Fascinating. Did you read the Ecology paper and associated studies?
A surplus no more? Variation in krill availability impacts reproductive rates of Antarctic baleen whales
First published: 15 January 2023
doi.org/10.1111/gcb.16559Abstract
The krill surplus hypothesis of unlimited prey resources available for Antarctic predators due to commercial whaling in the 20th century has remained largely untested since the 1970s. Rapid warming of the Western Antarctic Peninsula (WAP) over the past 50?years has resulted in decreased seasonal ice cover and a reduction of krill. The latter is being exacerbated by a commercial krill fishery in the region. Despite this, humpback whale populations have increased but may be at a threshold for growth based on these human-induced changes. Understanding how climate-mediated variation in prey availability influences humpback whale population dynamics is critical for focused management and conservation actions. Using an 8-year dataset (2013-2020), we show that inter-annual humpback whale pregnancy rates, as determined from skin-blubber biopsy samples (n =?616), are positively correlated with krill availability and fluctuations in ice cover in the previous year. Pregnancy rates showed significant inter-annual variability, between 29% and 86%. Our results indicate that krill availability is in fact limiting and affecting reproductive rates, in contrast to the krill surplus hypothesis. This suggests that this population of humpback whales may be at a threshold for population growth due to prey limitations. As a result, continued warming and increased fishing along the WAP, which continue to reduce krill stocks, will likely impact this humpback whale population and other krill predators in the region. Humpback whales are sentinel species of ecosystem health, and changes in pregnancy rates can provide quantifiable signals of the impact of environmental change at the population level. Our findings must be considered paramount in developing new and more restrictive conservation and management plans for the Antarctic marine ecosystem and minimizing the negative impacts of human activities in the region.
Paradoxically, krill abundance in the Atlantic sector of the Southern Ocean has likely declined since the end of commercial whaling (Atkinson et al., 2004)
Climate change also looms large in the Southwest Atlantic, where sustained surface warming has led to ecological shifts over the past several decades (Whitehouse et al., 2008). The whale aggregation and fishing activity reported here was associated with a large spring phytoplankton bloom following an exceptionally warm austral spring and a near-record low in regional sea ice extent.
Whitehouse, et al. 2008.
"Rapid Warming of the Ocean around South Georgia, Southern Ocean, during the 20th Century: Forcings, Characteristics and Implications for Lower Trophic Levels." Deep-Sea Research Part I: Oceanographic Research Papers 55: 1218- 28.
The Southern Ocean is known to have warmed considerably during the second half of the 20th century but there are few locations with data before the 1950s. Thus the long-term warming around South Georgia is substantial-more so than documented previously for the circumpolar warming of the Southern Ocean. Furthermore, the combination of a regional decline in ice extent and strong upstream warming likely explains a significant part of the strong seasonal variation apparent in the warming trend.
Simple abundance and growth rate relationships with our long-term temperature data appear to show declining habitat suitability for E. superba. [Antarctic krill]
Our dataset indicates significant warming throughout the top 100 m of the ocean in the vicinity of South Georgia. Overall, a simple regression of near-surface (0-10 m) temperatures with year indicates an increase of ?1.54 ?C. When spatial and temporal variations are incorporated in the model, this equates to a skewed warming of ?0.9 ?C over the January months and ?2.3 ?C for August (Table 1). These warmings are thus very significant, exceeding even the strong warming of the near-surface circumpolar