Discussions regarding the impact of predators – particularly Cormorants and Goosanders – upon fish stocks are not uncommon in my family. This is due to one particular family members prevailing interest in angling, and, more often than not, such debates end in a stalemate. One which often culminates in claims that piscivorous birds clear out river systems of salmonid species, and rebuttals from myself regarding the speculative (likely, untrue) nature of such assertions. As such, this is one case of human/wildlife conflict with interests me greatly – and, at times, frustrates me just as much.
Now, you may have noticed that this week the BBC reported on calls from the Dee Salmon Fishery Board for a cull of bird species perceived to be impacting upon stocks of Atlantic salmon in the River Dee, Scotland. The report based, it would seem, on a new study entitled: Smolt migration through the River Dee and harbour.
The study at the heart of the report involved the live-capture and tagging of salmon smolts from the upper and lower Dee catchment and the use of telemetry to track them on their journey downstream. Of the 101 fish tracked over the course of the study, it is reported that many failed to survive their journey downstream: the authors reporting a mortality rate of 70% in fish tagged in the upper catchment, and a not so significant 13% from the lower. With total in-river and estuarine mortality reported at 48%. In order to explain this, the authors are quick to point the finger at predators, citing Goosanders as the likely cause of high mortality in the upper catchment, in particular.
It is widely accepted that Goosanders do consume young salmonids, often in large quantities. Indeed, a notable study by Feltham (1995) concluded that annual predation by Goosanders on salmon accounted for between 3-16% of smolt production for one river in Eastern Scotland, and another study by Riddington et al (2004) stated that 3-5% of migrating smolt were taken by sawbill ducks in the Sprey catchment during 2003. There have, however, been serious questions raised about the methods/findings of such reports and, equally, other studies have concluded that sawbill predation has a minimal impact on salmon production. It is thought that predation levels vary between river systems based on salmon abundance – to be expected, really given the theory of abundance-based prey selection – and, simply put, no one can state conclusively the impact of sawbill predation on fish stocks. People may speculate based on personal observation or anecdote but the truth is, no one knows – not myself, not conservationists and not the Dee Salmon Fishery Board.
Much like the RSPB, it would seem, I have some serious reservations about the assertions made in the Dee report. Namely, its apparent fixation on predation and ignorance of other issues also likely impacting upon salmon productivity. Among these, habitat degradation, pollution, climate change, parasites and invasive species. As well as predation by alternate predators, including fish. While interesting, the study seems to compliment the inherited dislike of avian predators present in anglers far too much for my liking, and thus appears designed solely for use as lobbying material tailored to fit the needs and aspirations of the angling community. Many of whom view anecdotal evidence of sawbill predation on salmon as evidence enough to act.
It should be remembered that river managers and sporting bodies can already conduct lethal management to protect stocks under license, once all other methods of deterrence have failed. For more information on this, see the criteria set out by SNH.
I also find it hard not to take issue with two additional factors mentioned in the report. The first being the possibility that tagging makes young fish more susceptible to predation. Well, yes, this should be considered. It is known that predators tend to focus their attention on slow, injured or impaired prey, and in all likelihood, fitted transmitters are not going to help when it comes to predator avoidance. Mainly due to the extra weight they add. This report, while open to the possibility of interference from fitted tags (and handling) does seem to play it down a little bit. Further study will certainly help iron this one out but I agree with the suggestion made in the report that it is possible that these levels of mortality may be higher than that occurring in the untagged smolt population. Aka, there is a possibility that Goosander predation on smolts is, in fact, not the monumental issue it is made out to be.
The second issue is the mention of the unusually dry conditions during the Spring of 2017. Credit to the report for acknowledging the impact this may have had on the study, and I’m sure repetition of the experiment during 2018 will help clarify its impact.
I really cannot help but feel that calls for cull in this instance are unjustified. Not just because of the doubtful relationship between sawbill predation and fish loss, and the issues I have highlighted in the current report, but also because of the state of the UK’s sawbill population as a whole. Yes, Goosanders are increasing; though their remains only around 3100-3800 pairs breeding in the UK, with another 8000 or so birds joining them in Winter. They are not incredibly numerous in Britain at all, when compared to other European countries. Our other resident sawbill, the Red-breasted Merganser, clocks in at only 2800 breeding pairs and around 12,800 birds in Winter – mostly around the coast.
I am not at all opposed to species management for conservation purposes when it is grounded in scientific fact. Culling for conservation is sometimes necessary, and can undoubtedly be beneficial when trying to protect vulnerable wildlife populations – foxes and Capercaillie, corvids and waders, mink and Water Voles. In this instance, however, there is just too much speculation and too little hard proof. Until such is provided, there is utterly no reason at all why angling interests should be allowed to up the slaughter of waterbirds, and add to the number already killed both legally and illegally each year in the UK.
Ps, and on a purely speculative note – it would make no sense at all for a niche predator such as a Goosander to limit the viability of its prey populations. That would be like me consuming my entire weekly shop on Monday, and wondering why, by Saturday, I find myself starving.
Feature image courtesy of Martin Cooper, licensed under Flickr Creative Commons. View here.