Population trends, reproductive success, and organochlorine chemical contaminants in waterbirds nesting in Galveston Bay, Texas
The effects of environmental contaminants on the reproductive success of olivaceous cormorants (Phalacrocorax olivaceus), Laughing gulls (Larus atricilla), and black skimmers (Rhynchops niger) nesting in Galveston Bay, Texas were investigated from 1980 through 1982. Populations of cormorants and gulls have remained stable in recent years, but skimmer numbers may have declined. Cormorants produced 1.9 to 2.8 young per pair in nests that remained active throughout the season. Gull and skimmer reproduction was seriously limited by storms and predation. DDE and polychlorinated biphenyl (PCB) residues in carcasses and eggs generally were below levels associated with chronic poisoning and reproductive problems in most species of birds. Possible exceptions were four black skimmer carcasses with DDE concentrations that range from 20 to 92 microgram/g and one olivaceous cormorant carcass with 20 microgram/g DDE. The generally low levels of dieldrin, chlordane, HCB, toxaphene, and heptachlor epoxide were below those know to affect avian reproduction and survival. Mean DDE residues in cormorant and gull eggs were less than 1.8 microgram/g. The mean DDE residue in skimmer eggs was less than 3.3 microgram/g: 12% of the skimmer eggs contained at least 12 microgram/g DDE, a level known to cause reproductive problems in some avian species. DDE in the eggs of all species declined from 1.5- to 7-fold over the past decade. Polychlorinated biphenyl was detected in all gull and skimmer eggs and in 90% of the cormorant eggs. Residues of PCB in cormorant and skimmer eggs in 1980- 82 were lower than in 1970, but the frequency of occurrence in gull eggs increased. Polychlorinated styrens were recovered in 45% of the cormorant carcasses at levels below 1 microgram/g. Eggshell thicknesses in all three species were similar to those of eggshells collected before the widespread use of DDT.