Lough Ennell Survey

Lough Ennell Survey

Inland Fisheries Ireland is currently carrying out a fish stock survey on Lough Ennell to assess the current status of the fish populations in the lake. The survey will take place between the 19th September and 22nd September 2017 and will involve netting throughout the lake. The survey will involve Inland Fisheries Ireland staff from IFI Limerick and research staff from IFIs Citywest Headquarters and will include a total of two boat crews.

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. Samples for genetic analyses of brown trout and pike will also be taken. The information collected will assist IFI in managing, conserving and monitoring any changes in the fish stocks of the lake.

The survey crews will be very visible on the lake 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 to avoid snagging in the nets.  

Why survey Lough Ennell

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, 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 Ennell. 

A fish stock survey, using survey gill nets and other methods (e.g. fyke net) is standard international practice for carrying out such a census. Lough Ennell was last surveyed as part of IFIs brown trout monitoring programme in 2007. 

Survey Timing

Since 2016 most lake fish monitoring surveys are undertaken from June to the end of September. The timing of sampling is linked to an understanding of the life history strategies of the target species. In most circumstances sampling should be carried out towards the end of the growing season when juveniles are of a sufficiently large size to be susceptible to capture and distinguishable from similar species (European standard). Temperature also affects the efficacy of passive gear such as gill and fyke nets, and netting should coincide with peak activity periods. Therefore, fish stock assessment in Ireland should, ideally, be carried out between July and late September.

Are new / alternative fishing methods being explored?

IFI continue to explore new fish monitoring methods that will enable us to quantify the stock levels in our fisheries, such as hydroacoustics. 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.

Survey catches relative to stocks in the lake

It must be stressed that this netting exercise is a survey of all fish stock species, not a culling exercise in relation to pike, trout or any other species. Data from previous surveys 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. 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 aging and possible subsequent genetic or other use.