Notes for use of this data.
These data represent a snapshot of the fish distribution along a river at the time of survey. Surveys capitalise on the natural distribution of fish during the hours of darkness.
The value shown for fish density represents the number of fish per 1000 m3 of water sampled. The colour of the circle denotes the fish density as described in the map legend. The volume of water sampled by the acoustic beam broadly depends on the channel profile and boat speed If the total volume sampled is less than 1000 m3 then the calculated fish density does not represent the actual fish density. Consequently within the data set a value of 0.0 would indicate insufficient sample volume. Conversely a fish density value of 0.1 or higher indicates that sample volume is sufficient. In most cases insufficient sample volume is a consequence of channel width, depth and / or extensive weed growth.
Fish size research shows that the strength of a return echo relates to the size of the object producing the return.
The equipment used during surveys is capable of recording return echoes from objects less than 3 mm in size. Objects providing return echoes of this size could include reed stems, weed fronds, fish fry and suspended debris. To avoid erroneous fish counts and density value, return echoes are filtered during post processing. Fish density value reflects this filtering and the return echo strength chosen ensures that the minimum theoretical fish size counted is 100 mm.
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Jeff Compton Bsc Msc
Environment Monitoring officer (A&R) Fish
Dragonfly House, 2 Gilders Way, Norwich, NR3 1JB