Fish counter

Automatic fish counters are measuring the number of fish passing along a particular river in a particular period of time. Usually one particular species is of interest.

One major species Studied by fish counters are Atlantic salmon . This species is of interest in its ecologically vulnerable status and anadromous lifestyles.

Methods of operation

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

Resistive counters

A resistive counter is associated with an in-river structure, such as a Crump weir. [1] The resistivity of a fish is less than that of water. So, as fish cross this barrier, They pass embedded electrodes , 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 be measured by a Wheatstone bridge , or other means, to detect the size and direction of the fish.

Fish counters of this kind are used Widely in Scotland to census populations of Atlantic salmon, Where comparison with closed circuit television shows around a 97% detection rate.

Optical counters

An optical counter is also associated with an in-river structure. However, rather than pass electrodes, in an optical counter the fish of some of the 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 to minimize the disturbance of the fish as they do not see the light when they pass through the counter. When a fish swims through the net of light beams, the resulting silhouette is used to count and estimate the size of each fish. Each individual image is stored in the control unit so that the counting can be verified afterwards.

Some systems such as the Riverwatcher use the infrared scanner to trigger a digital camera to capture between 1 and 5 pictures or a short video clip of each fish. The computer then automatically links to the images for other information such as passing hour, speed, silhouette image, temperature etc.

The camera is installed in a tunnel and provides a constant supply of light. It is possible to get good pictures of the fish 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 aussi Distinguish the size of fish more than other Accurately counter type and n are Useful PARTICULARLY WHERE a mixture of species inhabit a river (for example salmon rivers Where mix with sea trout ).

The key disadvantage of optical counters is the small penetration of the water, restricting their use to narrow river features or in-river structures, such as fish ladders.

Hydroacoustic counters

Hydroacoustic counters operate using the principles of sonar . 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 (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%, although one study found detection rates as low as 3%. Careful planning and pre-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 certain amount of time, and are not accurate in any other way (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 to funnel the fish through the counter.

Recent advances in automated hydroacoustic monitoring systems. These systems include intelligent monitoring and real-time data processing, and proper operation and publication of status and results (eg fish counts) on a routine basis.

Siting counters

In river structures

Resistivity and (particularly) optical fish counters require in-river structures to the fish through the detection aperture of the counter. Fish ladders and Borland fish are effective structures for this purpose and 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 we are designed to ensure rapid planar flow over the detector.

Siting within the river system

When monitoring anadromous fish such as the Atlantic salmon it is important to remember that a species may return to a particular breeding ground its life. This means that within the larger rivers a number of quite distinct populations can cross together, in aggregate. A population which uses a particular tributary may collapse while the numbers are not clearly affected. Issues with the management of that particular tributary and population go unnoticed. It is important in these situations, therefore, that individual populations, rather than the species in aggregate, can be detected.

Alternative methods

The results of an alternative technique can be supplemented, confirmed, or replaced by alternative techniques, varying in accuracy, cost, complexity, and skew effects.

  • Electrofishing
  • traps
  • Net and rod counts
  • Redd counts (disturbances in gravel caused by mating activities of some fish)
  • Closed circuit television


  1. Jump up^ Crump weir


  • Eatherley, DMR, Thorley, JL, Stephen, AB, Simpson, I., MacLean, JC & Youngson, AF (2005). Trends in Atlantic salmon. 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.

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