Spring salmon on the River South Esk, Scotland: Scottish Marine and Freshwater Science Vol 7 No 10

Executive Summary

Throughout the text, "spring salmon" and "spring fish" are used interchangeably to refer to an Atlantic salmon Salmo salar (L.) that either enters a river and / or is caught in either net or rod fisheries before the end of May in any given year.

Background and aims

There has been a decline in the abundance of Atlantic salmon at sea across much of its geographical range over the last four decades. This rate of decline has varied among individual stock components as indicated by reported rod catches across months of the year. Thus, although rod catch as a whole has shown an increase over the last 50 years, the numbers of spring salmon in the catch have declined markedly. The decline in rod catches of spring salmon has abated in recent years in many Scottish rivers. However, catches of spring fish in the River South Esk have continued to decline.

The decline in spring salmon catches in the River South Esk has stimulated extensive debate about the potential for management interventions. In 2010, the Esk District Salmon Fishery Board applied to the Scottish Government for measures that were designed to reduce overall exploitation on the spring component of the salmon stock and also the sea trout stock in the River South Esk. Marine Scotland Science advised that application of their Rod Catch Tool confirmed a significant decline. They further advised investigation of possible causes to establish the need and potential for mitigating actions. Scottish Ministers acknowledged that there may be issues with these stocks raising particular concerns in view of the status of the River South Esk as a Special Area of Conservation for salmon. In view of the shortage of information available locally, Ministers instructed Marine Scotland to carry out a project to investigate the spring component of the River South Esk salmon stock.

The ensuing three-year investigation addressed the following questions:

• 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.

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.

Conclusions

In conclusion, considering the early running component of the River South Esk salmon stock, the majority of fish spawn in the upper catchment, and it is encouraging that densities of salmon fry in this area are generally as good as or better than expected. The limited available rod fishing effort data suggest a decline in angling effort for spring fish coinciding with a reduction in CPUE in recent years. Between 8 % and 29 % of the spring salmon catch from the coastal net fishery adjacent to the River South Esk was estimated to be destined to spawn in the River South Esk.