Seals – How Are They Affected by Human Activity?
By Tessa Broholm
There are 33 species of pinnipeds alive in the world today, most of which are seals. Other spectacular animals within this category include walruses and sea lions.
All pinnipeds are semi-aquatic marine mammals. This means they must spend part of their life on solid ground. Seals need to be out of the water during mating and breeding seasons.
Seals are carnivores, meaning they must eat other animals to survive. Most species consume fish, but there are some exceptions. For example, the leopard seal will hunt penguins, or even other seals!
The largest threats to seals are humans and the byproducts of our activity. Historically, hunters targeted seals for their fur and hunted several species to extinction. Today, seal hunting is much better regulated to help preserve seal populations.
A byproduct of human activity is greenhouse gases, which contribute to climate change. One result of climate change is the disappearance of sea ice. Some pinniped species rely on sea ice to raise their pups, and without it they are in trouble. This post will focus on some of the less obvious threats to seals that are still a byproduct of human activity.
Plastic In The Food Chain
Microplastics and Bioacculumation
Microplastics are particles of plastic smaller than 5 mm in size. This categorizes microplastics as environmental contaminants. Many other posts on this blog discuss microplastics and how they affect the ocean. Read More.
Seals are not immune to the impacts of microplastics. Seals do not eat microplastics but they are at risk of bioaccumulation. Bioaccumulation is the gradual accumulation of substances within an organism. This happens when the organism absorbs the substance faster than it eliminates it. One way to think of this is like filling a bathtub with a slow leak. Even though water will drain out of the bathtub, the tub will still continue to fill up with water.
Figure 2 shows an example of bioaccumulation. Organisms at the bottom of the food chain, like shrimp and small fish, eat microplastics. At low concentrations, microplastics are not dangerous to these individuals. The problem is when a predator eats a lot of prey. For example, if a seal eats a lot of fish, it will also eat the microplastics inside the fish. This high concentration of microplastics can be toxic or fatal to the seal.
A study of the feces of the fur seal in its eastern Pacific range found microplastics in over half of the seal specimens (Donahue et al., 2019). This is part of an ongoing evaluation on the threat of bioaccumulation in top predators (Donahue et al., 2019).
Help limit microplastic pollution by reducing your own personal use of plastic products. You may also want to consider:
- Sparking conversations within your community on how to reduce plastic consumption
- What proper waste channels are available to you
- Setting community standards for businesses that use certain products
Entanglement in Man-made Debris
Seals are also threatened by entanglement, primarily in materials sourced from fishing industries. Debris can trap seals. Young seals seem especially vulnerable to entanglement.
Early intervention for entrapped individuals has demonstrated a reduction in entanglement related mortality. Unfortunately, seals in remote areas are still threatened by discarded debris.
If you are at the beach or out on the water and see any kind of debris, please consider picking up and disposing it. Any debris pose a serious threat to the health and livelihood of seals.
If you have the time, consider joining Ocean Blue Project for a beach cleanup, or create your own clean up! To volunteer or learn more about other opportunities to get involved, follow this link: Ocean Blue Project Volunteers. These opportunities are fun, feel good, and most importantly, help us protect the environment.
Author Bio: Tessa Broholm is a biologist who works in conservation. She has worked with amphibians and birds, but she loves all wildlife.
Boren, L. J., Morrissey, M., Muller, C. G., & Gemmell, N. J. (2006). Entanglement of New Zealand fur seals in man-made debris at Kaikoura, New Zealand. Marine pollution bulletin, 52(4), 442-446.
Donohue, M. J., Masura, J., Gelatt, T., Ream, R., Baker, J. D., Faulhaber, K., & Lerner, D. T. (2019). Evaluating exposure of northern fur seals, Callorhinus ursinus, to microplastic pollution through fecal analysis. Marine pollution bulletin, 138, 213-221.
Hofmeyr, G. G., Bester, M. N., Kirkman, S. P., Lydersen, C., & Kovacs, K. M. (2006). Entanglement of antarctic fur seals at Bouvetøya, Southern Ocean. Marine pollution bulletin, 52(9), 1077-1080.
National Geographic. (n.d.) Seals. Accessed at: https://www.nationalgeographic.com/animals/mammals/group/seals-pinnipeds-walruses-sea-lions/
Nelms, S. E., Galloway, T. S., Godley, B. J., Jarvis, D. S., & Lindeque, P. K. (2018). Investigating microplastic trophic transfer in marine top predators. Environmental Pollution, 238, 999-1007.