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The Effectiveness of Habitat Enhancement on Salmon and Trout Stocks in Streams in the Corrib Catchment.
Gargan, P.G, O'Grady, M.F, Delanty, K1, Igoe, F2, and Byrne,C3.
1Central Fisheries Board, Mobhi Boreen, Glasnevin, Dublin 9, Ireland
2Shannon Regional Fisheries Board, Ashbourne Business Park, Dock Road, Limerick, Ireland
3 Carlow County Council, Athy Road, Co. Carlow, IrelandAbstract
As part of the Tourism Angling Measure Programme (TAM), considerable investment was made in carrying out habitat enhancement on brown trout (Salmo trutta L.) and Atlantic salmon (Salmo salar L.) spawning and nursery streams in the Lough Corrib Catchment, Western Ireland. The programme spanned the period 1995 – 1999 and was designed to improve the angling product for both species. A range of techniques were employed to repair damaged habitat, caused primarily by either sheep overgrazing or the physical alteration of channels by arterial drainage. Works included timber or rock bank revetments, inputting of weirs, rubble mats, lateral scour pools etc. In order to measure the success of the habitat enhancement programme on juvenile salmonid production, a number of control and experimental channel sections were selected and monitored over a five year period. The effectiveness of the habitat enhancement programme on juvenile trout and salmon stocks from a range of differing channel types is examined over the period.
Introduction
The Lough Corrib catchment is the largest and most important wild salmonid catchment in Ireland. Lough Corrib is the premier brown trout fishery in Ireland and also has a prolific salmon run while Lough Mask and Lough Carra are important brown trout fisheries. Over the period 1994-1999, the Tourism Angling Measure (TAM) of the Operational Programme for Tourism (Part funded by the European Regional Development Fund) invested €16m in the development of inland fisheries in Ireland. Under this programme, the Western Regional Fisheries Board, with technical assistance from the Central Fisheries Board, carried out extensive habitat enhancement programmes throughout the Corrib catchment, with a total investment of €2.5m. Over the period 1997-2000, electro-fishing of juvenile salmonid habitat was undertaken on selected stream sections throughout the catchment to monitor the effectiveness of the habitat enhancement programme. This paper outlines the results obtained, in broad terms, in relation to both Atlantic salmon (Salmo salar L.) and brown trout (Salmon trutta L.) populations.
Corrib Catchment
Lough Carra drains via the short Keel River (1.4 km) into the north-east end of Lough Mask. The Cong Canal flows southerly for 5.7 km connecting Lough Mask to Lough Corrib. The River Corrib drains Lough Corrib and runs for 7.7km before entering the sea in Galway City, fig 1. Salmon have access to all streams entering Lough Corrib, with the exception of the Cong Canal, which due the porous limestone nature of the bed, causes the canal to become dry in sections and run underground in summer. No salmon are present in the Lough Mask/Carra sub-catchment.
The catchment geology of the Corrib Catchment to the east of Lough Corrib, Lough Mask and Lough Carra is dominated by Carboniferous limestones whereas the catchment geology to the west and north-west of Lough Corrib and to the west of Lough Mask consists of Silurian quartzite, schists and gneiss, with smaller outcrops of granite. The local geology associated with each of the sites monitored in the present study are outlined (table 2). The electrofishing method used at each site, month of sampling and the type of habitat enhancement work carried out is shown in table 2.
Materials and Methods
Selection of Sites for Fish Stock Assessment.
Habitat enhancement work was undertaken on a wide range of streams on the Corrib Catchment. Thirteen streams were selected (8 entering Lough Corrib, 4 entering Lough Mask and one entering Lough Carra) to monitor the effectiveness of the enhancement works on salmonid stocks. One or more paired sites were selected on each of the thirteen streams, one site termed "a control site" where no enhancement work would take place and a second site, termed "an experimental site", where enhancement work would be undertaken. Prior to the implementation of the enhancement programme, all paired sites were electro-fished. These fish density data reflects stock densities prior to enhancement works being undertaken. On completion of enhancement works, both the control and experimental sites were monitored for a number of years, the experimental sites reflecting the effectiveness of such works on salmonid densities. Care was taken to ensure that works within the individual experimental sites chosen broadly reflected the type of enhancement works carried out in the particular sub-catchment in question.
