Liffey Fish Stock Survey 2005

Fish Stock Survey



River Liffey, Fish stock survey from upstream of Ballymore Eustace (Driver’s Ford) to disused Railway Bridge at Harristown, September 2005.
W.S.T.Champ
Central Fisheries Board,
Swords Business Campus,
Swords,
Co. Dublin
Summary
The Eastern Regional Fisheries Board was commissioned by Ballymore Eustace Trout and Salmon Anglers Association, with financial assistance from KELT - Kildare Leader Group, to carry out an investigation of the fishery in the Upper Liffey from Driver’s Ford (u/s Ballymore Eustace) to the disused railway bridge at Harristown. The survey was carried out from 12th to 15th September with the assistance of staff from the Central and Shannon Regional Fisheries Boards.

Quantitative surveys of the trout and salmon stocks were completed at two locations on the main river and on two tributary streams (the Lemonstown and the Brook-of-Donode). The Liffey upstream of Ballymore Eustace, although lightly stocked (about 8 young salmon and 8 trout per 100m²), showed a significant improvement in fish numbers and water quality compared to an earlier survey at this location in 1987. The second location, a shallow glide/riffle area about 2km downstream of the town bridge (at the ‘Horseshoe’ (Ferny Inch)) carried a very good stock of juvenile salmon (over 79 per 100m²). The Lemonstown stream section contained 45 trout and 19 young salmon (64 salmonids per 100m2); the Brook-of-Donode supports a similar salmonid population density (62 per 100m²) trout constitute the bulk of the stock (57 per 100m²) but young salmon (5 per 100m²) also occur in this small tributary.

The remainder of the river from Driver’s Ford to the old railway bridge at Harristown was fished qualitatively using 2 to 5 boats over three days. Section 1, the Ford to the ‘Horseshoe’; Section 2, the ‘Horseshoe’ to the ‘gas pipe crossing’ at Liffeydale/Cloughanstown and Section 3, from the ‘gas pipe’ to the railway, were each divided into sub-sections when fishing was interrupted at shallows. Details of fish encountered, instream and bankside features were recorded throughout. Mature bankside trees the branches of which frequently extended out over or trailed in the water, fallen timber in the river and deep water militated against effective fishing with the electrical apparatus. Trout (one possible individual to 3lb (1.36kg)) and juvenile salmon dominated the fish community. Young salmon were common at every shallow but no adult salmon was encountered. Only occasional perch and roach were noted and they are not a significant element of the fish community currently.

Trout especially, but salmon fry and parr also to a lesser degree, appear to be growing more quickly than previously. This was particularly evident in the waters around Ballymore Eustace where water clarity and quality generally has improved considerably in the vicinity of the Wastewater Treatment Works (the improvement in water clarity was evident down to the ‘gas pipe crossing’ and beyond) compared to 1987. However, in September, the Eastern Regional Fisheries Board, using the EPA macroinvertebrate quality rating system, still assessed the water quality of the river as slightly polluted (Q3-4) to moderately polluted (Q3), at four locations from Driver’s Ford to the railway bridge.

Several sections of particularly good salmon spawning gravel were noted throughout the BTSAA waters and it is possible that spawning occurs at almost every riffle. While the gravels in some areas were loose, suitable deposits in many riffles and shallow glides were compacted due to extensive growths of water crowfoot (Ranunculus sp.).

Several deep holding pools for salmon occur along the 8km of BTSAA waters but no adults were encountered. It is most probable, had such fish been present, they would have been recorded in the survey. Details of the salmon spawning redd counts provided by the Eastern Regional Fisheries Board show salmon spawning occurs consistently, albeit at varying levels, in the BTSAA fishery every year. Very good stocks of juvenile salmon occur at some locations and the species is well distributed at every shallow from Driver’s Ford to the railway bridge. It appears that adult salmon had just not arrived in this part of the river on this occasion at the time of the survey but adults were present in September 1987 when the stretch upstream of Ballymore Eustace was examined. Information received from the fish counters on the Liffey show a decline in salmon runs recently relative to the 1980s and 1990s. The numbers of adult salmon returning to spawn in the river Liffey are substantially below the Conservation Limit.

Conclusions and Recommendations.

The River Liffey, in the vicinity of Ballymore Eustace, is significantly cleaner than it was when this section of the river was surveyed in 1987. Juvenile trout and salmon are growing faster now than previously. Stocks of trout and juvenile salmon have improved considerably in the Ballymore Eustace area since 1987. This improvement is thought to be a direct consequence of the substantially improved discharge upstream of the town. However, stock densities of trout and salmon upstream of Ballymore Eustace are still low. This is not unusual for locations in rivers where the flow is regulated, especially considering the proximity of the survey site to the hydro-dam upstream at Golden Falls

No adult salmon were encountered in the 8.0km of channel surveyed (Water Treatment Works to the bridge at Harristown). Juvenile salmon were widespread and the river was well stocked with salmon fry at some locations (the ‘Horseshoe’ or Ferny Inch) and good numbers were noted at several other locations. There was some evidence that juvenile salmon were less numerous towards the lower end of the BTSAA section. Salmon spawning is noted annually throughout the BTSAA waters and redd counts vary from year to year; numbers are well down currently relative to the late 1980s. It appears that, in addition to fewer numbers ascending the Liffey, the run may be occurring later in the year. While the adult salmon run in the Liffey is not meeting the Conservation Limit some sections in the Ballymore Eustace area are quite well spawned and juvenile salmon stocks are good in parts. Small numbers of salmon also spawn in the tributaries in the Ballymore Eustace area as evidenced by modest numbers of juveniles in the surveys.

The trout stocks are low in the two sections surveyed quantitatively in the main Liffey. One of these locations was, however, well stocked with juvenile salmon. Two tributary streams in the Ballymore Eustace area were also surveyed quantitatively at two locations and these were moderately well stocked with young trout. These tributaries are especially important trout nursery areas.

The river holds only a moderate stock of trout throughout the 8.0km surveyed but some sections (moderate depths alternating with shallow glide/riffle) were better stocked than the many long deep stretches. These latter areas were difficult to fish effectively because of the extent of overhanging/trailing branches in many sections. Reasonable numbers of fish over 0.34kg (0.75lb) were noted and the biggest trout was about 0.9kg (2.0lb). The BTSAA section of the Liffey is poorly supplied with tributaries and thus associated spawning and nursery areas for trout are limited. The flow in the Liffey is regulated for hydro-electric generation and such measures are known to depress fish stocks but otherwise the river channel has not been physically modified (drained). The Liffey channel, therefore, is natural but the presence of the hydro-dam and the lake upstream does deprive the upper channel of gravel renewal from the headwaters. This is not an impediment to salmon spawning currently.

The numbers of salmon and trout in the river are dependant to a very significant degree on the availability of adult spawning fish. For salmon this is related to salmon management generally and is governed by factors affecting Liffey fish in that part of their life cycle outside of the River catchment (while at sea) but also by the hydro-dam at Leixlip. The salmon appear to be running the Liffey later in the year and in fewer numbers, these aspects are being investigated on a national basis by the Standing Scientific committee of the National Salmon Commission.

The numbers of spawning trout could be increased by introducing a catch and release policy. Some anglers may be operating such a practice individually but the killing of trout should be discouraged as much as possible. The current bag limit of four trout could be reduced to two or even one fish per angler per day. The objective is to increase the number of adult fish surviving to spawn and the more severe the restriction the sooner this will be achieved. The conservation measure should be maintained for a period of at least three years.

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