If Mute Swans could talk I reckon they would say a huge thank you to the British angling community that in 1987 gave up using toxic lead as fishing weights, albeit through legislation. I say that because a rather excellent scientific research paper has just been published that shows how the legislation banning lead fishing weights has resulted in an increase in the British population of Mute Swans. The research was undertaken by scientists from the Wildfowl and Wetlands Trust (WWT) and the Universities of Exeter and Oxford. The toxic effect of lead on bird populations has been well established, no more so than in the Mute Swan. In 1985 The Nature Conservancy Council (NCC) estimated that up to 4000 Mute Swans were dying annually due to the ingestion of lead fishing weights. The UK Government announced in July 1985 “that it would be prepared to control the sale and import of lead shot for fishing from January 1987 if voluntary measures failed.” The Control of Pollution (Anglers’ Lead Weights) Regulations 1986 came into force on the 1st January 1987 – banning the import and supply of lead fishing weights (between 0.06g and 28.35g).
The fact that lead is toxic to birds, wildlife and people is nothing new. The toxic effects of lead and its consequences are well established whether it be through petrol, paint or lead ammunition – take your pick from the 100s of scientific papers that are cited in the proceedings of the Oxford Lead Symposium or the Peregrine Fund conference. This latest research highlights how effective legislation has been in bringing about the population recovery of one of the most severely affected species.
What this newly published research does is take long-term population data from the Wetland Bird Survey (WeBS) for wintering Mute Swan (from 1974 until 2013). We know from other studies that the Mute Swan population in Great Britain has very little interchange with other populations from Ireland or continental Europe. This is important in the context of this latest study as it means that there is minimal chance of toxic lead being ingested elsewhere, as would be the case for Whooper or Bewick’s Swans that migrate to the UK from Iceland or Russia (amongst others). The authors analysed the environmental variables that might explain the population trend of the Mute Swan – winter temperature, food resources (the area of winter wheat and oilseed rape, as they are known to be the key food resource for Mute Swans), water quality and the presence or absence of the legislation banning lead weights in fishing.
The results clearly show that the best explanation for the increase in the population size of the Mute Swan is the introduction of The Control of Pollution (Anglers’ Lead Weights) Regulations in 1987. The population size pretty much doubled between 1986 and 1999, followed by a period of population stability. The following graphs (Figure 1) provide a good illustration of the results.
Figure 1. Mute swan population trend relative to changes in the potential explanatory variables (shown in grey). Plots show the temporal changes in mute swan population size in Great Britain in relation to (a) periods in which lead weights were unregulated (black circles) and regulated (open circles), (b) mean winter Central England Temperature, (c) the total area devoted to winter wheat and oilseed rape crops, and (d) the Total Reactive Phosphorus (TRP) values for British river (graph and header taken from Wood et al 2019).
The other important finding in the paper was that the mortality rate of Mute Swans attributable to lead poisoning changed significantly after the use of lead weights was regulated. This was based on the post mortem of dead birds and is illustrated in the following graph (Figure 2). Furthermore, the proportion of dead swans with lead weights within their gizzards or intestines also showed a significant decline once the regulations were introduced.Figure 2. Lead-induced mortality of mute swans before and after the regulation of lead ﬁshing weights. Following the regulation of lead ﬁshing weights in 1987 a signiﬁcantly smaller proportion of mute swans died of lead poisoning (graph and header taken from Wood et al 2019).
The fact that lead is toxic to birds is not really up for debate, but the clear implications of this latest piece of published research is the effectiveness of regulation in alleviating the negative effects of lead poisoning on a population of birds. The angling community should be applauded for adapting their pastime to the needs of the wildlife that share their environment. I’m not aware of any data that shows a decline in those participating in fishing in Britain due to the implementation of regulation, in fact I’m told the pastime is more popular than ever. We know that other user groups of toxic lead have been less willing to comply with regulation when it has been implemented. Maybe it’s time for the Government to do the right thing and implement the suggestion of its own working group – “a phase out of the use of lead ammunition and a phase in of the non-toxic ammunition alternatives” – after all regulation seems to work.
Thanks to Drs Kevin Wood and Ruth Cromie for providing me with the graphs to use in this blog. The full paper citation is as follows: Wood, K.A., Brown, M.J., Cromie, R.L., Hilton, G.M., Mackenzie, C., Newth, J.L., Pain, D.J., Perrins, C.M. & Rees, E.C. (2019). Regulation of lead fishing weights in Mute Swan population recovery. Biological Conservation 230, 67-74.
The research paper can be viewed here until the 6th February.
You can read about the work of the Wildfowl and Wetlands Trust on lead poisoning here.
Photo credit: Mute Swan by Paul Boxley used under license via Wikicommons