Scottish Marine and Freshwater Science Vol 7 No 10
Report of a three year project carried out by Marine Scotland to investigate the spring component of the River South Esk salmon stock. The following questions were addressed: • In which geographic region of the River South Esk do spring salmon spawn? • How does the status of the juvenile salmon stock vary within the River South Esk and how does this relate to spawning regions used by spring fish? • Is it possible to assess to what extent changes in rod fishing effort have accounted for the declining trend in rod catches of spring salmon? • What proportion of the catch from the coastal net fishery adjacent to the River South Esk was destined to spawn in the River South Esk? The outcome of this investigation is summarised in this report.
Data and Resources
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UK Open Government Licence (OGL)
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Spawning locations of spring fish Between 2012 and 2014, 245 spring salmon were sourced from either the coastal net fishery adjacent to the River South Esk (in 2012 and 2013), or the lower reaches of the river itself (in 2013 and 2014). A miniature radio transmitter was inserted into each fish so that they could be tracked to their spawning locations. Genetic samples were taken from each of these fish, and many other salmon from the nets, with the aim of relating genetic variation to geographic population structuring within the South Esk catchment. Altogether, 24 spring salmon were tracked successfully until spawning time over three years. Of this sample, 20 fish (83.3 %) spawned in the upper catchment in either Glen Clova (16 fish) or Glen Prosen (4 fish). The other 4 fish (16.7 %) spawned in the middle reaches of the river. It was not possible to use genetic tools (single nucleotide polymorphisms - SNPs) to identify geographic origins within the catchment due to relatively low levels of genetic differentiation between the geographic areas investigated. The status of the juvenile salmon stock Multiple-pass electrofishing was conducted in the River South Esk in 2013 and 2014 in sites from both the upper and lower catchment. These data were supplemented with historical data gathered in 2004, 2005 and 2011. The observed density of salmon fry at each site was compared with a national average abundance associated with broadly similar conditions. Model residuals (observed - expected) were examined to see whether sites were performing better or worse than expected in comparison with the national level. Nine out of the thirteen sites surveyed in the upper South Esk during 2014 remained above expectation, providing reasonable evidence that the upper catchment as a whole was well populated by salmon fry relative to the average national expectation. Furthermore, sites in the upper catchment were generally more productive for salmon relative to expectations than those in the lower catchment. In the upper catchment, sites in Glen Clova were generally better than those in Glen Prosen. In the lower catchment, sites in the Pow Burn and the Lemno Burn were notably poor for salmon fry, whereas high densities of salmon fry relative to expectation were present in the Noran Water. Spring rod fishing effort The six rod fisheries that contribute most to the spring salmon rod catch on the River South Esk were contacted and information on fishing effort was requested. Effort data, in the form of bookings from 2002 to 2012, were received from one fishery proprietor. “Low definition” data for 2002 to 2006 indicated whether or not a given beat was let on a given day. “High definition” data for 2007 to 2012 also included information on the number of rods booked on each beat day let. Trends in catch per unit of effort (CPUE) varied from trends in reported catch. It was also possible to identify trends in the high definition data which were not apparent in the low definition data. It is concluded that while measuring fishing effort may increase the reliability of fishery data as a proxy for stock abundance, such data need to be of a sufficiently high quality. Geographic exploitation of the coastal net fishery Radio tagged spring salmon sourced from the coastal net fishery in 2012 and 2013 were used to estimate the proportion of the catch destined to spawn in the River South Esk. Tagged fish were detected in several major rivers in the northeast and east of Scotland including the Spey, Don, Dee (Aberdeenshire), North Esk, South Esk and Tay. The proportion of the catch destined to spawn in the River South Esk was estimated at 8 % to 25 % in 2012, and at 11 % to 29 % in 2013. Genetic methods (SNPs) could not be used to determine the representation of different rivers stocks in the nets. This was because there is not enough differentiation detectable among the large east coast rivers.
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Marine Scotland Science
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