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The transfer of energy within a food chain: Why do large whales feed on small plankton?

October 28, 2019/in Conservation, Ocean News /by aanstett

By Meagan Ando, SRC intern

The ten-percent rule toward energy transfer among levels of a trophic system is one that has been used to study ecosystems’ energy dynamics for a long time. But, in order to understand it, one must have a basic understanding of a food chain (Figure 1). Food chains describe the transfer of energy from its source in plants, through herbivores, up to carnivores and onto higher order predators (Sinclair et al. 2003). These different “levels” are known as trophic levels, which is properly defined as the position within the food chain or energy pyramid that an organism can be found. But how much energy is passed along through each level? This is where the ten-percent rule comes in.

Figure 1: An example of a food chain. The first trophic level consists of primary producers gathering energy from the sun, which will be passed up to herbivores, then multiple levels of carnivores (source: nau.edu).

Food webs are often pretty short, which confused many scientists for a long time. Ever wonder why such a large whale feeds on such small planktonic organisms, such as krill? The evidence for the evolutionary advantage of this strategy lies within the definition of the ten-percent rule. When energy is passed along throughout an ecosystem from one trophic level to the next, only 10% of the energy that the first organism receives will actually be passed along. The way in which to study this phenomenon has certainly presented it’s difficulties, as it is clearly impossible to actually visualize the transfer of energy. However, the primary means for determining what marine organisms eat is to study their stomach contents, which is exactly what Reilly et al. 2004 did.

It was known that the International Whaling Commission (IWC) along with the Commission for the Conservation of Antarctic Marine Living Resources (CCAMLR) shared a common curiosity in the idea of the feeding ecology of Baleen whales. This was significantly due to their interests in efforts to place management decisions within an ecosystem context (Reilly et al. 2004). The most efficient way for them to determine their prey sources was to estimate krill consumption by various species of Baleen whales in the Southern Atlantic region during the summer feeding season in the year 2000. In order to successfully draw these estimates, inferences had to be made pertaining to how frequently the whales actually filled their stomachs. This included diurnal change in the forestomach content mass, which ended up producing estimates of 3.2-3.5% of body weight per day (Figure 2) (Reilly et al. 2004). To follow through with the energy tests, four ships participated in the survey to weigh the stomach contents of whales that were unfortunately killed for commercial or research whaling.

Figure 2: Daily consumption rates determined by the four models pertaining to various baleen whales (Humpback, Fin, Right, Sei, and Blue) (Reilly et al. 2004).

In total, 730 cetacean sightings were recorded which included 1,753 separate individuals. It was determined that 83% of the annual energy intake for the whales in this region occurred during this
120-day feeding span in the summer season. The range of total consumption was 4-6% of the standing
krill stock (Reilly et al. 2004). This percentage was derived from the fact that the initial stock included approximately 44 million tons of krill, of which the whales consumed somewhere between 1.6 million and 2.7 million tons (Reilly et al. 2004). These numbers allowed the scientists to make connections between food consumed and the total amount of energy a whale needs to carry out daily bodily functions to survive. It also allowed them to draw conclusions based on where they feed to better protect threatened animals as well as to tweak quotes set for the commercial exploitation of krill, as it is their main food source.

With all of this in mind, it still may not make sense as to why such a large animal would feed on some of the smallest organisms in the ocean. Blue whales, which can be 20-30 meters long, feed on shrimp-like krill that are a mere 2-3 centimeters long. As stated above only ten percent of the energy obtained from one trophic level gets passed along to the next trophic level. For this reason, ecosystems with longer food chains are proven to be, on occasion, less stable than those whose food chains are shorter (Sinclair et al. 2003). Therefore, it is more advantageous for the whale to eat animals on a trophic level in which there is more energy available to be taken in. Hill et al. 2018’s textbook Animal Physiology describes this concept in more depth. In it, they contrast two different possible mechanisms by which a whale can obtain food. One is for the whale to eat fish that are somewhat smaller than themselves. These fish can potentially eat fish that are slightly smaller than themselves, and so on. In this case, there are many trophic levels that the energy will have to pass through before reaching the whale. To apply the ten percent rule directly, we can say that the primary producer produces 10,000 units of energy obtained from the sun. The crustaceans that feed on the producer will generate 1,000 units of energy, from which the small fish that feeds on them will produce only 100 units of energy. The larger fish that feeds on this fish will produce only 1 unit of energy, which may not be enough to sustain the large whale. This is why Baleen whales have evolutionarily evolved into suspension feeders, using Baleen plates to take in large amounts of water and sift through to find small krill. The Baleen whales can eat organisms much smaller than themselves, which can cut down the trophic levels between primary producer and the whale itself, making the energy available to the whale population 1,000 units, as opposed to only 1. In summation, shortening the food chain will in turn increase the food energy available to the whales by a factor of 1,000 (Figure 3) (Hill et al. 2018).

