Declining scaup populations: A retrospective analysis of long-term population and harvest survey data

We examined long-term databases concerning population status of scaup (less er [Aythya affinis] and greater scaup [A. marila] combined) and harvest sta tistics of lesser scaup to identify factors potentially limiting population growth. Specifically, we explored evidence for and against the general hyp otheses that scaup populations have declined in association with declining recruitment and/or female survival. We examined geographic heterogeneity in scaup demographic patterns that could yield evidence about potential limit ing factors. Several biases exist in survey methodology used to estimate sc aup populations and harvest statistics; however, none Of these biases likel y accounted for our major findings that (1) the continental scaup breeding population has declined over the last 20 years, with widespread and consist ent declines within surveyed areas of the Canadian western boreal forest wh ere most lesser scaup breed; (2) sex ratios of lesser scaup in the U.S. har vest have increased (more males now relative to females): and (3) age ratio s of lesser scaup in the U.S. harvest have declined (fewer immatures now re lative to adults), especially in the midcontinent region, We interpreted th ese major findings as evidence that (1) recruitment of lesser scaup has dec lined over the last 20 years, particularly in the Canadian western boreal f orest; and (2) survival of female lesser scaup has declined relative to tha t of males. We found little evidence that harvest was associated with the s caup population decline. Our findings underscore the need for both improvem ents and changes to population survey procedures and new research to discri minate among various hypotheses explaining the recent scaup population decl ine.