Unweathered crude and refined oils are known to be very toxic to the embryos of aquatic birds (ALBERS 1977, 19781 SEARO et ale 1978, WHITE et al. 1979, McGILL & RICHMOND 1979, and others) but the toxicity of weathered petroleum is not as well established. The toxicity of Prudhoe Bay crude oil and No. 2 fuel oil to embryos of mallard ducks (Anas platyrhynchos) decreased only after 3 weeks and 2 weeks of weathering on a large container of fresh water (SZARO et al. 1980). Ten microliters (uL) of 4 week-old Libyan crude oil caused a significant increase in mortality among embryos of Louisiana herons (Hydranassa tricolor) but the same amount of fresh oil did not cause a significant increase in mortality. However, embryos of laughing gulls (Larus atricilla) were not significantly affected by I0 uL of 4 or 8 week-old weathered Libyan crude oil nor i0 UL of fresh oil (MACKO& KING 1980). Crude ell (i0 uL) recovered from the water surface (age unknown, but presumably slightly weathered) near the IXTOC-I oil spill site in the Gulf of Mexico did not significantly reduce the survival of mallard embryos by day 18 of incubation (D. HOFFMAN, unpublished data).
