The dynamics of hunted Icelandic goose populations: A reassessment of the evidence

Abstract

1. Migratory geese are dominant terrestrial herbivores in Arctic and north temperate regions. They are important quarry species and also conflict with agricultural interests through winter foraging. We investigated the dynamics of the populations of greylag Anser anser and pink-footed Anser brachyrhynchus geese that breed in Iceland and winter in Britain. 2. We estimated seasonal and annual survival of both populations based on colour-ringing programmes in Iceland 1996-2000 and in Britain since 1987. We used these results, along with hunting bag statistics from Iceland, in a set of models to assess the British autumn counts as well as the impact of hunting. 3. After a rapid increase, the pink-footed goose counts stabilized after 1992 at around 230 000. The greylag goose counts increased slowly to c. 100 000 around 1990 and then declined to around 80 000. Both populations showed stable counts from 1996 to 2000, when the proportion of young recorded in autumn was 17.5% for greylag geese and 18.2% for pink-footed geese. Hunting bag statistics have been compiled since 1995 in Iceland; during 1996-2000, means of 36 608 greylag geese and 13 991 pink-footed geese were reported shot annually. 4. Mean annual adult and first-year survival estimates were 0.727 and 0.472 for greylag geese, and 0.814 and 0.394 for pink-footed geese. The lower survival of juvenile pink-footed geese was probably due to higher natural mortality, whereas the generally lower survival of greylag geese could be explained by higher hunting pressure. 5. Simple modelling showed that the results from autumn surveys and bag statistics were incompatible for greylag geese. For the population to remain stable with the estimated survival rate, the proportion of juveniles in autumn must be 30%. We suggest that problems in separating juveniles from adults in October-November caused the lower proportion of juveniles recorded. Also, the number of greylag geese counted in autumn and the number reported shot in Iceland cannot both be correct; the bag statistics imply a population twice as large as the number counted. Large numbers of greylag geese may winter in areas not covered by the autumn census, but overreporting of the hunting bag may also have occurred. No such discrepancies were found for pink-footed geese. 6. Synthesis and applications. We recommend that a stratified sampling programme be established for estimating the size and age composition of both goose species. The age composition of greylag geese can profitably be studied in Iceland in August, when juveniles are easily recognizable. Ringing should be continued as a tool for monitoring survival, and a hunting bag survey should be implemented in Britain. In general, the reliability of population surveys should always be tested with independent data.