Mute Swan population recovery following the regulation of lead angling weights in Great Britain
By Kevin Wood
Mute Swans Cygnus olor are at high risk from lead poisoning as they live in habitats where recreational coarse fishing occurs (Fig. 1). Swans consume the discarded lead weights used by anglers whilst feeding or whilst ingesting grit, which they do to aid their digestion of vegetation. Following findings that lead poisoning was responsible for 4,000 Mute Swan deaths per year in Great Britain and a 15% decline in the national population size between 1956 and 1978, the import, sale, and use of lead weights in the sizes deemed most likely to be eaten by swans (0.06 – 28.35 g) were all banned from January 1987 onward across Great Britain. In a recent study, we assessed how effective these regulations had been in halting and reversing the decline in Britain’s Mute Swan population. Here, we share the results of that study (Wood et al. 2019).
Figure 1 Mute Swans may ingest lead angling weights whilst feeding or searching for grit. (c) Kevin Wood
In the first part of our study, we used 39 years of data from a national monitoring programme (the Wetland Bird Survey or ‘WeBS’) to assess trends in Mute Swan population size in Great Britain between 1974 and 2012. We found that the observed changes over time were best explained by the regulation of lead fishing weights, with a large and rapid increase following the restrictions on lead (Fig. 2). Lead regulation accounted for 82% of the between-year variation in Mute Swan population size. In stark contrast, other variables such as arable food supplies, river habitat quality, or winter air temperatures, had little effect on swan population size. Following the rapid increase in the late 1980s and 1990s, the size of the Mute Swan population in Great Britain has remained relatively stable since the early 2000s.
Figure 2 The trend in Mute Swan numbers in Great Britain, which rose sharply following the regulation of lead angling weights.
In the second part of our study, we used post-mortem data from Mute Swans collected from around Britain to assess the changes in the percentages of Mute Swans known to die from lead poisoning in the periods before and after the lead regulations were introduced. We found that prior to the regulations on lead angling weights, lead poisoning was the biggest single source of mortality amongst the swans, accounting for 34% of all the individuals that we analysed. Crucially, the percentages of individuals dying of lead poisoning dropped following the introduction of the regulations, from 34% to 6% (Fig. 3). This large reduction confirmed that a decrease in swan mortality caused by lead poisoning was the likely driver of increased population size in the years following the regulations.
Figure 3 Following the regulation of lead fishing weights from January 1987 onward, post-mortem data showed a drop in the percentage of Mute Swans found to have died of lead poisoning.
The results of our study showed that legal restrictions on lead angling weights succeeded in alleviating, although not completely eliminating, the impact of poisoning on Mute Swans. Our data showed that lead poisoning still accounts for 6% of Mute Swan deaths even after the restrictions on lead fishing weights. These cases could be due to the continued use of legal sizes of lead fishing weights that were not covered by the regulations (e.g. weights below 0.06g and above 28.35g), the persistence of lead weights deposited before the regulations came into force, non-compliance by anglers, and lead poisoning caused by lead from other sources such as ammunition used in shooting.
Putting comprehensive legal restrictions on the use of toxic substances, to prevent their release into the environment, can provide an effective conservation mechanism for reducing negative effects of human activities on wildlife populations. At a time when many policy makers prefer to rely on voluntary actions or market forces to achieve change, our findings highlight that legal regulations on human activities can offer an effective way of alleviating human impacts on wildlife.
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 results in mute swan population recovery. Biological Conservation 230: 67–74. DOI: 10.1016/j.biocon.2018.12.010