Degree of Threat: C : Not very threatened throughout its range, communities often provide natural resources that when exploited alter the composition and structure over the short-term, or communities are self-protecting because they are unsuitable for other uses
Comments: PESTICIDES: DDE, DDT, PCB, EDDT, dieldrin, oxychlordane, heptachloroepoxide, mercury, selenium, PCDDs, PCDFs, and EPCB found in eggs; not correlated with eggshell thining or mortality (Wallace and Wallace 1998). Chick with deformed bill typical of high concentrations of pollutants found on Farallon Island, California (Hobson and Carter 1988, cited in Wallace and Wallace 1998). Regional variation of DDT and DDE residue levels found in prey. Deaths of some captive birds were attributed to high concentrations of DDT in food source. OILING: Small numbers died after oil spills. Oiling damages plumage that results in hypothermia or waterlogging. Ingested oil has variety of lethal and non-lethal effects. Over 50 percent of recovered birds 1971-1985 were suspected to have died of oiling. Numbers may be higher due to tendency of this species to sink. FISHING NETS: Mortality from coastal fishing nets may have exceeded sustainable levels in Monterey, California in early 1980s (King 1984 cited in Wallace and Wallace 1998). HABITAT: Local declines or disappearance due to loss of habitat. DISTURBANCE: Vulnerable to disturbance by boats, low-flying aircraft, dogs, or humans especially during incubation period. Mortality has resulted from military bombing practice and missile tests on some California Channel Islands. Flushing adults may break eggs and increase chance of nest predation. Repeated disturbance may cause total colony desertion. Local decline attributed to disturbance caused by collection of common murre (URIA AALGE) eggs from 1850s to 1900s (Ainley and Lewis 1974, cited in Wallace and Wallace). Incidental injury and death reported by sportsfisherman (Wallace and Wallace 1998). PREDATION: Nest predators include western gull (LARUS OCCIDENTALIS), Heerman's gull (LARUS HERMANNI), northwestern crow (CORVUS CAURINUS), common raven (CORVUS CORAX). Western gull may prey on deserted eggs or young. Rats (RATTUS spp.) known to take eggs on Isla San Esteban (Wallace and Wallace 1998). EL NINO-SOUTHERN OSCILLATION (ENSO): Reduced reproductive output during and after periods of ENSO. ENSO occurs when warm waters prevail causing food levels to decline. Will abandon nest sites mid-season if food sources decline. In 1983, all nests were abandoned at two Oregon colonies and at least 49 percent of nests failed or had reduced fledgling success at another colony (Hodder and Graybill 1985, cited in Wallace and Wallace 1998). Periods of high mortality also attributed to food shortages related to ENSO. Additional reported deaths due to poisoning after ingesting fish affected by red tide (the dinoflagellate GYMNODINIUM BREVE) or northern anchovies containing valves of the diatom PSEUDONITZSCHIA AUSTRALIS. Also, one death reported to lodging of a plainfin midshipman in the throat. COMPETITION: Expanding fisheries industry and increase in the California Sea Lion (ZALOPHUS CALIFORNIANUS) populations may be suppressing recovery from effects of 1982-1983 EL NINO SOUTHERN OCCILATION (ENSO). Expansion of groundfish fisheries may reduce food availability to cormorants (Wallace and Wallace 1998). PARASITISM: Not known to occur.