Impact Type: Deposition of pollutant
Salmonid species are likely to be harmed by pH levels below 5.0, while levels below 4.0 are lethal (Alabaster & Lloyd 1980). In waters acidified by atmospheric pollution (i.e. rather than naturally organic acid waters) aluminium toxicity is the principal problem. Salmonids are most vulnerable at their early life stages in the nursery areas.
Atlantic salmon spawn in the upper tributaries of rivers where episodic events of acidified precipitation can have serious impacts on recruitment. The degree of stream acidification will depend on deposition rates and the Acid Neutralising Capacity (ANC) (Harriman et. al. 1995a) of the catchment. ANC may be reduced by afforestation (e.g. Rees & Ribbens 1995). Critical deposition loads can be calculated for individual sites. Recent reports on surface water chemistry in the UK indicate a reversal of acidification (e.g. Jenkins 1999).
Impacts can vary greatly between development stages and different same-species populations (Rosseland 1986). It is difficult to highlight general trends on any scale - in some cases entire populations can be wiped out or there may be degrees of population reduction. The ability of individual fish to find refuges of better quality water is significant. For freshwater fish, reproductive failure seems to be the most important factor in population loss. Salmonoids are particularly vulnerable when undergoing physiological changes in preparation for sea to river migration (or vice versa) particularly when these changes coincide with spring snow melt.
Critical loads are calculated for individual sites using the Henriksen model. The model requires a relationship between abundance or presence/absence of a selected species and ANC. In the UK, the ANC is set at 0 µeq/l, at this level there is a 50% probability that the population status is not healthy. There is some evidence that a variable ANC depending on individual site conditions would be more appropriate. Current ANC levels may be too low to protect salmonoid stocks. Lien et al. (1996) suggests from an extensive study in Norway that a tolerance level of ANC = 20 µeq/l would be more appropriate.
|Habitat/ Ecosystem Type||Critical Load/ Level||Reliability||Indication of exceedance||Reference|
Calculated on a site basis.
|not relevant/applicable i.e. the approach is not relevant for this species or habitat||
The calculation is based on ANC and species presence on a site-by-site basis. The effect of exceedance is reduced recruitment leading to small populations of irregular age structure or elimination.