Effects of a Danish action plan on reducing shotgun wounding of Common Eider Somateria mollissima

Abstract

Capsule The incidence of embedded shotgun pellets in Common Eider females declined from 34.1% to 5.5% from 1997 to 2009–2011 after the implementation of an action plan to reduce wounding. Males, which were exposed to hunting outwith the area of the action plan, showed a lower rate of decline.

Hunting using shotguns inevitably causes non-lethal wounding of birds that are hit by pellets but not instantly killed. A proportion of these birds eventually die as a direct consequence of their injuries (Newton 1998), but lightly crippled individuals may survive with embedded pellets, dependent on the extent of the injury. It has been shown that wounding can affect body condition of Common Eider Somateria mollissima (Merkel et al. 2006) such that birds experience difficulties maintaining adequate body-fat stores in midwinter. This may have potential fitness consequences, such as lower reproductive success and survival (Fournier & Hines 1994, Oosterhuis & van Dijk 2002), and ultimately affect population size, where population growth rates of long-lived species like the Eider are generally more sensitive to changes in adult survival than to changes in reproduction (Sæther & Bakke 2000).

To assess the extent of wounding among Eiders, Danish X-ray investigations in 1996–1997 detected shotgun pellets in 34% of adult females and 35% of adult males (Noer et al. 1996a, Holm et al. 2011). These percentages were very similar to the infliction rate of Common Eider in Canada in the same time period (29% of males and females: Hicklin & Barrow 2004). To this figure should be added an unknown but not insubstantial proportion of seriously wounded individuals that do not recover, and it was ultimately estimated that for every Eider that was shot and retrieved in Denmark, another was wounded (Noer et al. 1996a). These findings made wounding of game an issue in Denmark, as the Danish Game Act explicitly states that management of hunting shall be based on a ‘wise use’ concept, based upon ethical as well as ecological principles, and that hunters must not inflict unnecessary suffering upon game species. Because it was considered possible to hunt in ways that lower the probability of wounding, such rates as those observed failed to comply with both the letter and the spirit of the legislation. The main reason for game becoming crippled is because hunters shoot at too long range. The increasing dispersal and decreasing velocity of shotgun pellets with distance, especially at long range, leads to the increased risk of wounding of the target and adjacent individuals. To bring down an Eider, the bird's vital areas must be hit by pellets and the chance of doing this is inversely proportional to the distance between hunter and target (Van Dyke 1981, Noer et al. 1996a).

A national action plan was implemented in Denmark in 1997 to reduce wounding, granting hunters an initial trial period during which to reduce the proportion of crippled animals on a voluntary basis. Hunters were told that if major reductions of numbers wounded did not occur, regulation of hunting opportunities or protection might be necessary to achieve improvements. Since 1997, hunters have been subject to information campaigns from both the Ministry of Environment and the Hunting Association to improve their hunting techniques to minimize crippling. Serious management decisions hinge on the outcome of this trial and this article assesses the extent to which voluntary changes in hunter behaviour have succeeded in reducing wounding rates in Common Eider by monitoring the number of free-flying Eiders inflicted with shotgun pellets between 1997 and 2011. Lead shot was banned in Denmark in 1996 and only non-toxic shot has been legal for Danish hunters since the action plan was implemented.

Female Eiders that died from avian cholera during incubation in 2001 were collected at their breeding colonies at Isefjord, Hov Røn and Stavns Fjord (Pedersen et al. 2003) (Fig. 1). Outbreaks of avian cholera among Eiders have only been reported at breeding colonies (Gershmann et al. 1964, Swennen & Smit 1991, Christensen et al. 1997), because the aggregation of birds in breeding colonies facilitates transmission (Pedersen et al. 2003). There is no indication that crippled females are more likely to die from avian cholera, as nearly all individuals (80–95%) in a colony normally die during an outbreak (Pedersen et al. 2003). Scientific collection of male and female Eiders was also undertaken using a rifle or #BB shotgun pellets (4.6 mm) during January–March 2006, 2009 and 2011 at Horsens Fjord and Svanegrunden (Fig. 1). The Danish Nature Agency gave permission to use #BB shotgun pellets, which are normally illegal for hunting in Denmark. Due to their larger size, #BB pellets are easily distinguished from ordinary (legal) shotgun pellets in X-ray images. Finally, incubating females were collected on the nests at Saltholm in 2011 and euthanized under a licence from the Danish Nature Agency, in collaboration with the National Centre for Wildlife Health, which investigated the parasite load of the female Eiders (Chriél et al. 2012). Data from 2009 and 2011 were pooled to increase sample size. All individuals were sexed, X-rayed and assigned as first-year or older (adults) by plumage characters. Eiders are long-lived, and the corresponding low proportion of young in the population causes sample sizes for first-years to be too small for statistical reliability. Therefore, only older birds that had survived at least two hunting seasons were used in the investigation.