Figure 3: Shorter food chains deplete the energy available to whales less that longer food chains. (Hill et al. 2018).

By better understanding the way in which whales, or any animal for that matter, obtains energy through food, we can further implement new methodologies to better protect them. For example, now that it is known that krill play an extremely important role in the survival of the Blue Whale, agencies can implement new ecological management strategies to be sure that krill populations are not significantly affected by anthropogenic impacts. They may seem like invisible creatures floating in the ocean, but to Baleen whales, they mean a whole lot more.

Works cited

Hill, Richard W., et al. 2018. Animal Physiology. Sinauer Associates/Oxford University Press. Nau.edu. “Life on the Food Chain.” The Food Chain.

Reilly, S., Hedley, S., Borberg, J., Hewitt, R., Thiele, D., Watkins, J. and Naganobu, M., 2004. Biomass and energy transfer to baleen whales in the South Atlantic sector of the Southern Ocean. Deep Sea Research Part II: Topical Studies in Oceanography, 51(12-13): 1397-1409.

Sinclair, Michael, and G. Valdimarsson. 2003. Responsible Fisheries in the Marine Ecosystem. Food and Agriculture Organization of the United Nations 8: 125-131.

Krill’s Rapid Decline; Small Scale Manifests a Larger Scaled Result

December 22, 2015/0 Comments/in Conservation, Ocean News /by aanstett

By Casey Dresbach, SRC Intern

Krill (Euphausia superba) are small crustaceans found in all of the world’s oceans. They rely on small phytoplankton, single-celled plants as their food source that drift near the ocean’s surface. The tiny primary consumers rest at the bottom of the ecological marine food pyramid yet are key to the diets hundreds of different animals ranging from terrestrial to aquatic organisms. Because these organisms are situated at the lowest trophic level, they are forced to support all life in the above levels. If they were to be extinct in a day, there would be an off balance in the interconnected food web and populations between the ocean and parts of terrestrial regions would spiral down their declines.

A zoomed in image of an abundance of Krill. In terms of biomass, they are the most successful in terms of proliferation in the world’s ocean.

A few of the organisms that feed on Krill include sea birds, fish, baleen whales, and penguins. Their relative small size has little do with the large impact they have on the organisms several times their relative dimension. In terms of their numbers, one might question how such small organisms can support a large baleen whale. Krill gather into dense swarms that can have from 1,000 to 100,000 individuals per cubic yard (1 cubic meter), and a swarm can extend from 30 feet (10 meters) to almost 4 miles (6 km) in length. With this strategy they are able to concentrate areas of distribution and make reproduction fairly easy and attainable with such proximity.

The seven characteristics of life include the several aspects an organism must attain to survive as an individual. The most relative to Krill specifically is, “living things adapt to their environments. The most “primitive krill” had to adapt to a specific habitat and only then the survival of the fittest could come into play and those who were best suited would grow and reproduce and hence manifest a greater population. Their strongest adaptation supports their best suiting habitat, sea ice. Their relationship with sea ice is beneficial; it acts as both shelter and a feeding ground for larval and juvenile krill in the winter and in Antarctic regions year round. Temperature is favorable to the key stone species as well as green and blue algae. Krill have evolutionarily become accustomed to this habitat and have thus amplified in such regions.

Krill habituate on sea ice primarily for favored temperature, but also as an easy locus for their algal food source.

There are two critical aspects leading to a sharp decline in the keystone species. Sea ice loss is of significant concern to krill and the several species it feeds. Climate change is to blame for the gradual melting of the habitats. An increase in temperature – having much to do with global warming – is shrinking the sea ice in both the summer and winter. Krill struggle to adapt to such a difference in temperature and cannot survive. Their failure to survival has led to a decrease in their numbers by about 80% since the 1970s, according to several experts. The second agent to point a finger at is the excessive amount of krill fishing in Antarctic waters. Krill are targeted for their supplementary advantages they can offer pet food and food fertilizer. But most recently, have been found to be a great source of omega-3 fatty acid supplements that ultimately make their way into the pharmaceutical vitamin world or the cosmetic industry.

