Atlantic Salmon in SE England

A Genetically Distinct Population of Atlantic Salmon

Researchers from the Department of Biosciences, University of Exeter, and the Salmon & Trout Research Centre, Game & WildlifeConservation Trust (GWCT), have described the chalk stream salmon (Salmo salar) of Dorset and Hampshire, UK as a sub-species candidate. With a sub-species, these salmon could be eligible for special protection.  This population of Atlantic salmon spawn and grow to smolts in streams that originate in Early Cretaceous chalk aquifers¹ of Dorset and Hampshire².

The investigators found negligible genetic variation among the Dorset and Hampshire salmon, but collectively they differ genetically from other Atlantic salmon populations.  The hypothesis is that these salmon spawn and return to southern England chalk streams generally but not to any specific river.  The research team evaluated the river² populations of the Avon, Frome, Itchen, Piddle, and Test (Hampshire Basin, Fig 1.)  The Environment Agency (EA) sampled the Avon, Itchen, and Test, and the GWCT sampled the Frome and Piddle during ongoing management programs and surveys. The investigators compared their data with sampling results from NW France, SW England, and Norway. The researchers obtained international data from the SALSEA-MERGE³ project.

Fig 1. Geologic map SE England & Channel. By Woudloper (Own work) [GFDL ( or CC BY-SA 3.0 (], via Wikimedia Commons.  Modified by StreamSide News Jan. 2018.

What The Unique Genetics of Chalk Salmon Means
Geneticists use the term admixture to describe the mixing of otherwise distinct populations.   Stahl G (1987), as referenced in Ikediashi, C. et al., surmised that for two Salmo salar populations to maintain separate lines, they had to have ” fewer than one migrant per year between them.”  The chalk stream salmon of southern England are then genetically distinct, and the subgroup may merit a sub-species designation because the genetically different from other northern European subgroups, including English non-chalk salmon.

The chalk stream salmon’s early return to sea at one year of age (instead of the typical 2 t0 3 of other populations) is probably the result of porous aquifers feeding watercourses.  The flow volumes and temperatures are stable, and the pH alkaline.  The distinctive geologic and water characteristics bring the adults back from the sea to estuaries that are spatially relatively close (at least the Frome, Itchen, Piddle, and Test).   Thus, rivers of relatively short length sharing similar geology provide the homing signals for spawning fish.

The chalk salmon of southern England then must mix reproductively by returning to a specific region instead of an exclusive river.  Ikediashi et al. suggest managing the fish population as a whole rather than by administrative areas (i.e., EA Region Wessex, and EA Regions Solent and South Downs).  A subspecies designation would facilitate elevation to protective status and a change in management schemes.

¹The Chalk aquifer of the North Downs (Research Report RR/08/02, British Geological Survey) describes the aquifer underlying the chalk hills on the north edge of the Weald (Fig. 1).  A sister report (Jones, H K, and Robins, N S (editors). 1999. The Chalk aquifer of the South Downs. Hydrogeological Report Series of the British Geological Survey.) describes an analogous aquifer south of the Weald.  The study area for the chalk stream salmon is south and west of the Weald.

²The name, Frederick Halford (the priggish perfecter of upstream dry-fly technique) may be familiar to you. G.E.M. Skues (inventor of modern nymphing), and Frank Sawyer (the Riverkeeper on the Avon who perfected Skues techniques, especially with small nymph imitations) may be likewise familiar. Then you have probably heard of the rivers in the Exeter study.

³Advancing understanding of Atlantic Salmon at Sea: Merging Genetics and Ecology to resolve Stock-specific Migration and Distribution patterns.

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