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:
http://www.mediafreedominternational.org/2010/11/18/salmon-are-losing-distinct-genetic-characteristics/
http://www.woodlands-junior.kent.sch.uk/riverthames/tributaries.htm
http://www.freefoto.com/preview/2039-05-8/Narrow-boats-on-The-River-Avon--Bath-
No comments:
Post a Comment