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

Abstract

Summary1. 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 colourringing 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 pinkfooted 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

The German wildlife information system (WILD): population densities and den use of red foxes (Vulpes vulpes) and badgers (Meles meles) during 2003–2007 in Germany

“…and willingness of hunters (Gaillard et al 2003;Kalchreuter 2003;Keuling 2007). Additionally, hunting bags might show completely opposite or wrong trends or dimensions, especially for migratory birds (Frederiksen et al 2004) or due to insufficient harvest rates (Grauer and König 2009). Therefore, live monitoring seems to be necessary to develop strategies for conservation and sustainable use of game populations (compare also Strauß et al 2008).…”

A Capture–Recapture Model with Double-Marking, Live and Dead Encounters, and Heterogeneity of Reporting Due to Auxiliary Mark Loss

“…Brownie's model; Brownie 1985) can provide "real" estimates of survival probabilities. However, because recovery rates are often low, the precision of survival estimates and/or the power to test hypotheses based solely on recoveries is low (Frederiksen et al 2004;. The mixture of both live recaptures and dead recoveries in the same model has demonstrated its capabilities to attenuate the previous limitations (Lebreton, Almeras, and Pradel 1999).…”

Understanding changes in bird populations – the role of bird marking

“…Age ratios from direct observations in the field can be used to measure the productivity of species such as geese. However, population modelling has shown that, while age ratios of Pink-footed Geese Anser brachyrhynchus recorded in Britain in autumn are consistent with other measures of the dynamics of this population, those for Greylag Geese A. anser substantially underestimate productivity (Frederiksen et al 2004). This highlights the importance of validating such data against independent information wherever possible.…”

“…Pioneering work on neotropical migrants has shown that, where long-distance migrants are reasonably site faithful during the winter, much useful information can be obtained by conducting mark-recapture studies on the breeding and wintering grounds (Sillett & Holmes 2002). Where birds are resident, or populations can be followed through most of the year, estimation of seasonal survival rates should be more straightforward (Frederiksen et al 2004) and RAS projects can be structured to provide this type of information (Robinson et al 2009d). Nevertheless, such studies remain rare and should be a priority for future work.…”