7773

• UCREFRP Catalog Number: 7773
• Author: Many
• Title: Hydroecologie Appliquee
• USFW Year: 1993
• USFW - Doc Type: 2‎(5)‎
• Copyright Material: YES

The Enhancement of Drained Salmonid Rivers in Ireland 17
4a. A Works Schedule In Space
and Time
Studies of the effects of drainage on
salmonid stocks in Irish rivers imme-
diately post-works indicate a virtual
collapse of fish stocks immediately
post-works (Toner et al., 1985;
McCarthy, 1977 and 1983). Con-
sequently the works schedule in a
catchment where all channels are to
be excavated should be designed so
that the lower third of each channel
of fishery value is excavated first with
the machines moving upstream there-
after. The length of excavated channel
should be no more than one third of
total channel length in any one year.
In this way at least some macrophyte
and macroi nverteb rate recolonisation
of dredged areas will take place as
the machines move upstream. Con-
sideration should also be given to
scheduling works in relation to a
whole catchment in an order which
will allow some key spawning and
nursery grounds and adult trout feed-
ing areas to remain unaltered and
functioning in fisheries terms for at
least three years while the areas
drained initially are recovering. The
availability of a comprehensive pre-
drainage census of fish stocks (Sec-
tion 2 and fig. 4) provides one with a
basis for rational planning in this area.
Two Irish catchments studied by the
authors were drained in such a
piecemeal fashion for engineering and
logistic reasons. A review of the fluc-
tuations in salmon stocks in both of
these watersheds pre-, during, and
after works illustrates the advantages
of such an approach - a comparison
of adult salmon stocks returning to
these catchments over the relevant
period when compared with geo-
graphically adjacent undrained rivers
illustrates no significant reduction in
salmon numbers as a consequence of
works. Investigations have shown that
the staggered works programme in
these catchments, while not exclu-
sively responsible, has contributed to
the maintenance of good salmonid
stocks in these catchments during and
after works (O'Grady, 1991(a) and
O'Grady et al. 1993).
4b. Controlling Silt Loads
Dredging operations can sometimes
generate significant quantities of sus-
pended solids which at works stage,
and thereafter, can damage salmonid
stocks (Darnell et al., 1976). The
physical survey of bed and bank sub-
strates in a catchment pre-drainage
and data on channel gradients in com-
bination with hydrological information
should allow the engineer to estimate
the number and location of silt traps
which are likely to be required in any
particular channel to limit silt move-
ment. The maintenance of these traps
needs to be reviewed regularly at
works stage and their continued use
may be required in certain areas after
works are complete until bank stabil-
isation is evident.
4c. The Value of Fencing Chan-
nels
In Ireland it was the policy of the
drainage board (Office of Public-
Works) to fence off all dredged chan-