Associations between toxic blooms of Pseudo-nitzschia and sea lion strandings

After the lecture on diseases in marine mammals, I was interested to see what the latest findings were coastal harmful algal blooms (HABs) (including diatoms such as the genus Pseudo-nitzschia) and the production of domoic acid (DA), stemming on from one of the referenced studies (Bargu et al., 2002). The Californian sea lion in Monterey Bay (a coastal area off California) is one of the top predators in this region, but also may be most affected by HABs, as they predate on intoxicated species, which results in them becoming distressed, and hence stranded, along the coastline. Acute and chronic DA toxicosis both result in different neurological and behavioural signs in sea lions, such as seizures and disorientation in acute poisoning cases. Previous literature has shown that these toxic blooms are increasing in abundance and frequency, and simultaneously sea lion strandings are now exceeding previous numbers from Monterey Bay. 

The authors aimed to find associations between the presence of blooms and sea lion strandings along the coastline over the years 2004 to 2007, looking for potential links between acute and chronic DA poisoning, the abundance of toxic cells and concentrations of particulate DA in Californian coast waters, as well as in the faeces of stranded animals. 

DA-producing Pseudo-nitzschia diatoms can be the predominant group within the phytoplankton at certain times during the year, but not always to the level of abundance that typically forms blooms. This study did succeed in determining some correlations between some of the different parameters. For example, the highest concentration of particulate DA in spring 2007 was linked to the highest number of sea lion strandings with acute signs, and to the highest concentration of DA in faeces. Sea lions are most likely being exposed to varying levels of DA in their prey throughout the year, often at sublethal amounts, but because there is sometimes a time lag between the accumulation of toxin in the animals and stranding events, not many links have been made between chronic signs and toxic blooms. The combination of multiple acute cases could contribute to this observed increase in the number of chronic cases. 

All of the sea lions that stranded in the spring of 2007 tended to either be adult or sub-adult males. This could be because the normal pre-set of their breeding season is in May, so the number of males may be higher in Monterey Bay during this time. Therefore it seems like males are the most affected gender, because they are found in the Bay at similar times to the blooms. I wondered when reading this whether the females are less affected because they are in different areas during the same period of time, and if they feed on different, uncontaminated prey species, as they mention that they have a wide distribution of 30-60 km when foraging for prey. 

The clinical signs of low and repeated doses of DA are less well known in younger sea lion classes (pups, yearlings and juveniles) than in adult and sub-adult populations. Although it is unknown why, younger age classes appear to need higher levels of toxin in their systems to exhibit the same signs and symptoms as affected adults, or be detectable in their faeces (82%), so adults tend to be more susceptible to the toxins than juveniles. Perhaps the effects are less obvious, as their nervous systems are less well developed, or they exhibit different clinical signs at a young age that are currently not linked to DA poisoning? In any case, as a result of these findings, it may mean that the number of affected sea lions is an underestimate, due to the lack of recognised signs of poisoning. 

Similarly to Bargu et al. (2002), the authors have looked at a single entity within a food chain than studied the transfer of the toxin between species. It would be interesting to determine the extent to which DA accumulates up this food chain, depending on prey the predators feed on. I think linking studies of species at different trophic levels could give a more holistic view of the negative effects these HABs have on certain food webs. In conclusion, the presence of the toxin-producing cells in Monterey Bay may result in toxins accumulating in local pelagic food webs, but the survivability of intoxicated individuals may vary depend on the duration of the bloom in this region, as well as how much toxin the cells produce. They state that it is crucial to look at the stranding trends related to the season, sex and age class of these animals, so that the true potential impacts of domoic acid on sea lion populations can be quantified and detected. 

Bargu, S., Goldstein, T., Roberts, K., Li, C., and Gulland, F. (2012) Pseudo-nitzschia blooms, domoic acid, and related California sea lion strandings in Monterey Bay, California. Marine Mammal Science, 28 (2): 237-253