Monday, 5 September 2011

Exeter Monday: Salmon in the River Thames: The result of restoration or recolonization?

There are some new fish in town, and researchers want to know where they came from. The fish are Atlantic salmon (Salmo salar L.), and the town is London—or, more accurately, weirs located in the River Thames at the boundaries of London. It’s hard to find salmon in the Thames these days, and it’s even harder to find untagged salmon, which likely originate not from restocking efforts but from the wild. If this is the case, it may mean that the animals are finally repopulating the Thames after having been extirpated from the river in the 19th century—an impressive feat considering that teams of conservationists have been unable to achieve this goal despite decades of efforts. 

 (Atlantic salmon, Salmo salar L.)

The River Thames is one of the most famous polluted habitats in the Western world; urbanization, coupled with the industrial revolution, left the river foul and uninhabitable not just for the Atlantic salmon, but for many animals. Runs of salmon up the Thames were well-known as far back as 1215, when they were mentioned in the Magna Carta; fish numbers were previously so high that there was a fishery on the river until the early 19th century. But once the waters became polluted, the salmon gradually disappeared; the last record of a "natural" Thames salmon was made in 1833.

Over the years there have been various attempts to restore the species to the river, not just because it is an important keystone species and can be used as a bioindicator, but also because it can provide substantial economic benefits thanks to its popularity among avocational and vocational fishermen alike. Ultimately, none of the previous attempts met with much success. During the intense rehabilitation efforts performed in the 1970's and 1990's, the river was stocked predominantly by a mixture of captive-bred fish from Scotland and "supportively-bred" (in other words, with only one generation of captive breeding) fish from two rivers in Ireland. Despite these efforts, the number of adults in the Thames diminished steadily until 2005, when no fish were captured at all.

 (A map of the Thames and its major tributaries. For more information on the river, please visit the source page for this image.)

It was perhaps a bit of a surprise, then, when untagged adult salmon began to appear in 2006. Two were caught that year, followed by 5 in 2007 and 9 in 2008. Collaborators from several institutions—the University of Exeter, the Marine Biological Association of the United Kingdom, the Environment Agency for England and Wales (Thames Region), the Universidad de Oviedo, and the Westcountry Rivers Trust—pooled their efforts in order to identify the origins of these mysterious piscine strangers. They collected scales from the 16 untagged adults caught since 2006, as well as from 10 adults captured in 2003. Then they genotyped all 26 fish, using two different types of analyses and two different types of software—just to be sure.

All but one of the 2003 fish was genetically related to salmon from northern Europe—in other words, from the Irish stock population that was used in the rehabilitation effort; the singleton was, strangely enough, determined to have French origins. However, fish captured since 2006 were overwhelmingly of southern English origins—with the exception of one individual determined to hail from Scotland. Despite the two “outliers” from France and Scotland, these results overwhelmingly indicate that fish are relocating to the Thames from nearby rivers—the Avon and the Itchen, to be exact—where salmon populations are still viable. In other words, the fish are reintroducing themselves.

(The River Avon--an apparent source of Thames River salmon.)

There are, of course, other potential explanations, but none that holds much weight. For instance, these are not likely to be extremely old fish leftover from before 1994, when officials stopped repopulating the Thames with southern English fish. Likewise, these are probably not the offspring of introduced fish that hung around and formed a local, long-term breeding population; extensive surveying has failed to produce any evidence of salmon reproduction in the Thames—not to mention, this could only happen if the northern fish were breeding with local fish, which is highly unlikely. 

The scientists who conducted the study take their results as support of the “If you build it, they will come” style of population restoration. Theirs are not the first findings indicating that natural populations of animals are happy to recolonize areas from which they have been extirpated—provided that we humans clean up the habitat first. Thus, the researchers stress that managers should emphasize restoration of ecosystem and function, rather than reintroduction of apex species—predators whose hunting habits have widespread impacts on ecosystem stability—and clean-up of only the immediate habitat.

As with so many conservation tales, the end of the salmon story has not yet been written. Regardless of whatever small successes there may be with salmon in the Thames, there are still general declines in the number of salmon returning to UK rivers from the sea. This is likely attributable to both overfishing and climate change. Within the Thames itself, an extra concern is the influx of storm sewage releases after heavy rain; these reduce levels of dissolved oxygen and leave the salmon gasping for breath. Furthermore, water levels can fluctuate quite a bit in the Thames, possibly leading to flows inadequate for breeding efforts.
So, while the salmon have overcome several hurdles, there appear to be more yet to come. Whether it’s with the help of conservationists and managers or on their own, let’s hope the salmon populations eventually end up where they belong—so to speak.


Griffiths, A.M., Ellis, J.S., Clifton-Dey, D., Machado-Schiaffino, G., Bright, D., Garcia-Vazquez, E., and Stevens, J.R. 2011. Restoration versus recolonisation: The origin of Atlantic salmon (Salmo salar L.) currently in the River Thames. Biological Conservation, online advance publication.

Thanks to the following websites for providing the images used in this post:


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