Evidence of domoic acid exposure in harbour seals from Scotland: a potential factor in the decline in abundance?

The exposure of marine mammals to the toxins associated with harmful algae can be lethal. Domoic acid
(DA) is a biotoxin produced by the Pseudo-nitzschia group of diatoms many of which are now a common
component of the Scottish phytoplankton community (Stobo et al., 2008). DA is a potent excitatory
neurotoxin that has caused large-scale mortality of marine mammals. We found harbour seals (Phoca
vitulina) in Scotland are exposed to DA. Low levels, likely from recent exposure, were measured in the
faeces and urine of live captured adult animals (using a direct competitive enzyme linked
immunosorbent assay) and exposure was highest during August–September 2008 (7/32 of the faecal
(22%) and 11/29 (38%) of the urine samples were positive). Median concentrations in positive faeces and
urine were 25 ng/g and 6 ng/ml respectively. One positive pregnant female was subsequently found dead
with 10 ng/ml DA in her amniotic fluid but the contribution of DA exposure to the cause of death could
not be established. However, the highest levels in the study were found in anonymous faecal samples
collected in September 2009 on the east coast of Scotland (up to 397 ng/g). Further studies are urgently
needed to determine the importance of DA exposure to the population dynamics of Scottish harbour
seals in light of the recently reported major population declines.