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Click on the image to view a short clip of DIDSON footage. |
A DIDSON is an acoustic camera which gives near
video quality images for inspection and identification of objects
underwater (www.soundmetrics.com).
When applied to fish biology it can be used to count fish (either
manually by the operator or by using motion detection software supplied
with the unit). In this case the footage was recorded and saved onto
external hard drives and reviewed manually.
Under high flows, hatches 11 to 16 on the Great Weir (which are not
fitted with Resistivity Counters) are opened to maintain the upstream
water level. The proportion of fish which pass through these hatches
and therefore undetected is referred to as leakage. To estimate leakage,
a DIDSON acoustic camera was temporarily deployed on two occasions
in 2007 to evaluate the proportion of fish passing through the hatches
without counters. |
| August 2007
As the flows were not particularly high in August (but the DIDSON
was available) a trial was staged and hatches 17 and 18 were closed
and 11 and 12 were opened. This meant that roughly equal amounts
of water were passing through the hatches with counters on (19 and
20) as the hatches without counters (11 and 12).
During the study, 20 fish (including 2 Sea Trout) were found to
move upstream through the counters on hatches 19 and 20 (10 fish
per hatch). During the same period, 29 fish were observed moving
through hatches 11 and 12 on the DIDSON footage (14.5 fish per hatch). |
December 2007
During this trial, hatches11, 12, 17, 18, 19 and 20 were open fully.
16 fish were detected moving upstream through the counter hatches
17, 18, 19 and 20 (4 fish per hatch). During the same period, 16 fish
were observed passing through hatches 11 and 12 on the DIDSON footage
(8 fish per hatch). |
Date of trial |
No. of fish per hatch (counter
hatches) |
No. of fish per hatch (no counter
hatches) |
August 2007 |
10 |
14.5 |
December 2007 |
4 |
8 |
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This data indicates that the counter is potentially missing a significant
proportion of fish under high flows. Due to this we are currently installing
extra resistivity fish counters on hatches 11 and 12 (as soon as the water
level drops sufficiently). We are also using the DIDSON to further evaluate
this leakage in 2008.
In order to allow for leakage we monitor which hatches are open and calculate
it proportionally on a day by day basis. For example, if 12 fish go through
the 4 hatches with counters on and 2 hatches without counters on are open,
we estimate that 6 fish will have gone through these other hatches. In
2007 this increased the run estimate by 19.68%.
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