Endocrine-Modulating Substances in the Environment: The Wildlife Connection

Historical observations, first publicized in Rachel Carson's Silent Spring, demonstrated biological effects of persistent, bioaccumulative pollutants on wildlife. These effects included disruption of reproduction and, in some situations, responses mediated through the endocrine system. The substances that caused these effects were mainly highly chlorinated halocarbon compounds, such as DDT (and metabolites), other organochlorine pesticides, polychlori-nated biphenyls, poly chlorinated dibenzo-p-dioxins, and poly chlorinated dibenzofurans. In contrast, responses of fish to industrial discharges and pulp mill effluents have implicated more water-soluble compounds. Characterizations of wildlife exposures require consideration of temporal and spatial factors that they exacerbate or ameliorate responses. Likewise, effects of endocrine-modulating substances m ay appear at subsequent stages of development, not at the time of exposure. Consistent with the declines in environmental concentrations of persistent, bioaccumulative substances, populations of several wild bird species, including bald eagles, cormorants, herring gulls, and Caspian terns, have increased, recovering from declines noted in previously polluted areas during the 1960s and 1970s.

Food consumption patterns of the Vulnerable Socotra cormorant Phalacrocorax nigrogularis indicate minimal overlap with fisheries in the eastern Arabian Gulf

Historically cormorants have come into direct conflict with fisheries because of their feeding habits. The Socotra cormorant Phalacrocorax nigrogularis is a regionally endemic seabird restricted to the Arabian Gulf and coastal Oman, and is categorized as Vulnerable on the IUCN Red List. The species is perceived as being detrimental to local fisheries and is therefore persecuted. We studied the diet and estimated the number of Socotra cormorants breeding on Siniya Island, Umm Al Quwain, United Arab Emirates, to determine their impact on local fisheries. Regurgitated fish loads were collected during the breeding seasons of 2011–2012 and 2012–2013, and the biomass of fish consumed by the Socotra cormorant population was estimated. The diet comprised seven species of fish, with sailfin flying fish Parexocoetus mento and blue-stripe sardine Herklotsichthys quadrimaculatus dominating in 2011–2012 and anchovies (Encrasicholina spp.) dominating in 2012–2013. Biomass of fish loads was higher in 2011–2012 compared to 2012–2013, although mean biomass of fish loads did not differ. Dominant fish species in the cormorant's diet were either used as baitfish or were non-target species in the fisheries. The low overlap between the diet of the Socotra cormorant and the fish landed by the fisheries suggests that the cormorant's impact on fisheries is minimal. Conservation strategies to protect the birds and their feeding areas could safeguard populations of this species from further declines.

Tracking the History and Ecological Changes of Rising Double-Crested Cormorant Populations Using Pond Sediments from Islands in Eastern Lake Ontario

In the Laurentian Great Lakes region, the double-crested cormorant (Phalacrocorax auritus) has seen a thousand-fold population increase in recent decades. These large colonies of birds now often conflict with socioeconomic interests, particularly due to perceived competition with fisheries and the destruction of terrestrial vegetation in nesting habitats. Here we use dated sediment cores from ponds on islands in eastern Lake Ontario that receive waste inputs from dense colonies of cormorants and ring-billed gulls (Larus delawarensis) to chronicle the population rise of these species and assess their long-term ecological impacts. Modern water chemistry sampling from these sites reveals drastically elevated nutrient and major ion concentrations compared to reference ponds not influenced by waterbirds. Geochemical tracers in dated sediment cores, particularly δ¹⁵N and chlorophyll-a concentrations, track waterbird influences over time. Fossil diatom assemblages were dominated by species tolerant of hyper-eutrophic and polluted systems, which is in marked contrast to assemblages in reference sites. In addition to establishing long-term ecological impacts, this multi-proxy paleoecological approach can be used to determine whether islands of concern have been long-term nesting sites or were only recently colonized by cormorant or ring-billed gull populations across the Great Lakes, facilitating informed management decisions about controversial culling programs.

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 Stouffer's 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.

Double-crested cormorant (Phalacrocorax auritus) nesting effects on understory composition and diversity on island ecosystems in Lake Erie

The context for this study is the management concerns over the severity and extent of the impact of cormorants on island flora in the recent past on Lake Erie islands. Accordingly, this study sought to quantify the nesting colonies' influence on coarse woody litter and how nest densities and litter depth may influence the herbaceous layer, the seed bank composition and viability across the extent of three Lake Erie islands. The data for this study were collected from 2004 to 2008 on East Sister Island and Middle Island using two main strategies. First, herbaceous layer surveys, cormorant nest counts, soil seed bank cores, and litter depth measurements were executed using a plotless-point quarter method to test island-wide impacts from nesting activities (data were also collected on a third island, West Sister Island as a reference for the other two islands). Secondly, a sub-sample of the entire plot set was examined in particularly high nesting density areas for two islands (Middle Island and East Sister Island). Kruskal-Wallis tests indicated that there are subtle changes in the herbaceous diversity (total, native and exotic) and seed bank composition across the islands. The sub sample set of the plots demonstrated that Phalacrocorax auritus nest density does influence litter depth, herbaceous species abundance and diversity. Cormorant nesting pressures are restricted to areas of high nesting pressures and competition. However, there remains a risk to the interior herbaceous layer of the island if the effects of nesting pressures at the edges advance inward from this perimeter.

