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SFCC Introduction to Scale Reading - 2017


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SFCC Introduction to Scale Reading - 2017

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SFCC Introduction to Scale Reading - 2017

  1. 1. SFCC Introduction to Scale Reading
  2. 2. The Trout, Frost & Brown The scale lies in a pocket in the Dermis (inner layer of the skin). Embedded scales Dermis
  3. 3. The Trout, Frost & Brown The part facing towards the tail projects above the surface of the fish but does not pierce the Epidermis (outer layer of the skin)
  4. 4. The Trout, Frost & Brown The anterior, and much larger part is deeply embedded in the fish’s skin: like the root of a tooth in the gum This “root” part of the scale does not get worn and abraded like the extruding part and is therefore the bit of the scale that is used for reading
  5. 5. The Trout, Frost & Brown A scale is an oval plate with more or less concentric ridges which are more numerous and continuous on the anterior, embedded (and, in all but the youngest and smallest fish, greater) part, than on the posterior, exposed, part of the scale Posterior, exposed part of the scale. Some of the skin is still adhering on this scale. This gets worn down and is unreadable Anterior, embedded part of the scale. This is the part that is read These ridges , some of which are widely and some narrowly spaced, are called circuli. SCALES AS TISSUE SAMPLES With the arrival of new genetic techniques, scale samples now have to be regarded as tissue samples that can produce DNA as well as being sources of data on growth patterns and ages. It is actually the bits of skin and fish glaur (mucus) sticking to scales that can produce DNA, so clean scales are of less use for this than “dirty”
  6. 6. Softer material Hard “ridges” are left at the edge of each plate as softer material is re-absorbed. The upper side of a scale is therefore ridged while the lower layer is smooth. A scale consists of a fibrous, non-calcareous material laid down in thin plates below one other: Each new layer grows over the whole of the base plus some extra The Trout, Frost & Brown A scale is thus made up of plates with ridges at their edges
  7. 7. The Salmon, J.W. Jones These periods have been termed “Summer” and “Winter” growth respectively, though the correspondence with these seasons is not absolute. Some populations are known that make their best growth in Winter, when their food is seasonally abundant. It has long been accepted that the bands of wide rings (circuli) represent periods in the life of the fish when growth was rapid and that the bands of narrow rings represent periods of less rapid growth. Winter bands Summer bands
  8. 8. The ridges on one side of the scale mean that impressions can be taken
  9. 9. This means that scales do not have to be individually cleaned before reading as the impression in the Acetate shows up the pattern of the scale through any overlying dirt. Some scale readers prefer to read the actual scales rather than impressions.
  10. 10. Scale. Impression The pattern of widely and narrowly spaced ridges / circuli can be read to show the seasons that a fish has lived through. Reading
  11. 11. The Trout, Frost & Brown Scale “papillae” appear when Fry are around 3cms in length and the scales are complete when the fish are around 50mm in length. The scales show first along the lateral line, then spread dorsally and ventrally above and below it, the region posterior to the Dorsal Fin being the first to be colonised. Trout and Salmon are born “naked”, without scales.
  12. 12. The “oldest” scales with the most complete record of seasonal growth are therefore found in this area, just behind the Dorsal Fin, on the diagonal from the Dorsal to the Pelvic fins . Practical Note: Near spawning time, the skin of male Salmon & Trout becomes very thick and it can be impossible to scrape scales off without doing damage. Not getting scales from one sex can bias results. In such cases, it is sometimes possible to get scales off the wrist of the tail, which also have the full life history on them.
  13. 13. 1 Write the details on the front of the packet in pencil or waterproof ink. It is best to do this before putting the scales in as it is much easier to write on the packet when it is dry. * * If the fish is being opened up, make a note that the sex has been identified in this way. Outside the breeding season, sex can be unidentifiable from external features. The convention is that Mature Males are recorded as MM but Mature Females as FF. This avoids any confusion that could arise from Mature Females being recorded as MF TAKING SCALES . A: FROM DEAD FISH 2 The scales are taken on a diagonal from the back of the Dorsal fin to the front of the Anal fin, half way down towards the Lateral Line 3 Run the knife backwards across the area (with the scales) to remove some of the glaur 4 Then run the knife forwards (against the scale) to displace them. They will stick onto the blade of the knife. Place the blade into the scale packet, press down on it and withdraw the blade, wiping the scales off into the inside of the packet. 5 Put the packet somewhere dry. Scales will rot if kept damp, but if dried, can last for a very long time.
  14. 14. 1 This is done in the same way as for dead fish, but gently, and limiting the number of scales taken to about half a dozen TAKING SCALES . B: FROM LIVING FISH 2 Mature males at spawning time, when scales are under thickened skin, are difficult to get samples from. They can sometimes be taken more easily from the Wrist of the tail Tweezers can be used to take them individually (but this only really applies to anaesthetised fish). On large fish they can sometimes be drawn out of their skin pockets with the point of a knife – put the point on the extruding part of the scale, press down and then draw the scale towards the tail 3 Large and thick scales such as those of Grayling need to be removed individually from their skin pockets with the point of a knife as above.
  15. 15. A bit of HISTORY Revolution 1: early 19th century – Parr are the young of Salmon Revolution 2: late 19th century – Scales can tell the age & growth of fish Revolution 4: 1960s – Electric fishing allows sampling of juveniles Revolution 3: 1939 – Fertilisation in the wild is highly efficient Revolution 6: The present – Genetic data There have been six major revolutions in Salmon management & biology Revolution 5: 1960s – The marine feeding grounds located
  16. 16. The first scientific application of scale reading to salmonids was by Dahl, in Norway, around 1910 who summarised the matter as : “It is therefore quite clear that the summer-zones and the winter-bands on the scales of trout are respectively formed in the corresponding seasons, which affect their growth, and that these zones indicate annual epochs in the life of the fish. If, therefore, we count the number of these zones, we can ascertain the age of the fish.
  17. 17. During the last three years or so we have been gradually and with increasing certainty realising that a study of the scales of the salmon yields a most valuable addition to our knowledge of the fish’s life The first popular account of scale reading in English was in W. L. Calderwood’s The Life of the Salmon, published in 1910. He introduced the new technique by writing:- And outlined its basis:- … while summer feeding and growth is in progress the lines or ridges on the scales are added in greater numbers than at other times, and that between the ridges the spaces are greater, and that while the more moderate feeding of winter is in progress the ridges are few and close together. The result is that each summer and each winter leaves its indelible trace on the scale
  18. 18. The study of the scales has prominently called attention to the infrequency of spawning amongst heavy fish. It comes as a surprise, for instance, to learn that very many of the large spring fish of the Tay – fish almost invariably about 20lb in weight – have never spawned ….. From the study of the scales alone do we gain this information as to the infrequency of spawning, and the consequent benefit to our stock of salmon in preserving most religiously the breeding fish which enter our rivers. The first great revelation from scale reading was that salmon were not frequent and repetitive spawners, as Calderwood put it: The legal protection given to Kelts in 1857 was based on the idea that Salmon were repeat, annual, spawners & therefore if they were not killed they would come back bigger each year. However, there was a continual decline of numbers of Grilse over the latter half of the 19th century, though MSW numbers were maintained. This gave the impression of the fish getting bigger – showing that protection of kelts “worked” It was scale-reading that eventually showed what was really happening.