Junk Food Hypothesis in Stellar Sea Lions

Stellar Sea Lions in the wild are on the decline, especially in areas with lower variety in nutrition. They are eating more fish that contain fewer calories, which is essentially starving the Stellar Sea Lions (Fritz, 2005, p. 1244). A study was done in captivity to see the affects this had on resting metabolism. They observed the animal for 7 days prior to switching its diet to make a control group, then switched her diet and observed for another 6-7 days (Fritz, 2005, 1244). They then compared her resting metabolism and noticed a change in body mass. The sea lion had consumed more food, but lost weight due to the Pollock being less nutritional and fatty. If the stellar sea lions don’t get enough fat in their diet, their blubber layer starts to decrease, and they no longer have an efficient thermal layer for thermal regulation. This could also be because they are eating more, so they are probably more active and use more energy. This means that even less nutrients are being stored and are being used instead.

This also affects the reproduction of the species. If the seals are not getting enough nutrients for themselves, they will not have the excess nutrients needed to produce young. When food is scarce, the seals must go on extended foraging trips which means that the pups were left to fast for longer periods (Rosen, 2000). If there is not enough food, rather than lose body fat and/or tissue, their resting metabolic rate slows down, and the seals are less active (Rosen, 2000). Most studies have been done during the spring and summer months due to how adverse Alaska winters can be (Maniscalco, 2023). This is a problem as the winter months are the most critical. This can be challenging for adult females as they can be both lactating and pregnant during this critical time (Maniscalco, 2023). If the adult females are unable to consume and produce enough to lactate and support her young, then the survival rate of her pups will plummet as they will not get the nutrients needed to survive. It is especially important that they get enough fats in their diet to produce their layer of blubber to properly thermoregulate, especially once they start to experience their first winter months. Food is also scarcer during these winter months, so putting on a nice layer of blubber is essential for not just thermoregulation but also for having enough stored energy to survive.

Another problem that the seals face is not being able to consume enough when on a lower calorie diet (Calkins, 2013). There have also been attempts to correlate fishing activities with the decline of stellar sea lions (Calkins, 2013). None have been successful, but it is important to consider. The fish that are valuable to sea lions are also valuable to us which makes it probable that fishing activities play a role in the decline of stellar sea lions. Activities such as fishing have been affecting ecosystems in major ways for as long as we have been doing it. Sea surfaces are warming in the Southern Pacific Ocean, which negatively affects productivity (Montalva, 2022). This leads to less phytoplankton being produced, and with that less fish as it is a main food source. Due to this, seals are competing for food, causing agnostic interactions between sea lions and fur seals in Northern Pacific Patagonia (Montalva, 2022). This means that not only are both species eating fish with less nutrients, but they are also competing for it. More energy is needed to find and catch the lesser abundance of fish, meaning they are exerting more energy and storing even less than before. This and all the other factors more than likely stress the seals out. I think that this and the many other factors and example given proves that the “Junk Food” Hypothesis is true and is relevant. There are many factors, but what the main factors are seem to be unknown so far.

Works Cited

Calkins, Donald G., et al. “The Pollock Paradox: Juvenile Steller Sea Lions Experience Rapid Growth on Pollock Diets in Fall and Spring.” Journal of Experimental Marine Biology and Ecology, Elsevier, 11 Feb. 2013, www.sciencedirect.com/science/article/abs/pii/S0022098113000154.

Fritz, Lowell W., and Sarah Hinckley. “A critical review of the regime shift-“junk food”-nutritionalnstress hypothesis for the decline of the western stock of Steller sea lion.” Marine MammalnScience 21, (2005): 476–518.

Rosen, David A S, and Andrew W Trites. “Pollock and the Decline of Steller Sea Lions: Testing theJunk-Food Hypothesis” Canadian Journal of Zoology 78, (2000): 1243-1250.

Maniscalco, John M. “Changes in the Overwintering Diet of Steller Sea Lions (Eumetopias Jubatus) in Relation to the 2014 – 2016 Northeast Pacific Marine Heatwave.” Global Ecology and Conservation, Elsevier, 7 Mar. 2023, www.sciencedirect.com/science/article/pii/S2351989423000628.

Montalva, Felipe, et al. “Lower Marine Productivity Increases Agonistic Interactions between Sea Lions and Fur Seals in Northern Pacific Patagonia.” Academic.Oup.Com, Dec. 2022, academic.oup.com/cz/article/68/6/657/6521008.