- Fisheries Research
- National Bass Programme
- Freshwater fish species
- National Salmon Monitoring Programme
- National Salmonid Index Catchment
- Pike Research Programme
- Water Framework Directive
- Water Quality Monitoring
- Fish tagging
- AMBER Project
- Compass Project
- Eel Monitoring Programme
- Environmental River Enhancement Programme
- Brown trout genetics and EREP
- Habitats Directive and Red Data Book Fish species
- Other Projects and Programmes
- SMOLTRACK Project
- Understanding Brown Trout Conference
- Owenriff Project
- Pike Research Programme
Pike Research Programme
Background to the study
The ecology of many Irish lakes has changed significantly since the 1960s, when these systems were reasonably pristine and the fish community was dominated by brown trout, perch and pike.
Agricultural runoff has resulted in on-going nutrient enrichment, leading to algal blooms. This effect has subsequently been complicated by the invasion of zebra mussels, which filter the water but don’t reduce underlying enrichment.
The fish community has also shifted strongly, especially since the arrival of large and fluctuating roach populations in some lakes, which may provide an important new prey resource for pike. These important ecological changes are likely to have implications for the management of the designated trout lakes.
Inland Fisheries Ireland formulated a scientific research programme to answer some key questions that would inform policy options for the future.
Archived IFI data on pike ecology with empirical research on pike feeding and on the feasibility of transferring pike between Irish waters was combined. A cutting-edge mathematical model of pike-trout interactions was developed. This model will take account of existing knowledge relating to the focal species, including population dynamics, life-history strategies, feeding ecology, behaviour and physiology. The model will be designed to simulate the populations of pike and trout in a lake specified by available input data and will be validated using available survey-based time series data from Irish lakes.
This research was supported by additional field work which looked at the seasonal variation in the diet of pike. Genetics samples of pike will be taken from all waters where pike are recorded during routine IFI surveys on lakes and rivers (on-going), for future analysis.
The main finding of the report is that pike in Irish waters may have changed their diet preferences in the last few decades. The finding is based on new research carried out on Lough Conn, County Mayo and Lough Derravaragh, County Westmeath in 2016 and provides an insight into the dietary habits of pike.
Previous dietary research carried out in the 1960s and 1970s in Lough Derravaragh and Lough Sheelin (located across Westmeath, Meath and Cavan) indicated that pike preferred to eat brown trout and perch. However, this latest research reveals that pike have changed their prey preference and now predominately eat roach. Researchers in Scotland and England have found similar changes in pike diet occurring in Loch Lomond (Scotland) and Lake Windermere (England). It is thought the changes in diet are due to the invasion of roach in these waters with pike now preferring to eat roach over brown trout.
The research examines whether pike and brown trout can co-exist in the same habitat. Using statistical models, it found that pike and brown trout could live co-exist within relatively large deep lakes with strong stream connectivity, however, in small, low-complex systems pike introductions could potentially have a devastating impact on resident brown trout populations.
The research also looks at the practice of pike removal and the impact it has on brown trout stocks. The findings suggest that pike removal may only be effective in protecting brown trout populations in systems where trout are the only available prey but will have little effect in systems where other prey, such as roach, is available.
Frequently asked questions
What is the significance of this research?
This research is important as it gives an insight into the behaviour of the pike species and provides updated information around their relationship with brown trout. The changing food web and altered preferences of predators in the water systems highlights the need for continued monitoring and updated data to inform effective management strategies.
Who initiated this research and why?
This research was initiated by the Board of Inland Fisheries Ireland. The Board identified the research as an important step in the review of the pike and trout policies.
This project aimed to answer some on-going questions relating to the diet of pike and the interactions of pike and brown trout in lakes in Ireland. These questions are central to the informed management of pike and brown trout populations.
Who carried out this research?
This research was conducted by scientists working across the National Research Programme at Inland Fisheries Ireland. Assistance was also provided by Inland Fisheries Ireland staff from across the organisation.
Who funded this research and how much did it cost?
The funding was provided through voted exchequer funding to Inland Fisheries Ireland via the Department of Communications, Climate Action and the Environment. The overall cost of this research over a two year period was €203,000.
How will this document impact current management practises?
This document is a scientific report and will be considered by the Policy Review Group which is in place to review ‘Pike management in designated Wild Brown Trout Fisheries’ in the first instance. The policy group will provide recommendations to Inland Fisheries Ireland. These recommendations may progress through to updating the existing Pike and Trout policies as considered appropriate.
