Automatic fish counters are automatic devices for measuring the number of fish passing along a particular river in a particular period of time. Usually one particular species is of interest. One important species studied by fish counters are

Atlantic salmon The Atlantic salmon (''Salmo salar'') is a species of ray-finned fish in the family Salmonidae. It is the third largest of the Salmonidae, behind Siberian taimen and Pacific Chinook salmon, growing up to a meter in length. Atlantic salmon are ...

. This species is of interest owing to its ecologically vulnerable status and

anadromous Fish migration is mass relocation by fish from one area or body of water to another. Many types of fish migrate on a regular basis, on time scales ranging from daily to annually or longer, and over distances ranging from a few metres to thousan ...

lifestyles.

Methods of operation

Fish counters can be divided into three principal types: resistive counters, optical counters, and hydroacoustic counters.

Resistive counters

A resistive counter is associated with an in-river structure, an example constituting a Crump weir. The

resistivity Electrical resistivity (also called specific electrical resistance or volume resistivity) is a fundamental property of a material that measures how strongly it resists electric current. A low resistivity indicates a material that readily allows ...

of a fish is lower than that of water. So, as fish cross this barrier, they pass embedded

electrodes An electrode is an electrical conductor used to make contact with a nonmetallic part of a circuit (e.g. a semiconductor, an electrolyte, a vacuum or air). Electrodes are essential parts of batteries that can consist of a variety of materials de ...

, and the difference in resistivity disturbs the field established in the vicinity of the electrodes, altering inter-electrode resistance. With three electrodes these disturbances can then be measured by a

Wheatstone bridge A Wheatstone bridge is an electrical circuit used to measure an unknown electrical resistance by balancing two legs of a bridge circuit, one leg of which includes the unknown component. The primary benefit of the circuit is its ability to provid ...

, or other means, to detect the size and direction of travel of the fish. Fish counters of this type are used widely in

Scotland Scotland (, ) is a country that is part of the United Kingdom. Covering the northern third of the island of Great Britain, mainland Scotland has a border with England to the southeast and is otherwise surrounded by the Atlantic Ocean to the ...

to census populations of Atlantic salmon, where comparison with closed circuit television shows around a 97% detection rate.

Optical counters

A
optical counter
is also associated with an in-river structure. However, rather than pass electrodes, in an optical counter the fish interrupt some of a number of vertically arranged beams of light. The pattern of beam-breaks can be used to determine the size, profile, and direction of motion of the fish. Infrared light is used for minimizing the disturbance of the fish as they will not see the light when passing through the counter. When a fish swims through the net of light beams, the resulting silhouette image is used for counting as well as estimating the size of each fish. Each individual image is memorized in the control unit so that the counting can be verified afterwards. Some systems such as th
Riverwatcher
use the infrared scanner to trigger a digital camera to capture between 1 and 5 photos or a short video clip of each fish. The computer then automatically links the images to other information contained in the database for that individual fish such as size, passing hour, speed, silhouette image, temperature etc. The camera is installed in a special tunnel that contains both the camera and lights providing constant light, and same distance from the camera for the fish. That way, it is possible to get good images of the fish regardless of time of day. The performance of optical counters has been determined by studies, under various conditions, to be greater than 90%. Optical counters can also distinguish the size of fish more accurately than other counter types and so are particularly useful where a mixture of species inhabit a river (for example rivers where salmon mix with sea trout). The key disadvantage of optical counters is the small penetration of the beams through the water, restricting their use to narrow river features or in-river structures, an example being fish ladders.

Hydroacoustic counters

Hydroacoustic counters operate using the principles of

sonar Sonar (sound navigation and ranging or sonic navigation and ranging) is a technique that uses sound propagation (usually underwater, as in submarine navigation) to navigation, navigate, measure distances (ranging), communicate with or detect o ...

