Scott C. Barras

Management effects on breeding and foraging numbers and movements of double-crested cormorants in the Les Cheneaux Islands, Lake Huron, Michigan

ABSTRACT The yellow perch fishery of the Les Cheneaux Islands (LCI) region of Lake Huron, MI suffered a collapse in 2000, attributed in part to the increase of double-crested cormorants (Phalacrocorax auritus) in the region. A management program involving egg-oiling and lethal culling was initiated in 2004 to reduce cormorant foraging on yellow perch in the LCI. Counts of cormorant nests, nests oiled, cormorants culled, and aerial counts and telemetry surveys were used to evaluate management. Management contributed to a 74% reduction of cormorants on breeding colonies from 2004 to 2007. Cormorants used the LCI area significantly more (P<0.05) than surrounding areas. Aerial counts of foraging cormorants declined significantly (P<0.05) over the entire survey area but not within the LCI proper. However, aerial counts of cormorants in the LCI were five-fold less than cormorant counts in the same area in 1995. Reduced cormorant numbers were attributed in part to the elimination of cormorant nesting on a large colony due to the introduction of raccoons. Although the numbers of cormorants using the LCI did not decline, flocks were significantly smaller and more dispersed after management began. The reduced number of cormorants from 1995 levels and more dispersed foraging likely reduced predation on fish stocks including yellow perch in the LCI. Our findings indicate that the relationship between reduction in cormorant breeding numbers and reduced cormorant foraging in a given area is complex and may be influenced by density dependent factors such as intraspecific competition and quality of the forage base.

Evaluation of Aerial Cluster Sampling of Double-Crested Cormorants on Aquaculture Ponds in Mississippi

Abstract Concern over increasing numbers of double-crested cormorants (Phalacrocorax auritus) and their impacts on channel catfish (Ictalurus punctatus) aquaculture has resulted in increased need for quantitative information to develop and evaluate depredation management efforts. We evaluated aerial surveys in a stratified cluster sampling (SCS) design to estimate and monitor abundance of cormorants on catfish aquaculture ponds in the Yazoo River Basin of Mississippi, USA (hereafter Yazoo Basin). Twice monthly abundance estimates and coefficient of variation during winter averaged 8,128 (n = 29, SE = 1,233) and 33% (n = 29, SE = 0.02), respectively. Counts of cormorants on catfish aquaculture ponds between survey years were correlated (r = 0.87, n = 28). The correlation between diurnal counts of cormorants on ponds and cormorant night roost counts was 0.64 in 2000–2001 and 0.58 in 2003–2004 (n = 20 in both years). A priori estimates of sample size indicated an average increase in sampling effort of 39% during peak periods of cormorant use would be necessary to detect a ±15% change in cormorant abundance on aquaculture ponds at α = 0.05 and β = 0.80. The sampling design we used has the potential to be an effective tool for providing quantitative information on cormorant abundance on catfish aquaculture ponds in the Yazoo Basin. However, increased sampling effort would be necessary to obtain desired levels of precision. The SCS design we evaluated represents only one of many possible survey methods, and we recommend additional evaluation of this method and related survey methods.

Reducing Impacts of Double-Crested Cormorants to Natural Resources in Central New York: A Review of a Collaborative Research, Management, and Monitoring Program

Abstract. Double-crested Cormorants (Phalacrocorax auritus) have used central New York waters for breeding and stopover habitats during migration since 1984. In response to public concern over Oneida Lake, the United States Department of Agriculture Wildlife Services and the New York State Department of Environmental Conservation initiated an integrated research, management and monitoring program aimed at mitigating cormorant impacts to fisheries and other natural resources in 1998. The history of this program was reviewed and efforts to reduce negative impacts of the Double-crested Cormorant population in central New York described. Management was successful, as demonstrated by a substantial decrease in cormorant use of Oneida Lake during spring, summer and fall seasons, and the apparent recovery of certain sportfish populations. Research identified cormorant movement patterns within and among water bodies and documented cormorant responses to hazing and other management techniques. The cormorant management program in central New York was intended to keep cormorant use of Oneida Lake at a level that prevents unsustainable impacts to fisheries populations.

Geographic segregation and evidence of density-dependent changes in sex ratios in an abundant colonial waterbird.

Demographic information, such as geographic segregation of sexes and sex ratio data, is needed to develop, model and evaluate conservation and management strategies for wildlife. A variety of physiological, behavioral and environmental factors can influence segregation of sexes and sex ratios, many of which originate with density-dependent processes. Departure from 50:50 sex ratios of double-crested cormorants (Phalacrocorax auritus) collected during control efforts in breeding and wintering areas across their eastern range of the USA were evaluated using using a Z-test as well as Stouffers weighted Z-tests. In addition, a specifically-designed randomization test was used to evaluate density-dependent effects on primary sex ratios in cormorants from egg collections and colony nest count data over a 21-year period. Cormorants collected from breeding colonies were strongly male-biased, whereas cormorants collected from feeding flocks were slightly biased toward females. Cormorants were partly segregated by sex on the wintering grounds, with significantly more males found in areas with intensive channel catfish aquaculture. The null hypothesis that females produced a balanced sex ratio independent of number of nesting cormorants was rejected: more male embryos were produced during rapid population growth, whereas at maximum nesting number more female embryos were produced. Once populations stabilized, the sex ratio was more equal. This examination of sex ratios indicates that different management methods and locations result in sex-biased culling of cormorants. Sex-biased culling in cormorants could make population reduction efforts more efficient and reduce overall take. We suggest further research to examine density-dependent effects on primary sex ratios documented here.