There are no recommendations for action arising from this report. Why is this?
This is a scientific report which is produced to inform management, not to recommend a course of action.
What is the status of the Policy Review Group which is reviewing ‘Pike Management in Designated Wild Brown Trout Fisheries’?
The Policy Review Group remains in place and will now reconvene to complete its work. The terms of reference for the group can be found at www.fisheriesireland.ie/PikeReviewTOR .The Policy Review Group will provide a timeline regarding their work and the recommendations will be made public.
The review group was set up in 2016. Why has it taken so long to conclude this review process and when can we expect to see some outcome?
This has proven to be a very complex issue and it has not been possible to reach agreement between the stakeholder groups. The Policy Review Group embarked on a comprehensive public consultation process and reviewed a large number of written and oral submissions. The group have been awaiting this research which it will now consider in due course.
I thought the group had disbanded because some members removed themselves, is this not correct?
The review group remains in place. Participation of external parties on the group was by invitation and sought to ensure robust and respectful discussion of the issues. One group voluntarily ceased their involvement however full participation is welcomed at this time.
It appears that the report suggests that stock management in some lakes is no longer necessary while it is necessary in smaller lakes, is this the case? If so, will Inland Fisheries Ireland’s plans for 2019 leave out these lakes?
While that would seem like a reasonable synopsis, there is no ‘one size fits all’ solution as we are dealing with biological systems and any management practice will take account of the scientific report.
Will a policy change be required for the 2019 season?
Should a policy change be required, Inland Fisheries Ireland would expect to have it in place to inform management practices for 2019.
Why have you been carrying out stock management activities on pike?
Stock Management is undertaken on certain waters which are managed by Inland Fisheries Ireland as wild brown trout fisheries. Such waters are identified in Inland Fisheries Ireland’s pike and trout policies (Pike policy available at www.Fisheriesireland.ie/PikePolicy and trout policy available at www.fisheriesireland.ie/BrownTroutPolicy.) These stock management operations are informed by previous scientific research, are based on best practice and carried out in accordance with Inland Fisheries Ireland’s standing operating procedures for stock management.
But how can you justify continuing stock management practises given the latest research?
A review of pike management in designated wild brown trout fisheries is currently underway following an extensive public consultation last year. The Policy Review Group will provide recommendations to Inland Fisheries Ireland which will be considered and, if appropriate, implemented. It is anticipated that any required policy is likely to be in place before the commencement of the 2019 season.
Were pike killed in order to carry out this research?
No. Standardised electrofishing (using an electrical current to attract fish so they can be caught and later released alive) was the main method used to capture pike. Gastric lavage, a non-lethal method, was used to obtain stomach content samples of anaethetised pike.
However, euthanized pike (110) from the pike stock management programme on Lough Conn and seven euthanized pike from the whole lake fish stock survey on Lough Conn in August 2016 were used to supplement and verify the results.
Were gill nets used during this research?
In order to minimise disturbance to resident fish stocks, to ensure a broad size range of samples and to allow a live release of all captured fish, electrofishing was the primary method used to capture pike.
However, whole lake fish stock surveys using standardised gill nets were undertaken on both Loughs Conn and Derravaragh in August 2016 and July 2017 respectively. A survey of this type is standard international practice for carrying out such a census of fish populations in lakes.
What can you tell me about roach?
Roach is a coarse fish which is widespread in European waters and found in many Irish waters. They occur on the bottom or midwaters of a water body and migrate within freshwater only. Roach are considered an invasive species and their populations have the capacity to expand rapidly.
Does this mean that roach are beneficial in trout fisheries?
No. Roach are highly invasive and have greatly impacted upon populations of game and coarse fish in Ireland.
As roach are an invasive species and may compete with trout for food and habitat, will Inland Fisheries Ireland implement a roach removal programme?
Any future fish population management programmes will be based on scientific, economic, financial considerations alongside resource availability. Management programmes are prioritised in the overall Inland Fisheries Ireland development plan. Roach removal from lakes where roach are already established would be virtually impossible however where there are new introductions of invasive fish species to any waters, a removal programme could be considered subject to above factors.
Does this research prove that pike and brown trout can co-exist in the same habitat?
