"Autumn"
grilse and salmon
Approximately from August onwards the behaviour of
new entrants changes. Instead of racing to the hills,
fish entering the Tay have a more leisurely attitude.
These fish tend to be bigger than those earlier in the
summer. Nowadays most weigh perhaps 6 or 7 pounds in
August increasing perhaps to 7 to 10 pounds by October.
This class of fish are also grilse, though this is not
always appreciated by anglers. It is only that they
have grown more having been that bit longer at sea.
A bigger class of fish also exists at this time, usually
15 to 20 pounds, which are the 2 sea winter salmon.
Generally speaking the fish which enter the Tay in
the autumn spawn in more lowland tributaries of the
Tay or in the main stem of the Tay itself. These fish
are not heading for the headwaters so there is not the
headlong rush of earlier days.
Having said that there are still local differences.
For example, the River Almond which enters the Tay at
Perth, still flows out of highland country and its population
tends to be made up of what might be described as a
late summer (August-September) grilse while in the lower
Tay, the lower Earn and the Eden, some fish which spawn
in these areas may actually enter freshwater very late
in the year, i.e. in the late autumn or winter. In fact
some fish are caught every year in the lower Tay in
January and February which have only recently entered
the river (sometimes even sea-liced) and are on the
point of spawning.
Read more on autumn salmon, click here
Run timing - populations and genetics
Tagging experiments have shown that most salmon can
and do navigate back to the tributaries where they were
born. When salmon enter the Tay they do not merely select
a spawning tributary at random but return to approximately
where they, their parents, their grandparents....etc
were born.
The different patterns of run timing seen in a river
like the Tay represent different behaviour patterns
exhibited by different sub-populations of fish which
have a considerable degree of isolation from each other.
Some years ago the Freshwater Fisheries Laboratory
conducted an experiment whereby juvenile salmon from
spring salmon parents from the River Tilt and from late
summer grilse parents from the River Almond were stocked
into separate areas of the the River Braan near Dunkeld.
The emigrating smolts were tagged and it was found that
the Tilt offspring still returned in spring and the
Almond fish still returned in late summer despite both
being reared in a different environment.
This and similar experiments have shown that the run
timing of salmon is a strongly inherited characteristic.
The salmon in the Tay are not therefore just part of
one simple population but comprise number of different
populations with different traits.
Why the differences in run timing?
Why should fish spawning in the upper reaches of the
Ericht in Glen Shee differ in run timing from fish spawning
in the lower Tay or the Eden?
To some extent these differences can be explained by
adaptation. The Ericht or the Tilt, for example, are
relatively difficult tributaries for salmon to ascend.
There are waterfalls and rapids to be negotiated and
also in the case of the Tilt a hydro dam. Winter comes
quickly to those tributaries and the salmon must spawn
early. It would be physically impossible for fish entering
the Tay in October or later to penetrate the upper Ericht
or Tilt in time for spawning. But by running during
the summer when the water is warm spring salmon or summer
grilse can surmount obstacles.
Is there a relationship between juvenile growth
and run timing?
Since throughout Scotland spring salmon tend to be
produced in headwater areas of rivers and autumn fish
at the lower end, it has generally been found that spring
salmon are derived for slower growing juveniles than
autumn fish. That is not surprising of course, since
highland burns like the Tilt are colder and much less
fertile than lowland rivers like the Isla, Earn or Eden.
Some years ago it became fashionable to consider that
perhaps run timing was directly caused by the rate of
growth as parr.
While this simplistic picture may be true in some rivers,
it does not actually hold true in the Tay.
It is the case that the rivers Tilt and Lyon do predominantly
produce 3 year old smolts, but surveys by the TDSFB
have shown the headwaters of the Ericht to mainly produce
2 year olds. The upper Tummel also produces 2 year olds,
and the headwaters of the Dochart which are noted for
the earliest running of all fish in the Tay, produce
particularly fast growing parr for the uplands. The
upper Almond also produces many 3 year old smolts yet
it produces a relatively late run of grilse.
Therefore, on the Tay, there is no consistency to this
theory. If, on the other hand, run timing is more likely
some form of adaptation to distance from the sea or
difficulty of access, then the fact that the more distant
headwaters also produce slow growing fish, may just
be sheer coincidence. That seems to be much more likely.
The importance of understanding run timing
That we now have a much better understanding of the
factors which control the run timing of salmon (although
to be fair many people had long assumed this) is vitally
important. Only once it is appreciated how the different
runs be maintained can management be properly tailored
to maintaining this wonderful diversity. Individual
populations need to be managed separately.
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