Linking larvae and adults of Apharyngostrigea cornu, Hysteromorpha triloba, and Alaria mustelae (Diplostomoidea: Digenea) using molecular data

Because the taxonomy of trematodes is based on adults, the larval stages of most digeneans cannot be identified to species based on morphology alone. Molecular data provide a means of linking larval stages to known adults. We obtained sequences from the barcode region of cytochrome oxidase I (CO1) from adult and larval parasites of fish, frogs, birds, and mammals across North America. Sequences from adult Apharyngostrigea cornu, Hysteromorpha triloba, and Alaria mustelae (Diplostomoidea: Digenea) from definitive hosts matched those of meta- and mesocercariae from fish and frogs. These data provided new information on the distributions of all 3 parasite species. Metacercariae of A. cornu, which have not been previously reported in North American hosts, were found in Notemigonus crysoleucas, Pimephales notatus, and Catostomus commersonii in the St. Lawrence River. Metacercariae of H. triloba are reported in Canadian waters and in N. hudsonius for the first time. Alaria mustelae is reported for the first time in frogs from Quebec, Canada, and an additional species of Alaria was detected in California. Sequences of internal transcribed spacer rDNA from a subset of specimens support the same species boundaries indicated by CO1 divergence. There was little divergence in CO1 sequences from an unidentified diplostomid species sampled at a large spatial scale.

Sulfur isotopes link overwinter habitat use and breeding condition in Double-crested Cormorants

North American Double-crested Cormorant (Phalacrocorax auritus) populations have increased greatly. Both breeding and overwintering ground factors have likely contributed to these increases. However, demonstrating how overwintering conditions may affect breeding birds has not been possible because of the difficulty in linking breeding birds to their wintering grounds. Here, we demonstrate the utility of stable sulfur isotopes to elucidate overwintering habitat use by cormorants breeding on Lake Erie. Sulfur isotopes in feathers grown on overwintering grounds provided insights into the degree to which birds used freshwater vs. marine environments. The proportion of birds utilizing freshwater habitats increased through time. This change may have reflected increases in freshwater aquaculture (i.e., catfish) in the U.S. south. Examination of body condition in birds returning to breed on Lake Erie indicated that those individuals that solely used marine habitats for at least a portion of the overwintering period were in poorer condition than birds using freshwater. Enhanced foraging opportunities at aquaculture facilities may improve the fitness of individuals that have returned to breed after overwintering at such locations. This study is the first to demonstrate a linkage between overwinter habitat use and breeding ground parameters in Double-crested Cormorants. These results underscore that factors throughout the Mississippi flyway are likely acting together to regulate cormorant populations.

Effect of In Ovo exposure to an organochlorine mixture extracted from double crested cormorant eggs (Phalacrocorax auritus) and PCB 126 on immune function of juvenile chickens

Organochlorine (OC) contaminants including polychlorinated biphenyls (PCBs) and p, p'-dichlorodiphenyldichloroethylene (DDE) have been associated with immune modulation in wild fish-eating birds from the Great Lakes. The objective of this study was to evaluate the immune function of juvenile chickens after in ovo exposure to PCB 126 or an environmentally relevant OC mixture extracted from eggs of double crested cormorants (Phalacrocorax auritus) from Green Bay, Lake Michigan, USA. Fertile white leghorn chicken (Gallus domesticus) eggs were injected before incubation with 0.55-1.79 ng TCDD equivalents (TEQ)/egg PCB 126 and 1.2-4.9 ng TEQs/egg of cormorant egg extract into the air cell in two separate experiments. After hatching, the immune function was tested using in vivo phytohemagglutinin (PHA) skin response in 11-day-old chicks, antibody titers to immunization with sheep red blood cells (SRBC) in 28-day-old chicks, and, at necropsy, thymus and bursal mass and cellularity. PCB 126 decreased antibody titers at all doses and decreased the thymus and bursa index but not cellularity at 1.79 ng TEQ/egg. The cormorant egg extract caused no significant alterations in immune function even though it has been demonstrated as immunotoxic in chicken embryos. However, twofold to threefold increases in total anti-SRBC titers in 28-day-old chicks exposed to 1.2 or 2.4 ng TEQ/egg of cormorant extract were similar to elevations in anti-SRBC titer observed in Caspian tern (Sterna caspia) chicks from a highly OC-contaminated site in Saginaw Bay, Lake Huron. Posthatch exposure to OC through fish consumption in addition to in ovo OC exposure might be associated with the immune modulation reported in wild birds. Chicks in this study might have begun to compensate for embryonic immunotoxicity by the ages at which we studied them.