The report suggests that relatively large, deep lakes with strong steam connectivity offer a greater chance of coexistence for pike and brown trout in Ireland. However, it also concludes that pike introductions to small water systems have the potential to have negative results on resident brown trout populations.
How does Inland Fisheries Ireland ensure the quality of its research?
Inland Fisheries Ireland scientists provide data and analysis to national and international bodies to support fisheries management within this important sector and results are frequently published in quality, peer reviewed scientific journals. A series of scientific papers have been compiled in parallel to this report and these have been submitted to scientific journals for peer review.
Where and when did you carry out this research and why were those locations chosen?
Inland Fisheries Ireland carried out a seasonal diet study in a managed lake (active pike removal) Lough Conn, County Mayo and an unmanaged lake, Lough Derravaragh, County Westmeath where trout, roach and pike are present with the aim of describing the relative importance of trout, roach and other prey species in the diet of pike in the two systems.
Are the findings transferable to other lakes, for example Lough Corrib?
The overall picture in the lakes studied here is of systems that show strong ecological changes since pike management was initiated in the 1950s. While pike do consume trout (small seasonal peaks in both lakes coincide with the period when juvenile trout move out of streams) pike diet seems to have shifted significantly from trout and perch to roach where the latter species has invaded. This shift means that pike removal may no longer be a very effective trout enhancement tool in larger lakes where the fish community now includes established populations of roach and other potential prey species. The findings are therefore transferable to other large deep well connected lakes with similar fish species, particularly established populations of roach and other potential prey species. These larger well-connected systems also provide habitat diversity and potential spatial refuges that can better support on-going coexistence of pike and trout. A different situation exists in smaller lake systems, where pike may have appeared relatively recently and where brown trout remain the only available prey fish. In these situations, coexistence of pike and trout is unlikely. The mathematical modelling uses real world data and the model outputs have general applicability.
How does IFI ensure that the surveys associated with this research are conducted to the highest standards?
e.g. Has the study been carried out in accordance with any recognised standard, e.g. CEN 14757 or “O’ Grady method”?
e.g.. Would the study of the preferred diet of pike be enhanced through monthly sampling using the CEN 14757 or O’ Grady sampling methods”?
e.g. Are the research findings consistent with other research programmes findings?
e.g. Does pre-existing research demonstrates that pike prefer to consume trout, despite the presence of an abundance of coarse fish?
e.g. Has this study been peer reviewed?
Sampling methods appropriate to the project were reviewed and trialled prior to project start; these included the European standard Sampling of fish with multi-mesh gillnets (CEN 14757), the European standard for Sampling of Fish with Electricity and the O’ Grady method. Angling was also reviewed as a method.
Experience of using both the CEN 14757 and O’ Grady method has shown that these methods would not provide a large enough sample and length range of pike specimens for the study, therefore Sampling of Fish with Electricity (CEN 14011) was considered the most appropriate method for the project as the aim was to collect as many pike as possible for dietary analysis. In addition gastric lavage was identified as an appropriate minimally invasive non-lethal method of collecting the dietary samples. This method has been used elsewhere by fisheries scientists for collecting dietary information of pike species.
Electric-fishing is a well-established technique used by fishery biologists all over the world for sampling fish in freshwaters. It is generally the most non-destructive, effective and cost efficient means of sampling freshwater fish. The method is considered to be relatively benign when used effectively and it is the least invasive of the methods considered. Fishing effort, fishing equipment and protocols were the same on each sampling occasion. All sites selected were representative of main pike habitats within each lake. Angling was also used on some occasions during the study. These methods are similar to methods used in published studies and habitats are similar to those where pike were sampled in the historical period.
Most peer reviewed pike dietary studies published in Ireland to date were undertaken prior to the expansion of roach in the 1970’s. All of those studies also predate the establishment of CEN 14757 standards and O’Grady methods for sample collection methods. A recent study examined the short term diet of pike in several waters and the authors highlighted the need to describe the longer term seasonal diet of Irish pike and also whether the colonisation of roach has influenced pike diet. In response to this IFI initiated this new targeted seasonal study on two large lakes in 2016, Loughs Conn and Derravaragh to provide an up to date understanding of dietary habits of pike.
In parallel to the report a series of scientific papers have also been compiled and these have been submitted to scientific journals for peer review and publication.
Will you be carrying out further research on pike?
Inland Fisheries Ireland’s coarse and pike research and monitoring programmes are ongoing.
