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Large predators are back: Humpback whale population recovers from whaling and affects the ecosystem

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Environ beat

Intense whaling activities starting from the late 1700s caused a sharp decline in the humpback whale population in the 20th century. The Western South Atlantic (WSA) humpback whale number dropped from nearly 27,000 to only 450 within 100 years. 

To protect the whale population, exploitation was prohibited in the 1960s. Recently, a team of researchers from the School of Aquatic and Fishery Sciences reported a significant increase of WSA humpback whale population. The finding, published in the journal Royal Society Open Science, suggested the population has recovered close to the pre-whaling status from nearly going extinct. 

The model that researchers built to study whale populations incorporates observed data, abundance estimation, and genetic information of WSA humpback whales. Researchers can estimate the overall population size, trends in population growth, and the growth rate of whales and lots of other species.

According to lead author Alex Zerbini, a former Ph.D. student at the UW and the current contractor of NOAA Marine Mammal Laboratory, the data researchers used include catch records from the International Whaling Commission and survey data from scientists across the world. 

“This model is density-dependent,” Zerbini said. “It essentially means the growth of population depends on how many animals are in the population. With large numbers, the population grows slower.” 

Co-author John Best, a Ph.D. student in the quantitative ecology and resources management program, explained that by analyzing carrying capacity, which is the maximum population the habitat can hold, the model helps to project future whale population based on previous years’ numbers. 

The model includes a struck-and-lose rate which shows whales that are injured because of whaling and later died. The team is also looking at integrating other human-caused whale deaths, like being injured by ships, to improve the model. 

“We tried to account [uncertainties] in the modeling,” co-author Grant Adams, a current UW Ph.D. student in The Punt Lab said. “With pre-modern fisheries, we aren’t completely sure how many whales were removed.”

Researchers inferred human protections have driven humpback whales to a natural state. As these large predators become more abundant, other animal species in the food web will be affected.

“The krill is the base of the food web in the Atlantic ocean,” Zerbini said. “In addition to being food for whales, it is also food for many animals. The potential competition increases.”

WSA humpback whales primarily consume krill from the ocean near South Georgia, the South Sandwich Islands, and the Scotia Sea. Although competition is a natural phenomenon, a rapidly changing environment in these feeding regions creates larger impacts on the ecosystem than expected. 

“Temperature in the ocean is increasing,” Zerbini said. “Krills appears to be moving away from the current habitat and to the south. There are less krills close to the island.”

Zerbini explained that islands are important habitats to fur seals and penguins who need the land to breed. If climate change causes krills to migrate and whales consume the leftover, seals and penguins will have insufficient food and are negatively impacted. 

Humpback whales also chase after krills to their new habitats which affect the food web in those areas. By monitoring and understanding interactions between these species, scientists can create management plans for other whales in Southern and Northern hemispheres.  

“With the [whale] population being in a larger size,” Adams said. “The population is more buffered from environmental change like climate change.” 

Reach reporter Sunny Wang at Twitter:  @sunnyqwang64

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