THE GREAT DIVERSITY OF THE TAY SYSTEM RUNS
The River Tay has one of the most diverse populations of Atlantic salmon in the world. In more northern latitudes salmon tend to enter rivers during a concentrated period of the year, but in the River Tay salmon enter all year round. Some salmon enter the river in the late autumn perhaps only days before they are ready to spawn. Some, on the other hand, enter the river a full year before they spawn.
Some salmon enter the River Tay a full year before they will eventually spawn. Such fish may start to appear as early as October. Decades ago a lot of salmon used to enter the Tay over the winter months. Numbers of these fish have declined dramatically but some still do exist. The majority of these very early fish are / were 3 sea winter fish, fish which had spent a relatively long time in the sea and frequently weighed in excess of 20 pounds. These large early fish tended to be associated with certain tributaries of the Tay, in particular the River Dochart and to a lesser extent the Lyon and Tummel. Historically, this type of fish supported a considerable fishery on Loch Tay. Having passed up through the River Tay during the winter these fish were already in Loch Tay when the season opened in January.
Nowadays the main first significant runs of fish into the River Tay take place from late February / March and continue through April and May. Nowadays most of these fish are 2 sea winter fish, weighing anything from about 7 to 10 pounds in March, slightly bigger perhaps by May. Several decades ago many of the fish in the Tay at this time were also 3 sea winter fish. These spring salmon are again associated with particular tributaries. Today, they are the dominant type of fish in the Dochart, they also run the Lyon, the upper tributaries of the Tummel - the Tilt, Garry, Errochty and upper Tummel - and especially the River Ericht. Early in the spring when the water temperature is low the migration of these fish can be slow and they can be caught for example even in the lower Tay around Stanley where rapids and falls slow them up. But later in the spring they move faster and often scarcely stop between the sea and getting into their home tributaries. Over the summer and autumn these fish then press on upwards into the headwater areas where they will spawn. It is notable, that certainly at the present time, the great majority of the 2 sea winter spring salmon entering the Tay are females.
"Summer" grilseIn the early summer the first of the grilse begin to arrive in the River Tay. An occasional one might appear in May, with more in June, leading to much bigger numbers in July. These grilse tend to be relatively small, perhaps only 3 or 4 pounds in June.
The grilse which appear at this time of year behave quite similarly to the latter part of the spring run. That is, these grilse are also heading for highland tributaries, especially the Ericht and the Tummel, and unless perhaps very low water prevents them, they will make rapid progress to that end. These fish tend to shoot through the lower Tay without even being seen but start to figure in catches in the middle Tay or in the tributaries themselves. These fish run right into the headwater areas among the mountains as the spring salmon do. As the majority of the spring salmon are female, the majority of the summer grilse are male, the springers and the summer grilse spawning together.
"Autumn" grilse and salmonApproximately 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 there are some fish are caught every year in the lower Tay in January and February which have only recently entered the river and are on the point of spawning.
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.