Trout size survey River Avon 1962

vanilla-header

vanilla-menu-plus-social-media

Home page
About us
Instruction
Guiding
Locations map
Gift vouchers
Prices
Articles
Contact us
Accommodation
Links

What do the scientists tell us about the size of fish in a typical Dartmoor river?

John Gierach's book "Fly Fishing Small Streams" is a beautifully constructed mixture of 'how-to' information and mental approach to small stream fishing. Gierach has a refreshing approach to the increasing tendency to equate fishing quality with the size of fish caught. For example, ".. let me introduce an idea - just something to kick around: Maybe your stature as a fly fisherman isn't determined by how big a trout you can catch, but by how small a trout you can catch without being disappointed, and, of course, without losing the faith that there's a bigger one in there". (From 'Fly Fishing Small Streams").

We are lucky because Dartmoor's rivers teem with small trout, which at times can be free rising and liberate childish delight in all of us, and I have written several essays on the size of trout in Dartmoor's rivers. But my comments were based on the size of fish caught by anglers. What do scientists tell us about the size of fish in a typical Dartmoor river?

Recently my attention was drawn to an electro fishing survey carried out in 1962 on the River Avon in South Devon. Nine sites were sampled, a stop net was placed across the downstream limit of each survey section, electro fishing was repeated until the operators were satisfied that no more fish remained, the catch was transferred to an oxygenated holding tank and each fish measured to the nearest quarter (1/4) inch. Full report available here ...

The results are shown in this figure.

length frequency chart

The results show that:

most trout were less than 8 inches long. In 1962 all trout below 8" had to be returned, but anglers could take home fish over 8"
there were very few fish over 10" - in fact they were as 'rare as hen's teeth'.
but one fish was 18 inches in length - which echoes Gierach's point that we shouldn't lose the faith that there's a bigger one in there

The next graph shows the growth rate of wild brown trout in our area.

fish of 12 to 14 inches take six years to reach that length
our trout are about 7 inches long when they spawn for the first time at 3-4 years of age

(Data from Frost and Brown, "The Trout", published by Collins, London 1967, Appendix III, row 22)

Combining these results shows that:

less than half of the trout stock were capable of reproduction
most trout were younger than 3 years and less than 7 inches long.

According to the Wild Trout Trust "A typical female brown trout produces about 2,000 eggs per kilogram (900 eggs per pound) of body weight at spawning".

But

mortality is especially high in the period between hatching from the egg to establishing a territory
only 20 fish reach their first birthday

trout survival 0-4 years of age

So, the next time you hear someone complain about the small size of the fish they are catching, remind them that an eight inch fish is probabaly three years old and is one of a group of maybe only six fish that remain from a clutch of 1000 eggs!

Sources:

Frost and Brown, "The Trout", published by Collins, London 1967 (appendix III, row 22)
The Wild Trout Trust 'Trout Facts'
Watson, "The Trout: A Fisherman's Natural History", published by Swan Hill Press, Shrewsbury, 1993