Density-dependent growth of double-crested cormorant colonies on Lake Huron

By analyzing 20+ years of data, we found that the nesting colonies of double-crested cormorants ( Phalacrocorax auritus (Lesson, 1831)) in the North Channel and Georgian Bay of Lake Huron exhibit density-dependent population regulation. This conclusion is based on four lines of evidence. First, a time series of nest counts at specific colonies (1979–2001) showed density-dependent growth based on randomization tests of the time series. Second, the per capita rate of change in colony size declined with increasing colony size over a 10-year period. Third, a Ricker model of aggregate nest counts showed that population growth of nesting double-crested cormorants stabilized in recent years (through 2003), with K, the carrying capacity parameter, being 11 445 nests in the North Channel and 10 815 nests in Georgian Bay. Fourth, a colony area index showed near complete coverage of coastal areas by adult nesters coinciding with overall declines in population growth. High rates of population increase of double-crested cormorants on Lake Huron have largely come to an end, but changes in fish abundance may result in changes in carrying capacity.

An assessment of PCBs and OC pesticides in eggs of double-crested (Phalacrocorax auritus) and Pelagic (P. pelagicus) cormorants from the west coast of Canada, 1970 to 2002

Eggs of double-crested and pelagic cormorants were collected between 1970 and 2002 from colonies in the Strait of Georgia, BC, Canada, and assayed for concentrations of organochlorine (OC) pesticides and polychlorinated biphenyls (PCBs). Double-crested cormorant eggs from the early 1970's contained up to 4.1 mg kg-1 p,p'-DDE and 12.5 mg kg-1 sigmaPCBs. Corresponding values for pelagic cormorant eggs were 1.5 mg kg-1 p,p'-DDE and 3.9 mg kg-1 sigmaPCBs. Egg tissue concentrations of the dominant OC pesticides and sigmaPCBs dropped mainly during the 1970's, with minor declines thereafter. The data suggest that contaminant levels in cormorants have now stabilized at low levels throughout the resident population. Small but significant latitudinal gradients in several OC pesticides and PCBs indicated that areas of the southern strait were more contaminated than areas of the less populated northern strait. Interspecific differences in contamination may indicate that pelagic cormorants have a reduced capacity to metabolize chlordanes, DDT and PCBs compared to double-crested cormorants. Alternatively, the two species may have more divergent prey bases than previously thought. During the 1980's, TCDD toxic equivalents, largely contributed by polychlorinated dibenzo-p-dioxins (PCDDs) and dibenzofurans (PCDFs), were correlated with physiological and biochemical alterations. Also, from 1989 to 1990, four deformed cormorant chicks (two of each species) were found during nest visits; none were found between 1991 and 1995. The relative rates of deformed chicks were 6 per 10,000 for double-crested and 16 per 10,000 for pelagic cormorants. The findings of deformed chicks were coincident with the period of highest PCDD and PCDF contamination; however, the sample sizes were too low to derive a substantive connection. Colony-wide productivity of double-crested cormorants was poorer in the southern colonies where PCBs in particular were elevated. While of concern, these lines of evidence are insufficient to conclude that chlorinated hydrocarbon contamination was a dominant contributor to population declines. It seems more probable that ecological variables, particularly changing prey and predator dynamics, drove the reductions in population size.

Estimation of effects of dioxins and dioxin-like PCBs on wildlife population--a case study on common cormorant

We presented a method for quantitatively evaluating the effects of chemical pollutants in the environment on a wildlife population. We expressed the effects of exposure to dioxins and dioxin-like PCBs in Tokyo Bay sediment on a common cormorant (Phalacrocorax carbo) population in two ways. One was the changes in the intrinsic growth rate, and the other was the changes in the gross population size. The effects of exposure to the compounds were estimated by using the method of population ecology and available field data. Common cormorant population at Shinobazu Pond in Tokyo, Japan during 1974-1986 was selected as the target population. Intrinsic growth rate or gross size of the population based on the calculated residual level of dioxins and dioxin-like PCBs in the period was estimated to decrease to 89% or 85% of that without exposure to the compounds, respectively.

Allochthonous inputs: integrating population changes and food-web dynamics

Most ecosystems are recipients of allochthonous materials that enhance in situ productivity. Recent theoretical and empirical studies suggest that low to moderate inputs can stabilize food webs. However, depending on the trophic levels that use the resource, food webs can become unstable as inputs increase. Where large amounts of agricultural resources are transferred to natural habitats, trophic dynamics change: trophic cascades can occur and rare or uncommon species can become invasive. Rates of change in species abundances can also be amplified by the effects of changes in legislation and management practices on subsidized consumers.