In an effort to better investigate the long-term changes in the physical environment and how they pose a threat to the ecological marine food web, Wayne Z. Trivelpiece, Jefferson T. Hinke, Aileen K. Miller, Christian S. Reiss, Susan G. Trivelpiece, and George M. Watters conducted a study in Antarctica. Sea ice in the West Antarctic Peninsula (WAP) and Scotia Sea are hypothesized to affect penguin populations directly. The specific species included the Adélie penguin (Pygoscelis adeliae) and the Chinstrap penguin (Pygoscelis antarctica). The Adélie favor ice packed habitats in the winter whereas the Chinstrap favor ice-free water to thrive. They performed a series of analytical studies to ultimately come to the conclusion that it is not solely the melting of the sea ice causing the decline in numbers, it’s the interplay of the melting and the limited availability of krill in the ocean to nutrient the penguins. These researchers compared large-scale changes in krill populations and physical conditions in both the Scotia Sea and the WAP. The relative abundance and recruitment showed a direct correlation among suitable environments including temperature as a key factor. Since krill were responding poorly to the temperature, they were dying out, and were absent for the penguins to eat.

Researchers Wayne Z. Trivelpiece, Jefferson T. Hinke, Aileen K. Miller, Christian S. Reiss, Susan G. Trivelpiece, and George M. Watters compared relative krill abundance in both the Scotia Sea and WAP in regards to several variables including: temperature, per-capita recruitment, and sea ice extent.

If global warming continues to dominate, sea ice will continue to melt and krill will continue to lose their habitat. Without sufficient krill, penguins among many the krill’s many other predators will decline (Wayne Z. Trivelpiece, 2010)e too. Understanding how krill demography is affected by changes in the physical environment poses an outlook to predicting future changes in marine ecosystems. A single or double occurrence that happens at a small scale can have a pleiotropic affect on something grander, i.e. the marine food web.

Bibliography

(n.d.). Retrieved November 11, 2015, from Ice Stories Exploratorium:

http://icestories.exploratorium.edu/dispatches/big-ideas/krill/

Jonathan B Shurin, D. S. (2006, January 7). All wet or dried up? Real differences between aquatic and terrestial food webs. Retrieved November 11, 2015, from The Royal Society Publishing: http://rspb.royalsocietypublishing.org/content/273/1582/1

Wayne Z. Trivelpiece, J. T. (2010, November 10). Variability in krill biomass links harvesting and climate warming to penguin population changes in Antarctica . PNAS .

“Blue Whales (Balaenoptera musculus) optimize foraging efficiency by balancing oxygen use and energy gain as a function of prey density” Blog Review

December 8, 2015/0 Comments/in Conservation, Ocean News /by aanstett

By Dana Tricarico, SRC Intern

Blue whales are thought to be the largest animals to have ever lived, and despite this, maintain their energy and weight through the foraging small crustaceans known as krill. Researchers at NOAA Fisheries West Coast Region, as well as Oregon State University and Stanford University looked further into their foraging behavior to find out how this species maximizes energy efficiency. In order to do this, researchers kept in mind that the densest krill patches tend to be deeper in the ocean and therefore, require more oxygen recovery time at the surface as well as less time to forage at those depths. In order to determine how blue whales feed while still maintaining their energy, these researchers compared the foraging of 55 tagged adult blue whales off the coast of California. To see the density of the krill intake from these tagged whales, acoustic surveys were used.

Blue whales are thought to be the largest animals to have ever lived on this planet.

A humpback whale, a relative of the blue whale

The researchers found that their feeding pattern is done by a unique form of feeding called “lunge-feeding,” i.e. High speed acceleration during feeding. This form of feeding leads to engulfment of large volumes of water filled with their prey. When krill were spread out, the blue whale’s tendency was to decrease the number of lunges per dive in order to retain their oxygen levels. In contrast, increased lunge-feeding was done when aiming for the dense krill patches despite the high oxygen use associated with deeper dives. Jeremy Goldbogen, co-author of the study, explains this trade off by saying, “the increase in the amount of energy they get from increased krill consumption more than makes up for it.” In other words, despite using a great deal of energy and oxygen, the benefits of the foraging in dense krill patches outweigh the negatives. Specifically, dense patches of krill can be classified as about 100 to 200 individuals in a cubic meter of water. Anywhere below that amount, blue whales will invest less effort.

The theoretical responsive of the blue whale foraging technique created by looking at both the depth of the prey and the prey density at that depth. The right hand side of the picture shows high prey density while the left hand side shows low prey density. Different parameters are looked at in order to calculate foraging performance including time (t), bottom time, surface time (s) and number of lunges (l), which are marked by black circles.

At present, blue whales are considered endangered by the International Union for Conservation of Nature. Researchers of this paper state that by learning more about these foraging techniques, it can help determine how to protect these species. As Friedlaender explains, “If they are disturbed during the intense, deep-water feeding, it may not have consequences today, or this week, but it could over a period of months. There can be impacts on their overall health, as well as their fitness and viability for reproduction.” This study helps show that foraging techniques in these top predators is not random, but instead is planned depending on prey densities. Perhaps protecting them during their deep water feeding of these dense krill patches can save them further endangerment and better learn how to help them recover from threats in the future.