Where can I get a copy of this report?
The report is available for free download from this page
Who can I contact for further information?
Detailed Scientific FAQ
What were the main scientific objectives addressed in this study?
- Describe the seasonal diet of pike in Loughs Derravaragh and Conn, and assess any dietary change since colonisation by invasive roach.
- Develop statistical models to predict the probability of coexistence between pike and trout in Irish lakes.
- Investigate whether invasion by roach has changed the likely effectiveness of pike removal as a tool for brown trout enhancement in Lough Sheelin.
- Develop a mathematical modelling tool that could project the likely outcomes of a set of candidate fisheries management strategies, quantify uncertainty and support objective decision making.
What were the main scientific findings?
- Invertebrates were common in the diet of pike in both study lakes, but pike also fed on fish from very early stages in their life history. Roach were the most important fish species consumed by pike in both lakes. Other fish which appeared in the diet of pike in both lakes included perch, stickleback, trout and pike. The greatest proportion of trout in the diet occurred in April on Lough Derravaragh and May-June on Lough Conn. This observed peak in trout predation coincides with the period of downstream migration of river trout to lake environments.
Prey selectivity indices indicated that there were more roach and less perch in pike stomachs than would have been expected from the relative abundance of these species in the lakes, while the number of trout in pike stomachs reflected lake abundance.
Comparison of current pike diet with data from the 1960’s and 1970’s indicated a profound shift in the diet of pike in Loughs Derravaragh and Sheelin: perch and trout were the dominant fish prey in the early period, while roach are now most important.
- Statistical models suggested that relatively large, deep lakes with strong stream connectivity are likely to support coexistence of pike and trout. However, pike introductions to small low-complexity systems have potential for strong negative impacts on resident trout populations
- In Lough Sheelin, the shift in pike diet from trout to roach was associated with contrasting effects of pike removal on survey abundance of trout in the following year. Pike removal had some positive effect on trout in years of ‘low’ roach abundance, but little effect at ‘high’ roach abundance.
- A mathematical model was developed to express key features in the population dynamics of trout and pike, including predation by pike on trout and on alternative prey species. Management Strategy Evaluations using this model supported empirical evidence that the likely effect of pike removal on trout populations will change strongly with the abundance of alternative prey, e.g., invasive roach. The model showed that angling probably has a stronger impact on trout than pike predation.
Can you give me a simple summary of the results?
The overall picture in the lakes studied here is of systems that show strong ecological changes since pike management was initiated in the 1950s. While pike do consume trout (seasonal peaks coincide with the period when juvenile trout move out of streams) pike diet seems to have shifted significantly from trout and perch to roach where the latter species has invaded.
This shift means that pike removal may no longer be a very effective trout enhancement tool in larger lakes where the fish community now includes established populations of roach and other potential prey species. These larger well-connected systems also provide habitat diversity and potential spatial refuges that can better support on-going coexistence of pike and trout.
A different situation exists in smaller lake systems, where pike may have appeared relatively recently and where brown trout remain the only available prey fish. In these situations, coexistence of pike and trout is unlikely.
Outputs from the mathematical model show ‘considerable uncertainty’ in the estimates of exploitable brown trout biomass. Does this uncertainty invalidate the results?
Any forecasts of natural systems have associated uncertainty. The important factor is to quantify and acknowledge uncertainty, and convey this to stakeholders. The size-based modelling approach applies cutting-edge concepts in population modelling, and quantifies uncertainty arising from underlying ecological assumptions. The level of uncertainty observed in the current model outputs suggests that general trends can be accepted, i.e., the relative effects of different management strategies, but absolute levels of predicted brown trout biomass are less reliable.
There is no mention in the executive summary of the impact of pike on migratory salmonids – what is the impact here?
The seasonal diet study found that the greatest proportion of trout relative to other fish prey in the diet of pike occurred in April on Lough Derravaragh and May-June on Lough Conn. This peak coincides with the period of downstream migration of river trout to lake environments. This observation agrees with evidence that pike are flexible and opportunistic feeders, targeting whichever prey is most available and that lake entry is believed to be a pike predation bottleneck for salmonids in natural lakes with hydropower barriers and reservoirs. Further study is required to quantify the potential effect of such peak predation events on brown trout populations.
Is the complex statistical and mathematical modelling used in this report obscuring the results?
