WINTER SURVIVAL OF IMMATURE CANVASBACKS IN INLAND LOUISIANA

In spite of management efforts, population indices for canvasbacks (Aythya valisineria) declined during 1955-93. Since 1985, Catahoula Lake, located in central Louisiana, has attracted more wintering canvasbacks than any other site in North America. Canvasbacks are attracted to Catahoula Lake because of its consistent flooding regime and abundant below-ground plant foods, but are at risk of lead (Pb) toxicosis because of the high density of spent Pb shot contained in sediments. To estimate survival rates and determine sources and timing of mortality, we radiomarked 172 immature canvasbacks during winters 1991-94. Half of our sample had blood Pb concentrations greater-than-or-equal-to 0.2 parts per million (ppm) at time of capture and were considered Pb exposed. Blood Pb concentrations of others (unexposed) were initially at background levels (i.e., Pb < 0.2 ppm). Year had no effect on canvasback survival (P = 0.357), but survival was positively related to body mass (P = 0.014) and negatively related to the interaction of blood Pb concentration and body mass (P < 0.001). Winter survival estimates for Pb-exposed immatures were lower (P less-than-or-equal-to 0.045) than those for unexposed immatures in 1992-93 (0.569 vs. 0.815, respectively) and 1993-94 (0.578 vs. 0.923, respectively) but did not differ (P = 0.34) in 1991-92 (0.747 vs. 0.857, respectively) when Pb-exposed canvasbacks had greater body mass and lower blood Pb concentration. Causes of mortality were gunshot (31%), Pb toxicosis (16%), drowning in commercial fishing nets (6%), and unknown (47%). Survival estimates for Catahoula Lake canvasbacks, especially Pb-exposed individuals, were lower than those previously reported for immature females wintering in coastal Louisiana or Chesapeake Bay. Most immatures that were initially exposed to Pb apparently survived the winter; however, subsequent survival and performance of annual cycle events may have been affected because exposed birds weighed less than unexposed birds. We recommend that Pb exposure in waterfowl be periodically monitored and managers give priority to reducing Pb shot availability at concentration areas.