. A fish is insonified by a sound source and reflections from the fish are detected by an underwater microphone. The reflection occurs because of the sudden change in impedance to sound waves within the fish, particularly at the

swimbladder The swim bladder, gas bladder, fish maw, or air bladder is an internal gas-filled organ that contributes to the ability of many bony fish (but not cartilaginous fish) to control their buoyancy, and thus to stay at their current water depth w ...

(90% of the reflection). Hydroacoustic counters do not require in-river structures, but require skilled installation and operators. Without skilled installation at ideal sites hydroacoustic counters can be inaccurate. Studies typically indicate detection rates of 50% to 80%, though one study found detection rates as low as 3%. Careful planning and pre-siting study must be used to determine effectiveness. The lack of a requirement for any in-river structure makes the counters an attractive proposition. Generally used for short-term or seasonal studies, some situations require a long-term count which is accurate in absolute terms, not only in relative change (for example, no hydroacoustic sensors are routinely used in the detection of Scottish Atlantic salmon). In these instances resistivity or optical sensors tend to be preferred. Such methods usually require significant habitat modification, such as construction of a

weir A weir or low head dam is a barrier across the width of a river that alters the flow characteristics of water and usually results in a change in the height of the river level. Weirs are also used to control the flow of water for outlets of l ...

to funnel the fish through the counter. Recent advances in automated hydroacoustic monitoring systems has allowed continuous monitoring for periods exceeding 18 months. These systems include intelligent monitoring and real-time data processing, ensuring proper operation and publication of status and results (e.g. fish counts) on a routine basis.

Siting counters

In river structures

Resistivity and (particularly) optical fish counters require in-river structures to direct the fish through the detection aperture of the counter.

Fish ladder A fish ladder, also known as a fishway, fish pass, fish steps, or fish cannon is a structure on or around artificial and natural barriers (such as dams, locks and waterfalls) to facilitate diadromous fishes' natural migration as well as movemen ...

s and Borland fish passes are effective structures for this purpose and occasionally a natural restriction within the river may be used for a similar purpose. However, for most counters a custom in-river structure will be required. One of the most effective such structures is the Crump weir, a triangular profile weir designed to ensure rapid planar flow over the detector.

Siting within the river system

A species of anadromous fish, such as the Atlantic salmon, may return to a particular breeding ground throughout its life. This means that within the larger rivers a number of quite distinct populations may cross a counter together, in aggregate. A population which uses a particular tributary may collapse whilst the overall numbers are not clearly affected. Issues with the management of that particular tributary and population therefore go unnoticed. Counters should be placed to count individual populations, rather than the species in aggregate, in order that population collapses and recoveries can be detected.

Alternative methods

The results of automatic fish counters can be supplemented, confirmed, or replaced by a number of alternative techniques, varying in accuracy, cost, complexity, and skew effects. *

Electrofishing Electrofishing is a fishing technique that uses direct current electricity flowing between a submerged cathode and anode. This affects the movements of nearby fish so that they swim toward the anode, where they can be caught or stunned.

* Traps * Net and rod counts *

Redd Redd is a Turkish rock band established in 1996 by tenor opera singer Doğan Duru and guitarist Berke Hatipoğlu under the name ''Ten''. They used to play at bars until they set up their own studio in 2004. Their first album, titled ''"50/50" ...

counts (disturbances in gravel caused by mating activities of some fish) *

Closed circuit television Closed-circuit television (CCTV), also known as video surveillance, is the use of video cameras to transmit a signal to a specific place, on a limited set of monitors. It differs from broadcast television in that the signal is not openly t ...

Notes

References

* Eatherley, D.M.R., Thorley, J.L., Stephen, A.B., Simpson, I., MacLean, J.C. & Youngson, A.F. (2005). Trends in Atlantic salmon: the role of automatic fish counter data in their recording. Scottish Natural Heritage Commissioned Report No. 100 (ROAME No. F01NB02). * The Estimation of Individual Fish Size Using Broadband Acoustics with Free-swimming Salmonids, Thomas J. McKeever. Masters Thesis in Aquaculture, University of Newfoundland, 1998. {{Fisheries and fishing Aquatic ecology Fisheries science Counting instruments