The
fish passes at Pitlochry Dam acts as something
of a temperature barrier. Salmon cannot readily
ascend the fish pass until the water temperature
is around 10 degrees centigrade. Therefore,
in the early spring salmon tend to accumulate
in the river downstream of the dam and ascend
when conditions are suitable. This is illustrated
by the results from a radio-tracking study
conducted in 1996 (which just happened to be
a particularly cold spring) which are presented
below. Salmon were caught by angling just downstream
from the dam, fitted with radio-tags and their
progress was then monitored. The results show
that on average fish tagged in late April did
not ascend the ladder until mid June, but those
tagged in late June moved through the ladder
much more quickly.
Date on which radio-tracked salmon successfully ascended Pitlochry Dam fish ladder according to the date they were tagged, spring 1996. The data were obtained from Gowans et al. (1999).
Therefore the monthly counts for Pitlochry do not necessarily represent the exact months in which the salmon actually entered the Tay. Also, because some spring salmon may not pass through the counter until such times as there are summer salmon and grilse also present, it is not possible to use this counter to give an exact figure for the total number of spring salmon ascending the Tummel. The relatively low count in May 2005, for example, could be attributed to cold weather while the high count in April 2003 was a result of very warm weather.
Another complicating factor at Pitlochry seems to be that in the summer there have been in recent years problems in counting grilse. While the counter seems to be highly accurate for salmon, it may only count about 50% of small grilse, but maybe this will improve because a new counter was installed in 2005.
So, bearing this in mind, summer counts should probably be increased by about 1000 bringing the average counts for recent years to more like 5000 or 6000. As grilse have tended to become smaller in the last decade or so than they were in the 1970s, this correction to the counts may not be so valid earlier. If this is the case then, leaving out the high period in the 1970s, Pitlochry counts over the last ten years have been above the average of the 1950s, 60s and 80s.
While that is an encouraging finding, the effect of removing the net fisheries in the 1990s must be factored in. Since the final removal of the last of the nets in 1996 Pitlochry counts have been a bit higher than they were in the 1980s, but had the nets been removed much earlier it is quite likely that recent counts would actually be at an historic low.
So, bearing in mind changes in netting, the true abundance of Tummel fish probably increased from the 1950s to a peak in the early 1970s, but since then it has declined. Though in reality the true abundance will have been at its lowest over the last ten years, it may be fair to say that it has been steady over this period and at present there does not seem to be evidence of a continued decline for the time being.
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