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River Frome salmon population
The Frome Atlantic salmon population is one of the most
monitored in the world, with the East Stoke counter dating back over 50
years. Though this is helpful historically the problem has been that salmon
run data only measured returning fish with no corresponding data on the
smolts that left the river. All output but no input as the scientists like
to say.
However, since 2005 an annual cohort of Frome smolts have
been inserted with a unique micro tag which allows the station to
monitor when each individual departs to sea and when it returns i.e. Fred
out, Fred in. In 2020 8,000 were tagged as part of this long-term research
which should aid separately analysis of the changes affecting survival that
occur in freshwater and those that occur at sea.
It would be good to have a silver bullet of an explanation
for the decline of our salmon runs since the 1990’s but sadly this report,
as with all others, has not yet found the definitive
explanation. There are a few bits of good news (Frome smolt run up 40%
in 2020) but as yet, to labour the analogy, no smoking gun as to the why.
Understanding grayling survival
The grayling population on the River Wylye has declined
dramatically in the past 15 years, 75% lower than it was in 2003 with an
even larger decline in bigger specimens. The report by Jessica Marsh
examines whether more frequent summer and winter low flows are the cause
with the consensus being yes.
I picked out two sections of the report; one which drives me
bonkers and the other confirms what many of us have long thought. The
second first. Marsh writes, “Interestingly, we found no negative
impact of large trout abundance on grayling survival estimates, suggesting
that the two species are well adapted to coexisting in the same geographic
area.” She goes on to say that essentially good trout habitat is good
grayling habitat and vice versa.
However, when examining the reasons for low flows Marsh
falls back on the lazy explanation: climate change. “Low flows in
summer and winter seem to be becoming more frequent in the River Wylye,”
she writes, “ suggesting that its grayling population might be vulnerable
to climate change.”
Well, I’m sure grayling are vulnerable to climate change but
on the Wylye (and most other rivers) it will be over abstraction and
pollutants that do for them long before climate change.
Do beavers affect brown trout?
Bearing in mind that the UK government announced this week a
12-week consultation into the wild release of beavers this is a timely
piece of research which examined a site in northern Scotland comprising of
two streams that feed the same freshwater loch, one stream modified by
beaver activity whilst the other was unaltered.
This, for me at least, was a fascinating study because it
was as far as I know the first and only attempt to concurrently compare
like with like. The conclusions are not overwhelmingly surprising.
Bug life: in the slower water above the beaver dam there were more
invertebrates but the population altered to favour insect life that
preferred slower moving water such as midges. The population of
invertebrates preferring faster water decreased.
Brown trout: the same slower water favoured the older trout but reduced
the number of younger trout. This research took place over a single year
(2016) which suggests that over time the trout population would inevitably
decrease.
Migration: the trout in the study do move between the loch and the
river so it was considered ‘possible’ that beaver activity ‘reduced the
propensity for individuals to migrate to the loch.’ Further research is
ongoing.
It seems to be pretty clear to me that the introduction of
beavers will simply replace one habitat (fast flowing rivers) with another
(wetlands). Which is better? Well, I personally prefer the landscape we
currently have but ultimately it is a choice that our government, driven by
a zealotry for so called rewilding I don’t fully understand, will be
visiting on a section of countryside near you very soon.
You may read the full report here .....
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