Arctic Charr

In 2021 our seasonal assistant, Sam Poultney, developed and undertook a project on Arctic charr. This project was undertaken in conjunction with Queen Mary University London (QMUL), who provided eDNA expertise.

Arctic charr (Salvelinus alpinus) in spawning colours from Windermere, U.K. Courtesy of I.J.Winfield, Centre for Ecology and Hydrology.

Arctic charr, Salvelinus alpinus, are the true aboriginal freshwater fish in Scotland, found in their current locations after closely following the retreat of the British-Irish ice sheet Charr are the most northerly freshwater fish, have a broad Holarctic distribution and exhibit both anadromous and resident forms. Scottish populations’ exhibit residency in lochs with some migrating into inflow burns to spawn. Charr have complex abiotic requirements - a low tolerance of temperatures above 16°C and a lethal oxygen saturation level of <3mg L¯¹. With observed and predicted increases in surface temperatures due to climate change, plus the associated knock on effect on hypolimnion O₂ levels;  charr existence will be on a knife edge, as isolated lochs/lochans prevent the ability for populations to mitigate loch based changes.

There is no or very limited population monitoring of charr in Scotland with many waterbodies having only outdated or anecdotal references. Historic information averages around 25 years with some last known records going to back to the 1940’s. Due to their extreme morphological diversity and high intraspecific genetic diversity, they should be recognised as a significant contribution to impoverished northern aquatic biodiversity and have a place in future monitoring programmes. This current study aims to update the records on charr lochs within the Laxford and Naver Hydrometric Areas in which the last known record is more than 10 years old. 

Methods

Traditionally, population monitoring would involve destructive methods such as gill netting. However with advances in technology, population monitoring can now be done without killing fish, either by hydroacoustic surveys or via environmental DNA (eDNA) sampling. Working in cooperation with QMUL, a select number of lochs were surveyed using eDNA.

Firstly, a number of lochs underwent a depth survey to highlight the deepest areas where charr might be found during the summer stratification period. This first round of surveys produced accurate depth records for both known and unknown lochs, adding to the new discoveries from this project. Data was collected either by a GPS based hydroacoustic chart plotter (Fig. 1a) and processed by third party cloud software or via a handheld GPS and depth sounder using an inflatable pack raft. Once the bathymetry was established, the eDNA sampling occurred from both angler boats (August) and shore-based (November) locations, whereby 2L of water or more was pumped through a filter for future lab analysis to enable the detection of charr. In addition, some lochs were sampled by gill net. This involved setting up to three 30m multi panel nets, perpendicular to the shoreline, for a period of 12hrs, to gain data on population structure and individual growth rates, which cannot be collected through eDNA. Charr otoliths, fin clips and muscle tissue were also sent off to a PhD student based at the University of Glasgow to aid in their current project.

Fin clips were collected from salmon, trout and charr during the annual electrofishing programme and this projects gill nets. This species specific genetic material will form the basis for the lab based filter analysis. Any DNA collected on the filters during the water filtering process can be matched to the fin clip data for a positive result. The lab analysis is due to be undertaken by a master’s student based at QMUL sometime in January, with results expected sometime in early summer. If suitable, an academic peer reviewed journal article will be written up and submitted on the current data.

Results

There are a few preliminary results that can be presented here. Out of the 28 high priority waterbodies selected for investigation, a total of 20 lochs underwent a bathymetric survey (Table 1). This was carried out during an unseasonal dry summer resulting in datasets at or slightly above “base-flow” levels. Only one loch, Loch Urigill, doesn’t have accurate bathymetric data due to the boat design/hydroacoustic mounting issues, so Bartholomew (1910) depth data was used as a replacement. For the remaining seven lochs, data wasn’t collected due to time constraints, weather conditions and/or equipment issues. Due to the different levels of accuracy between methods, resulted in two different outputs; Fig. 1b shows the pack-raft output with depth points approximately every 10m and Fig. 1c shows the highly detailed boat based GPS chart plotter data.

Figure 1: An example of the methods and results of the bathymetric investigation completed as part of the Charr Project 2021 a) the hydroacoustic survey equipment insitu on a pioneer 12 rowing boat. b) the map output for the pack-raft based survey method, with data points approximately 10m apart. c) the hydroacoustic map output, with limited data gaps in the survey coverage (black).

In terms of NORDIC gill netting, three lochs were surveyed (Table 2). The surveys were completed on 08/08/2021; 20/08/2021 and 22/08/2021 respectively. Importantly, only Loch a’Garbh-bhaid Mor captured Arctic charr, with 16 individuals ranging from 95-176mm and an average weight of 36.38g. This updates the current known record for this loch from 1940 to 2021. Brown trout was captured throughout the three lochs. 

Summary and recommendations

While waiting for the results of the eDNA analysis only one loch, a’Garbh-bhaid Mor, can be updated from 1940 to 2021 and from “uncertain” to “extant”. The results from the bathymetric survey will also be useful for future datasets, even shedding light on lochs with previously unknown depth profiles.