Ecology and Management of Avian Botulism on the Canadian Prairies

23time. Our experiment differed in that we marked carcasses at regular intervals throughout thesummer <strong>and</strong> monitored the proportion <strong>of</strong> marked carcasses found over the course <strong>of</strong> the entireclean-up. In spite <strong>of</strong> the perception that the clean-up efforts were more intense due to continuoussearches <strong>of</strong> the lake, the proportion <strong>of</strong> carcasses found was similar to previous reports.Intensive clean-up <strong>of</strong> well defined areas has the potential to improve pickup rates. Although weachieved a pickup rate <strong>of</strong> 61% under these conditions, 51 carcasses/ha still remained after cleanup,likely because <strong>of</strong> the high initial density <strong>of</strong> carcasses <strong>and</strong> the heavy vegetation in the area.Another thorough clean-up <strong>of</strong> the area found only 33% <strong>of</strong> the marked carcasses, with 48carcasses/ha remaining. On both occasions, the clean-up crew thought they had thoroughlysearched the area even though the second search took half the time. The fact that similar carcassdensities remained after each search suggests that searcher fatigue may play a role in achievingbetter detection rates.Scavenging appears to play an insignificant role in carcass removal during large botulismoutbreaks that occur in heavily vegetated wetl<strong>and</strong>s. In three <strong>of</strong> the non-clean-up wetl<strong>and</strong>s, whereradio-marked carcasses were placed, none were removed by scavengers <strong>and</strong> all developedmaggots <strong>and</strong> decomposed (Table 3). In other lakes, where carcass density was lower as a result<strong>of</strong> clean-up efforts <strong>and</strong> low levels <strong>of</strong> botulism mortality, scavenging rates varied from 0-13%.Scavenging appeared to be promoted by low carcass densities <strong>and</strong> higher carcass visibility,which resulted from light vegetation <strong>and</strong> expanses <strong>of</strong> open or thinly vegetated shorelines.Coyotes, raptors, crows <strong>and</strong> other scavengers were frequently observed when shorelines werebeing searched for carcasses.Rates <strong>of</strong> maggot development on radio-marked carcasses revealed that carcasses took an average<strong>of</strong> six days to develop to the stage <strong>of</strong> most intense fly larvae activity. Even a few carcassesoccurring on a botulism-prone wetl<strong>and</strong> have the potential to precipitate an outbreak, becausesediment from wetl<strong>and</strong>s with a history <strong>of</strong> avian botulism are likely to contain spores <strong>of</strong> type C C.botulinum (Wobeser et al. 1987) <strong>and</strong> 60-100% <strong>of</strong> carcasses on wetl<strong>and</strong>s during our studyproduced botulism toxin (Part VI; T. Bollinger, unpublished data), which is similar to previouslyreported rates <strong>of</strong> toxin development (Haagsma et al. 1972; Duncan <strong>and</strong> Jensen 1976). To preventoutbreaks on these wetl<strong>and</strong>s, regular searches would need to occur at least every four to six daysto prevent carcasses from developing maggots. Waterbird mortality commonly occurs onwetl<strong>and</strong>s due to such causes as inclement weather <strong>and</strong> hailstorms (Stout <strong>and</strong> Cornwell 1976), aswell as disease <strong>and</strong> trauma affecting juveniles in nesting colonies (Part V; Soos <strong>and</strong> Wobeser2006). Intensive searches <strong>and</strong> clean-up <strong>of</strong> entire wetl<strong>and</strong>s would need to occur approximately 10times over the course <strong>of</strong> the summer, to ensure that these periodic or low-level occurrencemortality events do not escalate into large botulism outbreaks.Carcass clean-up on large, vegetated wetl<strong>and</strong>s was largely unsuccessful. Only 7% <strong>of</strong> carcasseswere removed from Whitewater Lake, an 8,200 ha wetl<strong>and</strong> <strong>of</strong> which 4,500 ha are heavilyvegetated. Effective clean-up was not possible, due to the large area <strong>of</strong> vegetated wetl<strong>and</strong> to besearched on a regular basis <strong>and</strong> the high cost <strong>of</strong> doing so. Based on the results <strong>of</strong> our intensiveclean-up study, approximately ½ hr/ha would be required to find 50-60% <strong>of</strong> the carcasses which,