Brown trout genetics and fisheries management in Ireland


In 2011 three brown trout genetics research programmes were completed. The programmes in question were as follows;


There were a number of objectives in relation to these projects;

  • To establish the “status quo” in relation to the genetic diversity of trout stocks in each of these areas – similar projects, already completed for the Mask and Corrib catchments, illustrated that there were identifiable  separate genetic subpopulations in all sub catchments which allowed one to assess the contribution of individual sub catchments to the adult “mixed stock” in each lake.  In the case of the Boyne and Suir Catchments it was hoped that the studies would enable one to assess the contribution of individual tributaries to the main stem stock and establish the extent, if any, to which there were individual main stem populations.
  • All sub catchments in the Boyne and the main stem itself, downstream to Navan, were subject to an arterial drainage scheme largely carried out in the 1970’s. In contrast arterial drainage programmes in the Suir Catchment have been very limited in extent and largely confined to the Clodiagh and localised reaches in the Thurles area. It was hoped that a comparison of these two genetic studies (Boyne and Suir) would shed light on the extent, if any, to which arterial drainage might have altered (simplified) genetic diversity in the Boyne trout stocks.
  • If the data from these studies provided one with the same complexity of detail evident in the Corrib and Mask Catchments then the results would prove invaluable in fishery management terms particularly in relation to the Boyne where large scale river enhancement programmes are in progress.
  • All of the Lough Ennell spawning and nursery streams were subject to arterial drainage programmes in the 1950’s.In the 1970’s a persistent major cultural eutrophication problem lead to a virtual collapse of wild trout stocks in this fishery. When this problem was resolved in the late 1970’s major trout stocking programmes took place, both directly to the lake and into all the spawning and nursery streams. Subsequently in the 1990’s a major stream enhancement programme took place to improve the morphology and ecology of the channels thereby increasing trout production. While this programme was successful, leading to a doubling of the adult trout stock in the lake in recent years, two outstanding questions remain unanswered;

A – Did the drainage programme in the 1950’s lead to a simplification of the genetic nature of trout stocks in the various sub catchments?

B – Could the large scale stocking programmes to both L. Ennell directly and its stream catchments have altered the genetic nature of the trout stocks in this area?

It was hoped that this study would help to answer the above questions.

Materials and Methods

A total of either fifty 1+ year-old trout were captured by electrofishing in each tributary sub catchment. A scale sample was retained from each individual fish for genetic analysis. Each group of fish from each sub catchment were collected in small groups from at least three different geographical locations in each sub catchment to ensure that the trout sample were from a wide range of parents. Trout fry were excluded from the sample because one could not assume that they had dispersed a significant distance from a redd and might therefore be offspring from a limited number of adults. Larger older trout (≥ 2+ year old fish) were also excluded from the sample on the premise that some fish of this size might have been born in a different tributary.

In the case of these three studies (Boyne, Ennell and Suir) none of the fish in tributary samples were captured upstream of impassable barriers, a factor which could seriously influence their “genetic makeup”.

Anglers from clubs fishing on the Boyne and Suir main stems kindly agreed to collect details of their catch and forward same to I.F.I. as part of this study. Scale envelopes were issued to anglers. They were asked to place a scale sample from a fish in an envelope and record the fish’s length and capture location on each individual envelope. Each angler was supplied with plastic “picnic knifes” and asked to use one knife to scale each fish, thereafter discarding the knife and using another for the next fish – this was designed to avoid cross contamination of genetic material.  The adult trout sample collected by anglers was compiled between May and August in 2011 when the adult trout were feeding. Samples from earlier or later periods of the year were avoided to ensure that anglers were not sampling shoals of fish which might be adjacent to their natal stream. When the angling caught samples from the Boyne and Suir were returned to I.F.I. the subsample of these selected for genetic analysis were from the widest possible geographic range within each channel.

In the case of the L. Ennell study the mixed stock sample of adult fish from the lake were obtained from an archive of fish scale material in I.F.I. which had been compiled in the course of fish stock surveys on this lake in recent years.

The subsequent genetic analysis of all of fish samples was carried out without the geneticists having any knowledge of the location or nature of the samples. This ensured that the analysis was entirely objective.