Organochlorine concentrations, eggshell thickness, and hatchability in seabirds off central California

We measured eggshell thickness in seven species of seabirds breeding at Sou theast Farallon island (SEFI) and Ano Nuevo Island, off the central Califor nia coast, in 1993. Eggshell measurements of the Common Murre (Uria aalge) from 1993 were significantly thicker than those of eggshells from SEFI meas ured in the early 1970s (by 10.9%), but thinner than those of eggshells fro m 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 co llected in 1979 (by 0.3 to 5.8%). In four species, Brandt's Cormorant (Phal acrocorax penicillatus), Common Murre, Pigeon Guillemot (Cepphus columba) a nd Rhinoceros Auklet (Cerorhinca monocerata),we measured levels of six chlo rinated hydrocarbons in the eggs and related them to eggshell thickness. Af ter adjusting for the effects of other compounds, all six compounds had neg ative correlations with eggshell thickness when species were pooled, althou gh 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, alt hough the small levels of this and the other compounds in the seabird eggs may negate the biological meaning of these results. No other compounds (inc luding DDE and PCB) had significant negative correlations. Hatching success of six species at SEFI showed no significant changes during the period 197 1-1994. Based on these results wt conclude that organochlorine contaminatio n and eggshell thinning do not currently pose a problem for seabirds off th e central California coast.