Résumé
Understanding whether hunting mortality is additive to or compensated by
other mortality sources is at the heart of managing harvested populations.
Long-lived species are expected to exhibit hunting mortality additive to
other sources of mortality, making them ideal candidates for population
management through sport harvest. Previous studies on these
processes have focussed on density-dependent natural mortality
compensating for hunting mortality, but when harvest occurs in distinct
periods of the year, heterogeneity in hunting vulnerability between
individuals could also lead to compensatory mortality between these
periods. We explore this new idea using the case of the greater snow goose
(Anser caerulescens atlantica), a harvested species whose population
became overabundant in the late 20th century. To control this
population, wildlife agencies liberalized hunting regulations with
unprecedented actions such as special hunting seasons implemented in
spring 1999 in Canada and in winter 2009 in the USA. To determine the
relative impact of each measure on survival, we estimated survival of
adult geese on a seasonal basis using 30 years of capture-mark-reencounter
data in a joint live-and-dead-encounter multievent model. We also used
this quasi-experimental set-up to evaluate possible compensation in
hunting mortality between seasons. We found that both special
hunting seasons decreased goose survival in the seasons and periods in
which they were implemented. However, survival increased during the spring
hunting season after the establishment of the special winter hunting
season in the USA in 2009. There was a negative relationship between
annual spring and winter mortalities, suggesting that the increase in
hunting mortality in winter was compensated by a reduction in spring
mortality after 2009. Synthesis and applications: To our knowledge, we
report the first documented instance of hunting mortality in one season
being compensated by a reduction in hunting mortality in a subsequent
season. We suggest that heterogeneity in hunting vulnerability among
individuals, possibly linked to the presence of juveniles, may explain
this phenomenon. A better knowledge of seasonal patterns and relationships
between mortality components is needed to improve our understanding of
population dynamics and management of harvested populations.