Spare the Snail-Eaters: Design a Net That Lets By-Catch Escape

Investigation question: Can your team design a net that keeps the large fish a village needs while letting at least 8 of every 10 small snail-eating fish slip through, using no more than 20 strands of string?

Scientific purpose

Students take on the Lake Malawi design problem: fishing families need the large fish, but the small fish that eat disease-carrying snails must survive. With dry pasta as fish (rigatoni for the large target species, ditalini for the small snail-eaters), groups measure their fish, sharpen the starting criteria and constraints into measurable statements, build a string-mesh lift net on a cardboard frame, test it in three standard hauls, modify it from the results and retest. The class then pools every design into one graph of percent kept against mesh opening, the same kind of selectivity curve fisheries scientists use when they set mesh rules.

Why this helps students with this chapter

Mesh size is the lesson's own solution, and it is easy to say and hard to get right. Students find that the right opening is set by the sizes of the animals, that string thickness and slipping strands change the real opening, and that the two criteria pull against each other: smaller holes keep more of the target and more by-catch, while bigger holes release by-catch and lose some of the target. Writing criteria with numbers before building forces precision, testing and rebuilding makes iteration real, and pooling class data shows a pattern no single group could see.

Standards connections

MS-ETS1-1 (practice, stating criteria and constraints precisely, including impacts on people and the environment): Students start from supplied criteria (keep the large fish, release the small fish) and constraints (string budget, build time, durability), add one measurable criterion or constraint of their own, and use measured fish sizes to define the opening range a solution must hit. Because the core criteria are supplied, this is practice toward the PE.

MS-LS2-5 (practice, comparing competing designs by how well they maintain a by-catch species): The class compares many net designs by how well they spare the small snail-eating fish while still feeding families, a design problem about maintaining a species whose survival protects people from disease. It is a single, simplified criterion set, so it is practice toward evaluating competing solutions for biodiversity and ecosystem services.

MS-ETS1-2 (practice, evaluating competing designs with shared test data): Every design is tested the same way and judged against the same two numerical criteria, and the class graph lets students compare designs systematically. The test procedure is supplied, so this is practice.

Roles (rotate each session)

• Measurer and designer: Measures the pasta, proposes the spacing, marks tick lines on the cardboard border and checks the spacing with a ruler as strands go on.
• Builder: Tapes strands snugly across the window at the marked spacing and keeps count against the 20-strand budget.
• Hauler: Runs every haul the same way: same pour height, same straight lift, five gentle shakes counted aloud.
• Data manager: Counts kept and escaped fish of each kind, fills the tables, calculates percentages and posts the group's results on the class chart.

Materials

Estimated cost: 8 to 11 per class (estimate) (an estimate; A 1 lb box each of rigatoni and ditalini, about $1.50 to $2 each, supplies every group with spares; a roll of cotton twine is about $3 to $4; printing a recording sheet of about 3 pages per student is about $2 to $3 at 2 to 3 cents a page; cardboard comes from shipping boxes; tubs, rulers, tape and scissors are classroom stock. The pasta can be bagged and reused for several classes.)

Safety

• Wheat allergy or celiac disease: dry pasta sheds flour dust. Students with a wheat allergy use the plastic-bead version described in the low-cost alternative, or take the data-manager role and wear gloves.
• The pasta is not food once it has been handled: no eating it, and wash hands after the lab.
• Only the teacher uses the craft knife, before class. Students use blunt-tip scissors to trim string.
• Dropped pasta is slippery on hard floors and breaks into sharp little shards; sweep it up rather than stepping on it, and never throw pasta.

