Lough Corrib Survey

What is the survey about?

The last major survey of adult fish stocks in L. Corrib was carried out in 1996, some 15 years ago. In the interim there have been a number of significant ecological changes in the lake – two additional species are now well established (the aquatic plant called Lagarosiphon and Zebra mussels).  An additional fish species (Roach), first introduced to the Corrib in 1982 has now had an extended period to become established in the fishery. Currently there is considerable debate in relation to the status of trout stocks in the fishery. The proposed fish stock survey in 2012 would establish;

• The current status of the adult trout population, numerically, compared to 1996.
• The relative strength of individual year classes of trout present.
• Comparative growth patterns of adult trout presently compared to times past.
• The trout’s dietary habits in 2012, compared to 1996.
• This survey will also allow one to update the comprehensive existing genetic data base on the trout population
• Changes in the population dynamics of the other fish species present (perch, pike and roach), since 1996, will be evaluated. Research on other Irish lakes has shown that the establishment of Zebra mussel populations can result in the decline of Roach and Pike stocks.
• It provides an opportunity to reaffirm the status of the Char stock in the lake. Regrettably the 1996 survey suggested that this species had become extinct at that point in time.
• The survey will be used to update the information on the distribution of Lagarosiphon in the lake.
• Information compiled on the pike stock will “feed into” an on-going national Ph.D. study of this species.

Will there be any other benefits to the L. Corrib survey?

The fish stock assessment being carried out on L. Corrib will provide an opportunity to collect adult trout genetic samples, as was done in 2008. The results of which will contribute to the over-all management of the L. Corrib brown trout fishery. Genetic analysis of the trout samples will pin-point the origins of the adult lake brown trout stock and will also determine the percentage contribution being made by individual sub-catchments. Such information is invaluable in fishery management terms as it can help identify the less productive parts of the catchment which then could be addressed through river enhancement programmes.

What did the 1996 survey show?

Relatively poor trout stocks and a significant population of large pike.

Who is carrying out the survey?

Staff from Inland Fisheries Ireland are carrying out the survey. Both research staff from the IFI office in Swords, Co. Dublin, and field staff based around Lough Corrib will be involved in the fieldwork.

When is it happening?

It is intended to start the survey on Monday, February 27th and, weather permitting, to finish the sampling programme in two and a half weeks.

What does it involve?

The survey will involve the collection and processing of fish samples from a total of 250 sites – the same locations which were sampled in the 1996 survey. All trout captured alive in the survey nets will be scaled, measured and released alive. Pike ≥ 85cm in length will also be released alive. All other fish captured will be retained for analysis. In the 1996 survey a total of 130 trout were retained (dead fish in the nets) and the balance (358 trout) were released alive.

What percentage will the trout captured represent in terms of the stock in the lake?

Circa 0.1% of the adult trout stock of which approx. 88% will be released alive.

Where will the nets be set?

Nets will be set in the same locations that were used in 1996. 250 locations around the lake will be used. All nets are set to fish on the bottom, but in shallow areas (<2m depth) they will be fishing close to the surface. Anyone boating on the lake during this time is asked to keep an eye out and steer clear of marker buoys, especially in shallow areas.

What is done with the fish that are caught?

Any trout that are alive will be measured and released again. Pike over 85cm will also be measured and released. All other fish caught will be retained for processing. This involves measuring length, weight, etc. and stomach analysis to see what they are feeding on.  A genetic sample will also be taken from all trout samples.

How is this survey different from the survey in 2011?

The 2011 survey was a general fish stock assessment carried out under the Water Framework Directive (WFD), to give an overall assessment of all the fish species in the lake. It also used different netting techniques, which are used in all WFD surveys. The upcoming survey is targeted specifically at analysing adult trout stocks in the lake, and uses the same survey techniques as the 1996 survey so results can be comparable.

How many survey nets will be out every night?

A total of 25 gangs of survey nets will be set throughout the lake every night during the survey.

How will I know where nets are so I don’t drive my boat into one?

All nets will be marked at either end with a brightly coloured marker buoy. Each buoy will have IFI SURVEY printed on it.  Nets are set to fish in the bottom 2 metres of the water column, but in shallow areas this means they will be close to the surface. Please keep a look out for marker buoys during this period, and steer well clear.

What happens after the survey? Is there a report?

Yes. There is a lot of post-processing of fish, and data analysis to be carried out. This work is usually done over the autumn/winter months, when fieldwork has been completed, so the full report will not be available until this has been done. In the meantime, an interim report giving details on the numbers of fish caught, species composition, etc. will be uploaded to www.fisheriesireland.ie for public viewing. The reports from the previous WFD surveys are available to view/download on www.wfdfish.ie

What are the findings of the survey used for?

Fishery managers need good data in order to manage fish stocks. The results of this survey will allow IFI staff to determine if trout stocks are healthy, and if not, what actions need to be taken. The overall aim of fishery management on Lough Corrib is to provide quality game angling for local and visiting anglers.

What measures are being taken to stop the spread of invasive species?

Biosecurity is very important, and survey boats are not exempt from this. All equipment (boats , engines and other equipment) brought to L. Corrib to assist the survey will be  cleaned, treated chemically and/or frozen as appropriate to ensure that no biological material of any nature is introduced to the lake.

In order to ensure that the survey nets do not contribute to the spread of Lagarosiphon in the lake nets which pick up any weed, while fishing,  will be brought ashore, cleaned,  and blast frozen overnight to kill any weed fragments in the nets  before being reset in the lake.

Following the completion of this survey all nets and other equipment will be frozen and/or chemically treated as appropriate before being used for survey purposes on other waters.