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. 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%, 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 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 ladders 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