Fish scales have growth rings which show the age of the fish, like rings in tree trunks. For salmon and sea trout, their scales can show at what age they migrated to sea (as a juvenile), the length of time they spent at sea, whether they have spawned previously and approximately when they returned to the river.  More about the growth and life cycle of Dee salmon can be found here (a brief guide to salmon).

The age of a salmon is described in terms of the number of winters it has spent at sea. 1 Sea Winter (SW) salmon are commonly termed 'grilse'; grilse make up a large portion of the Dee's summer and autumn runs of fish. 2 SW salmon occur throughout the year and are the most common age of salmon caught on the Dee. Lower in numbers are the 3 SW salmon, which are almost exclusively spring salmon. The older the fish, the larger they tend to be as they have spent more time feeding at sea. The scales of a salmon (opposite) demonstrate clearly the benefits in growth of going to sea.

Scale from a 3 SW salmon                                  

Scale sampling can therefore provide information about the stocks of fish in the river and detect changes in stocks over time. This information can provide an early detection system for population changes and stock robustness. The information can be used to advise fisheries management, for example, through identification of the most vulnerable spring salmon in the rod catch, which can then inform the river's Conservation Code.

In 2008 the RDT set up a scale sampling programme on the river to sample adult salmon and sea trout in the rod catch. This is ongoing and is providing us with information for management, based on the efforts of ghillies and angling club representatives who collect the scales. Fish are sampled along the length of the river, from Aberdeen to near Braemar. Each year 8–10% of the salmon rod catch is sampled, giving us an excellent representation of the whole of the Dee's rod catch. The information from this scale sampling undergoes both internal and external quality control and is published twice a year – all reports are on the website and the latest report (Summer 2010) can be accessed here.                      

Scales from a grilse (left) and a 2 SW salmon (right)

Findings (2008 – 2010)

1. The spring salmon rod catch on the Dee is comprised primarily of 2 SW salmon (average of 78%), with the remainder being 3 SW salmon (19% average) and very small proportions of repeat spawners (2%) and grilse (0.2%).
2. 3 SW salmon average 16 lb in weight, 2 SW spring salmon average 8 lb, 2 SW summer salmon average 11 lb and 1 SW salmon (grilse) average 5 lb.
3. 3 SW salmon are at their peak abundance in February and decline by April.
4. A small proportion (average 6%) of Dee salmon caught in the month of May is in fact summer salmon (this is determined by the presence of fast 'summer' growth at the edge of the scale).
5. 22% of the Dee's rod catch between 1st June and 15th October comprises spring salmon. For the month of June, spring salmon comprise an average of 55% of the rod catch.
6. Grilse dominate the rod catch in the months of August and September, comprising approximately two thirds of the rod catch.
7. The majority of the Dee's spring salmon (two thirds) smolted at 2 years old and one third at three years old. Summer and autumn salmon show evidence of faster juvenile growth – lower numbers smolt after 3 years (24%) and a small proportion (1%) smolt after only 1 year.
8. The proportion of salmon that were 3 year old smolts is significantly greater in the Upper Dee rod catch and probably reflects slower juvenile growth rates in the upper catchment.
9. The majority of Dee sea trout are 1+ SW fish. A small proportion, 4 – 12%, comprises previous spawners.

Support

Sampling in 2010 was supported by funding from Rivers and Fisheries Trusts of Scotland (RAFTS) and the Scottish Government. Marine Scotland Science provides cross checking of some scales and advice on our sampling.