Teacher preparation and setup

1. Cut a 12 cm × 12 cm window in the center of each 20 cm × 20 cm cardboard square with a craft knife on a cutting mat, before class and away from students. Make 3 frames per group.
2. Pre-cut the string into 18 cm strands and bundle 45 per group so strand counting is easy.
3. Measure a few rigatoni and ditalini from your boxes across their narrowest width (the tube diameter). If the ditalini are wider than 10 mm or the rigatoni narrower than 13 mm, swap in another shape so the two sizes differ by at least 4 mm.
4. Count 10 rigatoni and 20 ditalini into a cup for each group (the standard haul), plus a few spares of each.
5. Draw the class chart and a blank class graph: clear mesh opening from 0 to 24 mm on the x-axis, percent kept from 0 to 100 on the y-axis.
6. Make one demonstration frame with strands about 12 mm apart so you can show the taping method, but do not test it in front of students.

Investigation design

Student procedure

1. Measure your fish. Measure the narrowest width (the tube diameter) of 3 rigatoni and 3 ditalini to the nearest millimeter, record them and find each average. Answer q1.
2. Sharpen the problem. Read the starting criteria and constraints. In q2, add one criterion or constraint of your own, written so it can be checked with a number or a yes/no test.
3. Plan design 1. Choose the spacing between strands in each direction (center to center), work out how many strands it needs on a 12 cm window (no more than 20), sketch the grid, and predict the percent of each fish it will keep (q3 and the sketch).
4. Build design 1 in 12 minutes or less. Mark tick lines on the cardboard border at your spacing. Tape one set of strands across the window, pulling each snug, then a second set at right angles to make a grid. Tape only on the cardboard, never across the window. Record the strands used.
5. Haul test. Lay the net flat in the bottom of the empty tub. Pour the haul cup (10 rigatoni, 20 ditalini) onto the window from about 5 cm above, and push any fish resting on the cardboard border onto the window. Lift the frame straight up about 15 cm, hold it level, and shake it gently side to side 5 times, counting aloud. Fish still on the net are kept; fish in the tub escaped.
6. Count and record the kept and escaped fish of each kind, return all 30 to the cup, and repeat for three hauls in all with the same hauler, pour height and number of shakes. A ditalini stuck inside a rigatoni counts as kept.
7. Calculate the percent of large fish kept (large kept ÷ 30 × 100) and the percent of small fish released (small escaped ÷ 60 × 100), and check each criterion and constraint.
8. Modify. Use your results to change one thing (the spacing in one or both directions, the number of strands, or how the strands are fixed), build design 2 on a fresh frame, write your evidence in q4, and run three more hauls.
9. Measure the clear opening of your better design: the gap between the edges of neighboring strands, not center to center, at three places, then average. Post the opening, strands used and both percentages on the class chart, and plot the class data on your graph: percent of large fish kept and percent of small fish kept against opening.
10. Clean up: bag the pasta for reuse, strip tape from the frames, sweep up broken pieces and wash hands. Answer the analysis questions on your own.

Expected observations

• These are predictions; the activity has not been physically tested. With ditalini about 8 mm and rigatoni about 15 mm wide, clear openings under about 8 mm keep nearly all the small fish; openings of roughly 10 to 13 mm should release most small fish while keeping nearly all large fish; openings above about 15 mm let rigatoni slip through end-first during the shakes.
• Planned and measured openings differ: string 1.5 mm thick makes the clear opening about 1.5 mm smaller than the center-to-center spacing, and strands taped loosely sag or drift.
• A few small fish stay even on good nets, wedged where strands cross, resting on a sagging strand or caught inside a rigatoni tube; a few large fish escape over the frame edge if the lift tilts.
• A square grid at 12 mm spacing needs 9 strands each way (18 in all) on the 12 cm window. A rectangular grid of about 11 mm by 20 mm (10 + 5 = 15 strands) can also stop the rigatoni and release the ditalini, because a fish passes only if both sides of the opening are wider than the fish.

Likely difficulties

• Uneven spacing from measuring each strand by eye. Have the measurer mark tick lines on the cardboard border first, then tape strands to the ticks.
• Loose strands that sag under the pasta make the middle openings bigger. Pull each strand snug before taping its second end.
• Inconsistent hauls: a higher pour or harder shaking changes the results more than the mesh does. Keep one hauler per design and count shakes aloud.
• Ditalini hiding inside rigatoni. Count them as kept, since real small fish also get trapped in a catch, and note how many.
• Running short of time for design 2. Groups can move strands on the same frame instead of starting fresh, as long as they record the new spacing.

