The fish assemblages associated with asphalt volcanoes in the Santa Barbara Channel, California, USA

Asphalt volcanoes and other extensive hard tar seafloor accumulations are known from the Gulf of Mexico, off Angola, and in the Santa Barbara Channel (SBC), southern California. There are two major volcano features in the SBC: Il Duomo (about 20 m high, cresting at a bottom depth of 115 m) and Il Duomito (about 15 m high, cresting at 185 m). While the geology of these features has been well studied, their fish assemblages have not been well documented. Using an autonomous underwater vehicle, we surveyed the fishes that live on the volcanoes and on the surrounding mud. We observed 1836 fishes, which represented a minimum of 43 species. Of these species, at least 23 (53.5%) were of rockfishes (genus Sebastes). Most of the species were entirely or primarily associated with a specific habitat type, either asphalt or mud. The most abundant primarily asphalt-associated species were greenblotched, swordspine, greenspotted, and flag rockfishes, and bocaccio, cowcod, and bank rockfish. The most abundant mud-associated species included Pacific sanddab, slender, Dover, and English soles, and unidentified poachers. A few taxa (shortspine combfish, greenstriped rockfish, and spotted ratfish) were found both on the mud and over the edges of the asphalt. We observed relatively low densities of all fishes on the volcanoes compared to densities on many southern California natural reefs within the same depth range. We suggest that this is due to (1) a lack of cobble fields around the volcanoes, creating limited habitat for either dwarf species or juvenile fishes, (2) a lack of complex habitat (volcanoes are relatively smooth) creating very limited habitat for adults that need to shelter, and (3) fishing pressure targeting economically important species. Given the unique nature of these tar volcanoes off California, we also suggest the state consider protecting this habitat and the organisms within it.