A comparison of trends in population size and life history features of Atlantic salmon (Salmo salar) and anadromous and non-anadromous Brown trout (Salmo trutta) in a single catchment over 116 years

Abstract

We use a long time series of catch abundance from a recreational fishery over 116 years to look for population trends in Atlantic salmon, and anadromous (sea trout) and non-anadromous (brown) trout for a single catchment, Loch Lomond, west central Scotland. Year strongly predicted variation in catches but catch effort did not meaningfully increase explained variation. Salmon showed periods of increasing and decreasing trends, for sea trout and brown trout there was an overall declining trend. Since 1952, Lomond salmon population trends differed from both wider Scotland and southern Europe, indicating that the Lomond population is partially buffered from drivers of change in salmon populations more widely. In contrast Lomond sea trout showed a similar declining trend to that of populations from the wider west of Scotland over this period. The Lomond populations showed some evidence of shorter-term cycling patterns; the drivers for which are unknown. Body size in salmon and sea trout declined but increased in brown trout; salmon returned to freshwater later, and the relative proportion of all caught trout that were anadromous increased across the time series. This study shows a long and protracted period of fundamental change to populations of these two species over 116 years.

Annual numbers of Atlantic salmon (Salmo salar) and anadromous brown sea trout (S. trutta) leaping at the Pots of Gartness, Loch Lomond, Scotland

“…Against this background of general decline a few rivers in Scotland still possess significant salmon and sea trout "runs", one of which is the Endrick Water, which feeds into Loch Lomond, ca. 30 km to the north of Glasgow (Adams et al, 2022). About 10 km from where the Endrick Water enters Loch Lomond at Gartness a series of rock strata cross the river which result in waterfalls and pools known as the Pots of Gartness (NS501867) (Fig.…”