I thought it might be interesting to look at the official EA redd counts on the Severn from 1975-2004. The picture this presents is of an increasing salmon population over the period as a whole (but with a peak in the early '90s). This is very much at odds with the views of many anglers who look at the long term decline in the total rod catch over the same period and have lived through the experience of massively reduced spring catches and therefore assume that the population has been declining.
The simplest explanation of how we can have two apparently contradictory measurements of the salmon population is that over the period more fish were entering the river either later in the season, or after the season had ended. This explanation would fit with broader trends on other UK salmon rivers over the same period.
A stable population in numbers terms, but with a change in the balance of runs from earlier to later in the season, would drastically depress the rod catch by half at the very least. The reason for this is that early run fish are known as better takers and available to the rod fishery for longer, therefore a much higher proportion of them are landed compared to back end fish.
The most comprehensive study of monthly catch rates of Salmon in a river with different runs
of salmon at different times of year was made in the 1990’s on the Welsh Dee, where there is a fish trap at
Chester that allowed a substantial sample of Salmon to be caught and tagged. The rates of capture of tagged
fish by anglers upstream of the trap were:
MAR 39.4%
APR 38.2%
MAY 24.9%
JUN 19.9%
JUL 21.7%
AUG 17.9%
SEP 10.8%
OCT 5.5%
from which it can be seen that early running fish had much higher rates of capture than later run fish.
Three other factors magnify the impact of the switch to later runs on the Severn rod catch.
1. The impact of the spring bye laws. This makes it illegal to use bait before June 16th yet this was the method which historically accounted for the vast majority of fish from the middle and upper river. In 2007 the EA estimated that the fall in the mean spring catch of 157 since 1999 was not due to a reduced spring stock but down to the impact of the spring bye laws. I.e. bait bans and reduced effort.Angling effort has fallen by roughly two thirds as a result of the bye laws.
2. The Severn season finishes earlier than 95% of salmon rivers in England and Wales. As the balance of runs has switched from spring to summer and autumn this has had a massive impact on the Severn rod catch. Rivers like the Tyne, Lune and Ribble catch a majority or close to that of their annual catch in October.
3. The nature of the Severn means that in a normal year the vast majority of the summer and early autumn run stay out in the estuary or don't move far into the lower river.http://severnsalmon.blogspot.co.uk/2012/02/how-industry-shaped-severn-salmon-runs.html
If we look at all these factors in combination then the puzzle of the two sets of apparently contradictory data is solved.
Many thanks to Tony Bostock of the Severn Rivers Trust for supplying me with this data.
