Run timing changes: evidence from netting catches on Tweed and Spey

Despite the onset of social media the river reports in the Trout & Salmon magazine are still an important source of information and there are usually some revealing insights into thinking across the country. I note for example that this month the Tweed correspondent is still hanging on to some hope that the back-end run will re-appear this year. History suggests that he will be disappointed.

As evidence to support that assertion I am going to present below a series of slides showing the catches from the Sandstell netting station, which was operated at the mouth of the Tweed and the Raik nets on the Spey. The Raik nets were mainly net & cobble nets which operated in the lower few miles of the Spey mainstem. But before presenting those catches here is a graph showing the Spey rod catch split into spring salmon (to end of May), summer salmon and grilse from 1952 to 2017 (Data from Marine Scotland Science and Spey Fishery Board (since 2002)).

Trends in Spey rod catch 1952 to 2017. The most stable component of the stock has been the summer salmon. The spring catch passes all three NASCO tests with 2016 and 2017 well up the list of recent catches. It was only the wet September of 2017 that saved the autumn component from failing all three tests. This failure of tests 2 & 3 show that the reduction in the autumn catch is not a recent phenomenon. The reduction in the autumn catch is mainly as a result of the decline in the grilse catch. There is a difference of almost 3,000 between the recent five year average grilse catch and the peak of the early 1990s.

The collapse of the spring catch in 1981 was matched with an upsurge in the grilse catch. The grilse catch exceeded the spring salmon catch from 1982 until 2009. It is as if they can’t be in the same room at the same time. [/caption]

Below are the decadal catches from the Tweed and Spey netting stations with the catches split into salmon and grilse and expressed as a percentage of the total by month. The whole series will be presented one after the other with additional comment at the end. The Tweed catches are shown in the left graph and the Spey on the right. The sequence starts in the 1850s and ends in the 1980s.

There are some minor differences between the two rivers but the overall pattern is similar over a period of 140 years. The most noticeable change is the dominance of spring catches in the 1920 and 1930s. There was evidence of an increasing spring catch from 1900 and this lasted until the 1950s. This increase in the spring catch was offset with a decline in the grilse catch. This shows that when changes in run timing occur they tend to be long lasting and, based on this analysis, occur over large geographical areas. Tony George in his seminal PhD on cyclical changes in run timing in Scotland considered that the rivers from the Findhorn to the Tweed followed the same trends concurrently and this analysis supports that.

When I was making these graphs it occurred to me that whilst the Spey Raik net catches in the Spey in the 1970s showed low spring catches the spring rod catches in the 1970s were great (see below and first graph).

Until I had completed this analysis I was working under the impression that it was the nets that provided the best sample of the run and that the rod catch was biased for a variety of reasons. The netting effort data shows that in 1972 there was a 25% reduction in the number of crews operating in the spring months and this maintained until the 80s. This 25% reduction in effort is likely to have had a lesser impact on catches as no doubt the least profitable crew/station would have been cut first. What I take from this is that if you want to understand the characteristics of the run at any particular time you have to utilise all the data available.

Salmon run timing changes occur in a predictable cyclical way (or at least it has until now).
When changes occur they can last for decades.
The main change is a swing in the relative proportions of spring salmon and grilse, the two are never present in abundance at the same time.
These changes occur over large geographical areas at similar times, although there are always differences from river to river.
We have been losing the grilse run for a number of years; there will be year to year variation, but based on historical evidence there is little prospect of an upturn in grilse catches in the the immediate future.
The stage is set now, will the spring run deliver?

Thanks to Dr. Ronald Campbell, Tweed Foundation, for access to the Tweed nets data.

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