Harvest, survival, and abundance of midcontinent lesser snow geese relative to population reduction efforts

Abstract We assessed the effectiveness of an extensive and unprecedented wildlife reduction effort directed at a wide‐ranging migratory population of geese. Population reduction efforts that targeted several populations of light geese (greater snow geese [ Chen caerulescens atlantica ], lesser snow geese [ C. c. caerulescens ], and Ross's geese [ C. rossii ]) began in 1999 in central and eastern North America. Such efforts were motivated by a broad consensus that abundance of these geese was causing serious ecological damage to terrestrial and salt marsh ecosystems in central and eastern parts of the Canadian Arctic and subarctic regions along Hudson Bay. Starting in February 1999, special conservation measures (or, in the U.S., a conservation order) were added to the respective federal regulations that permitted hunters to take snow geese (in parts of Canada and the U.S.) and Ross's geese (in parts of the U.S.) during specified harvest periods outside of the hunting season. These measures were accompanied by increase or removal of daily kill and possession limits and by permissions to use previously prohibited equipment for hunting these species in certain regions of the continent. The intent was to reduce adult survival through increased hunting mortality, which was judged to be the most cost‐effective approach to reversing population growth. Our principal goal was to assess the effectiveness of reduction efforts directed at the midcontinent population of lesser snow geese, which was thought to be the most serious threat to arctic and subarctic ecosystems of the 3 light goose populations. Our multiple objectives included the estimation and detection of change in the response measures of total annual harvest, harvest rate, survival rate, and abundance, using the 1998 hunting period (defined as 1 Aug 1998 to 31 Jul 1999) as a point of reference. We used information about hunter recoveries of leg‐banded snow geese and estimates of regular‐season harvest to estimate 1) conservation‐order harvest and total annual harvest, 2) geographic and temporal distribution of recoveries by age class, 3) survival and recovery probability, and 4) abundance of snow geese each August using Lincoln's () method. We also modeled population growth to infer the form of population response to management efforts. Toward that end, we also proposed a method of estimating conservation‐order harvest and tested for differences in band‐reporting rate between Canada and the United States. Overall, the balance of evidence favored the conclusion that the midcontinent population has continued to grow during the conservation order, although perhaps at a reduced rate. We suggest that annual rate of population growth $({\hat {\lambda }})$ , derived from estimates of annual population size in August, likely provides the most reliable inference about change in the midcontinent population. There was a decline in annual survival probability between these 2 periods from about 0.89 to about 0.83 among snow geese from the southern‐nesting stratum (south of 60°N latitude), thought to compose about 10% of the midcontinent population. However, we detected no change in the much larger northern‐nesting stratum (north of 60°N latitude), where annual survival remained at about 0.87 from 1989 to 2006. Thus, the conclusion that this population continued to increase during the conservation order was largely consistent with the finding that a weighted‐survival probability for midcontinent snow geese essentially did not change between the period preceding (1989–1997) and during (1998–2006) the conservation order. Consistent with high survival rates were low harvest rates, which increased from 0.024 during 1989–1997 for northern geese to only 0.027 during 1998–2006 and from 0.031 to only 0.037 for southern geese. Despite the initial increase associated with the conservation order, harvest rates declined during the conservation order for geese from both strata. We suggest that the higher harvest rate evident for southern geese was related to their earlier fall migration and thus earlier exposure to harvest pressure. Migration by more abundant northern geese was later and resulted in a higher ratio of geese to hunters. Additionally, there was more harvest of southern geese in areas north of the Canadian prairies than there was of northern geese. Total annual harvest increased due to the conservation order but failed to exceed 0.75 million adults in any year during the assessment from 1989 to 2006. Harvest of both age classes exceeded 1 million in only 2 of 9 annual harvest periods since the conservation order started. These lower‐than‐expected harvests of adult snow geese combined with their low harvest rates of ≤0.048 during the conservation order suggested an August population size in excess of 15 million adult snow geese since 1998. We suggest that abundance of midcontinent snow geese was seriously underestimated in the past, and that this underestimate may have contributed to an overconfidence with which suggested harvest levels could achieve a goal of reduced survival and population reduction. Overall, all 3 populations of light geese now exceed numbers present when the conservation order was initiated. We are confident that the abundance and population growth rate of midcontinent snow geese (as well as by Ross's and greater snow geese) currently exceeds the ability of existing numbers of hunters to exert harvest pressure that is necessary to impose sufficient additive mortality and thus effectively influence population growth. It remains unknown how much more or how much longer such populations can increase towards carrying capacity, which we assume to be determined by the standing crop of arctic foods that they exploit, before density dependence can measurably slow the population growth rate. Estimation of carrying capacity in the large northern nesting stratum is among the key research needs that we propose. The situation that has emerged requires a review of perspectives about impacts of midcontinent lesser snow geese in the arctic, whether initial goals behind population management are still relevant, and whether alternative options from the initial array of management tools should be exercised. © The Wildlife Society, 2011

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