A study links a decline in krill stock to whale killings in the last century. (Photo: Australian customs Service/ T Carter, aad.gov.au)
The krill paradox
Thursday, June 17, 2010, 01:30 (GMT + 9)
A scientist has published a study analysing whether the killing of several million large whales in the last century has resulted in lower krill populations in the Southern Ocean.
Several million large whales were killed between 1900 and 1970, and all of those whales were the primary predators of krill (Euphausia superba). Researchers are seeking the link between this plunge in whale populations and dropping krill stocks.
Jay Willis from the QMS Program, School of Zoology, University of Tasmania, Australia, hypothesised that krill have changed their behaviour – thereby increasing their mortality -- due to the absence of whales in their ecosystem. This change has resulted in a fall of krill abundance.
He reproduced a computer model of krill life history.
“I then extended the model as an individual-based model to show the effects of habitat choice on individual lifetime reproductive success and abundance,” he explained in a study published in the journal Evolutionary Ecology Research.
Willis found that predator invoked behaviour may result in a higher population abundance and, without the predator’s presence, natural selection may encourage behaviour leading to lower abundance.
This undoes the predictions made by mass balance ecosystem models. A large surplus of krill had been predicted as a result of the removal of 2 million whales within 100 years, he told.
“Other predictions included a large increase in other predators of krill; for instance, a yearly increase of 300 million penguins was hypothesized. If, however, the abundance of krill did not increase dramatically and neither did any of its other predators, this would represent a paradox,” Willis elaborated.
Based on the average whale’s diet, he found that, before 1900, whales consumed 175–190 million tonnes of krill per year, and by 1987 whales ate fewer than 43 million tonnes of the crustacean.
“Contrast this to the best estimate of total post-whaling krill biomass: ‘Extrapolations from acoustic measurements of krill abundance indicate a krill biomass of between 60-155 million tonnes,’” the study reads.
Admittedly, krill populations have been infamously difficult to measure, and there may have been a long-term decline, rendering these estimates mute. Either way, the study claims, the measurements are the result of decades of international collaborative projects collaborating to survey the abundance of krill with specifically designed equipment.
Willis noted that the case, consequently, cannot be proved either way because sufficiently accurate data are lacking.
“The present highest best guess of the standing stock of krill would be insufficient to feed the whale population before human exploitation. This critical point is the basis for the assertion of a paradox in this study,”Willis wrote.
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