Salmon scale readings

Each year the Spey ghillies collect DNA samples from some of the rod caught fish along with a few scales to allow the fish to be aged. Steve Burns has just finished aging the 2013 rod caught scale samples along with any other scales which we collected ourselves when the opportunity arose.

There were only 89 readable sets of salmon scales for 2013, a smaller sample than normal. Another facter to be borne in mind is that the scales were collected when available so the sample colected is not necessarily representative of the whole population. Many of the scale samples collected in 2013 were from fish caught in May with late season fish under-represented; much the same as in previous seasons.

Here the results of the 2013 salmon scale samples are presented along with a comparison with recent seasons and historic data from the Spey scale reading report produced by G.H. Nall in 1921 on behalf of the Fishery Board for Scotland.

The sea ages of the 2013 fish are shown in the graph below.

2013 fish sorted by sea age

2013 fish sorted by sea age. The blue bars show the one sea-winter fish, or grilse. The grilse normally enter the river from May onwards peaking in the last months of the season in the lower river. The green bars show the three sea-winter fish. Typically these larger fish normally enter the river in the early months of the season. Fish which run the river in the early months could of course be caught later in the season but scales from coloured fish are rarely collected by the ghillies. The remainder of the fish were two sea-winter. No scale samples from four sea-winter fish were collected in 2013. No really big fish were landed on the river in 2013; a slight disappointment after the good run of three sea-winter salmon in 2012.

For comparison here is the same graph but showing the fish from 2012.

2012 salmon showing sea age by month.

2012 salmon showing sea age by month. The broad pattern is the same as in 2013, although the run of three sea-winter fish was of greater importance that year (see below). A small number of three sea-winter fish were encountered in September also.

Spey

This graph shows the sea age of the fish sampled presented in a different way along with recent seasons and the 1921 data. It can be seen that the two sea-winter class is normally the dominant type of fish although it has not always been so. In the 1990s there were a couple seasons during the peak of the grilse run when that component matched or exceeded the multi sea-winter catch. Some variation in the strength of the one and three sea-winter classes can be seen, pariclularly the fish emanating from the 2009 smolt year class (see below). There appears to be little change between the distribution of the salmon year classes in 1921 and that pertaining in recent years.

It can be see from the graph above that the one sea-winter year class occurred much more frequently in our 2010 samples than in any pre- or succeeding years. This does actually reflect well what happened in that year as there was a big run of grilse. These grilse were from the 2009 smolt run, a year class where marine survival was good all across Scotland. The impact of that strong year class was also evident in 2011 and 2012.

Spey

This graph shows the sea age but sorted by year class along the horizontal axis rather than year. The big run of grilse in 2010 was followed by the excellent run of two sea-winter fish in 2011 and in 2012 the proportion of the run comprising three sea-winter was significantly higher than usual for the Spey. Unfortunately that year class was not suppported by another of similar strength in the following seasons.

So in a period of generally low marine survival the 2009 smolt year class did very well. As this was almost universal across Scotland (that same year class produced the record catch of 23,000 in the Tweed in 2010), the success of those smolts is likely to have been due to improved marine survival rather than the size of the smolt run. The reasons for the improved marine survival that year are not understood although interestingly it did coincide with the much improved sea trout run in 2010. The 2010 sea trout catch on the Spey was the best in the last ten years. The factors affecting marine survival of salmon and sea trout are almost certainly not the same, but good inshore feeding conditions during the smolt run would obviously benefit both species.

3.4% of the salmon aged in 2013 were previous spawners. All these fish were spring fish, one had been caught in February.

From the scales we can also derive the river (smolt) age of the smolts providing the scales are not replacements, in which case the early history of the fish will be missing. The graph below shows the river age of the fish sampled.

Spey

One year old smolts are not common in the Spey: not in our rod caught samples at least. Two year old smolts are by far the most frequent, comprising between 63% and 78% of the run in the years shown. The remainder are generally three year old with a few four year olds. The proportions of two and three year old smolts could be affected by environmental factors such as mean temperature. A cold year, or succession of yerss with below average temperatures, could result in more fish remaining in the river for an exta year. Fish densities also affect smolting age: higher densities and competition between juveniles could also results in an increase in the mean smolt age. Four year old smolts occur infrequently in the Spey. There was only one in the 2013 rod caught sample. Interestingly it was caught at Ballindalloch, the beat where the Avon joins the Spey. The Avon is a cold water tributary and the growth of the salmon fry  and parr is lower here than in any other part of the Spey. It is likely that the Avon will produce a higher proportion of four year old smolts than any other tributary or area of the Spey. Maybe it was waiting to run the Avon?

From this rather limited (and flawed) comparison there would appear to have been little change in the age composition of salmon run in the Spey in the years studied. Sea and river ages of the 1921 sample and the recent years are similar, although the recent samples are not as representative as the 1921 collection. The 1921 samples were taken from the Spey nets throughout the season. We have a vast archive of scales and other analysis have been undertaken. I will try and pull it all together sometime.

We are grateful to the ghillies and anglers who continue to supply scale samples. Understanding the age composition of the Spey salmon run is of fundamental importance.

 

 

 

 

 

 

 

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