This research utilises recent best-practice in statistical and mathematical modelling. Standard statistical methods (e.g. general linear models and Bayesian modelling) were applied to historical data and to data collected during this research in order to advance understanding of the factors determining pike-trout coexistence, the nature of pike diet on several lakes, and the likely effects of roach invasion on pike and trout ecology and management.
This research also utilises mathematical modelling methods from fisheries science to project the likely outcomes of a set of fisheries management strategies, quantify uncertainty and support objective decision making. This ecosystem-based modelling approach to fisheries management is considered best practice for inland and marine fisheries systems and supports management evaluation for numerous fish stocks globally.
The report states that most fish were captured in the marginal (littoral) zone, does this bias the results?
Pike are considered to be a littoral species, i.e., they typically inhabit the shoreline or other shallow areas of the lake. The sampling programme for this study aimed to catch as many pike as possible, and so sampling concentrated on the main (littoral) habitat where most pike are found. This sampling protocol is in contrast to long-term sampling programmes, e.g., for WFD reporting, which aim to provide standardised whole lake abundance indices that track relative abundance of given fish species through time.
The earlier data (1960’s and 1970’s) was also collected in the marginal areas and it is therefore directly comparable.
Historical studies in L. Derravaragh recorded only one species or prey group, how does this affect the analysis? How many prey items would typically be expected?
Because individual numbers of prey were not evaluated in the earlier period, the Frequency Occurrence metric was used to compare diet across eras. Effectively this means that additional smaller prey items (e.g. invertebrates if consumed with trout or perch) may be under represented to a small degree in the earlier period. No inter era difference in consumption of invertebrates was detected suggesting that impact upon results was unimportant.
Given the level of historical stocking of trout in these catchments – how would this affect the analysis?
This question was not specifically analysed but it can be speculated that pike might opportunistically target naïve stocked brown trout, and that this may have contributed to historical observations of trout in pike stomachs.
On Lough Conn, a large proportion of samples were taken from two locations (Enniscoe Bay and the River Deel), while on Lough Derravaragh most samples come from the Roach Hole and Inny inflow. Does this sampling pattern bias the feeding study?
The sampling programme for this study aimed to catch as many pike as possible for the feeding study, and so sampling concentrated on the habitats where pike were most likely to be found. This sampling protocol is in contrast to long-term programmes, e.g., for Water Framework Directive reporting, which provide standardised abundance indices based on whole-lake surveys to track relative abundance of given fish species through time.
Does Inland Fisheries Ireland have an opinion on the rationale for the negative selectivity ratio associated with perch?
Pike showed a positive feeding ratio for roach and negative ratio for perch. It may be speculated that perch spines make this species more difficult for pike to handle.
What were the main conclusions of the Inter-era comparison of pike diet on Derravaragh and Sheelin?
The main conclusion is that pike diet was dominated by perch and brown trout in the historical period but is now dominated by roach. The roach invasion can thus be inferred to have reduced pike predation pressure on trout. This effect may have changed the effectiveness of pike removal as a tool for trout enhancement.
Can you please outline in layman’s terms the results of the coexistence analysis?
This analysis explores abiotic factors that might influence the probability of successful coexistence of pike and trout in Irish lakes. The results suggest that larger lakes with a high level of stream connectivity are likely to support coexistence, while small and poorly-connected lakes have a very low probability of coexistence.
Note: The co-existence analysis is based on the presence & absence of trout and pike in an Inland Fisheries Ireland survey of 821 lakes.
Please explain in layman’s terms: “Removing top predators may have unanticipated and potentially negative effects on target fish stocks in systems experiencing multiple anthropogenic pressures”.
The ecology of the designated Irish trout Lakes has changed markedly since the 1960s, when these systems were reasonably pristine and the fish community was dominated by brown trout and pike. The lakes currently experience impacts from agricultural run-off, invasive species, angling and other human pressures.
These factors probably interact to influence the fish community and the relative abundance of particular species. In this complex environment, the effect of removing a predator such as pike is difficult to predict and may be negative in terms of its impact on trout stocks. The Inland Fisheries Ireland study suggests that pike removal may have benefited trout in the healthier lake systems of the past, but is likely to be much less effective in the current impaired situation.
Download the final report and briefing note:
- pdf Pike (Esox lucius) in Ireland: Developing Knowledge and Tools to Support Policy and Management (4.61 MB)
- pdf Pike report: Public Briefing Note (687 KB)