Organochlorine Concentrations, Eggshell Thickness, and Hatchability in Seabirds off Central California

Abstract

-We measured eggshell thickness in seven species of seabirds breeding at Southeast Farallon Island (SEFI) and Afiio Nuevo Island, off the central California coast, in 1993. Eggshell measurements of the Common Murre (Uria aalge) from 1993 were significantly thicker than those of eggshells from SEFI measured in the early 1970s (by 10.9%), but thinner than those of eggshells from SEFI measured before 1947 (by 3.2%). In five species, eggshell thickness measurements at SEFI in 1993 were thinner than those of eggs from Oregon collected in 1979 (by 0.3 to 5.8%). In four species, Brandt's Cormorant (Phalacrocorax penicillatus), Common Murre, Pigeon Guillemot (Cepphus columba) and Rhinoceros Auklet (Cerorhinca monocerata), we measured levels of six chlorinated hydrocarbons in the eggs and related them to eggshell thickness. After adjusting for the effects of other compounds, all six compounds had negative correlations with eggshell thickness when species were pooled, although only for oxychlordane was the correlation significant. Oxychlordane also had significant negative effects on eggshell thickness in Common Murre and Rhinoceros Auklet, after adjusting for the effects of other compounds, although the small levels of this and the other compounds in the seabird eggs may negate the biological meaning of these results. No other compounds (including DDE and PCB) had significant negative correlations. Hatching success of six species at SEFI showed no significant changes during the period 1971-1994. Based on these results we conclude that organochlorine contamination and eggshell thinning do not currently pose a problem for seabirds off the central California coast. Received 5 September 1998, accepted 26 August 1999.