Domoic acid in planktivorous fish in relation to toxic Pseudo-nitzschia cell densities

In at least two mass mortality events in Monterey Bay, California, planktivorous fish were implicated as vectors of the neurotoxin domoic acid (DA) from diatoms to sea birds and marine mammals. Whereas the transfer of DA from planktivorous fish to piscivorous predators has been well established, the relationship between toxin levels in plankton-feeding fish and the regional abundance of DA-producing diatoms has not been documented. Here we present data from an extensive field study in which cell densities of toxic Pseudo-nitzschia species and DA levels in anchovies and sardines were measured from samples collected weekly throughout Monterey Bay from 8 October 1999 to 18 October 2000. Four distinct blooms were documented with cell densities ranging from 3.2×103 to 5.0×105 cells l–1. DA was detected in fish viscera samples whenever toxic diatom densities reached ≥103 cells l–1 in surface waters, suggesting that anchovies and sardines regularly consume toxic diatoms when present. Fish contained DA levels above the regulatory limit (20 µg DA g–1 whole fish) only when toxic cell densities exceeded 104 cells l–1. DA was only detected in fish when toxic diatom species were also present in the water, suggesting that the toxin is quickly depurated and that fish are only dangerous vectors during the bloom period. Anchovies appear to be more potent vectors than sardines as they consistently contained more DA than sardines collected simultaneously. Maximum DA levels detected in fish were 1,815 µg DA g–1 in anchovy and 728 µg DA g–1 in sardine viscera samples. In fish with high viscera levels of DA, corresponding body tissues contained 0.2–2.2 µg DA g–1 (0.2±0.1% of the viscera level), suggesting that DA is not accumulated in edible body tissues to levels that threaten human consumers. Results from this study suggest that anchovies may be a valuable indicator species for assessing the risk of DA intoxication to piscivorous sea birds and mammals during toxic Pseudo-nitzschia blooms in Monterey Bay.