Castle Rock Refuge is half a mile offshore from Crescent City in northern California. This coastal rock covers approximately 14 acres, and rises steeply 335 feet above sea level. The refuge provides an important sanctuary for Aleutian Canada geese and nesting seabirds. It is, in fact, known as the second largest nesting seabird colony south of Alaska (after the Farallon Islands). The estimated number of seabirds using Castle Rock has been as high as 150,000. It also has the largest breeding population of common murres in California; the latest estimate for common murres was 98,000 in 1986.

Over 21,000 Aleutian Canada geese roost on the island, flying off at dawn to feed in adjacent agricultural lands, returning in the evening. This growing population appears to be impacting the habitat suitability for seabird nesting.

Castle Rock Refuge also serves as a resting place for four species of pinnipeds; harbor seals, northern elephant seals, California sea lions, and Stellar's sea lions.