Post-whaling Recovery: The Beautiful Story Of Humpbacks

A population’s growth rate depends on the survival and reproduction of its individuals. According to a review of literature published in the Marine Ecology Progress Seriesin 2017, the recovery rate of humpback whale populations depends on prey availability, anthropogenic threats, hunting intensity and genetic diversity. Populations in the Northern Hemisphere tend to increase less quickly than those in the Southern Hemisphere. Researchers suggest that lower krill productivity and increased shipping traffic in the Northern Hemisphere may partly explain these differences.

Rapid climate change is affecting this region, with a 7°C spike in temperatures since 1950. As a result, ice cover duration has grown 80 days shorter in the past 40 years, which might allow humpbacks to spend more time in their feeding grounds, thereby favouring population growth. However, in the long term, reduced ice cover could have negative effects on krill production.

However, the eastern Australian population is surprisingly continuing to grow exponentially and does not seem to be approaching a plateau anytime soon. Although currently available data are inconclusive, researchers considered two explanations. First of all, the capacity limit of the ecosystem may be greater than that considered by the researchers, either because the pre-whaling size of the population was poorly estimated, or because today’s ecosystem is richer in resources than before.

Alternatively, local krill abundance could potentially allow the humpback population to increase beyond the plateau, leading to a significant decline in the future toward the true capacity limit. In grey whales, significant mortality events might be associated with the same phenomenon of exceeding the capacity limit.

While humpback numbers are rapidly trending upward, blue whale and fin whale populations are not enjoying the same growth rates. Whaling represents the most intense exploitation of animals by humans in history. It is therefore impossible to draw parallels to predict the recovery of whale populations. “While we know that populations of mammals recover from hunting and other forms of over-exploitation, the dynamics of how this occurs is not well understood,” explains to phys.orgMichael Noad, lead author of the study on eastern Australian humpbacks. This population has been monitored since the 1980s and the data collected during the species’ recovery should therefore enhance scientific knowledge of this phenomenon.

According to Ari Friedlaender, co-author of the study of humpback whales off the Antarctic Peninsula, the success of this species can be attributed to a multitude of factors. In winter, humpbacks tend to congregate, which favours mating between males and females. On the other hand, blue whales as well as fin whales are more scattered, which might limit breeding opportunities, especially when populations are small. Humpback whales also reach sexual maturity faster than other species and birth intervals are shorter, which bodes well for their recovery.