Fish Stock Assessment
A semi-quantitative fish stock assessment was carried out using electrofishing. In shallow stream sections in eleven sub-catchments, fish were removed using bank-based electrofishing equipment, consisting of a portable generator (220/240V) with an appropriate control (D.C converter) unit attached. In two larger sub-catchments, boat electro-fishing was undertaken using a portable generator and a control unit. The sampling area at each site was isolated using stopnets to ensure no escapement of fish upstream or downstream during the electrofishing operation. Two fishings were carried out in the contained area in an upstream direction from the bottom net. Fish from each pass were held in bins of water, sorted and processed separately. All fish were measured for fork length within 1cm length groupings. A set of scales from brown trout and salmon were taken from a representative range of sizes for back-calculation of length at age and examination of growth pattern at each site. All fish were returned alive after sampling.
The reach length electrofished at each site varied but was generally greater than 30m of stream channel length and included one natural stream sub-unit, ie one riffle/glide/pool sequence or one riffle/glide sequence where pools were absent. Control and experimental sites were fished on the same day or one day later and sites were fished annually at the same time of year, usually within ten days of previous annual surveys.
Data Analysis
Minimum fish density values, based on two fishings, were calculated (Crisp, Mann and McCormack, 1974). Life stages of salmonids were selected for salmon fry (0+ year old), salmon parr (1+ year old), trout fry (0+ year old), trout parr (1+ year old) and older trout (2+ year old and greater). Non-parametric statistics, the Wilcoxon Signed Rank Test, was applied to each life stage of both species. Data for the Lough Corrib sub-catchment were treated separately as were combined trout data for the Lough Mask/Lough Carra subcatchment. Where more than one control or experimental site was monitored on any stream, a mean density value was obtained for each stream prior to analysis to avoid pseudoreplication (Cooper and Barmuta, 1993). Mean data from control and experimental sites on each stream from both sub-catchments were compared to test for differences prior to any enhancement work being. Then data from control and experimental sites after enhancement works were undertaken were also compared.
Results
The location of control and experimental sites electrofished during the present study are set out in fig 1. Data obtained from control and experimental sites from the Lough Corrib and Lough Mask/Carra sub-catchments, prior to commencement of the habitat enhancement programme are presented, (Appendix 1,3). There were no significant differences in stock density for each life stage for both salmon and trout with the exception of trout parr in the Lough Corrib catchment, (table 3,5).
Minimum salmonid densities from paired control and experimental sites for all years fished after enhancement works were undertaken are presented (Appendix 2,4). Statistical analysis indicate that there were significant differences in stock density for certain life stages (table 4,6). In the Lough Corrib sub-catchment, no significant difference was found for salmon fry (p=0.492) while a highly significant difference was observed for salmon parr (p=0.001) with a mean stock density value for salmon parr at experimental sites (0.118) higher than the value obtained at control sites (0.055), (table 4).
No significant difference was recorded for trout fry (p=0.215) or older trout (p=0.095). A significant difference was recorded for trout parr (p=0.026) with the mean value (0.127) higher at experimental sites, post enhancement (table 4).
Results of a Wilcoxon Signed Rank Test for the Lough Mask/Carra sub-catchment revealed no significant difference for trout fry (p=0.333) or older trout (p=0.08). A significant difference was recorded for trout parr (p=0.013), with a higher mean value recorded at experimental sites, post enhancement, (table 6).
Discussion
Baseline survey data complied for the Corrib Catchment which was used to design these enhancement programmes clearly highlighted two key problem areas – the physical alteration of channels by arterial drainage in the Lough Carra, eastern Lough Mask and eastern Lough Corrib sub-catchments, and the impact of overgrazing by sheep in drainages on the western side of Lough Mask and Corrib. In broad terms, the negative impacts of both of these problems on riparian zones and channel form are very similar - ie the thalweg is either poorly defined or has been lost, both lateral scour pools and that the number and/or quality of pool areas in straight channel reaches has been reduced. In summary, all of these channels were excessively broad and shallow and have lost their riffle/glide/pool sequence. In both the drained and overgrazed drainages the quality of the riparian zone has at least been impaired and, in many cases, severely damaged.
Previous survey data from the Lough Corrib and Lough Mask/Carra sub-catchments had shown that in almost all reaches of streams in these sub-catchments there were adequate quantities of gravels for spawning purposes and sufficient “holding” areas for the 0+ year old component of the salmonid stock. The fact that 0+ year old salmon and trout densities are not significantly different statistically, pre and post enhancement, is therefore to be expected and also shows that the gains made in the capacity of these channels to support older (>= 1+ year old) fish has not been at the expense of the fry component.
Pre-enhancement analysis for the parr life stage for trout and salmon indicated no difference in recorded densities from both sub-catchments except for parr densities from the Lough Corrib sub-catchment, which were greater at control sites. Data analysis post habitat enhancement, showed differences in stock density for trout parr (1+ year-old) from both sub-catchments and salmon parr (1+ year old). The mean density for these life stages indicate a higher density at experimental sites. Given the pre-enhancement results, it is reasonable to assume that the increases in parr stock densities recorded post-works at experimental sites are a result of the habitat enhancement programme. Both Igoe et al (1998) and O'Grady et al (2002a) have shown that trout that migrate as parr from streams to Irish lake catchments exhibit a greater survival rate to the adult trout stage than fish which migrate as fry. The finding that trout parr densities increased in the present study after completion of the habitat enhancement programme should result in a greater proportion of juvenile trout surviving to adulthood in both lake sub-catchments.
A significant increase in older trout stocks was not recorded from either sub-catchment up to three years after completion of the habitat enhancement programme. Where adult brown trout stocks are likely to recolonise some of the larger channels enhanced in the Corrib programme it is unlikely that we have seen the full benefit of such programmes to date. Hunt (1976) reports that in terms of enhancing adult brook trout (Salvelinius fontinalis L.) habitat, optimum stocks of fish were not present until the fifth and sixth years post-development.
The habitat enhancement programme undertaken at experimental sites involved either the inputting of vortex stone weirs, rubble mats, stone deflectors, or timber/rock bank protection. These works have had the effect of restoring a thalweg and increasing both the number and quality of pool habitat. The success of the habitat enhancement programme in restoring this habitat type is reflected in the significant increases observed in the parr life stages of salmon and trout. Previous similar enhancement programmes in Irish rivers have yielded similar positive results (O’Grady, 1991, O’Grady et al, 1991, Kelly, 1996, O’Grady et al, 2000 and O’Grady and Duff, 2000). In broader terms, internationally, it is clear that the implementation of such programmes can be successful for a range of salmonid species in both Europe and North America (Frankenberger, 1968, Hale, 1969; White, 1975; Hunt, 1976; Naslund, 1969 and Glen, 2002).
O’Grady et al (2002b) have shown that one Corrib sub-catchment (Glenglosh River), which was a part of this over-all programme, is still evolving post-works in physical and, probably also, in ecological terms. The same is likely to be the case in all of these sub-catchments and suggests that, over time, the habitat enhancement programme may lead to even greater increases in juvenile salmonid production.
Acknowledgements
The authors would like to acknowledge the contributions made by the management and staff of the Western Regional Fisheries Board, for their help, support and commitment throughout the programme. Thanks are also due to the staff and management of the Engineering Services section of the Office of Public Works (Headford) for their invaluable contribution to this programme. A special thanks is due to Don Duff for the expert advise given on project design provided so willingly, during the course of this habitat enhancement works programme.
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