When do salmon spawn?
In the Tay system salmon spawn over a long period. The first fish to spawn commence about the beginning of November or even right at the end of October, but fish will continue spawning somewhere in the district throughout November, December, January and perhaps some even into February.
While this is true at the district scale, locally there are big differences.
In highland headwater tributaries spawning starts earliest and finishes earliest. So, in the upper reaches of the Ericht system, the Tilt, upper Tummel, upper Lyon or in the Fillan say, the spawning is normally completely finished by mid November.
As you get lower and lower down the system the spawning season gets progressively later, so that in the main stem of the Tay the bulk of spawning does not actually commence until December and in the lower half some fish are still actively spawning in mid January when the season opens.
Relationship with run timing
The time when salmon spawn is connected with run timing. Those fish which spawn first are the spring salmon and the early grilse. Fish which enter the river late in the year also spawn late in the year. There is no evidence on the Tay of say spring salmon spawning late in the year, but this can happen. The chalkstreams of southern England were historically famous for spring salmon, but in those rivers, spawning was always much later than for Scottish springers.
Relationship with temperature
It has been shown that the time when salmon spawn seems
to be related to temperatures experienced during the
winter and the timing of the ultimate emergence of the
So, in cold upland streams, the eggs will take a relatively long time to hatch and so fish spawn early. However, in a warmer lowland stream, if fish spawn too early the eggs might advance so much during the late autumn that the fry might emerge to feed say in March when conditions may still be bad for them. Therefore, in warmer lowland streams, spawning is relatively late. This also applies to the chalkstream example cited earlier.
But there is yet another factor. In cold highland streams, it may also just be physically impossible for salmon to spawn much later than they do. Traditionally, by December, upland Scottish streams may just be too cold for salmon to be able to get there, although there is nothing of course to stop early running salmon spawning in lowland streams in they so wished.
The time of spawning is therefore closely linked to geography and to run timing of salmon and it is likely to be one of the adaptive features which has helped generate the differences in run timing we see across the district.