Banner image courtesy of Ron MacDonald
Grey seals are present at the Newburgh haul out site in large numbers (up to 2000 reported at the haul out site in 2019) and both grey or harbour seals are regularly seen in the Ellon area of the river.
The impact of the seals on the fish population is not yet clear. However the River Ythan Trust takes the view that as grey seals can swim fast enough to catch salmonids at every stage of their life cycle and they may be having an impact on the river’s fish population. Further study of the population is needed to assess the impact.
When the salmon netting operations ceased at Newburgh in 1997/1998 seal numbers, both common and greys, were almost zero – salmon netters had been controlling seal numbers there legally, for somewhere between 100 and 150 years. In 2019 Scottish Natural Heritage (SNH) stated on their website that up to 2000 grey seals haul out on the Newburgh site, an increase from zero to 2000 in around 10 years.
Callan Duck of the Sea Mammal Research Unit (SMRU) advised the diet of the seals in the estuary comprised mainly flatfish, also that if they concentrated on salmon and sea trout entering the estuary, there would be none left, he also stated that seals were lazy predators and that they would concentrate on fish that were easier to catch.
Using information on grey seal swimming speeds detailed by the Smithsonian National Zoo and Conservation Biology Institution, salmon and sea trout burst swimming speeds (salmon smolts, mature salmon, sea trout smolts, finnock (whitling) and adult sea trout, extracted from the Scotland and Northern Ireland Forum for Environmental Research (SNIFFER) extended field manual the River Ythan Trust determined that grey seals can swim fast enough to catch salmonids at every stage of their life cycle.
Visual observation of seals eating fish near or on the water surface – salmon and sea trout by the colour of the flesh, usually pinkish/red in colour, other fish generally have white flesh.
Collecting seal scats (poo), extracting hard fish parts in the scats and identifying the species of fish consumed. Perhaps the most important fish parts found in scats for identification of species, are otoliths, structures in the inner ear cavity that serve as a balance organ and aid in hearing. The shape of otoliths vary between species, therefore can be used for identification purposes.
Use of DNA from seal scats – where identification of otoliths is not possible (it has been suggested that otoliths from mature salmon may not be found because it is alleged that seals frequently do not eat mature salmon heads. There is also concern that delicate otoliths from juvenile salmon and sea trout may not survive the digestive process to be contained in scats.)
The River Ythan Trust looked at three studies looking into seal predation and diet, one carried out on the rivers Dee and Don between 1993 and 1996. Two others carried out on the Ythan estuary, the first one covering a period from July 2011 to June 2012, the second one during 2014. None of the three studies used DNA techniques for fish identification purposes.
The Dee/Don study found that visual sightings of seals (mainly commons) predating on salmon were mainly confined to the Dee. The few seal scats, from the Don estuary were not found to contain salmon otoliths.
The two Ythan estuary studies found no visual sightings of seals, greys, or commons, predating on salmonids. Both Ythan studies examined seal scats for the presence of salmonid remains, including otoliths – none were found.
Both Ythan estuary studies made significant comments as follows.
The first study 2011-12:
“A key potential source of bias lies in fact that otoliths of different species are digested at different rates and species with small or fragile otoliths (e.g. salmonids) may be underrepresented in scat samples due to complete digestion.”
The second study 2014:
“To summarise, the detection of salmonid consumption by grey seals would be more effective with underwater camera trapping and DNA analysis. With these methods, we could present clear evidence on possible salmonid predations.”
The Ythan Estuary: Based on the studies above, there is currently no scientific evidence that grey seals predate on salmonids within the estuary, however both of the Ythan studies have recognised that the use of DNA techniques on seal scats could help further future studies and provide greater clarification in this matter.
The river upstream of the Estuary: Although we have no confirmed visual reports of grey or harbour seals predating on salmonids, as is the case on the river Dee, both species are present in the Ellon area on a regular basis in late summer and autumn, when both salmon and sea trout are present – commonly appear at dusk and descend around first light.
The Ythan District Salmon Fishery Board is a member of Fisheries Management Scotland.
The River Ythan Trust is a registered charity
Scottish Charitable Incorporated Organisation (SCIO) number SCO 41269