Ocean Fisheries and Sustainability- Energetic Inputs and Energy Removal
Day _______________________________________
Learning Objective(s):

Understand that fisheries remove energy (in the form of biomass) from marine ecosystems.

Understand what constitutes a “sustainable” fishery.

Understand the trophic structure of marine food webs.

Understand the amount of energy it takes to produce and maintain top predators.

Understand people typically eat marine top predators, but not terrestrial top predators.
Grade 6 & 7 standards:

Analyze environmental risks caused by human interaction with biological systems

Propose possible solutions to address the environmental risks in biological systems.

Compare food chains in a specified ecosystem and their corresponding food web.
Abstract: Fisheries remove energy from marine ecosystem by harvesting target species. Many fisheries
also accidentally remove other animals and cause damage to marine habitats from their fishing gear. The
removal of target and non-target species, coupled with the possibility of causing habitat damage, has
profound consequences for maintaining energy flows in ecosystems.
Materials
three 10-20 gallon fish tanks
3 aquarium scoop nets
3 ladles/spoons
3 gill nets
aquarium rocks (~ 3 bags)
fake aquarium plants
fake aquarium animals (turtles, dolphins, whales, etc.)
krill (live or dead)*
live crickets (OPTIONAL to represent seabirds or other animals at the surface)
3 plastic trays
1 small box (for the plant/animal picture cards representing different trophic levels)
picture cards of animals
*frozen krill can be bought at Petco/Petsmart, etc for a few dollars. If you want to use live krill
(recommended), please contact me and I will let you know if/where they are available.
During snack
 Video of bottom trawl fishing: http://planetgreen.discovery.com/videos/blueaugust-oceana-destructive-trawling.html (This is NOT a YouTube video so it
should work in the classroom.)
Engage
10 minutes

Ask:
o
How many of you have ever eaten seafood?
o
What is your favorite kind of seafood?
o
How many of you would eat a whale, sea turtle, or sea lion?

Explain that when fishermen accidentally capture these animals, in a way, you
are also eating them!
o
So how many of you would eat shrimp if you knew you were also eating
dolphins, sea lions, whales, sharks, or sea turtles?

Demonstrate the gillnet fishing technique (the gillnets consist of two wooden poles
connected by a net) to all of the students, showing them (with your hands in the
water!) how other species (e.g., dolphins, turtles, sea lions) end up getting
entangled and captured in gillnets.

OK, now the students are ready to go fishing just like a commercial fisher! Before
starting the engage section you should have the three tanks set up: First, you will
have to put the aquarium rocks on the bottom of the tank, along with the fake plants
and animals, krill, crickets (if used), and fill the tank half way with water. It is
recommended to put each tank at a different table with the same amount of fake
plants and animals, krill, crickets (if used), and water.
o
Time to go fishing. EACH group will try the following two methods AFTER
the teacher demonstrates the gillnet method (they will not do the gillnet
method as its not intuitive and requires the teacher actually moving the
bycatch animals into the net):
1. The first method the students should use is the bottom trawl (aquarium
net). They should drag the net along the entire bottom of the tank,
picking up everything it its path. Here, the aquarium nets represent
bottom trawl fishing, in other words a “non-selective” fishing method
that catches everything in its path and damages bottom habitat. NOTE –
with the bottom trawl they are not trying to “catch” krill, but rather
simply dragging the net along the bottom and “digging into” the
substrate just like a real bottom trawl would.
2. Next, the ladles/spoons should be used to represent hook and line or pole
and troll fishing methods, in other words a “selective” fishing method.
NOTE – in this technique the students are actually trying to “catch” the
krill; i.e. a very “selective” method with little-no bycatch or habitat
damage.
Explore
25 minutes



