Wilson, et al. 2016. Southern right whale (Eubalaena australis) calf mortality at Peninsula Valdes, Argentina: are harmful algal blooms to blame? Marine Mammal Science: 32(2): 423-451. DOI: 10.1111/mms.12263
Trouble in Argentina
There’s something wrong at Peninsula Valdes, an important calving ground for southern right whales off the coast of Argentina.
Since 2005, whales have been dying in record numbers (view an interactive chart of whale moralities here), and no one can explain why. Most mysteriously, 90% of the mortalities have been young calves less than 3 months of age.
The whale deaths off the Patagonian coast are unusual for a few reasons. To start, such mass mortalities are rare in baleen whales. They become even more unusual when we consider that the deaths are spread throughout the entire breeding season (most mass mortalities cluster together in time) and recur for multiple years in a row. The fact that the mortalities are so heavily biased against newborn calves complicates the mystery
Determined to solve the puzzle, the International Whaling Commission conducted a workshop in 2010 (read the full report here) and proposed the following potential causes (1) a decrease in food abundance, (2) biotoxins produced by harmful algal blooms, and (3) infectious diseases.
Suspect #1: Algae
Knowing Peninsula Valdes has a history of harmful algal blooms, a team of scientists investigated if biotoxins contributed to whale mortalities. They analyzed the level of biotoxins in tissue collected from dead whales, and compared the timing of whale deaths to the timing of known harmful algal blooms and shellfishery closures. They also looked at whether whales could be exposed to biotoxins through their diet.
Trace amounts of biotoxins
A logical first step was to measure the levels of biotoxins, such as paralytic shellfish toxins and domoic acid, in the tissues of dead whales. Unfortunately, the authors did not find the conclusive, smoking gun they perhaps had hoped for – detectable levels of paralytic shellfish toxins and domoic acid were only present in 4% and 2%, respectively, of the analyzed samples.
Being unable to detect biotoxins in tissue samples does not necessarily mean that the whales were never exposed, or that it is not a contributing factor of the calf mortalities. Domoic acid levels are known to vary widely between individual algal cells and species. The toxicity of many biotoxins is also strongly influenced by environmental factors, such as upwelling events that can disturb algal blooms hidden below the surface, which would not have been captured in this study. The toxins may have also caused long-lasting damage to calves during their mothers’ pregnancies that lead to death soon after birth. Finally, toxins used by aquatic organisms dissolve easily in water, meaning they are quickly eliminated through the waste excretion system. For the same reason, they can leech out of the tissues of dead animals long before scientists may be able to find and test the carcass.
Syncing up mortalities, blooms, & closures
The next step was to see if the dates of known whale mortalities correlated with days when algae levels were high, and the dates when harvesting from shellfish beds were prohibited due to dangerous levels of paralytic shellfish toxins.
Algal abundance was historically low, but gradually increased from 2000 to 2012. The highest risk level was reach around 2007, and it did not decline until 2013 and 2014. Interestingly, this roughly follows the annual record whale deaths – years with particularly high mortality rates (>50 whales) occurred from 2007 to 2013, followed by a sharp decline in 2014 (only 23 whales).
So there was a positive relationship between whale mortality rates and the amount of algae present off the coast.
Shellfish closures regularly occur in September or October, and continue through the Southern Hemisphere summer to February or March. They are triggered when regulatory authorities detect high levels of paralytic shellfish toxins in shellfish harvests. From 2003 to 2012, the same window when algal abundance was steadily increasing year to year, the shellfish closures began earlier and earlier each year. And, in 2013 and 2014, when algal abundance dropped, the fisheries were able to remain open until later in the season.
Do whales eat biotoxins?
Aside from swimming through algal blooms or biotoxin-laced water, whales may also be exposed directly by swallowing algae, or a calf may be indirectly exposed through the milk of its exposed mother. To get at what whales were ingesting, the authors also looked at urine and fecal samples from both living and dead whales for zooplankton (a favourite snack of baleen whales). Interestingly, the zooplankton collected from these samples contained the remains of organisms known to produce biotoxins, suggesting whales may have also been exposed to biotoxins through their diet.
Correlation is not causation
An important caveat of this study is that all the findings are correlations, and that just because two things follow the same pattern does not mean that there is a cause-and-effect relationship (click here to see some funny graphs that drive this point home!). This means that while algal blooms may seem to contribute to calf mortalities, we can’t ignore other possibilities, especially since we can’t test experimentally whether algal blooms kill whale calves.
Algal blooms & ecosystem health
Harmful algal blooms are a global problem, causing fish die-offs, contaminating water supplies, and forcing the closure of beaches and fisheries. While some blooms seem to occur naturally, the severity and frequency of blooms is increasing due to human impacts, like fertilizer run-off in lakes and oceans and climate change. While the cause of mass mortalities off the Patagonian coast are still a mystery, we may be a step closer to understanding where Peninsula Valdes has gone wrong and, maybe, how to protect future generations of southern right whales.