Peter J. Ewins

Characteristics of a Rapidly Increasing Colony of Double-Crested Cormorants (Phalacrocorax auritus) in Lake Ontario: Population Size, Reproductive Parameters and Band Recoveries

The double-crested cormorant (Phalacrocorax auritus) is now a prominent component of the Great Lakes ecosystem. Lake Ontario supported 24% of the breeding population in 1991, and the largest colony—5,428 nests on Little Galloo Island (LGI). Increases at LGI averaged 36% per annum since colonization in 1974. This dramatic increase is attributed to three main factors: 1) reduced levels of organochlorine contaminants in the diet, particularly DDE which caused eggshell thinning, leading to egg breakage and total reproductive failure; 2) relaxation of human persecution, which kept overall Great Lakes cormorant numbers low earlier this century; and 3) increased availability of forage-base fish, particularly alewife (Alosa pseudoharengus). Cormorant increases at LGI since 1983 were correlated significantly with abundance indices of sexually mature alewife three years previously. Most cormorants first breed when 3 years old, and alewife are important in the diet in Lake Ontario. Since the DDT-era, alewife availability has probably been the key factor affecting cormorant reproductive output, fledging condition, and post-fledging survival on Lake Ontario, and hence subsequent recruitment rates. LGI cormorants winter along Atlantic coasts of the southeastern U.S. and in the lower Mississippi Valley. Improved over-winter survival due to exploitation of farmed channel catfish (Ictalurus punctatus), may also have increased recruitment rates of cormorants.

Geographical distribution of organochlorine contaminants and reproductive parameters in Herring Gulls on Lake Superior in 1983

As part of the Great Lakes International Surveillance Plan, 1978–83, egg contaminant levels and reproductive output were determined for Herring Gull colonies on Lake Superior in 1983. Since 1974, the Herring Gull has been widely used in the Great Lakes as a spatial and temporal monitor of organochlorine (OC) contaminant levels and associated biological effects. Most eggs contained a wide range of OCs, the main compounds being DDE, polychlorinated biphenyls (PCBs), dieldrin, heptachlor epoxide, oxychlordane, hexachlorobenzene and mirex. Levels of an additional ten OCs and five polychlorinated dibenzo-p-dioxin (PCDD) congeners were also determined for some sites. Overall, levels varied significantly among colonies, but there was no obvious relationship to spatial distribution of contaminants in sediments or fish species. OC levels in eggs had declined by up to 84% since 1974. Eggshells were only 8% thinner than before the introduction of DDT, and shell thinning was not a cause of breeding failure. Average reproductive output varied from 0.15 to 1.57 young per apparently occupied nest in 1983: at 56% of colonies the value was below that thought necessary to maintain stable populations. The main causes of failure were egg disappearence and cannibalism of chicks. Despite this, the population appeared to have been increasing at about 4% per annum. Reduced availability of forage fish during the early 1980s was the most likely reason for the poor reproductive output in 1983.

Geographical Distribution of Contaminants and Productivity Measures of Herring Gulls in the Great Lakes: Lake Huron 1980

Abstract The concentrations of DDE, PCBs (1:1 mix of Aroclor 1254:1260), mirex, HCB, QCB, and tetrachlorobenzenes in eggs of herring gulls are presented for nine colonies in Lake Huron in 1980, as part of intensive investigations of herring gull numbers, biology, and contaminant burdens in that year. Additional data are given for seven other organochlorine contaminants at two colonies. These data are analyzed in relation to eggshell thickness, clutch size and reproductive output, and population trends at sample colonies. Levels of all contaminants measured were markedly lower than during the 1970s, and eggshells were only 6% thinner on average than during the pre-DDT era. Levels of most organochlorine (OC) residues were highest in eggs from Saginaw Bay, which remains one of the most heavily contaminated sites on the Great Lakes. Discriminant function analyses provided reasonably good regional, but not colony-specific, separation of eggs based on suites of OCs. Reproductive output was relatively high, and we found little evidence of impaired reproduction attributable to OC contamination or burden in 1980. Lake-wide, herring gull numbers were increasing slightly, but decreasing at colonies on the main body of Lake Huron.

Organochlorine contaminant residues and shell thickness of eggs from known-age female ospreys (Pandion haliaetus) in Michigan during the 1980s

Eggs have been widely used as indicators of exposure to lipophilic contaminants in wild birds, but very few studies have examined the relationship between female age and egg contaminant concentrations. Organochlorine contaminant (OC) levels and eggshell thickness were determined for 25 addled eggs laid by female ospreys (Pandion haliaetus), 3–15 years old, in Michigan during the 1980s. A wide range of OCs was detected, but there was no significant variation in wet weight concentrations of any compound with female age or year. Similarly, eggshell thickness did not vary significantly with female age or year. Among PCB congeners, neither the degree nor the positioning of chlorine substitution on the biphenyl ring, nor the degree of hydrophobicity, had any significant influence on contaminant bioaccumulation as birds aged. Eggs from females sampled in more than one year indicated considerable annual variation in OC levels, presumably reflecting differences in recent exposure and/or the extent to which endogenous lipid reserves were used to form the egg. In general, OC concentrations in these female ospreys during the 1980s appeared to have reached a life-time equilibrium level by the age of first breeding (3–4 years usually). Therefore, eggs from any female osprey can provide a consistent indication of OC uptake, independent of the birds age.

Effects on Productivity of Shooting of Double-Crested Cormorants (Phalacrocorax auritus) on Pigeon Island, Lake Ontario, in 1993

Abstract At least 50 adult double-crested cormorants ( Phalacrocorax auritus ) were shot illegally in early June 1993 at a colony of 818 pairs on Pigeon Island, in eastern Lake Ontario. This constitutes the first documented case of killing of cormorants by humans in the Canadian waters of the lower Great Lakes since cormorant populations began to recover from the effects of organochlorine contaminants. Some birds had been shot at the nest, but others probably fell into the water and were not counted. Disturbance caused by the shooting undoubtedly resulted in additional losses of cormorant eggs and small chicks to predation by gulls (Laridae). Numbers of apparently occupied nests fell by 30% (from 818 to 570) after the shooting incident. Overall reproductive output was less than 0.3 young per apparently occupied nest, compared with a more typical value of 1.6 from a neighbouring island whose cormorants were not impacted. Further, 90% of the reproductive potential was lost at this colony in 1993, compared to only 35% at a nearby reference colony. The shooting resulted in the death of an estimated 3-6% of the adult cormorants at this colony, where breeding numbers have been increasing at an average annual rate of 36% since 1978.