Surprisingly large amounts of food – and thus more fertilizing excretions: Baleen whales eat considerably more marine animals than previously thought, according to a study. According to the researchers’ calculations, the role of sea giants in the marine cycle of matter has so far been underestimated along with the need for food. Because with their nutrient-rich excrement, the baleen whales promote the growth of phytoplankton considerably. The losses from whaling have probably changed the marine ecosystem significantly. A recovery in stocks could thus improve ocean productivity and carbon uptake, the scientists say.
Humpback whale, fin whale … and the super giant: the blue whale: The up to 33 meters long and 200 tons heavy marine mammals are the largest animals that have ever lived on our planet. The giant growth of the baleen whales enables an extremely effective diet: They swim in schools of small prey such as krill and suck them into their gigantic throat pouches. The animal then pushes the contents through its whiskers, which filter the food out of the water like a sieve. One thing is clear: the sea giants devour gigantic quantities of food and, through their excretion, contribute to marine nutrient recycling. But how much do baleen whales actually eat? Assessments of this have so far been based on rather weak data that have not been substantiated experimentally.
To obtain more precise information, the researchers led by Matthew Savoca from Stanford University collected data from 321 baleen whales from seven species that live in the Atlantic, Pacific and southern oceans between 2010 and 2019. For this purpose, the animals were equipped with GPS transmitters and measuring devices that made it possible to draw conclusions about their eating behavior. In addition, the scientists took small boats to the places where the whales were feeding and recorded the areas with echo sounders. This enabled them to recognize and assess the size and density of schools of krill and other prey. Based on the data obtained and the respective body sizes of the whales, the marine biologists then developed the new estimates of their food intake.
Surprisingly big whale appetite
According to this, an adult blue whale in the eastern North Pacific eats around 16 tons of krill per day, a North Atlantic right whale five tons of zooplankton and a bowhead whale around six tons of food. The populations of blue, fin and humpback whales in the northern Pacific ecosystem alone each eat more than two million tons of food per year, the calculations show. In the case of krill in particular, food intake is twice to three times higher than previously suspected, the scientists report.
Based on this new data, the scientists then performed a series of additional calculations. The focus was on the role of the whales as ecosystem engineers: With their nutrient-rich excrement, they help ensure that growth-promoting fertilizers are supplied to the CO2-absorbing phytoplankton. Iron is particularly important. Without the whales, this deficiency element sinks to the ocean floor more easily, which can limit productivity in certain parts of the ocean, the researchers explain. Using previous measurements of the average iron concentration in whale faeces, in combination with their new data, they calculated that the animals in the Southern Ocean alone recycle around 1200 tons of iron every year. As a result, the fertilization must originally have been much more intensive due to the once much larger stands.
Animal manure producers
In order to be able to estimate what amounts of food the baleen whales in the Southern Ocean converted before the era of industrial whaling, the researchers carried out research on the former populations. Before the slaughter, there were about a million more baleen whales than today. According to the calculations, by the beginning of the 20th century, these stocks would have to have eaten around 430 million tons of krill annually in the Southern Ocean. This is a number that looks amazing at first. Because this amount is twice as high as the estimated amount of krill in the region today. Paradoxically, at the time when many more whales were eating krill, there were significantly larger populations of these small marine animals.
As the researchers explain, the productivity-enhancing effect of whales on the marine ecosystem apparently outweighed their role as krill-eater at the time. This is also plausibly reflected in their calculations of the role of whales as iron suppliers: Before the losses from whaling, the animals fertilized the surface water in the Southern Ocean with around 12,000 tons of iron per year – ten times the amount of today. This could have decisively promoted algae growth, which formed the basis of biological productivity and was also associated with high carbon sequestration.
Scientists now see this as a further argument in favor of protecting the sea giants’ stocks, which have been severely impaired by the whaling era. “Our results suggest that whales’ contribution to global productivity and carbon depletion was likely to be comparable in magnitude to the forest ecosystems of entire continents,” said co-author Nicholas Pyenson of the National Museum of Natural History in Washington DC. “This system is still in place and if you help the whales to recover, you could restore the lost ecosystem functions and thereby also make a contribution to curbing climate change,” says the scientist.
Source: Smithonian, professional article: Nature, doi: 10.1038 / s41586-021-03991-5