Humpback whale energy hangs in the balance
For any animal, there are two key survival rules: don’t get killed by a predator, and don’t starve to death. For whales in the twentieth century, commercial whaling was the biggest threat to survival, and whale populations were decimated by unsustainable whaling practices. Large whales today have few predators, just the remnants of whaling activities and the odd seagull, so their main challenge for survival is consuming enough food to sustain themselves.
Humpback whales live a “feast and fast” lifestyle. For about three months every summer, they gorge on krill blooms in cold polar waters, building up fatty blubber stores to fuel a nine month, 10,000‒18,000 km round trip to warm subtropical waters, where they breed and calve. This is one of the longest mammal migrations in the animal kingdom. During this journey, the whales do not actively feed, relying entirely on their stored energy. If their stores do run out, there is no safety net, and they will become exhausted and end up beached somewhere along the coast.
Balancing energy gain with energy use is crucial for surviving this long migration. For my PhD research, I studied several factors that influence energy gain and energy use in humpback whales migrating along the west Australian coast, and whether humans have the capacity to upset this energy balance.
Getting enough energy for migration
Krill is the most abundant crustacean on earth, with about 500 million tonnes of Antarctic krill living in the Southern Ocean. This would suggest that there is an oversupply of food for humpback whales, but my recent research published in Polar Biology found that annual changes in the body condition of humpback whales were linked to fluctuations in krill abundance.
To estimate the body condition of whales, we delved into the historical whaling database. Whales were mainly hunted for oil and meat, and the oil was mostly obtained from blubber. So we used oil yields as an indicator of body condition, as the fatter the whale, the more oil the whalers would be able to extract. From these yields, we produced a time series of how body condition in humpback whales changed during part of the whaling era, between 1947 and 1963.
No data exist on krill abundance during the whaling era, so we took an indirect route to determine if whale body condition was affected by changes in prey abundance, via sea ice.
Sea ice is important for krill survival. It provides shelter over winter, and microbial communities living in sea ice are a food source for juvenile krill. Hence, in some areas of Antarctica, more winter sea ice means more krill the following summer. Around the east Antarctic region, where west Australian humpback whales forage, we found that winter sea ice cover is a good predictor of summer krill abundance.
Looking at past sea ice trends, we showed that the body condition of humpback whales is positively related to winter sea ice cover. This relationship is probably mediated by krill abundance: more sea ice means higher krill abundances, and thus fatter whales.
Energy use during migration
Once whales leave the Southern Ocean, they must conserve energy to ensure their stores don’t run out prematurely. A second paper I recently published in Conservation Physiology shows two ways that whales can conserve energy: by managing (1) swimming speed, and (2) the time spent resting versus travelling.
Swimming speed has a large influence on a whale’s energy use. To swim, a whale must produce enough power to overcome drag and move forwards. This means when a whale doubles its speed, the energy used increases eight-fold, so swimming at high speeds is energetically expensive.
On the other hand, swimming slow isn’t an advantage either. The slower a whale travels, the longer migration takes, and the more energy is spent on basic maintenance costs (basal metabolic rates). The optimal speed for migrating whales is a balance between the costs of swimming fast and the costs of swimming slow. We found this “just right” speed was 1.1 metres per second, which is close to speeds observed in the wild. It seems that energy conservation may be an important factor in the evolution of the migration strategies we see today.
Another energy-saving strategy we identified was managing the time spent resting compared to travelling. During the southbound migration when females are accompanied by calves, humpback whales stop in calm, sheltered embayments along the coastline for a few weeks before continuing their journey. The reason for these stop-overs is unclear, but scientists think it may be an important time for females to focus on feeding calves in these quiet conditions.
Our model of migration energetics showed that the optimal period of time for a female to spend resting is around 30 days. Like swim speed, there is a trade-off here. Spending too much time resting means a whale has to swim faster to cover the migration distance and arrive in the Southern Ocean in time for the summer bloom, incurring the high costs of swimming fast. Spending too little time resting, and a mother cannot feed her calf enough milk for it to grow, as she is physiologically limited in how fast she can lactate.
Energy conservation is an important part of a whale’s migration, and deviating from optimal conditions can result in higher energy use. Our results from investigating swim speed and resting indicate that migration patterns have evolved to minimise energy use, to ensure survival and reproductive success over the nine-month journey.
Disrupting the balance
Balancing energy budgets is vital for supporting the humpback whale’s “feast and fast” lifestyle. Our research shows that both energy gain and energy loss can be influenced by external environmental conditions.
In the Southern Ocean, future changes to sea ice dynamics, such as those associated with climate change, could reduce the amount of prey available for whales, meaning they will begin their migrations on diminished blubber stores. Krill fisheries also operate in Antarctic waters and may threaten the whales’ food source.
Along the migration route, other human activities could potentially disrupt optimal migration patterns in whales. Shipping, fisheries, mining, and tourism activities can disrupt nursing females or disturb whale behaviour, causing them to swim away in avoidance. While individual incidences seem minor, cumulative disturbance along the migration route can cause an increase in average swim speed, or reduce the amount of time a whale can rest, resulting in increased energy use.
It’s strange to think that such large, blubbery whales can run out of energy, but these animals do end up on Australian beaches “thin” from exhaustion. To protect the survival of these remarkable migrators, we must think beyond the direct harm caused by “scientific” or commercial whaling, and regulate other human activities that can disrupt their critical energy balance.
[Header image: Wanetta Ayers/Wikimedia Commons, public domain]