Acidification comes through a combination of acid precipitation and conifer
plantations.
Threats To The Trout Habitat:
The threat to habitat comes in a number of different forms. All of which are unfortunately linked to agricultural, Industrial and recreational needs. These threats can be studied individually and range from pollution e.g acid rain through to overshading by overhanging trees
4.1 The Threat To Habitat From Acidification.
Acidification comes through a combination of acid precipitation and conifer
plantations.
Acidity greatly affects trout in a number of different ways, particularly at the younger stages of growth. Survival of eggs from just after fertilization to hatching is usually high and independent of intergravel water chemistry . The survival of alevins (young trout) exposed for 28 days (before swim up) or 42 days (including swim up) was most strongly related to mean total aluminium content and to water pH. Acidification is predominantly a problem for East and West Ireland but not for the Midland regions; Fisheries in these areas have suffered a severe fish decline that may well be acid related.
This is due to atmospheric deposition of Sulphur. Many soft waters throughout the Irish uplands have increased in acidity since the mid-nineteenth century. There has been an increase in concentrations of metal, particularly aluminium which unfortunately is particularly damaging for alevins, it is believed that the aluminium affects respiration and the absorption of dissolved oxygen.(picture of a common acidifed lake)
Also of concern, particularly in small streams which are important brown trout spawning
and nursery areas is habitat destruction through small- scale drainage schemes for forestry which then leak pollutants into the streams and lakes. This is often more prominent after heavy rain falls and flooding which release large deposits of pollutant into the water courses. Upland drainage is also of concern as it causes accelerated run-off and hence higher spate flows which ir they occur at spawning time can wash out redds. The drainage of upland marsh/bog areas eliminates the slow-release, resulting in premature drying up of tributaries, hence reducing effective spawning and nursery areas.
Picture - Small stream ideal for reout spawning.
4.2 The Threat To Habitat From Stock
This chapter deals with a problem which is unfortunately an extremely common problem for the trout habitat, the seriousness of which cannot be over stressed, particularly on smaller waters. However it can be overcome by good habitat management. Where stream banks are unfenced and are grazed by cattle and sheep, great damage can result. Typically there is little vegetation on either the banks or within the channels. Often the banks have been broken down, so the stream is over wide and shallow. A good bank should be at a 30-45 degree profile to allow adequate cover for trout. These streams often lack any diversity of pools and riffles and offer little for trout.
4.3 The Threat To Habitat From Dredging
Stream dredging is normally carried out for land drainage purposes. Dredging tends to be done less frequently than in the past and there has been a change in emphasis from agricultural land drainage to urban flood protection. As a result the number of capital land drainage schemes have greatly decreased. This is fortunate as dredging is very damaging to fish habitats. Features like pools and riffle sequences can be removed, as are undercut banks and draping bankside cover. Also, by increasing the width of streams, current speeds are reduced, silt deposition results and spawning habitats are lost.
4.4 The Threat To Habitat From Channelisation
River channels have been modified in Ireland for many purposes including flood alleviation, agricultural drainage and reducing bank erosion. Channelisation is one method of river engineering. Other methods include deepening and banking. This usually means that the river has been artificially straightened by the cutting of a new channel. Consequently meanders, pools and riffle diversity have been removed. Other impacts due to channelisation can include erosion of river beds and banks due a higher velocity of water, this leads to a decrease in ecosystems and especially trout habitat.
4.5 Threat To Habitat From Overshading
Trees and scrub along the riverbank and within the floodplain are significant landscape features. They create a stable environment in terms of light, shelter and temperature within the river which are all important for trout survival. Trees and scrub also provide habitats for a range of animals and plants that would not otherwise find suitable conditions, they also provide a link to other terrestrial habitats in and beyond the water margins. Where trees are allowed to grow unchecked on the bank of a stream they will eventually prevent much light from reaching the water. The effects of this are to suppress lower growing vegetation on the banks and the weed and algae in the stream itself . By suppressing growth of riverside vegetation, banks are more easily eroded, so shaded streams are often wide and shallow with relatively little cover for trout. Often there is little aquatic vegetation or minimum number of invertebrates so there will be less food in the stream for trout to feed upon and fewer weeds to provide cover. As mentioned previously coniferous afforestation can release toxic metals such as aluminium into the water. It can also prove harmful through canopy closure which increases water absorption and can dry the surface of the plantations leading to greater surface run off which can concentrate pollutants.
4.6 The Threat To Habitat From Siltation.
Siltation is the build up of fine sediment on the river bed which can be harmful to many different forms of aquatic life. Increasing silt levels are more of a problem for lowland rather than upland streams, however it can still be a problem for upland streams adjacent to cultivated land such as recent forestry plantations, and can also be a problem where stock have access to the streams. Fine sediments smother fish eggs, fill in the crevices where insect larvae live, and fill in slack water areas where very young fish could find refuge.
4.7 The Threat To Trout From Riverine Impoundments.
Large dams, or weirs in a stream, whatever the reason for their being such as flow management or hydro electric schemes can have a profound effect on habitat. Riverine constructions usually " pond up" the water. This alters the flow of the river and stream. The water tends to be deep slow and consequently many cause a silty bed. This affects trout and indeed many other species of fish. Such constructions can also obstruct the spawning migrations of many other species of fish such as Salmon. Impounding dams ought to be removed if they no longer serve a useful purpose.
4.8 The Threat To Trout Habitat From Bank Erosion.
As mentioned earlier river engineering can lead to bank erosion as can other practices such as stock access to streams and rivers. Eroding banks do not have the cover value of well vegetated, stable banks. They release fine sediments into the stream and the natural scouring of deep pools can be prevented.
As can be seen in this chapter the threat to the trout habitat comes from a number of different sources ranging from industrial pollution through to over grazing by stock. However there are other threats to trout populations which are not directly linked to changes in the habitat and come from external sources, again unfortunately many of these threats are created by bad management.
