Blank page introduction Summary Conclusions and Recommendations 9

4.2 Lead ingestion by waterbirds

It has often been assumed that lead from spent shot and lost fishing sinkers is environmentally stable or inert and thus not a considerable source of environmental lead contamination and transfer. However, there is now sufficient evidence to conclude that ultimately all of the metallic lead in shot and sinkers will be transformed into particulate and molecular kinds of lead and will be dispersed through the environment to some degree. This process can result in local lead concentrations in soils and water far in excess of normal concentrations, which can form a serious risk factor for all trophic levels: lead transferred to soil and sediment invertebrates and terrestrial and aquatic plants will ultimately affect entire food webs (Jorgensen & Willems 1987, Scheuhammer & Norris 1995).

Tens or hundreds of years may be required for total breakdown and dissolution of lead pellets (Scheuhammer & Norris 1995). In addition, wetland bottoms are often too compact for pellets to sink into the sediment. Therefore, the vast majority of spent lead remains accessible for waterbirds (CWS 2001).

Lead shot ingestion and poisoning of waterfowl has now been documented in at least 21 countries, including Canada (Kennedy & Nadeau 1993), Australia (Kingsford et al. 1989), Great Britain (Mudge 1983), France (Pain 1990, Beck & Granval 1997), Spain (Mateo et al. 1998), The Netherlands (Lumeij & Scholten 1989), Japan (Honda et al. 1990; Ochiai et al. 1993), and the United States (Sanderson & Bellrose 1986; USFWS 1986). It has been known since the late 1800’s (Grinell 1894) that waterbirds ingest spent lead shotgun pellets that have been deposited on the bottoms of lakes and marshes, mistaking these pellets for food items or grit, which is selectively picked up from the bottom in order to facilitate the grinding of food in the gizzard. Once lead pellets are ingested, ionic lead is released as a result of the grinding action of the gizzard combined with the acidic environment of the digestive tract. If there has been ingestion of a large number (ten or more) of shot, acute lead poisoning rapidly ensues, and birds usually die within a few days. Because sinkers are generally much larger than shot pellets, a single lead sinker may induce acute poisoning. Victims of acute poisoning can appear to be in good condition, without pronounced weight loss. More commonly, birds die of chronic lead poisoning following ingestion of a smaller number (two to ten) of shot pellets. In these instances, signs of lead poisoning (green and watery faeces, drooping wings, anaemia, weight loss, atypical behaviour) appear more gradually, and affected birds die approximately two to three weeks after ingesting the shot, often in a very weak condition. In addition, many sublethally exposed birds probably die, even though mortality cannot be attributed directly to lead poisoning. Lead exerts sublethal toxic effects on many tissues, primarily the central and peripheral nervous systems, the kidneys, and the circulatory systems. The lesions caused in these tissues by lead exposure result in biochemical, physiological, and behavioural impairments. These impairments contribute to an increased risk of starvation, predation, and disease in affected birds; in addition, reproduction can be impaired or even obstructed (Demayo et al. 1982, Scheuhammer 1987, Eisler 1988, Scheuhammer & Norris 1995, Duranel 1999, Mézières 1999, VHJ 2000).

Mortality of waterfowl from lead shot ingestion becomes manifest either as large-scale die-offs or as less conspicuous, day-to-day mortality. Many instances of die-offs have been recorded in the United States (USFWS 1986) and Canada (Kennedy & Nadeau 1993). Although spectacular cases of mortality have drawn public attention to the issue of lead poisoning, these occasions are probably less important than the largely invisible losses of small numbers of birds on a daily basis. Sick and dying birds generally become increasingly reclusive. After death, carcasses are not likely to be seen, even by trained observers (Stutzenbaker et al. 1986). Carcasses are not often noticed unless the mortality rate surpasses the ability of predators and scavengers to efficiently remove them. Because of the difficulties of directly measuring the day-to-day mortality of waterfowl from lead poisoning, various indicators of lead exposure have been developed and used as indirect measures of the relative magnitude of lead shot exposure and poisoning (Scheuhammer & Norris 1995). The most widely used method has been the gizzard survey, which estimates the incidence of shot ingestion at local sites at the time of sampling. Based on gizzard surveys, it has been estimated that a total of about 15% of all dabbling ducks in Canada could be ingesting at least one shot pellet every year (Scheuhammer & Norris 1995). Similar considerations caused Sanderson and Bellrose (1986) to estimate that as many as 40% of North American waterfowl ingest shot during a single season of exposure. Several other studies have demonstrated that the incidence of embedded shot in apparently healthy, free-flying waterfowl frequently exceeds 20%, indicating that millions of migrating ducks and geese carry embedded shot (Scheuhammer & Norris 1995).

These are rough estimates of average ingestion incidence; statistics could vary considerably among different geographical locations, and among different species. In addition, these estimates are valid only for a period of about 20 days prior to sampling, because shot are either completely eroded or have passed through the digestive tract within that time (Dieter & Finley 1978). More reliable therefore are bone lead surveys; bone lead concentration is a good indicator of the relative degree of lifetime lead exposure because lead has a high affinity for mineralised tissue and easily accumulates in bone. Once deposited there, lead has an extremely long biological half-life. A duck that has ingested one or more lead pellets should, assuming it survives, exhibit an elevated bone lead level for the rest of its life (Scheuhammer & Norris 1995).