A major factor in the quality of fish in our lochs is geology. Acid rocks generally mean poorer trout fishing, alkaline mean better. This is why lochs in Caithness and parts of Sutherland hold more and bigger trout than those of the Cairngorms. There can be very localised variations though and a good indicator is the diversity of plant life. Some knowledge of mountain plants is useful as well as being interesting for its own sake.
Next time you are fishing Loch Boralee at Cape Wrath take a good look around you. You will find Mountain Avens (Dryas octopetala) growing on the outcrops near the island. This plant is only found on alkaline rock. OK, the loch is not exactly up in the mountains but in the far north of Scotland such distinctions become blurred; high alpine and arctic species are found at sea level.
There are loads of books on this subject, but perhaps the definitive work on British mountain plants is still Mountain Flowers by John Raven and Max Walters. This book was number 33 in the Collins New Naturalist series, published in the 1950s. It is as relevant now as it was then. It covers all of the UK, but as you might imagine Scotland, being the most mountainous part of the country, takes up most of the book.
Chapters include the nature and origin of our mountain flora, various regions of the country etc
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Not always easy to find but well worth looking for. I paid ?35 for my copy about 20 years ago. If you want to find out more about our mountain flora I would recommend this book.
For anyone interested in getting a copy see below.
http://www.antiqbook.co.uk/boox/bygone/01341.shtml