Acid deposition :: Brown trout

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.

Three categories of brown trout can be considered: the migratory form (sea trout), resident stream/river trout and resident loch trout. All these spawn in streams where episodic events of acidified precipitation can have serious impacts on recruitment. Populations of resident stream/river trout subjected to frequent acid events are therefore at great risk of elimination. In large lochs, acidification that may be lethal in tributaries is diluted and trout can also escape from polluted inflows. Medium and small loch populations however, can be subjected to prolonged acidification and reduced recruitment leading to small populations of irregular age structure or elimination.

The degree of stream acidification will depend on deposition rates and the acid neutralising capacity (ANC) (Harriman et. al. 1995a) of the catchment. Additionally, for lochs, morphometry and water retention time can be important. ANC may be reduced by afforestation (e.g. Rees & Ribbens 1995). Critical deposition loads can be calculated for individual sites. Organically stained waters are believed to provide some protection from the deleterious effects of acidification. 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 species, development stage and different populations of the same-species (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 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.