Acid deposition :: Atlantic salmon

Latin name: 

Salmo salar

Impact Type: Deposition of pollutant

Key Concerns:

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).

Additional Comments:

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.

Critical Load/level: 

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.



Alabaster, S.; Lloyd, R. 1980 Water Quality Criteria for Freshwater Fish
Harriman, R.; Bridcut, E.E.; Anderson, H. 1995 The relationship between salmonid fish densities and critical ANC at exceeded and non-exceeded stream sites in Scotland Water, Air and Soil Pollution 85 2455-2460
Harriman, R.; Christie, A.E.G.; Hornung, (Eds) M.; Skeffington, (Eds) R.A. 1993 Critical Loads: Concepts and Applications ITE Symposium 28 103-108
Jenkins, A. 1999 Environmental chemistry: End of acid reign? Nature 401 537-538
Lien, L.; Raddum, G.G.; Fjellheim, A.; Henriksen, A. 1996 A critical limit for acid neutralizing capacity in Norwegian surface waters, based on new analyses of fish and invertebrate responses Science of the Total Environment 177 173-193.
Rosseland, B.O. 1986 Biological effects of acidification on tertiary consumers: fish population responses Water, Air and Soil Pollution 30 451-460
Rosseland, B.O.; Staurnes, M.; Steinberg, (Eds) C.E.W.; Wright, (Eds) R.F. 1994 Acidification of freshwater ecosystems: implications for the future ES14 227-246

Species group: