A tale of two rivers: Fish monitoring in the North and South Esks

By Dr Iain Reid

Introduction

The Forth Rivers Trust have been carrying out electrofishing surveys for a number of years throughout all the rivers in the Forth Catchment and hold historical data for a number of sites (including several in the Esk catchment).  This year in response to concerns about the minewater discharge from Junkie’s Adit, and building on from invertebrate monitoring work we carried out earlier HERE, we opted to carry out some extra electrofishing surveys on the South Esk to see if the effects of the Adit could be detected in the fish population.

Methods

In order to assess the impact of minewater released from the discharge at Junkie’s Adit on fish populations within the River South Esk, a single pass electrofishing survey (covering an area of approximately 100m²) was carried out at 4 sites in the river (Fig 1).  The uppermost site (SEsk7 – Fig 5) was situated approximately 25m upstream of the discharge while the remaining three were at 100m (SEsk6), 560m (SEsk5) and 1285m (SEsk1)) downstream from the Adit.  An additional site on the North Esk (NEsk1) 50m downstream of Montague Bridge was also sampled to allow comparison of historical fish data which the Trust holds from 2011 between this and the SEsk1 site.

So that a fair comparison across sites could be made, only habitats with similar characteristics (i.e. the same type of flow and substrate composition) were selected, and the number of fish caught were standardised by the total area surveyed to give a measure of fish density (given as the number of fish per square meter).

NEsk1 and SEsk1 were surveyed on the 14/07/2020 while the remaining sites on the South Esk were surveyed on the 17/07/2020.  Fish were anaesthetised using benzocaine before being identified and measured.

Results

Fish densities on the South Esk were found to be much lower downstream of the Adit when compared both to an upstream site and the comparable site on the North Esk at Montagu Bridge (Figs 2 & 3).  Densities were shown to be lowest immediately downriver of the Adit and remained low at Laundry Bridge (over a Kilometre downstream of the Adit in the Country Park – Fig 3) being less than a third of those of the upstream site.  Densities on the North Esk at Montagu Bridge were found to be over 4 times higher than those found at Laundry Bridge, indicating that all the sites surveyed downriver of the Adit were underperforming compared with the sites upstream and on the north river.   Fish species richness was also lower in the south river with only one fish species recorded at the two sites immediately downstream (bullhead (Fig 6) and trout respectively) of the Adit versus three in the upstream site.  Highest species richness was found in the North Esk at Montagu bridge where five species, including Atlantic salmon were recorded.

We also compared this year’s data with historical records of juvenile salmonid (brown trout and Atlantic salmon) densities at Montagu and Laundry bridges (Fig 4).  Prior to 2020 salmon were consistently recorded on the South Esk at higher densities than those found on the north.  The 2020 surveys failed to find any salmon on the south river.

Discussion and interpretation.

The historical survey data suggest that salmon spawning conditions in the past were more favourable on the South Esk when compared to the North.  The fact that the 2020 survey failed to record any salmon at all in the four sites on the South Esk would support the view that the Adit discharge (potentially combined with other pressures such as sewage inputs) is having a negative (catastrophic) impact on the previously productive salmon spawning habitat on the South Esk.  This is further supported by the increase in recorded densities on the north from those of 2013, suggesting that the progressive fouling of the South Esk has caused a shift towards utilisation of the sub-optimal spawning habitat in the north river (it also being historically fouled by minewater from a different source).  Access to both rivers (and therefore spawning gravels) is limited by the presence of impassable weirs at Montague Bridge on the north and Dalkieth Weir in the south.  It is therefore possible that with the fouling of spawning habitat in the south, the available redd sites in the north are already saturated and that lost production cannot be compensated for. 

From the 2020 survey it is apparent that as the Adit is approached the density and diversity of other fish species appears to progressively decline before a sharp increase is recorded in the site above the Adit (Fig 3).  A total of five fish species were found in the North Esk while only three were found in the south (stoneloach and salmon being absent from the south river).  Overall fish densities in the South Esk remained very low with only a single bullhead being recorded at SEsk6, the site 100m downstream of the Adit.  The site above the Adit, SEsk7, was shown to have an approximately two to four-fold increase in fish density (Figs 2 & 3) and can be considered to be representative of a ‘control’ site free from the influence of minewater.  The trends reported here are further supported by invertebrate monitoring carried out by the Trust at sites SEsk5 – SEsk7 (INSERT HYPERLINK), which demonstrated statistically significant declines in invertebrate abundances and sensitivities to pollution associated with the Adit discharge, mirroring the results of the fish survey.

Conclusions/Limitations

The results indicate that the discharge from the Adit is having an adverse effect on the fish community in South Esk and that these effects can be felt at Laundry Bridge some 1.2km downriver.  Due to the limited nature of the data it was not possible to carry out any statistical analyses but the comparison of the upstream site on the south (SEsk7) and at Montague Bridge on the North (NEsk1) provides good reference states to glean comparative insight into the direct effects of the Adit.  When these sites are compared it is apparent that all sites downriver of the Adit are performing worse than those above the Adit and on the North Esk.

In conclusion the discharge at Junkie’s Adit appears to be having a negative impact on the fish community which is supported by historical comparisons between the north and the south rivers, and by comparison with an upstream control site.  These effects contribute towards a reduction in fish densities and the loss of two fish species, most notably Atlantic salmon, which previously dominated the fish community in the South Esk.  It is also worth noting that there here are several other pressures acting on migratory fish within the river, specifically in the south from frequent organic enrichment from the CSO downstream of the Sun Inn, from long standing mine drainage in the north at Ironmills Park, and from numerous barriers to migration in both rivers which prevent access to suitable spawning sites. 

Given that suitable migratory salmonid spawning habitat is already limited within the system (by the presence of impassable weirs at Montague Bridge and Dalkieth) it is clear that the fouling of the South Esk represents a significant loss of the total production potential of the river.  With this in mind, I’ll leave you with the words of Joy Adamson.

“Wildlife is something which man cannot construct. Once it is gone, it is gone forever. Man can rebuild a pyramid, but he can’t rebuild ecology, or a giraffe”.    

Joy Adamson