an intake and if required towards a fish pass.


Best Available Technology. A term used in Europe to describe the use of best available technology to minimize any adverse impact caused by an intake. A similar term is BATNEEC – Best Available Technology Not Entailing Excessive Cost, which takes into account the cost of installing, operating and maintaining the technology. FGS acoustic systems are considered Best Available Technology by the UK Environment Agency for the screening of intakes in estuarine locations.

Behavioral Barrier or Behavioural Barrier

Is a non-physical screen generating a stimulus that creates a response in the behaviour of the fish, which is used to deflect or guide the fish to safety. The stimulus is typically acoustic, electric, or light, but acoustic has been found to be the most effective. When a sound signal is combined with a light signal an enhanced or synergistic effect is created, so new barriers now typically comprise of a combined sound and light signal.

Best Practice Guide

The UK Environment Agency (EA) Best Practice Guide for the screening of intakes reviews all of the available technologies, and recommends acoustic deflection systems as being Best Available Technology for screening intakes in estuarine locations. This is based upon numerous independent trials of FGS systems that have demonstrated the effectiveness of the technology.


Best Technology Available. A term used in Section 316(b) of the US Clean Water Act that requires that the location, design, construction and capacity of Cooling Water Intake Structures reflect the best technology available (BTA) for minimizing adverse environmental impacts. Acoustic deflection systems supplied by Fish Guidance Systems can be used to help meet this requirement.

Cooling Water

Is abstracted by a variety of manufacturing and facilities and power plants to cool high-temperature processes, or in the case of power plants, to produce steam. When a large body of water is present nearby, be it a lake, river or estuary, the majority of existing facilities use once-through cooling, where the water is drawn from the water body and into the facility where it is used for cooling, and is then returned directly back to the river or estuary from which it was abstracted. These facilities can have a significant impact on the local fish population as well as migratory fish passing the intake, as significant numbers of fish can be drawn into the intake along with the cooling water. Legislation, such as Section 316(b) of the Clean Water Act in the US, seems to limit this impact and the result is that many new intakes are based upon recirculating or closed-loop systems.

Cooling Water Intake

Cooling water intake structures (CWIS) can take a variety of different forms, ranging from offshore intakes to those accessible from the land. Section 316b of the US Clean Water Act establishes requirements for cooling water intake structures (CWISs). Many existing intakes have been designed around the requirements for screening debris from entering the intake, but more recently the requirement for screening fish has been included, resulting in larger structures, with lower intake velocities. FGS can design a fish deflection system for existing cooling water intakes, which can be retrofitted to the intake, as well as help with the specification for new fish friendly water intakes.

Electric Barrier

Electric barriers have been used by the U.S. Army Corps of Engineers (USACE) to deflect Asian carp in the Chicago Sanitary and Ship Canal. The latest independent review by BARR Engineering Company (2013) considered all of the options for future screening of the Chicago Shipping Canal at Lock and Dam 1 and concluded that there are not only safety concerns over using electric barriers, but that electric barriers are more costly to install and operate. The conclusion and recommendation of the report was to install an acoustic / air bubble curtain / strobe light system based upon FGS’s SILAS technology.


Fish Entrainment occurs when fish, or other aquatic organisms, are drawn into an intake, but are small enough to pass through the intake screens and are drawn into the intake system.


Fish Entrapment typically occurs with offshore intake structures, which are connected to the shore via a long tunnel. Fish that enter the offshore intake and enter the tunnel as the flow exceeds their maximum swimming speed and are not able to escape. The fish are likely to eventually be impinged on the onshore screens, or entrained into the downstream facilities.

Environmental Protection Agency

The US EPA is the United States federal agency responsible for the writing and enforcing of environmental legislation and regulations in the US, such as Section 316(b) of the Clean Water Act.

Fish Barrier, Fish Disco, Fish Diversion, Fish Scarer and Fish Screen

Are a variety of names used by clients to describe acoustic fish deflection systems supplied by Fish Guidance Systems.

