Organochlorine and metal contaminant exposure and effects in hatching black-crowned night herons (Nycticorax nycticorax) in Delaware Bay

Pea Patch Island in Delaware Bay is the site of the largest heronry north o f Florida. From 1989-93, the popu lation of nine species of wading birds nu mbered approximately 12,000 pairs, but has recently declined to about 7,000 pairs. Because Delaware Bay is a major shipping channel and receives anthr opogenic releases of toxic substances from agricultural, industrial, and mu nicipal point and nonpoint sources, contaminant exposure and effects to the heronry have been an ongoing concern. In 1997, pipping (early hatching sta ge) black-crowned night herons (Nycticorax nycticorax) were collected from separate nests at Pea Patch Island and from a coastal reference site, Middl e Island in Rehoboth Bay, Delaware. There was no evidence of malformations or hepatic histopathological lesions in embryos, and their body and liver w eights did not differ between sites. Biomarkers of petroleum hydrocarbons, polyhalogenated contaminant, and metal exposure (cytochrome P450 induction and oxidative stress responses) did not differ (p > 0.05) between sites, al though activities of benzyloxy-O-dealkylase and ethoxyresorufin-O-dealkylas e were somewhat elevated in 3 of the 15 embryos collected from Pea Patch Is land. Concentrations of 21 organochlorine pesticides and metabolites were r elatively low at both sites, with p,p'-DDE values well below the threshold associated with eggshell thinning. Although total PCB concentration was mod estly elevated (p < 0.05) in Pea Patch Island heron embryos, levels of aryl hydrocarbon receptor-active PCB congeners, polychlorinated dibenzodioxins a nd dibenzofurans, and toxic equivalents were low and did not differ between sites. Concentrations of Cd and Mn in pipping embryos from Pea Patch Islan d were slightly greater (p < 0.05) than values observed in Middle Island em bryos, but levels of these and the other metals and metalloids (e.g., Hg an d Se) were below values associated with toxicity. In conclusion, it seems u nlikely that chlorinated hydrocarbon and metal contaminant exposure constit utes a direct threat to the reproductive success of black-crowned night her ons at Pea Patch Island. However, low-level exposure to these contaminants may constitute one of many stressors that in combination could adversely af fect the stability of the wading bird population at this large heronry.