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Foraging energetics and prey density requirements of western North Atlantic blue whales in the Estuary and Gulf of St. Lawrence, Canada

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By Nicholas Martinez, SRC intern

Pelagic predators throughout the world’s oceans face the same challenge: foraging for food in an environment where much of their prey are available in clusters, centralized around specific areas of the ocean. For this reason, many pelagic predators have unique ways to find these limited resources, all the while adjusting these foraging techniques so as to maximize energy gained for every unit of energy expended.

In a world where oceanic ecosystems are facing rapid change, the need for more research and implemented protective measures is rising. Many marine animals have been documented as having shifted their foraging habits because of a rapid decline in available resources. These resources, which have supported countless generations of predators, are now facing serious threats to their population size. For this reason, studies of the foraging habits of large marine predators allows insight into the hunting grounds that still remain for these animals. Understanding how often these species forage for food allows scientists to determine the health of a specific population of that species. A 2019 paper from Guilpin and colleagues, “Foraging energetics and prey density requirements of western North Atlantic blue whales in the Estuary and Gulf of St. Lawrence, Canada”, focused on the foraging efficiency of the North Atlantic blue whale in a quickly changing oceanic environment.

 

Figure 1. The blue whale (Source: NOAA Photo Library/anim1754/Wikimedia Commons)

Foraging efficiency is a ratio that compares the rate of energy consumption to the rate at which energy is expelled so as to provide insight into an organism’s ability to store energy. Understanding a blue whale’s capacity to store energy is crucial because there is a direct correlation between the animal’s energy supply and its ability to reproduce. Using 10 depth-velocity tags attached to the whales, the scientists were able to monitor the foraging behaviors of blue whales in the St. Lawrence Estuary. While blue whales are the largest living organisms on the planet, their food supply consists small invertebrates called krill, which can be found in large populations throughout the study site. This food source exhibits spatial and temporal variations and thus requires a specialized foraging technique in order to maximize the whales net energy reserve.

 

Figure 2. “Predicted change in blue whale foraging effort with time of day in (a) feeding depth (m), (b) dive duration (s), (c) number of feeding dives, (d) number of lunges d−1, and (e) number of lunges h−1. Dark grey ribbons represent the 95% confidence intervals around the predicted response from generalized additive mixed models. Shaded areas are for nighttime (grey), dusk and dawn (light grey), and daytime (white). Points are data observations” (Source: Guilpin et al. 2019, p. 213)

Here, the researchers found that during the day, the tagged blue whales performed fewer but longer feeding dives than at other times of the day (Figure 1). This suggested that the blue whales invested in fewer but longer dives so as to maximize the amount of energy they could store by minimizing energy expenditure (Figure 2). In addition, the authors found that the whales were performing more lunges per dive (accelerating towards the surface, trapping any krill in their mouth as they momentarily breach), showing that even while the whales were not deep diving, they were still feeding.

 

Figure 3. “Relationship between energy expenditure during feeding dives for 3 blue whale sizes (22, 25, and 27 m length) and (a) dive duration (s) … (b) maximum dive depth (m) … and (c) number of lunges per dive” (Source: Guilpin et al. 2019, p. 214)

Although the whales were observed to feed throughout the day, this did not necessarily mean that they were consuming enough krill to achieve a neutral energetic balance. In fact, this study found that only 11.7 and 5.5% of Arctic and northern krill patches contained densities that could sustain a neutral energy balance for the blue whales. This could be due to a decline of krill populations via environmental impacts, and if so, poses a great threat to blue whale populations. This information emphasizes blue whales’ constant need to forage for high densities of krill in order to maintain neutral energy balance or maintain a healthy energy storage suitable for reproduction. The discoveries made in this paper may therefore help predict the effects of climate change on both predator/prey densities and may also offer insight on potential krill fisheries and how they may or may not affect blue whale populations.

Work cited

Guilpin M, Lesage V, McQuinn I, et al (2019) Foraging energetics and prey density requirements of western North Atlantic blue whales in the Estuary and Gulf of St. Lawrence, Canada. Mar Ecol Prog Ser 625:205–223. doi: 10.3354/meps13043

Combining hard-part and DNA analyses
of scats with biologging and stable isotopes can reveal different diet compositions and feeding strategies within a fur seal population

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By Nicole Suren, SRC intern

Diet analysis of top predators is important in the study of ecology because it can help to illuminate the energetics and ecological interactions of that predator. One method of studying diet is using hard part analysis and DNA barcoding using the animals’ scats, while other methods include stable isotope analysis from blood plasma and an examination of behavior using satellite telemetry. Jenniard-du-Dot et al aimed to compare the effectiveness of these methods, and compile data from all four to obtain a full picture of the diets of 98 lactating females in an Alaska colony of fur seals. The hard part analysis consisted of straining the hard parts out of the seal scat and identifying the prey items they belonged to, and the DNA barcoding utilized the matrix of the scat to isolate the DNA of the various prey items. The two scat analyses were generally in agreement, with the more general classifications obtained from the hard part analysis confirmed and further specified by DNA analysis. While this information was useful, it only included data on what the seals had eaten in the last one to two days. To obtain longer-term data, the researchers used stable isotope analysis from blood plasma, which would give insight into the trophic level the seals had been feeding from for the previous one to two weeks. Finally, the seals were fixed with satellite tags for two months to examine where they were going to forage and what they might be eating there.

Results of the scat analyses. The percentage of the diet of the population of fur seals of many different species confirmed by hard part analysis and DNA barcoding are shown (Jeanniard-du-Dot, Thomas, Cherel, Trites, & Guinet, 2017).

 

Results of the stable isotope analysis. Levels of carbon (x-axis) and nitrogen (y-axis) isotopes show two trophic clusters, one signifiying an oceanic (pelagic) diet, and one showing a neritic (inshore) diet.

The overall conclusions were that there were two separate foraging strategies within the population: neritic (inshore) foragers and offshore foragers. While the scat analyses may have hinted at this by identifying some exclusively offshore species in the seals’ diets, the other two methods illuminated this phenomenon. The stable isotope analysis identified the inshore trophic levels versus the offshore trophic levels in the respective individuals, and the satellite telemetry showed two distinct foraging patterns, shown in figure 3.

The foraging tracks of 20 satellite tagged fur seals show two distinct foraging patterns.

These findings are not only important in building the life histories of an Alaskan population of fur seals. They support some previous research demonstrating that generalist feeders are often made up of subsets of specialist individuals. Furthermore, they demonstrate the importance of mixed methods in ecology, and how using different methods to examine different timescales and aspects of the life history trait being examined (like combining direct diet analysis and behavior) results in a very thorough study that gives the trait a wider ecological context.

Works cited

Jeanniard-du-Dot, T., Thomas, A. C., Cherel, Y., Trites, A. W., & Guinet, C. (2017). Combining hard-part and DNA analyses of scats with biologging and stable isotopes can reveal different diet compositions and feeding strategies within a fur seal population, 584, 1–16.

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

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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.

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.