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SFCC Scale Reading Uses - 2017


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SFCC Scale Reading Uses - 2017

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SFCC Scale Reading Uses - 2017

  2. 2. WHY DO WE READ SCALES ON THE TWEED ? INPUT 2D: ANALYSE THE CATCH COMPOSITION AND TRENDS OF EACH STOCK OF SALMON Rationale: Historic records show how the sizes and run timings of the fish have varied greatly over the years and give the context for assessing the present day situation. Establishing long- term trends shows if there are large-scale changes that cannot be countered, though could be managed. Variation outside known parameters from the past could be a warning sign of problems Analysis of catches from year to year for their composition shows which stocks (and areas of the catchment) are producing the fish that support the fisheries and how the ages and sizes of fish can change To understand what is going on
  3. 3. AN IMPORTANT PART OF THE TWEED FISHERIES MANAGEMENT PLAN Policy 2D.2 - Monitor Catch Composition (a) Continue collection of salmon scales from rod fisheries along the course of the main river and the larger tributaries and the netting station at Paxton and add the readings to the scales database (b) Use this database to analyse scale readings to show :- ( i) Geographical and temporal patterns and age structures ( ii) Lengths and weights of fish in relation to area, time of year and age (iii) The different stocks and age classes of fish being caught. This shows how dependent catches are on particular ages of fish and how success or failure of particular spawning or smolt years can be reflected in the catches of the resultant adults. (iv) Whether high or low catch years are related through generational cycles i.e. whether they are spaced at intervals of common salmon or grilse ages ( c ) Develop and improve expertise in scale reading through collaboration and co-operation with scale- reading personnel in Trusts and Agencies ( i)Work to ensure consistency of scale reading interpretation throughout the country ( ii)Analyse Tweed scale reading results with those from other rivers to find differences and similarities.
  4. 4. Scale Reading for Understanding: Example 1 – Tweed Spring Salmon MIDDLE TWEED: Catches < 1st July 0 500 1000 1500 2000 2500 3000 1970 1973 1976 1979 1982 1985 1988 1991 1994 1997 2000 2003 2006 The change back from Spring to Autumn phase was in 1968 when half the catches were again caught in Autumn. 1998: Spring Salmon “Catch & Release” policy begins Five yearly patterns of low catches could be seen in the Spring Salmon catches: but what is the significance of five years, if any ?
  5. 5. Scale reading showed that five years is the commonest age of Spring Salmon TWEED : Percent Spring Salmon that are 2.2 0 10 20 30 40 50 60 70 80 90 100 1970 1973 1976 1979 1982 1985 1988 1991 1994 1997 2000 2003 2006 PERCENT Low catches reflected lower than usual proportions of five-year old fish: The cycles of low Spring catches can be traced back to 1982 – so did anything special happen five years before, in 1977?
  6. 6. This is what happened in 1977! PEAK FLOWS AT LINDEAN, LOWER ETTRICK 0 100 200 300 400 500 600 1962 1967 1972 1977 1982 1987 1992 October November December Peak flow of 560cumecs on the 31st October, 1977, the largest ever recorded on the Ettrick & right in the middle of the Spring Salmon spawning season on the most important tributary BEFORE THE 1st of JULY 1st JULY to 31st AUGUST 1st SEPTEMBER ONWARDS RADIO-TRACKING RESULTS - Salmon tagged in the estuary :-
  7. 7. Scale reading therefore showed: 1) The five yearly low catches were linked, five years being the age of a cohort 2) That because so many Spring Salmon were the same age (5 yrs), one low catch year could lead to another. Too few fish of different ages (4 & 6 yrs old to “fill the gaps” – and fish of different ages might well spawn in different areas of the Ettrick (see next slide) 3) Therefore that such low catches indicated when stocks were too low to fully recruit the next generation i.e. showed when Conservation Limits were not being met. 4) That reduced catches could therefore be due to past problems, even 20 years before, and not to any immediate cause.
  8. 8. FRY LENGTH CATEGORIES (5mm) UPLAND, COLD: “room” for two cohorts of parr before smolting TOP OF ETTRICK BOTTOM OF ETTRICK AVERAGE FRY LENGTH AVERAGE PARR LENGTH AVERAGE SMOLT LENGTH LOWLAND WARM: “room” for only one cohort of parr before smolting 3.2 come from this zone 1.2 come from this zone 2.2 come from this zone AVERAGE LENGTHS OF FRY AND PARR IN THE ETTRICK WATER FROM TOP TO BOTTOM 6 year olds 5 year olds 4 year olds
  9. 9. Scale Reading for Understanding: Example 2 – Sea-trout (Next)