Demographic Reconstruction of Antarctic Fur Seals Supports the Krill Surplus Hypothesis

Summary

Scientists used advanced DNA sequencing (RAD sequencing) and population modeling to trace the genetic history of Antarctic fur seals at South Georgia Island, aiming to test whether increased krill availability led to larger seal populations. Their analysis uncovered a dramatic population crash during the commercial sealing era of the 18th and 19th centuries, when hunters nearly drove the species to extinction. However, the seals subsequently recovered to even greater numbers than before.The key finding was that the effective population size after the sealing period was roughly double what it had been historically. This supports the "krill surplus hypothesis" – the idea that industrial whaling removed so many krill-eating whales that it left more krill available for other marine predators like seals. With this abundant food source, seal populations were able to grow beyond their original size.

This graph shows temporal trends in Antarctic fur seal abundance and baleen whale harvesting. Fur seals were heavily hunted in the late 18th and early 19th centuries and were considered virtually extinct by the early 1900s. The seal population then experienced explosive growth during the second half of the 20th century, coinciding with baleen whale harvesting. Purple dots show fur seal abundance from empirical population estimates in the scientific literature. Whale data represents numbers of harvested baleen whales. Comparable whale population estimates from the same period are not available. We cannot show temporal changes in harvested seals because while sealing vessel records exist, the number of seals taken was often not recorded.

Key Findings

The genetic data showed that during the sealing period, the breeding population dropped to approximately 534 individuals. This was a severe population bottleneck that nearly caused extinction.

After sealing ended, the effective breeding population grew to about 29,319 individuals. This was roughly twice the size of the original pre-sealing population of around 12,506 individuals.

This study provided the first DNA-based evidence that removing whales from the ecosystem created more food for other marine predators. The extra krill allowed seals to thrive beyond their natural capacity.

The research demonstrated that analyzing population genetics can effectively test ecological theories. It showed how genetics can help us understand how ecosystems respond to human impacts.

Abstract

Much debate surrounds the importance of top-down and bottom-up effects in the Southern Ocean, where the harvesting of over two million whales in the mid twentieth century is thought to have produced a massive surplus of Antarctic krill. This excess of krill may have allowed populations of other predators, such as seals and penguins, to increase, a top-down hypothesis known as the 'krill surplus hypothesis'. However, a lack of pre-whaling population baselines has made it challenging to investigate historical changes in the abundance of the major krill predators in relation to whaling. Therefore, we used reduced representation sequencing and a coalescent-based maximum composite likelihood approach to reconstruct the recent demographic history of the Antarctic fur seal, a pinniped that was hunted to the brink of extinction by 18th and 19th century sealers. In line with the known history of this species, we found support for a demographic model that included a substantial reduction in population size around the time period of sealing. Furthermore, maximum likelihood estimates from this model suggest that the recovered, post-sealing population at South Georgia may have been around two times larger than the pre-sealing population. Our findings lend support to the krill surplus hypothesis and illustrate the potential of genomic approaches to shed light on long-standing questions in population biology.

Published in

Genes

2022

Authors

Hoffman, J.I., Chen, R.S., Vendrami, D.L.J., Paijmans, A.J., Dasmahapatra, K.K., Forcada, J.

Institutions

Department of Animal Behavior, University of BielefeldBritish Antarctic SurveyDepartment of Biology, University of York

Methods

Biological sampling DataCase

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