Impacts of acidification on brown trout Salmo trutta populations and the contribution of stocking to population recovery and genetic diversity.

Anthropogenic acidification in SW-Scotland, from the early 19th Century onwards, led to the extinction of several loch (lake) brown trout (Salmo trutta) populations and substantial reductions in numbers in many others. Higher altitude populations with no stocking influence, which are isolated above natural and artificial barriers and subjected to the greatest effect of acidification, exhibited the least intrapopulation genetic diversity (34% of the allelic richness of the populations accessible to anadromous S. trutta). These, however, were characterised by the greatest interpopulation divergence (highest pairwise DEST 0.61 and FST 0.53 in contemporary samples) based on 16 microsatellite loci and are among the most differentiated S. trutta populations in NW-Europe. Five lochs above impassable waterfalls, where S. trutta were thought to be extinct, are documented as having been stocked in the late 1980s or 1990s. All five lochs now support self-sustaining S. trutta populations; three as a direct result of restoration stocking and two adjoining lochs largely arising from a small remnant wild population in one, but with some stocking input. The genetically unique Loch Grannoch S. trutta, which has been shown to have a heritable increased tolerance to acid conditions, was successfully used as a donor stock to restore populations in two acidic lochs. Loch Fleet S. trutta, which were re-established from four separate donor sources in the late 1980s, showed differential contribution from these ancestors and a higher genetic diversity than all 17 natural loch populations examined in the area. Genetically distinct inlet and outlet spawning S. trutta populations were found in this loch. Three genetically distinct sympatric populations of S. trutta were identified in Loch Grannoch, most likely representing recruitment from the three main spawning rivers. A distinct genetic signature of Loch Leven S. trutta, the progenitor of many Scottish farm strains, facilitated detection of stocking with these strains. One artificially created loch was shown to have a population genetically very similar to Loch Leven S. trutta. In spite of recorded historical supplemental stocking with Loch Leven derived farm strains, much of the indigenous S. trutta genetic diversity in the area remains intact, aside from the effects of acidification induced bottlenecks. Overall genetic diversity and extant populations have been increased by allochthonous stocking.

Long-term trends in pH, aluminium and dissolved organic carbon in Scottish fresh waters; implications for brown trout (Salmo trutta) survival.

Over the past 30 years upland streams and lochs in Scotland have shown significant signs of recovery from acidification, particularly in terms of declining concentrations of non-marine sulphate (nm-SO(4)). Long-term index sites in central and southwest Scotland reveal a significant decline in the concentration of biologically important components, notably acidity (H(+)) and labile (toxic) forms of aluminium (Al-L), whilst dissolved organic carbon (DOC) and complexed forms of aluminium (AL-NL) have increased significantly. Although these improvements should increase the probability of brown trout survival, and have resulted in increased acid neutralising capacity (ANC) in fishless streams to values close to current critical limits, there is still a relatively poor correlation between ANC and current fish status. Site to site variability appears to be linked to the relative contribution of (H(+)), Al-L and DOC within the critical ANC (ANC(CRIT)) range. It is proposed that ANC(CRIT) should cover a range of ANC values as a function of Al-L concentration. Based on field studies an empirical range of critical ANC values are suggested which better predict the presence of a sustainable brown trout population.