Structured debrief

1. Build the class graph together. Two patterns should appear: the percent of small fish kept falls steeply once the opening is wider than the ditalini, and the percent of large fish kept stays high until the opening nears the rigatoni width. The window between the two drops is the design target. Ask what would happen to that window if the two species were closer in size.
2. Compare planned center-to-center spacing with the measured clear openings. Many groups find the real opening smaller than planned (string thickness) or uneven (strands that slipped). Ask why real fishing rules specify mesh by measurement and why inspectors measure nets with gauges.
3. Connect to Lake Malawi and to the California swordfish story on slide 21: a mesh rule works when the target is bigger than the by-catch; when the by-catch is bigger, such as turtles and dolphins, the fix has to be a different design, like an escape grid or different gear.
4. Ask which criterion the group would relax if it had to choose, and who would bear the cost: the families who eat the large fish, or the people exposed to more snails. That is the tradeoff at the heart of the lesson.

Recording sheet: what students fill in

Lab 2 recording sheet: Spare the Snail-Eaters

A fishing village on a large lake needs its large fish for food and income. Small fish of another species eat the snails that carry the bilharzia parasite, so the village wants them left in the lake. Rigatoni = large target fish. Ditalini = small snail-eating fish. Your net is a lift net: it rests on the bottom, the fish gather over it, and it is lifted straight up.

Analysis questions: answer key

Individual evidence of learning

Each student submits the measurement table, the written criterion or constraint (q2), the design 1 sketch, both designs' haul tables and summaries, the class chart and graph, and their own answers to q5 to q12. The criterion statement and the design 2 justification (q2, q4, q6) are the evidence for defining and iterating; q7 and q11 are the evidence for evaluating designs that protect by-catch.

Assessment criteria

Accessibility adaptations

• Provide a mesh guide card printed with lines at 8, 10, 12, 14 and 16 mm that sits under the window, so students can tape strands along the lines without measuring each one.
• Students with limited fine-motor control can direct strand placement and spacing while a partner tapes, or take the hauler or data-manager role.
• Use high-contrast pasta or beads for students with low vision, and count kept fish by moving them into a labeled tray.
• Offer a calculator and a percent strip: with 30 large fish each one is about 3.3 percent; with 60 small fish each one is about 1.7 percent.

Low-cost alternative

Use plastic craft beads instead of pasta: round beads about 16 to 20 mm across as the large fish and pony beads (about 9 mm by 6 mm) as the small fish. Beads are reusable for years and safe for students with wheat allergies. Any two sizes work if they differ by at least 4 mm; students measure them first.

Model limitations

Pasta does not swim, bend or choose where to go, and every piece of one kind is nearly the same size, while real fish of one species range from tiny young to large adults; real mesh rules are set from data on what sizes a net keeps across that whole range. The lift-and-shake test stands in for fish swimming against a net underwater, where mesh shape, water flow, weeds and the fish's own behavior all change what escapes. The 20-strand budget and the 80 percent targets are teaching choices, not real regulations. This activity has not been physically tested by the authors, and pasta sizes vary by brand, so the opening that works best must come from each class's own data.

Teacher notes

The activity is untested, so treat the expected openings as predictions and let the class graph decide. Check pasta sizes before class: the design space depends on a clear gap between the two widths. The most common errors are measuring spacing center to center but reporting it as the clear opening (q5 addresses this), counting a ditalini inside a rigatoni as escaped, and computing the small-fish percentage as kept rather than released. For q9, walk through why the 40 percent applies to the whole haul. If time allows, extend with a third species, small shells standing for young large fish that should also escape, and ask whether one mesh can meet three criteria at once.