Fish ladder, fish pass or fish way

Is a structure typically installed on a dam, lock or weir to enable migratory fish, such as salmon and eels, to safely pass the obstruction. FGS Engineers can help design and evaluate the optimum fish ladder for a variety of applications.

Fish protection

Is becoming increasing important due to the increase in legislation, such as the European Water Framework Directive and eel Regulations as well as the US EPA Section 316(b) ruling on the protection of fish at cooling water intakes. Fish Guidance Systems has a number of behavioural systems available to help facilities to comply with this and future proposed legislation, ensuring that our clients can minimise the impact of their facilities on freshwater and marine fish.

Fish swimming speeds

Are important when designing a fish friendly intake and fish deflection system, as it is essential that the maximum intake water velocity does not exceed the maximum swimming speed for the fish. FGS has access to a significant database on the swimming speeds of a wide variety of fish species, which can be accessed when specifying a fish deflection system, but for many fish species a design velocity of 0.3 m/s is used.


High Intensity Lights. FGS LED based technology used to deflect fish, particularly eels, from water intakes. The underwater LED lights are similar to xenon strobe lights, in that they are operated at various flash rates to provide a strobe or flashing light signal. When combined with an acoustic system the LED lights enhance the deflection efficiency for a wider variety of fish species.


Fish Impingement occurs when fish, or other aquatic organisms, are drawn into an intake and become trapped on physical screens due to the intake velocity exceeding the fish’s ability to swim away and escape.

Physical screens

Can take many forms, including close-spaced bar racks, flat panel screens such as wedge wire screens, travelling band screens, rotary screens and nets.  Physical screens can prevent entrainment of fish, if the spacing between the bars is close enough, but impingement of fish will occur if the intake velocity exceeds the swimming speed of the fish.  This can happen when debris collects on the screen, causing ‘hot spots’ where the intake velocity is significantly higher than the intended design velocity.  There is therefore a need to operate and maintain cleaning equipment to ensure physical screens don’t block and potentially collapse.
Acoustic systems are typically installed as an alternative to physical screens, particularly on larger intakes, as they less expensive to install, operate and maintain however, increasingly they are installed in conjunction with a course trash screen and fish return. The acoustic system protects the physical screen from collapse as a result of sudden inundations of fish, such as shoals of herring and sprat, and also prevents these delicate fish species from coming into contact with the physical screens, as typically 90% of these fish will die within 24 hours of being entrained, even if the screen is fitted with a fish return.


(Predictive Image Source Model) is a proprietary acoustic model used by FGS to determine the optimum number and configuration of Sound Projectors required to screen a water intake. The modelling is usually carried out early in the design stage of a project, along with hydraulic modelling if no information is available on the water flows into the intake.


Synchronised Intense Light And Sound. Patented technology developed by FGS to provide enhanced fish deflection efficiencies for a wide variety of fish species

Sensory Deterrent Systems

Typically considered to be acoustic or light technology, which provide a sensory stimulus that affects the natural behaviour of a fish, enabling it to be deterred from water intakes or entering other waterways.

Water Intake

A structure used to abstract water from a river, lake, estuary or coast, so that the water can be used for a variety of uses, including drinking water, cooling water, irrigation and flood defence.  In the UK most large water abstraction is associated with power stations, petroleum and oil refineries and chemical manufacturing plants, which use coastal or estuarine intakes, but in continental Europe and the United States freshwater river and lake intakes are used, due to the size of the available water bodies, and distances from the coast.

Xenon tubes, xenon strobes, xenon lamps and strobe lights

Are all forms of xenon lights that have been used in the past to deflect fish.  Xenon technology has been replaced with Light Emitting Diode (LED) technology, as LEDs enable significantly more control over the signal, are safer to use and have much longer life than xenon strobes.  Research has shown effective light systems are not solely reliant on how bright a strobe light is, as the same results (if not better) can be achieved by using the right signal in a LED based system.

For further information on the systems available please contact FGS.