Abstract

Early in 2002, an unusual pattern of marine mammal strandings was noted initially in long-beaked common dolphins (Delphinus capensis) and was recognized soon after to also include a large number of California sea lions (Zalophus californianus) primarily along the southern California coastline. Many animals alive before or at the time of stranding exhibited seizures and head weaving, indicating severe neurological disease. Most stranded animals were in good body condition, suggesting the ability to forage until near the time of stranding and therefore an acute course of disease. These observations were compatible with previously reported outbreaks of domoic acid (DA) toxicity in California sea lions (Gulland et al., 2002), but the cetacean strandings added a new level of complexity to the mortality event investigation.

Although relatively few numbers of individual cetacean strandings regularly occur in southern California, the number of strandings for long-beaked common dolphins, short-beaked common dolphins (Delphinus delphis), and bottlenose dolphins (Tursiops truncatus) increased over two fold from a comparison year when the number of strandings was at an expected level. Additionally, strandings of six gray whales (Eschrichtius robustus), five Cuvier's beaked whales (Ziphius cavirostris), four Risso's dolphins (Grampus griseus), and one humpback whale (Megaptera novangliae) were observed in the area. While testing for DA in individual stranded animals was sporadic, DA was detected in urine, fecal, and blood samples from multiple California sea lions, long-beaked common dolphins, and short-beaked common dolphins, as well as one each from Risso's dolphin, Cuvier's beaked whale, gray whale, and humpback whale.

Time series cross correlation coefficients were estimated to test for temporal associations between marine mammal strandings and Pseudo-nitzschia spp. blooms. California sea lion and gray whale strandings presented the highest correlations with blooms two weeks after the peak in bloom activity was detected. This specific pattern in temporal correlations is highly suggestive of DA toxicity as it is consistent with the known Pseudo-nitzschia ecology and DA transmission through the food web. The highest correlations for short-beaked common dolphin and bottlenose dolphin strandings with blooms were observed at the same time as the peak in bloom was detected. In contrast, long-beaked common dolphin strandings had the highest correlations with blooms preceding the peak in bloom by up to four weeks. It is likely that long-beaked common dolphins forage further offshore than bloom activity monitoring sites.

For Cuvier's beaked whales, Risso's dolphins, and humpback whale stranding numbers were not large enough to examine temporal correlations with bloom activity. Detection of DA in these individuals may be an incidental finding, but positive DA test results indicate that exposure can occur in species that do not commonly inhabit and forage in the near shore areas where blooms are most easily detected. Both Cuvier's beaked whales and Risso's dolphins forage in deep pelagic waters mainly feeding on cephalopods (Clarke, 1996). The migratory humpback whale feeds mainly on krill and occasionally on planktivorous fish (Pauly et al., 1998). Although the humpback is thought not to feed during its migration, there are reports of feeding activity near the Baja California breeding grounds (Gendron, 1993). Little is known of the bioaccumulation of DA in many of these prey species, but Pseudo-nitzschia australis frustules have been detected in krill, which provides evidence for its possible role as a vector to humpback whales (Bargu et al., 2002). Our data on these more rarely stranding cetacean species are not adequate to confirm that DA toxicity played a major role in their strandings, but do indicate that exposure to DA has occurred.

Overall, the highest densities of temporally-associated strandings with bloom activity along the southern California coast occurred in Ventura and Los Angeles counties (Figures 1 and 2). After the first detection of blooms in the southern California region, variably high concentrations of DA were also detected in shellfish sampling stations, especially along the coast of Ventura and Los Angeles counties (Langlois, 2002). Extremely high levels of DA were detected briefly in January and late February, but the highest concentrations ever recorded were detected in April, which persisted through June (Langlois, 2002). It is unclear as to why Pseudo-nitzschia blooms may be more toxic or more frequent in this geographic area, but marine mammals may be at highest risk for DA toxicity near Ventura and Los Angeles counties if blooms follow similar geographic and temporal patterns in the future.

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	Figure 1. Geographic distribution of cetacean strandings temporally associated with the peak Pseudo-nitzschia spp. bloom activity along the southern California coast.
	Figure 2. Geographic distribution of California sea lion strandings temporally associated with the peak Pseudo-nitzschia spp. bloom activity along the southern California coast.

References

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