The Circle of Poo

Nutrients from whales’ poo fertilize phytoplankton, which are eaten by krill … which are eaten by whales. 

Published January 5, 2016

In the Southern Ocean, surrounding Antarctica, blue whales were nearly wiped out by industrial whaling from the 1920s to 1940s. But now, with whaling curtailed, the whales are surging back: their population growing by 7.3 percent per year. By 2066, blue whale numbers could be back to historic levels. For conservationists, this last-minute rescue of the largest animals on Earth is a good news story. For others, the blue whale’s return is a daunting prospect. As conventional thinking suggests, the resurgence of these massive creatures—with their equally massive appetites—is creating an ever-greater competition for the commercial krill fishery.

These worries are not entirely unfounded. In the Southern Ocean, shrimp-like krill sit near the base of the food web. Estimates place the total krill biomass in the sea at 379 million tonnes. Each year, commercial trawlers pull in up to 5.6 million tonnes, and during peak feeding season a blue whale can eat nearly four tonnes of krill a day. With a pre-whaling population of 239,000 in the Southern Ocean, blue whales will soon be consuming a decent chunk of the total krill. That would leave the commercial trawl fleet—mostly from Norway, South Korea, and China—with fewer krill to use in fish meal for aquaculture and aquarium feeds, in vitamins or pharmaceuticals, or as bait for sport fishing.

Countering this conventional thinking, a study headed by marine biologist Trish Lavery suggests that blue whales are not exploitive gluttons that will ruin the commercial krill fishery. Instead, the mammals are actually an important and previously overlooked contributor to marine productivity. Blue whales don’t decimate krill populations, they bolster them.

The Southern Ocean is short of iron, an essential nutrient for photosynthesizing phytoplankton. Blue whale feces, however, has an iron concentration more than a million times higher than the background seawater.

Since whales defecate near the ocean’s surface, most of this iron enters the water in the photic zone, where light can reach it. Each whale deposit, then, kicks off a burst of photosynthetic activity and triggers a phytoplankton bloom. In turn, these phytoplankton are the main food source for krill.

Imagine the iron acting like jet fuel to boost prey at the bottom of the food chain, a spike in phytoplankton that benefits predators all the way up the ladder—not just whales, but seals, penguins, squid, and fish. “By defecating in surface waters, blue whales essentially fertilize their own feeding grounds with the nutrients needed to sustain the growth of their krill prey,” Lavery and her colleagues write.

Recovery of blue whales to their historic abundance is “unlikely to reduce fishery yields and may, in fact, enhance ecosystem productivity in the Southern Ocean,” they report.

Lavery presented her findings at the recent conference of the Society for Marine Mammalogy, in San Francisco. The research earned her the society’s F. G. Wood Memorial student award.

Lance Barrett-Lennard, a whale researcher with the Vancouver Aquarium, attended Lavery’s presentation. “It seems very credible to me,” he says.

“As with lots of scientific theories like this, the devil is in the details. The amount of iron in whale feces, how it disperses once released, [and] how quickly photosynthetic plankton take it up” will all be important considerations in figuring out the ultimate balance between whales’ hunger and their contributions of their waste, he says.

“For now, we can say there is, at least, a strong possibility that blue whales stimulate rather than suppress plankton abundance,” concludes Barrett-Lennard.

The findings should assuage some fishers’ fears. At the same time, the research speaks to the incredible complexity of food webs, and shows how whaling affects more than just the whales.