The Environment Agency undertook a further Hydro Acoustic Survey on the River Thurne in Oct 2015 following the Prymnesium event in the Spring of 2015.
These surveys undertaking in darkness ensures that the fish which migrate out into open water at night can be captured by the sonar technology. The density is measured as fish per 1000 sq metres.
Click on the map to jump to a google map view of the survey or the attached full EA report at the bottom of the page.
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 total volume of water sampled depends on the beam shape, channel profile, beam frequency and boat speed. If the total volume sampled is less than 1000 m3 for a segment (approximately 1/10 of a sequence) then the calculated fish density does not represent the actual 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 but fish are almost absent. In most cases insufficient sample volume is a consequence of channel width 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 these surveys is sensitive enough to record return echoes from objects less than 3mm 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 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.
Data Sourced from the EA - Essex Norfolk and Suffolk team with thanks.