Spatial and temporal variation in abundance of willow ptarmigan Lagopus lagopus and rock ptarmigan Lagopus muta in Finnmark county, Norway : evaluation of methods for population monitoring

Abstract

Abstract: Sustainable harvesting requires reliable quantitative estimates of pre-harvesting population size, however, within low management budgets, good quality predictions are often difficult to obtain. For the popular game species willow ptarmigan (Lagopus lagopus) estimates of population density or relative indices are often obtained from line transect surveys, however, the quality of estimates could be sensitive to low sampling effort. Additional ptarmigan population data is hunting statistics, but the potential lack of convergence between population density and hunting bag remains a concern. The sympatric rock ptarmigan (Lagopus muta) is commonly given much less management attention, and the species is often arbitrarily expected to show synchronised dynamics with willow ptarmigan. In this paper we evaluate different population indices obtained from willow ptarmigan line transects surveys in late summer, and catch reports of subsistence snare trapping in late winter, and specifically assess these indices’ ability to predict subsequent ptarmigan hunting bags in eight municipalities in northernmost Norway. We use line transect data from 6–7 years of varying monitoring effort, snare trapping data from between 6 and 12 years in different municipalities, and hunting statistics from 7–8 years in all municipalities.

Generalised linear mixed-effect models indicated that only crude predictions of the subsequent willow ptarmigan hunting bags could be made from the line transect estimates under the current management regime. The snare trapping index made only a minor improvement of the prediction, and although the potential effects varied between municipalities, its inclusion as a predictor is not generally recommended in future ptarmigan management. The rock ptarmigan hunting bags could not be predicted by any of the available pre-harvesting indices. Variance component analyses revealed pronounced lack of systematic variation in all indices for both ptarmigan species, consequently any prediction of subsequent hunting bag outcome, should be highly conservative. Although we did not find clear tendencies of better predictions in municipalities with higher line transect effort, we suspect that the monitoring effort needed to provide useful pre-harvest indices is highly underestimated in the current management regime. Potentially unknown ptarmigan population processes and an elevated hunting pressure, should encourage management to increase, knowledge-based ptarmigan monitoring in the future.

Key words: willow ptarmigan; rock ptarmigan; sustainable harvesting; population density indices; line transect; snare trapping statistics; hunting statistics; population dynamics; population monitoring; game management; arctic ecosystems; Finnmark; Norway.