Inland Fisheries Ireland is currently carrying out a fish stock survey on Lough Derg to assess the current status of the fish populations in the lake.  The survey will take place between the 13th June and 1st July 2016 and will involve netting of over 200 sites throughout Lough Derg and Parteen reservoir.  Four different types of survey nets are being used.  Many of these survey nets are being set on the lake bed but a small proportion are being set as floating survey nets on the surface.  A hydroacoustic survey of the deeper parts of the lake is also being undertaken.

Bouy like these will mark IFI survey nets
Bouy like these will mark IFI survey nets

The fisheries research survey will be conducted by IFI under the supervision of Inland Fisheries research staff and will include a total of five boat crews with one of these working at night.

The survey will provide a range of information on the fish stocks in the lake, e.g. size distributions of fish captured, age and growth information for all species, diet of selected species, catch per unit effort (CPUEs) for each fish species, etc.  It will also provide information on the status of the rare and endangered fish species Pollan.  Samples for genetic analyses of brown trout and pike and other species will also be taken.

The survey crews will be very visible on Lough Derg over the next few weeks and all sets of nets will be marked with distinctive buoys labelled ‘IFI Survey’.   Any anglers or other lake users are asked to be vigilant if out and about on the lake over the next few weeks and to avoid snagging in the nets.

Information on the Lough Derg Survey (FAQ)

Why Survey Lough Derg ?

IFI is aware of major changes in the ecology of many of our freshwaters over the last four decades due to the spread of non-native species, the spread of invasive species and various anthropogenic pressures.  As the statutory body charged with the conservation of our waterways IFI need to stay informed on the status of fish stocks in all Irelands’ major fisheries, including Lough Derg.  It is also a requirement of the Water Framework Directive (WFD) surveillance monitoring programme that the fish stocks in Lough Derg are monitored every three years.  A fish stock survey, using survey gill nets and other methods (e.g. fyke nets, hydroacoustics) is standard international practice for carrying out such a census.  Lough Derg was surveyed previously as part of IFIs WFD monitoring programme in 2012 and 2009.

Survey Timing

The summer season is the most appropriate season for undertaking a full fish stock survey on a lake, as most fish species are active at this time and spawning of coarse fish species is normally finished, juvenile fish are also large enough to be captured.  Since 2005 fish surveys for Water Framework Directive Monitoring purposes have been undertaken during the June to early October period..

Are new / alternative fishing methods being explored?

IFI continue to explore new fishing methods that will enable us to quantify the stock levels in our fisheries.  A specialised boat fitted with hydroacoustic apparatus will be surveying the deep parts of the lake in tandem with the netting survey.  This method may, in time, offer an alternative method to survey gill nets but it will require a considerable amount of research and ground truthing, the latter which will be achieved using survey gill nets.

IFI is also bringing into operation electrofishing boom boats, which have the capability of catching fish in water up to 2 metres deep (and deeper in clear lake situations).  This method will complement other lake fishing methods but on its own will not be capable of generating the metrics required to assess the stock status of a large lake system.  One of these boats will be used in selected shallow areas of Lough Derg during the survey.

Survey Catches relative to Stocks

It must be stressed that this netting exercise is a survey of all fish stock species, not a management tool in relation to pike or any other species.  IFI are aware that Lough Derg is a valuable “mixed fishery” for Cyprinids, Pike and Trout.  Data indicates that the proportion of any species captured in such a survey is usually ≤ 0.1% of the stock of any species present.

Survey gill nets in tandem with fyke nets provide the most effective method for catching fishes in lakes.  The nets are generally set for a period not exceeding 24 hours but commonly are fished overnight to capture the active periods of fish movement, i.e. dawn and dusk.  Not all fish that enter survey gill nets are killed and many can be removed and returned alive to the water.  Research conducted within IFI over the years, commonly using tagging methods, has demonstrated that many fish that are released from survey gill nets survive for long periods and are available to the angler.

Some fish species (e.g. bream and large hybrids) can be relatively unaffected by survey gill nets as they become passive once they enter the net.  The proportion of fish captured in survey gill netting operations relative to the stock levels present in a lake is very small (usually ≤ 0.1% of the stock of any species present).  Every effort is made by IFI staff to release live fish to the water.

What happens to the fish that are killed?

All fish that fail to survive the survey gill netting operation will be used for scientific purposes.  Each fish will be measured, weighed, aged and have their stomach contents examined.  This information will be analysed to provide information on the age cohorts of each fish species present, their relative growth rates, their feeding patterns and other relevant information.

Fish scales will also be retained for possible subsequent genetic or other use.

Other studies being undertaken in parallel

Pike diet

As part of the on-going review of the pike policy a new research project has been initiated by IFI to assess the seasonal diet of pike in selected Irish waters.  A trial of appropriate methods to use in this new research project is being undertaken on Lough Derg in tandem with the fish stock survey.

Landlocked sea lamprey

IFI are collecting information on landlocked sea lamprey as part of the ongoing research on rare and endangered fish species.  IFI staff will be recording information on fish with lesions and other relevant information.


Many eels have been tagged in the Shannon catchment as part of the National Eel Management Programme.  During the fish stock survey the IFI survey team will be checking eels captured in fyke nets for tags and recording the relevant information.