It’s been just in the past few years that scientists have been able to construct an overview of what drives, and maintains the health of, marine ecosystems – and it is probably not what you’d guess if you’re playing Trivial Pursuit. This story starts when scientists tried to explain why the abundance of krill (small shrimp-like creatures that are near the bottom of the food chain) in the Southern Ocean is lower today, 30 years after commercial whaling ended and after 2 million whales had been killed in these waters, than it was during the “pristine” period before commercial whaling began.
How does this pan out? Krill are hard to estimate, especially in waters as notoriously rough and remote as the Southern Ocean, but an estimate in 2009 following 10 years of searching concluded that the total global biomass of this creature is 133 million tons. The stomach contents of more than 500 great whales killed during the industrial whaling period indicates that the estimated total population of 1.7 million whales prior to industrial commercial whaling in the Southern Ocean would have consumed 276 million tons of krill a year – that is, the feeding requirement of the whales known to have been killed is twice the present biomass of krill.
So what gives? Normally, if an apex predator is removed from the ecosystem, there’s a “predatory rebound”–that is, the prey population explodes. Moose, deer, and elk proliferatedacross the U.S. after the extermination of wolves and mountain lions. The loss of these predators set off a “trophic cascade”that resulted inbroad ecosystem changes. Krill are also consumed by seabirds, seals, squid, and fish, but no commensurate increase in the abundance of these animals has been noted. So something is different in the oceans than in terrestrial systems, where coyotes often fill the wolves’ space. In the oceans, there’s been no measurable “compensatory predation” on krill by other animals.
Thus, krill abundance has declined by as much as 50% since the start of commercial whaling, and actually continues to drop. Might this be due to global warming melting the ice sheets under which ice algae, a favored krill food, are found? Unlikely, since the ice sheets are nowadays somewhat larger than they were during the late 1900s. What appears to be true is that removing whales, with their nutrient-rich feces and urine, has impaired the productivity of phytoplankton, the krill’s food. In fact, it seems that whales and krill acting together were ecosystem engineers. Pelagic marine species (that is, open-ocean species like whales and krill) affect their environment in two ways: nutrient release (that is, defecation or decay) and turbulence from their passage through the water.
To add intricacy to an already abundantly complex situation, it appears that the behavior of krill has changed since the onset of commercial whaling. Once they were found in huge swathes at the ocean’s surface at all hours of the day; now they appear to spend daylight hours at depth, rising to the surface to feed at night. Very large schools of krill on the surface during daylight were reported by whaling vessels and scientific expeditions [‘constant records of krill in sight day after day’, ‘thick with [krill] like pea soup’, and ‘immense pastures’]; in the l950s and 60s, scientific observers mapped surface schools and concluded that krill only inhabited the top 10 m and rarely went below 40 m. This is definitely not the case now; there are no observations of large schools of krill on the surface during the daylight reported in recent scientific literature. In the Southern Ocean, they are now found on the surface only at night.
An intriguing piece of evidence is that krill are attracted to substances rich in iron. One experiment using, of all things, iron-rich Newcastle Brown Ale, described krill as having to be pried off the end of the pipette introducing the beer into their tank. It is likely that whale feces (and the iron these contain) attract krill to the surface, which in turn attracts whales to feed on the krill and produce more poop (whales invariably defecate at the surface), continuing the cycle.
If krill no longer come to the surface except at night, what is the likely impact of this on the marine ecosystem, especially when coupled with the decrease in krill biomass? Ecologists have only recently begun to recognize the importance of animals in the recycling of crucial elements such as phosphorous in maintaining the health of terrestrialecosystems. Whales bring phosphorus, nitrogen, and micronutrients such as iron from the ocean’s depths to the surface, inducing krill to remain near the surface as well as providing essential nutrients and food for efficient reproduction. This increased productivity feeds seabirds and anadromous fish (those thatspend part of their lives in marine waters, part in fresh). Imagine then a healthy interlinked biome where whales pump nutrients from the depths to the surface, with the resulting krill blooms feeding salmon. The fish swimupriver, where they are fed on by bears, otters, and eagles. These in turndefecate on the terrestrial side,which fertilizes plantsfor the herbivores that graze there. Sound farfetched? It has been demonstrated in near-stream soil, vegetation, and insects in British Colombia, Canada. Marine nutrients too have been found in trees far from streams in British Columbia, likely transported by salmon-eating bears. Indications are that it occurs in the Gulf of Maine, where whales and seals pump four times as much nitrogen into the ocean’s productive upper layer, the euphotic zone – that is, the layer of the ocean where light penetrates – as all the rivers in the area combined.
What does this mean in practical terms? It’s likely that humanity has shot itself in the foot on multiple levels. With the human population set to reach 11 billion by mid-century, we urgently need healthy and productive oceans to provide the protein for the roughly 3 billion people dependent on marine resources, and fish on the plate for we who consume it by choice. Just as much as we need to maximize mangroves and sea grass plains– the nurseries for many fish species – we need the great whales to move nutrients from the depths to the surface and then laterally outwards to enhance fish stocks. The destruction of the great whales is shocking in its extent: ecologists now estimate that whale numbers globally have been reduced by 66% to 90% o