Declining scaup populations : A retrospective analysis of long-term population and harvest survey data / Alan D. Afton and Michael G. Anderson.

Includes bibliographical references (pages 795-796).

We examined long-term databases concerning population status of scaup (lesser [Aythya affinis] and
greater scaup [A. mania] combined) and harvest statistics of lesser scaup to identify factors potentially limiting population
growth. Specifically, we explored evidence for and against the general hypotheses 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 limiting factors. Several biases
exist in survey methodology used to estimate scaup populations and harvest statistics; however, none of these
biases likely accounted for our major findings that (1) the continental scaup breeding population has declined
over the last 20 years, with widespread and consistent declines within surveyed areas of the Canadian western boreal
forest where most lesser scaup breed; (2) sex ratios of lesser scaup in the U.S. harvest have increased (more
males now relative to females); and (3) age ratios of lesser scaup in the U.S. harvest have declined (fewer immatures
now relative to adults), especially in the midcontinent region. We interpreted these major findings as evidence
that (1) recruitment of lesser scaup has declined over the last 20 years, particularly in the Canadian western
boreal forest; and (2) survival of female lesser scaup has declined relative to that of males. We found little evidence
that harvest was associated with the scaup population decline. Our findings underscore the need for both improvements
and changes to population survey procedures and new research to discriminate among various hypotheses
explaining the recent scaup population decline.