Figure 1. Spatial distribution of Danish sites where Eiders were collected for X-ray investigations: Horsens Fjord (1), Hov Røn (2), Svanegrunden (3), Stavns Fjord (4), Isefjorden (5) and Saltholm (6).

Based on an annual survival rate of 0.90 for adult female Eiders in Danish breeding colonies (Noer et al. 1996b), the expected development in infliction frequencies was calculated using the models by Noer & Madsen (1996) for scenarios representing declines in wounding of 0%, 25%, 50%, 75% and 100%, respectively. The annual survival of males is unknown, because no capture–recapture studies have been done. To test for differences in infliction rates between years, a Chi²-test with a Yates correction was used.

A significant decrease in female Eiders with embedded pellets was found between 1997 and 2001 (34.1%, n = 249 compared to 26.3%, n = 418) (χ² = 4.6, d.f. = 1, P = 0.032) (Fig. 2). From 2001 to 2009–2011 this proportion decreased significantly to 5.5% (n = 55) (χ² = 12.1, d.f. = 1, P = 0.001). The percentage of males with embedded pellets was 35% (n = 40) in 1997, 28.3% (n = 60) in 2006 and 22.0 % (n = 50) in 2009–2011, with no significant difference between years (χ² = 1. 9, d.f. = 2, P = 0.39). The difference between males and females in frequency of birds with or without embedded pellets in 2009–2011 was significant (χ² = 6.2, d.f. = 1, P = 0.013).

Figure 2. The percentage of wounded adult females (black dots) and males (white triangles) in 1997, 2001, 2006 and 2009–2011, respectively. The curves show the expected development if the rate of wounding fell by 0%, 25%, 50%, 75% and 100%, respectively.

Males collected in this study overwintering in Kattegat breed both in Denmark and in Sweden and elsewhere in the Baltic according to Noer et al. (2006). The latter are also hunted in Finland and Sweden where there has been no Action Plan to reduce wounding and where hunters have not been subject to information campaigns. It was therefore expected that infliction rates of males would be higher than for females, because females were collected from Danish breeding colonies, and had therefore been almost entirely subject to hunting within Denmark (Noer et al. 1996a). The small sample size of males in this study was probably the main reason why differences in male infliction rate did not attain statistical significance, In order to detect an effect at the observed frequencies, annual sample size would have been 150 Eiders or above.

As Fig. 2 shows, the infliction rate in 2009–2011 was lower than it would be theoretically, if wounding completely ceased from 1997 onwards. This implies that other factors must have contributed to explain the very low female infliction rate. First, because of the avian cholera outbreaks in the beginning of the century, many of the females which were found wounded in 1997 have been removed from the population, resulting in a lower infliction rate than we would expect if female survival was unchanged in the study period, according to the slopes in Fig. 2. Secondly, because of a decline in the Baltic Eider population generally (Desholm et al. 2002), the open season for shooting female Eiders was shortened in Denmark in 2004 to make the hunting more sustainable. Since then, the bag size has declined from 17 000 to 7000 adult females per year (Noer et al. 2009), which is also likely to have made a contribution to the difference in wounding rates between males and females.

Increased female mortality and the shortened hunting season may have resulted in a lower infliction rate in 2009–2011 than expected, making the results harder to interpret, but because it seems that both male and female infliction rates have declined, we conclude that a reduction in wounding to females may have contributed significantly to the overall result. The increased mortality and change in hunting are factors that first affected the population from 2001 onwards. Before this, infliction rates decreased by about 60% (Fig. 2). It is plausible that this number reflects the real decrease in wounding over the whole period and that the actual infliction rate for females in 2009–2011, therefore, should lie between 15% and 20%. So, even though Eiders are still being inflicted with shot in 2011, the reduction indicates that the Danish action plan has had a positive impact on the wounding of Eiders. The conclusion is supported by studies of other key species included in the action plan, which found that the percentage of wounding of Pink-footed Geese decreased from 36% to 17.8% between 1997 and 2005 (Noer et al. 2007) and Red Fox from 24.9% to 8.5% between 1997 and 2009 (Elmeros et al. 2012).

ACKNOWLEDGEMENTS

Thanks to Tony Fox & Thomas Kjær Christensen for valuable comments and proof reading on the manuscript.