Historically it was regular practice to drain the lake partially in winter to expose the aquatic weeds to frost and for clearing of silt This necessitated a fish trap in the stream below the lake, so that fish were not swept downstream and lost when the water was lowered. The dark blue section of the stream in the plan shown below indicates where the fish trap was created, beginning where there are Lily pads in the stream north of the northern-most bridge, and running downstream almost until the next bridge - as shown in the postcard image above, which dates from the 1920s.
Lowering the water level continued in modern times, but in autumn rather than winter. The lake was drawn down in late October to expose up to two metres of bed at the edges for the collection of drowned litter, to cut weeds on the face of the dam and for de-silting operations. It was considered important that these works be completed quickly, ideally in no more than three days, and the lake refilled before the first frost, which would otherwise kill Swan Mussels, tubifex worms etc. inthe lake, generally damaging the ecosystem and reducing fish and duck food.
The procedure for lowering the level of the lake was to drain the water at a slightly faster rate than it was entering at Wild Gardens Road. The drain pipe for the lake is about 15 yards out from the Promenade, between the Café and the Clocktower. Roughly 20 turns were needed on the valve to start to empty the lake and the water above the drain pipe had to be monitored. If a vortex had developed, the rate of emptying had to be reduced.
This part of the stream was a fish sanctuary 18 to 24 inches deep and with submerged waterweeds like Ceratophyllum and Lagarosiphon, later to be replaced by Elodea. A dam maintained the level in the trap, and there was a sluice to permit emptying the trap. There was a simple chicken wire screen to stop the fish escaping downstream. The fish were retrieved from here and transported back to the lake in water-butts on motor vehicles. The Parks Service carried out this process annually from the mid 1960s (and possibly earlier) until the early 2000s, when care of the lake became the responsibility of the boating contractor, Parkwood Outdoors.
The stretch of stream which used to be the fish trap has now deteriorated and can no longer achieve its original purpose.
Sources of Information
Much of the information on this page has been provided by Terry Davies, formerly Roath Park Superintendent and Horticultural Officer for the Cardiff Parks Service.
Other sources are: