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Outstanding Results Achieved On Rubble Mat Sites

Are Atlantic salmon using the rubble mats installed on the Mulkear River as planned? These and other questions cannot be answered simply through observation. To gather data, you need a method that brings the fish to you. Electrofishing is one of the best tools available, within the world of fisheries, to study anything instream development works aimed at improving fish habitats, such as an assessment of the success or otherwise of MulkearLIFE’s habitat restoration work on the Mulkear River.


As fish float to the surface they are quickly collected (Image: Ruairí Ó Conchúir)

Electrofishing involves sending electric current underwater at a set frequency. The frequency and voltage depend on a variety of factors – the size of fish, the size of the waterway, the conductivity of the water’s chemistry. The electricity will temporarily stun whatever fish are nearby, making them pop to the surface, and it’s done to help MulkearLIFE accurately count and measure fish populations.

MulkearLIFE and our work colleagues in Inland Fisheries Ireland have recently completed resurveying the rubble mats sites which were installed by our project partners, Limerick County Council, in 2011. Elevated water levels, due to the extremely wet summer in 2012, prevented any surveying work last year. Thus, this was the first opportunity MulkearLIFE had to evaluate these specific instream measures.


The 2011 rubble mats: now supporting exceptional aquatic life (Image: R. Ó Conchúir)

A rubble mat is an instream restoration technique used to create an artificial riffle. The rubble mat reduces the cross-sectional area of the river thereby increasing flow velocities at low summer flows. The faster flowing area on top of the rubble mat is quickly colonised by a range of aquatic vegetation and aquatic invertebrates. The fast flowing water also provides exceptional habitat for young salmonids. The main objective of the rubble mats is to enhance their numbers. The work will also benefit various lamprey species and other fish, kingfisher, heron, dipper and various aquatic invertebrates including white-clawed crayfish. To read more about Rubble Mats including the ongoing construction of rubble mats in 2013 please click here.


All salmon are measured and returned to the river (Image: Inland Fisheries Ireland)

MulkearLIFE is delighted to conclude that, following the electrofishing of the rubble mat sites in recent weeks, exceptional results have been achieved. There is now no doubt that the rubble mats are an outstanding success. Electrofishing survey work in 2011, prior to their installation of the rubble mats, clearly demonstrated that the proposed sites for the installation of rubble mats were not utilized by salmon fry. Indeed, only a very limited number of salmon parr used them. The picture in 2013 is totally different. The results for this season reveal that hundreds of salmon fry are utilising the six rubble mats that were surveyed. The average density is an amazing 0.72 fry/m². In the space of two years, the average salmon parr density has tripled.


Average salmon parr density on rubble mats has tripled since 2011 (Image: Fran Igoe)

Interestingly, a number species not captured in the 2011 electrofishing survey were captured in 2013 survey work. Eels and white-clawed crayfish were captured at each of the six rubble mat sampled for which pre-existing data was available. These two native fish species are utilising the interstitial spaces between the boulders as cover.


White-Clawed Crayfish now utilising rubble mat sites (Image: Glen Wightman)

MulkearLIFE can now state, with absolute certainty, that the rubble mats installed in 2011 are an overwhelming success. Not only have salmon numbers increased dramatically but the rubble mats are also greatly enhancing the overall richness of the wonderful biodiversity found in the Mulkear catchment. These electrofishing results clearly indicate the success of the restoration work, which has been a key objective of MulkearLIFE. The results are a significant project milestone and a testament to the hard work and dedication of the MulkearLIFE Team and our colleagues in Inland Fisheries Ireland together with our key project partners, the Office of Public Works and Limerick County Council, together with the National Parks and Wildlife Service, who act as a project co-financier.

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Mulkear LIFE, Inland Fisheries Ireland, Ashbourne Business Park, Dock Road, Limerick, Ireland
Phone: +353 (0)61 300 238   Email: limerick@fisheriesireland.ie