Explain
25 minutes

Students will pour their catch onto the plastic trays at their tables. Have them
separate their catch into species and determine which gear caught the most target
species (krill) and which technique caught the most non-target species (bycatch).
They should quantify the differences between the 2 fishing techniques they used
(bottom trawl = aquarium net; ladle = hook and line or poll and troll).
It is important that the students see the difference between each fishing technique,
and how much less bycatch is caught from the poll and troll method (ladle/spoon)
as opposed to the bottom trawl and gill net techniques. They should also note how
much less damage to the habitat is done.
Have the students will draw a picture of what each tank looks like before and after
the bottom “fishing” (trawling) exercise (i.e., using the aquarium nets). This is to
demonstrate the damage to the bottom habitat from the trawl vs. the hook and line
or poll and troll method.
Introduce and explain the following concepts:
o Food web structure and trophic levels
o All ocean animals rely on primary productivity, which forms the basis of the
food chain.
o In terrestrial environments, primary producers are plants, but in marine
systems, most of the primary production comes from algae.
o

Expand
20 minutes

Energy from primary production is transferred up the food web until it gets to
the top predators. For example, algae (phytoplankton) are eaten by tiny animals
called zooplankton, which are eaten by small fish, then bigger fish, and
ultimately by top predators such as killer whales or sharks at the top of the food
web.
o Energy transfer between trophic levels is not 100% efficient, so some energy is
lost at each trophic step. As a result, it takes even more biomass from the lower
trophic levels to produce top predators than would be expected if all the energy
of a prey item were converted to biomass of the consumer. This is why there
are very few top predators; because there is not enough energy to support a lot
of them.
o By removing top predators through fisheries we are removing a significant
amount of the energy they have accumulated from the ecosystem (you can also
discuss the ecosystem consequences of removing these top predators, such as
trophic cascades). If these predators were to die naturally, the energy in their
bodies would remain in the marine ecosystem because it would be eaten or
decomposed via consumers, detritovores, and decomposers.
o Many large marine organisms have life history strategies that entail delayed
reproduction (e.g., many species of sharks don’t reproduce until they are ~20
years old). Removing young individuals that have not yet reproduced means
the population cannot replace/sustain itself.
o Sustainable fisheries are ones that do not target large predators or capture other
animals as bycatch, since both of these practices removes large amounts of
energy from ocean ecosystems and have other impacts on species persistence
and the ecosystem.
Next, have the class work together to replicate a food web. The students will form
each trophic level. Have 8-10 students be the phytoplankton - they will be the base
of the food web (L1) and will stand by themselves distributed across one side of the
room. Another group of 6 students
L4
XXXX
will be the zooplankton - they will
nd
be the 2 trophic level (L2) and
L3
XXX XXX
will stand in pairs , linked arm-inarm, to the side of all of the
L2
XX XX XX
phytoplankton students (each pair
represents 1 organism). Another
L1
X X X X X X X X X X
group of 6 students will be
rd
carnivorous fish in the 3 trophic
level (L3) - they will stand in
triplets ), linked arm-in-arm, to the side of zooplankton students (each triplet
represents 1 organism. The last group of 4 students will be the 4th trophic level/top
predator (L4) - they will stand as one organism, linked arm-in-arm, to the side of
the carnivorous fish. This exercise will simulate the aggregation of energy up food
chains. It demonstrates that it takes a lot of organisms at the lower trophic levels to
make one top predator. See the diagram for visual (X’s indicate the students).
In their research notebooks, have the students design their own bycatch removal
strategy that reduces the catch of non-target organisms and cause less destruction to
the ocean environment.* OPTIONAL* Students will be asked to write a letter to
their congressional leader and principal about why they should eat sustainable
seafood, and why cafeterias at their school (and in AZ) should serve sustainable
seafood to their students.
Evaluate
10 minutes
Have the students grab three picture cards from the box (some will pick out three
different trophic levels, whereas others may pick all the same trophic levels – no
worries – they will have to figure it out. Then, have the groups pool their pictures with
the other students at their table to build a representative marine food web. Note that
there is an equal number of cards that represent: (1) primary producers (algae, plants,
phytoplankton), (2) primary consumers (zooplankton), (3) secondary consumers (small
fish), (4) tertiary consumers (large fish, turtles, seabirds, etc.), and (5) quaternary
consumers (apex predators; e.g. sharks, killer whales, etc.).