Red-pelaged harbour seals, in Humboldt County, California: why some seals rust

In this paper we present data on the occurrence of red-pelaged harbour seal s, Phoca vitulina, at a haul-out in Humboldt County, California. Density of red seals ranged from 4% to 17.5% (n = 21) and was comparable to that repo rted for San Francisco Bay, being the highest documented worldwide. The red colouration is the result of natural inorganic iron oxide/hydroxide pigmen ts (limonite), which are present in small patches on the beach sand of the seals haul-out at the mouth of the Mad River. Thorough analysis of the rive r water and ocean water excluded these as possible sources for the iron oxi de/hydroxide pigments. We found that the manmade reinforcement of the river bank adjacent to the haul-out consisted of greenshist boulders with a very high iron content. Weathering of these rocks produces red iron oxide/hydro xide particles which are then redeposited by rain or river action in small patches across the haul-out. Seals receive their red colouration through di rect physical contact with these patches. SEM examination showed that these particles can adhere lastingly to the seals' hair. Other studies provide r eason for concern that this situation may detrimentally impact the seals' h ealth, (C) 1999 Elsevier Science Ltd. All rights reserved.