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 Traps Net and rod counts Redd counts (disturbances in gravel caused by mating activities of some fish) Closed circuit television 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. Aquatic ecology Fisheries science Counting instruments