Brown trout genetics in the Suir Catchment

Analysis of the Suir data indicated the presence of seven genetic families of trout, six located in tributary sub catchments and one confined to the headwaters of the main stem upstream of Thurles (figure 7).There would appear to be two separate genetic families of trout living in the Tar sub catchment as were observed in the Kells Blackwater. Unfortunately the level of hybrids in the Tar sample is too high to allow one to see, whether or not, these two fishes are part of a sympatric population.

The distribution of the seven genetic families of trout found in the Suir Catchment
Figure 7.
The distribution of the seven genetic families of trout found in the Suir Catchment.

A more detailed analysis allows one to look at the relationships between individual sub catchments (Figure 8).

Relationships between individual genetic stocks.
Figure 8. Relationships between individual genetic stocks.

The Drish and Anner stocks are the most distinct individual stocks. All of the tributaries draining the western side of the catchment, except the Nier, are similar. There is a marked difference between these tributary stocks and those in the Boyne. Most of the Suir tributary stocks exhibit a higher level of hybridisation than was evident in the Boyne channels – the black portion in each pie diagram represents the level of hybridisation. The upper main channel cluster has not yet been fully analysed.

The Genetic  Composition of the Main Stem Stock

Genetic analysis of the angling caught adult trout from the main stem, with reference to the tributary data, allows one to evaluate the origins of this mixed stock (Figure 9).The general nature of the adult trout population in the Suir is very similar to that observed in the Boyne system. It is made up of a combination of fishes from the tributaries. In the Suir, unlike the Boyne, there is one main stem stock. However, this is confined to the upper reaches north of Thurles. In the Suir, like the Boyne, the contribution of trout to main stem stocks is very variable with the contribution from individual tributaries varying significantly and bearing no relationship to the size of individual sub catchments.
The contribution of the tributary populations and the upper main stem stock to the trout  population in the Suir
Figure 9. The contribution of the tributary populations and the upper main stem stock to the trout  population in the Suir itself. The unassigned fish (19.4%) were hybrids.

We know that two tributaries, which currently make no contribution to the mixed stock in the main stem (Ara and Anner), support substantial trout stocks. It is now clear that these are non-migratory in nature. The major importance of the Nier as a contributor to the main stem stock is surprising in that it is a relatively small sub catchment.

This is the first genetic study where we have fish in two sub catchments (Anner and Nier) which belong to the same genetic family (see Fig. 7) yet behave completely differently in relation to their movement in the catchment with Anner trout being sedentary while Nier fish migrate up and down through the system in large numbers. This is the first evidence we have from a genetic study to suggest that the migratory/sedentary nature of trout stocks is driven by ecological, not genetic factors. The Anner River drains a rich alluvial area and would be regarded as a naturally highly productive ecosystem. The Nier, in contrast, is a high gradient spaty river with extensive gravel shoals suitable for spawning but far more limited productivity than the Anner to support a resident fish stock

The distribution of fish of tributary origin in the main stem

The trout migrants from the tributaries to the main stem of the Suir are behaving in the same way as the Boyne fishes – travelling considerable distances, both upstream and downstream once they enter the main stem. The capture location of trout, of known tributary origin, as adults in the Suir are illustrated in Figure 10 for four sub catchments.

Trout in the Suir have the opportunity to travel greater distances than their counterparts in the Boyne simply because there is an almost continuous productive zone in the Suir, capable of supporting adult trout from Loughmoe, upstream of Thurles, downstream to Carrick-on-Suir. The maximum distances travelled by trout from their natal river through the Suir are documented in Table 2.

Multeen West
Figure 10. The capture location of adult trout in the Suir from four different sub catchments – Nier, Clodiagh, Multeen West and Tar.

Table 2. The maximum distances travelled by trout up and down the Suir main stem from their natal sub catchment.

A Summary of the Genetic Features of Brown Trout Stocks in the Suir Catchment

  1. Seven discrete genetic strains of trout were identified in the Suir Catchment  -  one in the upper main channel upstream of Thurles, one in the Drish and one in the Aherlow. A different family of fish was found in the Multeen East, Multeen West and Ara Rivers. Two different families of fish were found in the Tar. Another family were found in both the Anner and the Neir.
  2. A very high level of hybrids were found in all of the tributary populations except the Anner and the Drish.
  3. Gene flow seems to be significant in both an upstream and downstream direction for different genes within tributaries on the western side of the catchment.
  4. Gene flow, into or out of the Drish system, is very limited.
  5. Limited gene flow is evident from the Anner to the Neir system.
  6. Anner and Nier fish belong to the one family . Yet, the former are sedentary and the latter exceptionally mobile which suggests that migration from tributaries to the main stem is ecologically, not genetically driven
  7. The trout of main stem origin in the Suir seemed to be confined largely to the channel reach u/s of Thurles.
  8. The adult fish, of tributary origin, in the main stem were very mixed in terms of their distribution.  Aherlow fish travelled as far north as Thurles. Clodiagh fish were caught south of Clonmel.  Drish fish travelled north towards Templemore and as far south as Golden. Multeen fish were caught by anglers at intervals from a point near Twofordbridges all the way downstream to Kilsheelan.  Neir fish were the most widespread being caught by anglers from Loughmoe near Templemore all the way south to Kilsheelan. Tar fish were also widely distributed with individuals being caught as far north Twofordbridges and south to Kilsheelan.
  9. These data clearly illustrate that the trout population in the Suir main stem should be managed on a “federal basis” given its broad generic state.