Learning from long time series of harvest and population data - Swedish insights for European goose management

Goose management in Europe is faced by multiple challenges, as some species are declining and in need of conservation actions, while other populations have become very abundant, resulting in calls for increased harvest.

Sweden has long-term series of harvest data and counts of breeding and autumn-staging geese. We used national data (indices) for greylag goose, bean goose, and Canada goose to study shifts in temporal trends and correlative patterns, and to infer possible causal links between harvest and population trends. Our study provides an opportunity to guide management given the data collected within the present monitoring, as well as to suggest improvements for future data collection.

The populations of greylag and Canada geese increased in Sweden 1979–2018, but this long-term trend included a recent decrease in the latter species. Bean goose breeding index decreased, whilst staging numbers and harvest varied with no clear long-term trend. For Canada goose, our analysis suggests that harvest may affect population growth negatively. For bean goose and greylag goose we could not detect any effect of harvest on autumn counts the following year.

We find that the present data and analysis of coherence may suffice as basis for decisions for the current management situation in Sweden with its rather unspecific goals for greylag (very abundant ) and Canada goose (invasive species) populations. However, for management of bean geese, with international concerns of over harvest, data lack crucial information. For future management challenges, with more explicit goals, for all goose species we advocate information that is more precise. Data such as hunting effort, age-structure of goose populations, and mark-recapture data to estimate survival and population size, is needed to feed predictive population models guiding future Swedish and European goose management.

Methods
We based this study on data from three independent long-term monitoring programs in Sweden, providing annual data of: 1) breeding season abundance (1998-2017), 2) autumn staging counts (1978-2017) and 3) national harvest estimates (1978-2017). These datasets are here used as indices for breeding and autumn staging population development and changes in harvest levels respectively, and represent the available nation-wide monitoring of goose populations in Sweden.