The Castle Rock IBA occupies 27 hectares of land comprised of: bare rock/sand/clay. The IBA is located in the Oregon, Washington, Vancouver Coast and Shelf ecoregion. It is owned and managed as: state, and has the following primary uses: non-recreational fishing.
The Castle Rock IBA contains 9 seabird species and an estimated 121,330 birds. The colony is an IBA for the following species: Common Murre (108,318), Leach's storm-petrel (1,646), Brandt's cormorant (2,490), Western Gull (1,370), Rhinoceros Auklet (1,034), Cassin's auklet (5,638), Pelagic Cormorant (392), Pigeon Guillemot (360), and Tufted Puffin (82).Castle Rock is located 80 miles north of Humboldt Bay and about 20 miles south of the Oregon-California Border. It is half a mile of the coast of Crescent City, California. The island is the second largest in northern California, after south Farallon, ranging between 13-14 acres in area. The highest peak is 235 ft. and the island is part of the Castle Rock Wildlife Refuge. Castle Rock is dominated by cliffs, which rise to 235 feet above sea level and have grassy slopes. Each part of the island is characterized by different geological conditions. Greywacke and shale dominates the rocky east end of the island, while the south and west ends are largely barren cliffs. The north end of the island is a gradual slope, sloping down to the water at about a 30º angle. Not all the island is rock as the western and eastern slopes have soil and the flat portions of the island have rocky yellow-sandy subsoil. Waves have eroded cliffs on the southern side forming large caves. Near the east side a fault collapsed which formed a 100 ft. in diameter pit, which connects to the ocean by cave. The climate on Castle Rock is much like that of Crescent City. It is a Mediterranean climate with moderate temperatures, with heavy precipitation in the winter months and fog. The winds blow from the north and northwest.

Ornithological Summary

The Castle Rock IBA contains 9 seabird species and an estimated 121,330 birds. The colony is an IBA for the following species: Common Murre (108,318), Leach's storm-petrel (1,646), Brandt's cormorant (2,490), Western Gull (1,370), Rhinoceros Auklet (1,034), Cassin's auklet (5,638), Pelagic Cormorant (392), Pigeon Guillemot (360), and Tufted Puffin (82). Castle Rock hosts 9 breeding seabird species including; Leach?s Storm-Petrel, Brandt?s Cormorant, Western Gull, Common Murre, Cassin?s Auklet, Pelagic Cormorant, Pigeon Guillemot, Rhinoceros Auklet, and Double-crested Cormorant. Brown Pelicans roost on Castle Rock during migration. These populations constitute for 19% of California?s seabird population making Castle Rock one of the most important seabird colonies in California. The total number of seabirds averages around 100,000. Castle Rock is one of only five sites in the California Current System that hosts over 100,000 sea bids and is the largest, most diverse and densely populated site in the CCS with about 8,600 birds nesting per acre. Castle Rock supports the largest colony of Common Murre, which make up 90% of the nesting birds on the island and third largest colony of Brandt?s Cormorant in California. With the largest colony of Western Gull north of the Farallon Islands, 2% of California?s statewide population nests here. Castle Rock also has 2% of the statewide Pigeon Guillemot population and almost 50% of the California population of Tufted Puffin.

Conservation Issues

Becoming part of the Humboldt Bay National Wildlife Refuge, the threats to Castle Rock?s seabird populations has been greatly reduced as this limits human influence and destruction. Previously the island was home to introduced herbivores that destroy habitats and nesting sites. Recently however, these herbivores have been removed from the island. Vegetation changes on the island pose a threat to several nesting species, especially storm petrels as it reduced their burrowing habitat. Change is vegetation can also cause soil erosion, which also reduces burrowing habitat. The increase of population of Double-crested cormorant also poses a threat to the other native species on the island. Double-crested cormorant alter the composition of nesting sites and habitats and also compete for nest sites with other species. Because of their negative effect on the island, Double-crested cormorant have become a management problem. Because of Castle Rock?s proximity to the mainland, it is a risk for adverse effects from pollution.

Ownership

The Castle Rock IBA is owned and managed as: state. In 1979, Castle Rock was purchased from the Nature Conservancy. In 1981, Castle Rock was then incorporated into Humboldt Bay National Wildlife Refuge to protect it and close it to public access.

Habitat

The Castle Rock IBA is located in the Oregon, Washington, Vancouver Coast and Shelf ecoregion and contains the following habitat types: bare rock/sand/clay. The colony occupies a total land area of 27 hectares. Castle Rock is the most structurally diverse island off the California coast with over one thousand offshore rocks. The diverse structure includes deep topsoil, vegetated terraces, sheer rocks cliffs, talus slopes, protected sandy beaches and reef habitat. It is this structural diversity that allows Castle Rock to support such a large and diverse group of seabirds. The lush and rich waters surrounding the island also support high diversity of seabirds. The intertidal zones are lush and rich with marine sources, due to the upwelling from the California Current System. Although the vegetative composition has changed in recent years, information is lacking on native plants that historically inhabited the island. Two primary plant species cover the eastern flats or meadows, while the plains with deeper soils are dominated by coastal scrub. The protected parts of rocky areas support a very different species of plants also. The climate is Mediterranean with moderate temperatures, ample rainfall and many foggy days.

Land Use

The Castle Rock IBA is used for: non-recreational fishing. As part of the National Wildlife Refuge, Castle Rock is closed to public access and can only be visited by researchers. Because of bird sensitivity, even researchers presence is severely limited and very few researchers are able to visit Caste Rock.

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