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Throop Fisheries Enhancement at Glen’s Weir August 2011

Environment Agency
November 9 2011

Throop Fisheries Enhancement at Glen’s Weir August 2011
Dorset Stour at Holdenhurst

For the ninth year in a row the Fisheries Recreation & Biodiversity Team at
Blandford combined with the Avon & Stour Ops Delivery Field Team, has now
successfully completed the latest enhancement works on the Dorset Stour.
The works were at Throop Fishery and the project aims were the following
which will contribute to the lower Stour achieving good ecological status under
the Water Framework Directive:

• Increase flow and natural scouring effect over gravels allowing self
cleaning and mobilisation
• Enhance existing spawning areas and increase potential spawning
habitat upstream and downstream of Glens Weir
• Increase parr and juvenile barbel habitat
• Re-instate collapsed banks and re-profile weir pool to increase weir
pool habitat and dynamics
• Provide fry refuge areas
• Swim creation, modification and reduced cattle poaching

Project Background

Extensive dredging work in the 1970’s removed thousands of tonnes of gravel
from the lower Stour for flood risk purposes leaving a featureless linear
channel. The removal of these gravels eliminated miles of spawning habitat
for coarse species such as barbel, chub and salmonid species such as brown
trout, sea trout and salmon. These works contributed to the collapse of the
Dorset Stour as a salmon fishery and also impacted on species such as
barbel, which need varying flows and associated habitats to complete their
lifecycle successfully. Although in places the river has re-naturalised to some
degree, routine dredging works still take place further downstream for flood
risk purposes, as maintenance of the Christchurch flood alleviation scheme.
These past and ongoing works restrict the opportunity for these heavily
modified areas of river to naturalise.

The project was aimed at restoring and enhancing instream habitat in a
previously dredged area providing new spawning habitat, fry refuge areas and
juvenile habitat for multiple fish species. Species benefiting from these works
such as Barbel once reaching adulthood, can migrate up for 20km into
different areas upstream and downstream utilising different habitats before
returning in May to spawning areas. Projects such as this one contribute not
only to local habitat but to the whole biodiversity of the river itself.

The Environment Agency worked in partnership with The Barbel Society and
Ringwood & District Angling Association (RDAA) to gather data and highlight
areas where river restoration could take place on the lower Stour. Glen’s weir
at Throop Fishery was identified as a great opportunity not only to improve a
known spawning area for the local barbel population, but also to improve and
create further spawning area’s downstream, improve weir pool habitat and
provide fry refuge areas for multiple species. Pre-enhancement monitoring
work was completed (a separate report is available from the EA) and further
post project monitoring is taking place in partnership with RDAA and the
Barbel Society through angler catch data, electric fishing and redd count data.

The project was completed with the use of a 50’ 360 reach machine digger in
the expert hands of craftsman ‘Midge’ together with a 360 tracked dumper
driven by Mike Clements from Ops delivery. Nearly 600 tonnes of Portland
stone were placed into the collapsed bank, old weir and in channel as flow
deflectors together with gravel re-instatement and re-profiling. Fry refuge
areas were also created behind flow deflectors diversifying habitat for juvenile
and adult fish species. The area was then fenced off to prevent cattle
poaching and angler access into this area has been retained by installation of
a gate into the enhancement area.

The end result has lead to increased flows over a known spawning area by
placing of a block stone deflector above Glen’s weir, created a 30m riffle and
shallow glide downstream of the weir by re-using gravels previously dredged
from the bank side, and installation of another block stone deflector at the
tailrace of the weir pool. These combined works together with re-profiling of
the weir itself have increased flows over these spawning and juvenile habitat
areas and created holding features for larger adult species within the weir
pool itself. As flows are now concentrated in the middle of the channel, less
scouring of the bank will occur retaining clean gravels throughout the year.

“I am extremely happy with the finished result as over 100m of river habitat
has been restored for a multitude of fish species benefiting the whole reach at
Throop. I fully expect to see salmon spawning in these areas this winter and
in late spring, barbel and chub spawning over the newly re-profiled gravels.
Adult Salmon have already been seen lying in the re-profiled weir pool and
good catches of Barbel have already been taken from the tail race.”

“The Dorset Stour is often looked at as second best compared to
neighbouring rivers, but in terms of its biodiversity this is not the case. Slowly
but surely it is returning back into a fantastic river. My special thanks go to
Ops Delivery again as they have completed a grade ‘A’ job, also Chris Allport
and Brian Wilson from RDAA and Pete Reading from the Barbel Society for
their continued support and significant funding contribution. It has been a
fantastic collaborative project.”