1. Role of mesopelagic fishes in the marine ecosystem
Authors: Amanda Netburn, Scripps Institution of Oceanography; Danny Blas, Kearny High School
Why: Students have previously learned about the mesopelagic environment, the animals that live there, and human impacts on this environment. This lesson synthesizes much of the information from the previous lesson. The lesson first introduces students to the role of mesopelagic fishes in the marine food web, and then challenges them to consider the interactions between humans and the marine environment, introduces them to the idea of managing wild-catch fisheries, and invokes them to make their own management decisions based on different scenarios.
Summary: The terms “fishery management” and “ecosystem-based management” are introduced, followed by further descriptions of ecosystem-based management and how it is enacted. An example of ecosystem-based management is provided. There is a description of forage fish, and students are prompted to discuss how ecosystem-based management applies to mesopelagic fishes. The accompanying activity walks students through various scenarios in which they must make the management decisions.
Image credit:
Context for Use: This is the final lesson in a 5-lesson unit “Mesopelagic environment, adaptations and human impacts.” The unit was developed for a 9th grade Earth Science class, but would be applicable for any Marine Science or Environmental Science courses.
Misconceptions:
Mesopelagic fishes are not important to marine food webs
Wild- caught fisheries are not managed
Evaluation Tips:
The accompanying handout is required for the activity: Lesson5_EBMactivity
Students will work in groups of 2 to complete the ecosystem-based management simulation.

2. Forage fish are an important link in the ecosystem
The image shows the direction of energy flow from the plankton to the predators. The “forage fish” includes small fishes like anchovies and sardines, as well as squid and larger shrimp-like animals (neither squid or crustaceans are “fish” but they are included in this category). Forage fish eat zooplankton and are eaten by larger predators, such as sharks, large fishes, and marine mammals. They are therefore very important to transferring energy from low (e.g., zooplankton) to high (e.g., top predators) trophic levels. Common zooplankton (very small animals) are things like copepods and krill (small crustaceans related to shrimps and crabs), although include many different types of animals.
Ask students: Where do the plankton get their energy from? (Answer: The zooplankton get their energy from phytoplankton- the small photosynthetic algaes that harvest their energy from sunlight.
Image Credit:
Forage fish are small fish that transfer energy from zooplankton to large predators

3. MESOPELAGIC FISH ARE FORAGE FISH TOO!
For a long time the role of mesopelagic fish in transferring energy in the marine environment was ignored, but they are quite abundant and an important food source to many different top predators.

4. Predators
Prey
This is a food web that shows the role of a common species of mesopelagic fish, Stenobrachius leucopsarus (pictured in the middle) in the oceanic food webs. All animals shown below the broken grey line are prey to S. leucopsarus and those above are their predators. As you can see, these mesopelagic fish link a huge range of zooplankton to a huge range of top predators. Their loss in the system could potentially cause serious problems for the animals that eat them.
Note: It may seem odd that there is a seabird in this food web. A portion of this species of fish (and a number of other species) migrate into the surface waters during the nighttime where they feed. This is likely when they are fed upon by seabirds, which have been found to have mesopelagic fish in their stomachs.
Beamish et al. 1999

5. What will happen if populations of mesopelagic fishes decline?
Prompt the student to come up with answers to this question.

6. What will happen if populations of mesopelagic fishes decline?
It could cause declines of seabirds, tunas and sharks, and dolphins (because they don’t have enough food).
Could increase populations of zooplankton (because it would decrease predation on them).

7. What could cause mesopelagic fish populations to decline?
At present, mesopelagic fishes (at least in the US) are not under fishing pressure because we do not harvest them commercially. However, students learned in the previous lesson that there are threats to these animals that could affect their numbers.
Prompt the student to come up with answers to this question *Note: this is review from the last lesson, but important to emphasize here when thinking about why we may need to conserve these kinds of fish, even though they are not what fisherman are typically fishing for.

8. What could cause mesopelagic fish populations to decline?
eating microplastic
climate change, acidification, decreasing oxygen levels
pollution
oil spills
fishing
Plastics found in mesopelagic fish
The figures are photos of microscopic pieces of plastic found in the stomachs of some mesopelagic fishes.
Image credit: Davison and Asch 2011 (

9. Limit fishery harvests by government agencies
Fisheries management
Limit fishery harvests by government agencies
Traditionally, each species was managed completed separately from each other
Definition of fisheries management.

10. Ecosystem-based Management
Place-based approach to natural resource use that aims to restore and protect the health, function and resilience of entire ecosystems for the benefit of all organisms
It does not make much sense to manage individual species of fish without considering the availability of their prey, pressure from their predators, and population responses to environmental variability, which all, in addition to fishing pressure, affect how many fish are available at any one time. Scientists have recently come up with the concept of “Ecosystem-based management” which aims to take all these factors into consideration when setting limitations on how many of each species can be fished.
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11. Ecosystem-based Management
Takes into account the environment, the food web, and human interests (e.g., fishing, conservation)
Further elaboration on EBM. Emphasize that in EBM, conserving the environment is as important as economic and social interests, such as fishing. In EBM, it is not only important to conserve the environment, but also to preserve jobs and a “way of life” that is a important in fishing communities.
Image credits:
food web: Encyclopedia Britannica (
fisherman: Wikipedia Commons

12. Example of Ecosystem-Based Management
In 2013, sea lion pups were found stranding on beaches. They were starving.
This is an example of a time when EBM was used. In 2013, sea lion pups stranded on California beaches, and seemed to be starving. Scientists believe that they did not have enough of their favorite food, sardines available because of overfishing for sardines.
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More information:

13. Example of Ecosystem-Based Management
It turns out there was not enough of their favorite food, the forage fish sardines
This is an example of a time when EBM was used. In 2013, sea lion pups stranded on California beaches, and seemed to be starving. Scientists believe that they did not have enough of their favorite food, sardines available because of overfishing for sardines.

14. Example of Ecosystem-Based Management
It turns out there was not enough of their favorite food, the forage fish sardines
This is an example of a time when EBM was used. In 2013, sea lion pups stranded on California beaches, and seemed to be starving. Scientists believe that they did not have enough of their favorite food, sardines available because of overfishing for sardines.
Response: Managers decreased the amount of sardines that fishermen can take, so the population could recover

15. Conclusions Forage fish are a key link in marine ecosystems
Mesopelagic fish are forage fish that are threatened by human activities
Ecosystem-based management aims to restore and protect the health, function and resilience of entire ecosystems for the benefit of the environment and humans
Forage fish are key to transferring energy from the lower trophic levels (i.e., the plankton) to top predators in the oceans. They are a key food source for many of the animals that are important for commercial fishing and for animals with conservation concern (e.g., dolphins, sea lions).
The mesopelagic fish are a key forage fish, and are fed on by many important top oceanic predators.
It is important to take into account the whole ecosystem in order to conserve the resources of the ocean. Fishery management has traditionally ignored the environment, but ecosystem-based management takes into account the availability of prey as well as the environmental conditions.

16. Assignment You will become a fisheries manager
Scenarios are presented, and you have to make decisions to make sure that the health of the ecosystem and fisherman’s interests are well-balanced
The accompanying handout is required for the activity: Lesson5_EBMactivity
Students will work in groups of 2 to complete the ecosystem-based management simulation.
It is useful to go over the first couple of questions together as a class to make sure students understand what is being asked of them.