Geology topics

B.F. Blackwell

Publications and source records attributed to B.F. Blackwell.

3 recordsLinked to original sources

USGS2010

Assessing allowable take of migratory birds

Legal removal of migratory birds from the wild occurs for several reasons, including subsistence, sport harvest, damage control, and the pet trade. We argue that harvest theory provides the basis for assessing the impact of authorized take, advance a simplified rendering of harvest theory known as potential biological removal as a useful starting point for assessing take, and demonstrate this approach with a case study of depredation control of black vultures (Coragyps atratus) in Virginia, USA. Based on data from the North American Breeding Bird Survey and other sources, we estimated that the black vulture population in Virginia was 91,190 (95% credible interval = 44,520?212,100) in 2006. Using a simple population model and available estimates of life-history parameters, we estimated the intrinsic rate of growth (rmax) to be in the range 7?14%, with 10.6% a plausible point estimate. For a take program to seek an equilibrium population size on the conservative side of the yield curve, the rate of take needs to be less than that which achieves a maximum sustained yield (0.5 x rmax). Based on the point estimate for rmax and using the lower 60% credible interval for population size to account for uncertainty, these conditions would be met if the take of black vultures in Virginia in 2006 was < 3,533 birds. Based on regular monitoring data, allowable harvest should be adjusted annually to reflect changes in population size. To initiate discussion about how this assessment framework could be related to the laws and regulations that govern authorization of such take, we suggest that the Migratory Bird Treaty Act requires only that take of native migratory birds be sustainable in the long-term, that is, sustained harvest rate should be < rmax. Further, the ratio of desired harvest rate to 0.5 x rmax may be a useful metric for ascertaining the applicability of specific requirements of the National Environmental Protection Act.

Journal of Wildlife Management↗

USGS1997

Spring prey use by double-crested cormorants on the Penobscot River, Maine, USA

We analyzed 2 sets of data for Double-crested Cormorant (Phalacrocorax auritus) stomach contents (including esophageal contents) that were collected from April through June of 1986-1988 (N = 580) and 1992-1993 (N = 200) on the Penobscot River, Maine. Our objectives were to examine temporal and spatial variation in the spring diet and estimate the importance of Atlantic salmon (Salmo salar) smolts to the cormorant diet. We analyzed stomach contents relative to samples from 3 river sections: 5 mainstem dams collectively, above the head of tide, and free-flowing areas above and below the head of tide. Between years composition of taxa lists were compared (P = 0.05) relative to time and river section. We estimated taxon importance for data collected during 1992-1993 by ranking taxa according to 3 statistics: frequency of occurrence, mean percent volume, and numerical abundance. Data from 1986-88 were analyzed by frequency of occurrence only. Across the 3 river sections, the number of prey species recovered from cormorant stomachs increased from 15 in late April to at least 31 through May. Cormorants collected above the head of tide consumed 12 fish species (including freshwater, anadromous, and catadromous types), whereas birds collected below the head of tide consumed 28 freshwater and seasonally-available estuarine, marine benthic, and pelagic species. Salmon smolts were not recovered from stomachs collected in April, rare in stomach samples during the first week of June, and absent from the diet thereafter. In contrast, smolts were among the 5 most frequently occurring (1986-88) and highest ranking (1992-1993) prey taxa across the 3 river sections through May.

Waterbirds↗

USGS1997

Spring foraging distribution and habitat selection by Double-crested Cormorants on the Penobscot River, Maine USA

Restoration of the Atlantic salmon (Salmo salar) to New England rivers requires, in part, an understanding of the use of habitat and prey by potential and known predators. We examined variation in habitat use by Double-crested Cormorants (Phalacrocorax auritius), from April through June 1992-94 on ca. 288 km of the 2 primary tributaries and the mainstem of the Penobscot River, Maine. Our objectives were to quantify temporal and spatial variation in habitat use and test for selection of spring foraging habitats by cormorants. Cormorants forage during daylight hours only. To determine the distribution of foraging birds we conducted aerial surveys at intervals of <8 days. Counts from individual aerial surveys were analyzed relative to 9 river sections and used to examine selection of foraging habitats, such as free-flowing areas and the headponds and tailraces of dams. We defined selection as the difference between the proportionate length of each river section relative to the total river km available and the observed proportion of birds using each section. All birds observed from the aircraft within a river section were assumed to be foraging within that river section. Cormorants were first observed between 15 and 20 April in all 3 years. Individuals consistently selected (P < 0.05) against use of 2 large tributaries and sections >105 km north of the mouth of the mainstem. However, from late April through early June of all years, birds selected (P < 0.05) 4 of the 5 mainstem dams and the estuarine portion of the river. Cormorant use of dams remained at or above expected levels until the second week of June in all years. We suggest that cormorant selection for foraging areas adjacent to dams reflected a higher availability of prey, possibly due to delay and injury of migrating Atlantic salmon smolts. Cormorant selection of the estuarine portion of the Penobscot River was likely a response to a seasonally increasing availability and abundance of estuarine and marine prey, in addition to resident freshwater species and migrating smolts.

Waterbirds↗