THE TAY SALMON LIFECYCLE
When do Tay 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 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 fry. 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.
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.
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.