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

Part 6: Discussion & Conclusions

This investigation has shown that South Esk spring salmon spawn predominantly, but not exclusively, in the upper catchment. Of the 24 fish tracked successfully until spawning time over three years, 20 fish (83.3 %) spawned in the upper catchment. Of these, 16 fish were found in Glen Clova, suggesting that this area is an important spawning area for spring salmon. Electrofishing surveys showed that in general, sites on the River South Esk performed close to or better than the mean national expectation for sites with similar habitat characteristics indicating that the River South Esk is not performing worse than expected, and in many cases exceeds expectations. There was no evidence of sites in the upper catchment being markedly worse than those in the lower catchment. Indeed, the converse was true, with sites in the lower catchment generally performing worse than expected. Prior to 2013, electrofishing surveys in the lower catchment were scarce. Surveys carried out in 2013 and 2014 demonstrate poor densities of salmon fry relative to expectation in the Pow Burn and the Lemno Burn, although the Noran Water generally performed very well. Of the sites fished in the upper catchment, those in Glen Clova were generally performing better relative to expectation than sites fished elsewhere in the upper catchment. Although we cannot be sure that fry caught during electrofishing surveys were the progeny of spring fish, the results of the radio tracking work mean we can be confident that electrofishing surveys in the upper catchment were carried out in the general area where spring fish spawn. When considered together, these results suggest no unexpected problems with productivity of salmon fry in the upper catchment where the majority of spring fish spawn.

Effort data relating to the spring salmon rod fishery was scarce. However, effort data relating to an 11-year period (2002 to 2012 inclusive) were sourced from one of the six rod fisheries contributing most to the overall spring salmon rod catch on the River South Esk. These data provided evidence of a reduction in rod effort over the period for which data are available, notably a step-change in the percentage of beat days let from 2005 onwards. Catch per unit of effort ( CPUE) tended to be relatively stable across the time series as a whole, although there was evidence of a decline in later years (2007 to 2012). Examination of the high definition data revealed a significant decline in effort over the period 2007 to 2012, and this coincided with a significant decline in CPUE over the same period. Further evidence for a reduction in rod fishing effort came from a subset of "high definition" data available for the period 2007 to 2012 inclusive, which also showed a tendency for the number of rods booked per beat day let to decline, despite the number of beat days let remaining relatively constant over this period. Translating these results to the River South Esk as a whole requires a degree of caution, since these effort data were sourced from just one fishery. Nonetheless, the fishery providing the data is among those contributing most to the overall spring rod catch on the river. If the effort data available from this fishery are typical of other spring salmon rod fisheries up and down the river, then there is the possibility that declining rod catches of spring salmon on the River South Esk reflect a decline in rod fishing effort.

The coastal net fishery adjacent to the River South Esk was found to be highly mixed stock, catching fish which were subsequently detected in several major rivers in the northeast of Scotland (the Rivers Spey, Don, Dee, North Esk, South Esk and Tay). Given that tagged fish could have entered rivers other than those that were being monitored for tags, the number of rivers in which fish were detected should be viewed as a minimum estimate. The proportion of the catch from the coastal net fishery destined to spawn in the River South Esk was estimated at 8 % to 25 % in 2012 and at 11 % to 29 % in 2013. Therefore, up to an estimated 29 % of the spring salmon catch from the coastal net fishery may have been destined to spawn in the River South Esk.

It was not possible to use genetic tools ( SNPs) to identify geographic origins within the South Esk catchment because there was not sufficient spatial genetic differentiation between areas within the system. Similarly, SNPs could not be used to determine the representation of different rivers stocks in the nets because there is not enough differentiation detectable among the large Scottish east coast rivers using the techniques employed.

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.