Second North American Sea Duck Conference - Patuxent Wildlife ...
Second North American Sea Duck Conference - Patuxent Wildlife ...
Second North American Sea Duck Conference - Patuxent Wildlife ...
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Gregory J. Robertson<br />
SECOND NORTH AMERICAN SEA DUCK CONFERENCE<br />
USING WINTER JUVENILE/ADULT RATIOS<br />
AS INDICES OF RECRUITMENT<br />
IN POPULATION MODELS<br />
Canadian <strong>Wildlife</strong> Service, 6 Bruce Street, Mount Pearl, NL, A1N 4T3; greg.robertson@ec.gc.ca<br />
There is an increasing interest in using winter juvenile counts as indices of recruitment in species<br />
that show delayed plumage maturation, especially for species, such as dispersed breeders, where<br />
it is difficult to obtain good productivity estimates from the breeding grounds. For sea ducks that<br />
congregate in large flocks during winter, large amounts of productivity data could be potentially<br />
collected. To date, however, there has not been the needed mathematical work to assist in interpreting<br />
these winter juvenile/adult ratios. First, details on the precise ratios of juveniles to adults required for<br />
population models, and the influence of sex biases in the population, are presented. Next, a matrixbased<br />
population model is presented that is modified to allow the fecundity component to be measured<br />
in mid- to late winter. This model is simplified to a set of equations that relate adult survival, winter<br />
juvenile/adult ratios and population growth rates, and allows an assessment of population trend<br />
with only one survival rate and age ratio data. These models have the advantage of not requiring<br />
that age of first breeding be well known. As an example, age ratios and survival rates of harlequin<br />
duck (Histrionicus histrionicus) populations in British Columbia and Maine are presented. Models<br />
for both populations suggest recruitment of young is insufficient to compensate for adult mortality,<br />
which is contrary to observed trends in population numbers. Although some further methodological<br />
work is needed, such as better ways to estimate true adult survival, validation of true age and plumage<br />
for some sea ducks, and a further understanding of how to obtain unbiased estimates of juvenile/<br />
adult ratios in the field, these models may prove to be a useful tool to assess population trends when<br />
detailed demographic data are not available.<br />
NOV. 7-11, 2005 ANNAPOLIS, MARYLAND, USA<br />
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