A variety of different fish swim around coral in part of a coral reef.|An example of a food chain, from green algae to mollusks, sculpin, and then salmon, from “Waterford's Energy Flow through Ecosystems.”|A computer-generated image of a sea otter looking for food underwater.|

There's Always A Bigger Fish (Until There Isn't)

Sustainable fishing is now a common topic in both marine science and in discussions about food and the food industry. If you’re a seafood lover, chances are that you’ve heard about overfishing. You have probably given thought to where the fish on your plate comes from. However, you might not know exactly why sustainable fishing is so important. Let's take a closer look.

An example of a food chain, from green algae to mollusks, sculpin, and then salmon, from “Waterford's Energy Flow through Ecosystems.”
An example of a food chain, from “Waterford's Energy Flow through Ecosystems” (OpenStax).

Marine Ecosystems

All marine ecosystems are dependent on “food webs,” a series of interconnected relationships between organisms. Energy moves from the primary producers, such as algae or phytoplankton, all the way to the apex predators, such as sharks and killer whales, as organisms consume one another in a “food chain.” These food chains all connect into one big marine food web, with organisms all linked to each other through predation and consumption. Without all the necessary links in the chain, and all the necessary chains in the web, the health of the ecosystem is compromised.

The Atlantic White Shark Conservancy’s Food Chain Investigation video gives an example of a marine food web where each organism plays a vital role in maintaining a delicate balance in the ecosystem.

Keystone Species

However, some species play an even greater role in maintaining the state of their ecosystems. These keystone species shape the environment and interactions around them through their place in the food web. Keystone species can keep the population sizes of other marine species in check, which means that even more species can coexist in the ecosystem. They can also engineer the environment in a way that creates spaces for other species to go about their lives. Without keystone species, the entire ecosystem changes, which could lead to other organisms in the food web decreasing in number or even disappearing.

A computer-generated image of a sea otter looking for food underwater.
Sea otters are an example of a keystone species, as they have a major impact on the health of kelp forest ecosystems.

The virtual kelp forest tour from the California Academy of Sciences highlights sea otters as a keystone species. Sea otters keep sea urchin populations under control, which in turn keeps the kelp forests healthy and thriving.

Using the Keystone Experiment, an interactive from LabXchange, provides insight into how the keystone species concept was established. It also explains how the removal of a keystone species can drastically alter an ecosystem.

The Threat of Overfishing

With food webs and keystone species in mind, think about what would happen if overfishing caused a substantial decrease in the population of one or more species in a marine food web, especially keystone species. Some fishing methods have high levels of bycatch. This means that species other than the one being targeted are caught and killed as a result of fishing activity. Overfishing and unsustainable fishing methods leads to an increase in bycatch, which could have severely detrimental consequences for marine food chains and food webs. As the health of the entire ecosystem depends on the presence of keystone species and the functioning of these food webs, severe overfishing could lead to an ecosystem collapse.

Healthy Oceans: Sustainable Seafood from the California Academy of Sciences provides some great tips on how to choose your seafood sustainably. This is better for the environment - and better for you!

So the next time you’re craving fish, take some time to think carefully about what you order. This will help ensure that marine food chains, food webs, and keystone species can continue to keep ecosystems functional and thriving for generations to come.

To better understand the importance of marine ecosystems, take a dive into Marine Ecosystems and Food Webs. This free pathway can be shared with your entire class - or any budding marine biologists you may know.

Written by
Emily Spencer
Instructional Technologist

Emily Spencer is a member of the LabXchange curation team, and holds a bachelor’s degree in marine biology from the University of Cape Town. She is passionate about marine ecology and ecosystem interactions, as these interactions are what make the ocean such a mysterious and diverse place. She is especially fascinated by rock pools and kelp forests, of which there are plenty on the Cape Town coast.

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