Harvest, nasal-markers, and lesser scaup vital rates

Abstract

Since the mid-1980s, lesser scaup (Aythya affinis) population abundance has been approximately 20% below the North American Waterfowl Management Plan goal of 6.3 million lesser adult lesser scaup. Sustained harvest opportunity of this species is an important management goal in North America. We examined the relationship between harvest mortality and survival rates for a breeding population in southwest Montana from 2005 to 2016 when Adaptive Harvest Management was implemented for lesser scaup. We combined resighting, recapture, and hunter recovery to estimate survival and harvest rates using multistate capture-mark-recapture models in Program MARK. Nasal-marker loss rates were substantial and accounted for by allowing resight detection probability to decay with nasal-marker age. Adult female survival rates tended to be positively related to an interaction between annual Pacific flyway lesser scaup harvest and this value divided by the Pacific flyway daily bag limit, which we used as an index for population density (beta INTERACTION = 0.168, SE = 0.091). Estimated annual survival rates for adult metal-banded females averaged 0.66 (SE = 0.03) and 0.49 (SE = 0.02) for adult nasal-marked females, while first-winter survival rates averaged 0.39 (SE = 0.03) for metal-banded juveniles and 0.23 (SE = 0.02) for nasal-marked juveniles. However, this decrease in survival rates is not attributable to differing harvest rates between nasal-marked and metal-banded females (beta NASAL-MARKED, HARVEST RATE = 0.101, SE = 0.166). Body condition of nasal-marked females did not differ from unmarked females as their residual body mass was 1.6 g (SE = 10.4 g) less during the pre-breeding season and 3.5 g (SE = 6.5 g) less during the brood-rearing period. Nasal-marked females were found to have delayed initiating nests by 3.6 days (SE = 1.9 days) and laid 0.5 fewer eggs (SE = 0.3 eggs) relative to unmarked females while they hatched ducklings weighing 0.2 g (SE 0.2 g) more than unmarked females. Nasal-marked females were harvested 154.9 km (SE = 112.5 km) further from the study site than unmarked females, suggesting nasal-markers don't impede migration ability. Cumulatively, our results for nasal-marker impacts on vital rates suggest substantial individual heterogeneity among individuals exists in this population.