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Part 2: Salmon scale reading; 'Problem scales', Erosion, Spawning Marks and Smolts


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Introduction to Salmon Scale Reading

Here is part 2 of 2 featuring 'problem scales', erosion, spawning marks and smolts.

Developed with thanks to The Tweed Foundation

Published in: Science
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Part 2: Salmon scale reading; 'Problem scales', Erosion, Spawning Marks and Smolts

  1. 1. Introduction to Salmon Scale Reading • “Problem” scales • Erosion • Spawning marks • Smolts.
  2. 2. Not all scales conform to these simple patterns • Two examples of “problem” scales.
  3. 3. Where is there a winter? 18 /08 /2006 3lb, 50cm.
  4. 4. 26 /09 /2007 5lb, 59cm. Split winter - (Winter checks ) - Where’s the actual Winter band? ? ? ?
  5. 5. Erosion • When salmon return to fresh water they stop feeding • To provide energy, material is reabsorbed from body tissues, including the roots of the scales • The degree of erosion can show how long a fish has been in fresh water – but some can occur at sea after feeding stops.
  6. 6. 29 /09 /2007 4.5lb 61cm This means this fish has been in fresh water for some time. Half of the edge of this scale has been eroded away 1SW 2SW
  7. 7. 29 /09 /2007 10lb 74 cm This 70% eroded scale shows that this fish probably entered the river as a Spring Salmon, though caught in September, 1SW 2SW
  8. 8. 100% erosion makes this scale impossible to read fully. the scale is no longer complete to the edge 10 /11 /2007 14lb 86cm. 1SW 2SW
  9. 9. When scale reading, Erosion is the first thing to look for and assess as it tells you whether you are dealing with a fish newly in the river and therefore directly reflecting the part of the season during which it was caught (Spring, Summer or Autumn) or whether it is from earlier in the year – e.g. a Spring or Summer fish caught in Autumn and is not therefore reflecting the time at which it was caught.
  10. 10. Spawning marks • If a fish spawns in the river and returns to the sea to recondition, evidence of spawning remains on its scales.
  11. 11. 04 /09 /2007 9.25 lb 72cm. The spawning mark (SM) is the old eroded edge, with new marine growth beyond it The SM has replaced the old edge of the scale: it could have been open or closed or a full winter band at the end. All that is definite is that there was summer growth before it, which is denoted as “+” . 1. 1SW + + SM SM. ++
  12. 12. Smolts • Scales can be collected from smolts and read in the same way as other life cycle stages
  13. 13. 16 April 2007 131mm 2+ . With a lot of + growth (= “run-out” on the adult scale)
  14. 14. 16 April 2007 127mm 2. With no + growth (or very little) but average smolt length
  15. 15. 16 April 2007 101mm 2. With no + growth (or very little) but a small smolt
  16. 16. 11 April 2007 85mm 1+. :with a lot of+ growth but a very small smolt
  17. 17. Warning We should remember that information should be read from the image not that we should force information onto the image to match what we would expect for a given size of fish or time of year. Always be aware of which way information is actually flowing – it should be from the screen, not to it. It is better to have an “unreadable” category rather than have forced interpretations in your data.