This chapter discusses the threat to trout which are not linked to changes in the trouts environment and often come from external sources.
5.1 The Threat To Wild Brown Trout From Hatchery - Reared Brown Trout.
As can be seen in the pictures below, brown trout show large differences. It can be assumed that these differences are genetic and reflect local environmental adaptations such as diet, acidity and altitude. Within these regional areas where significant reductions have been in stock numbers due to loss of habitat or pollution, the solution decided upon by the anglers is to introduce large numbers of hatchery-reared trout. It is then assumed that the introduced trout will breed with the natural population and lead to a loss of genetic variation among the wild trout, which could reduce the hardiness of the trout. Population genetic monitoring of stocking programmes strongly suggests that releases of trout from exogenous hatchery strains cannot be recommended.
If stocking is a necessity then it is preferable that offspring of the original gene pool are used. Although it would of course be preferable to prevent a decline in habitat quality before the introduction of fish was a necessity.
Note the larger brown trout compared to the small silver one.
5.2 The Threat To Wild Brown Trout From Predation
The main predators for Trout stocks in upland Ireland is sawbill ducks, and mainly the Merganser and Goosander. Goosander diet varies with season and location. On the Shannon the spring diet was 43% Trout. This may seem a large amount of young fish, however, the effect of the losses upon the remaining populations is as of yet unknown and it has not been established to date whether the predation is additive to other causes of mortality. Idealistically habitat management should be able to create a population of trout which can support the avian predators. Pike are a trout predator but are fortunately not found in Upland Ireland, however their culling on lowland stretches should be implemented if trout are found to have evidence of Pike attacks.
There are two mammal predators of trout, the otter (Lutra lutra) and the mink (Mustela vison). Otters have slowly regained a foothold in Ireland after long periods of persecution and habitat degradation and are now a protected species. Although some anglers are certain that otters are a menace it is the authors belief that to sight an otter is a sign of excellent water quality and a healthy fish stock. It is also common knowledge that the otter prefers to feed on eels rather than Salmonoids. Otters also dramatically increase the aesthetic value of rivers and streams and should not be classed as a serious predator. Mink on the other hand should be controlled. Mink have become naturalised along many rivers of the British Isles since they first escaped fur farms in the 1950's . Unlike Otters, Mink take more than fish, they are damaging to many bird species and are listed a priority by game keepers as their ability to damage gamebird stocks is high. Mink are easy to trap, however they are well established throughout the British Isles and would not be easy to eliminate.
5.3 The Threat To Trout From Overfishing.
This is a very serious threat for wild brown trout in Ireland. Brown trout fisheries close to centres of population are those most likely to be subjected to higher angling pressure, which can lead to over exploitation of the spawning stock. The effects of this may be masked by stocking. The size limit for trout was 19 centimetres, to prevent the over- exploitation of trout the size limit was raised to 23 centimetres. The current size limit prevents trout from being taken before they have reached maturity and therefore allows for breeding to occur. If fish are taken at an immature age stock numbers will reduce significantly. This is more relevant to the more acidic uplands of Mainland Britan where it takes longer for fish to mature due to the harsh environment in which the fish live, but the fish also tend to mature at a smaller size. Therefore the argument could be made that 23 centimetres is to large a size. Currently the it is being planned to introduce regional differences.
The angler personally should also take responsibility for over - fishing. A large hen trout should be returned towards the end of the season (Aug. - Sept.) to allow the hen to spawn. A cock trout is obvious towards the end of the season due to the hook that grows as part of the lower jaw. All anglers should carry a measuring stick or ruler to measure the fish if in any doubt.