Salmon in the Thames are 'strays'

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The high-profile project to restore the presence of the Atlantic salmon (Salmo salar) in the River Thames basin by restocking it with hatchery-raised fish can be considered a failure, according to a study published in the most recent issue of the journal Biological Conservation.

The River Thames was historically well stocked with Atlantic salmon, this fact being mentioned as far back as the Magna Carta in 1215; a substantial fishery for this species existed on the river until the early 19th century. 

Increasing pollution of the river wrought by the industrial revolution and urbanisation of London led to the extirpation of this species from the Thames by the 1830s.

After previous unsuccessful attempts to restore salmon to the Thames, a lone individual was caught downstream of London in 1974 after significant improvements in water quality. This discovery reinvigorated attempts to restore salmon to the Thames and stocking of juveniles began in 1975.

In 1979 the Salmon Rehabilitation Scheme was established, which released tagged salmon (mostly from stock originating in Scottish hatcheries) into a number of Thames tributaries. The numbers of salmon recorded in the Thames gradually rose until reaching a peak of over 300 in the early 1990s. Since the late 1990s, this number has been declining until no salmon were again recorded in 2005.

The objective of the study by Andrew Griffiths and coauthors was to identify the origins of untagged salmon ascending the Thames between 2005 and 2008. The authors wanted to determine which of the two possible scenarios were more likely:

(i) that the salmon were the descendants of the stocked fish or
(ii) that the salmon were strays from other rivers.

Using 12 microsatellite loci in their analysis, the authors sampled 26 salmon (10 tagged and 16 untagged) caught from the Thames and compared them to data obtained from 3730 salmon, originating from 55 rivers in the southern part of the European range of Atlantic salmon (encompassing northern Spain, western France, Ireland, southern England and the Atlantic coast of Britain).

They found that all but one of the tagged fish was found to originate from areas to the north of the Thames (the sole exception being from northern France).

Conversely, all but one of the untagged fish were found to originate from southern England (the sole exception again originating from northern France).

The results therefore imply that the untagged salmon present in the Thames are most likely due to straying from proximate rivers, and not from the establishment of a naturally breeding population.

Although the stocked fish failed to found a reproducing population in the Thames, the results of the study imply that natural processes could establish a population of salmon in the Thames without the need for stocking if conditions on the river were to improve.

For more information, see the paper: Griffiths, AM, JS Ellis, D Clifton-Dey, G Machado-Schiaffino, D Bright, E Garcia-Vazquez and JR Stevens (2011) Restoration versus recolonisation: The origin of Atlantic salmon (Salmo salar L.) currently in the River Thames. Biological Conservation 144, pp. 2733–2738.

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