Reading Fish Scales Under the Microscope

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Reading Fish Scales Under the Microscope

Use scale rings and tank data to explain what makes bluegill grow.

Two ponds, one kind of fish

Read the passage and Table 1, then answer the questions.

Rings on a Scale

1Alder Hollow Nature Center has two ponds a short walk apart. The nature center, the people and the numbers here are made up, but they are based on real pond studies. This fall, fish biologist Gwen Talbot ran a pond study with sixth grader Mateus Oyelaran's science class. The team caught bluegill, a common pond fish, in nets. They measured each fish, gently took two scales from its side and let it go. The scales grow back.

2Sun Pond is shallow and open to the sky. Its water warms quickly in spring, and its weedy edges are full of insects. Few bluegill live there. Mill Pond is deeper and shaded by tall trees, so its water stays cooler, and many more bluegill share its insects.

3In the classroom, the students looked at the scales under microscopes. A scale grows as the fish grows, adding thin ridges around its edge. In summer the new ridges form far apart. Each winter they crowd close together and make a dark band. By counting the dark bands, the students could tell how many winters a fish had lived. Fish biologists use the same method to find the ages of fish in lakes and rivers, much as foresters count tree rings.

4Ms. Talbot also showed the class a test the nature center ran in its fish room. Young bluegill from Sun Pond parents, young bluegill from Mill Pond parents and young largemouth bass, a larger kind of sunfish, hatched in the same season. All were raised in identical warm tanks and fed all they would eat for 5 months.

Table 1. Average Bluegill Length by Winter Bands
Winter bands on scale	Sun Pond (mm)	Mill Pond (mm)
1	65	45
2	120	80
3	160	105
4	185	125

1.

On average, a Sun Pond bluegill with 3 winter bands was mm longer than a Mill Pond bluegill with 3 winter bands.

2.

Between 1 and 2 winter bands, the average Sun Pond bluegill grew mm, and the average Mill Pond bluegill grew mm.

3.

Use paragraph 3. Why do the ridges on a bluegill scale most likely crowd together each winter?

A. The fish eats the most in winter, so its ridges pile up faster than in summer.B. The fish sheds its old scales each winter and grows back smaller ones.C. Mud packs into the scale in winter and fills the space between its ridges.D. Cold water slows the fish's growth, so its new ridges form close together.

4.

Use paragraph 2 and Table 1. Which explanation for the difference between the two ponds fits best?

A. Sun Pond fish were measured in spring, after a summer of growing more than the others.B. Sun Pond's warm water and plentiful insects shared by few fish let each fish grow faster.C. Mill Pond fish ate more insects each, so they used their energy to swim instead of grow.D. Mill Pond fish with the same number of bands were younger, so they had less time to grow.

The fish room test

Use paragraph 4 and Table 2 to answer the questions.

Table 2. Young Fish After 5 Months in Identical Warm Tanks
Young fish	Parents from	Average length (mm)	Shortest to longest (mm)
Bluegill	Sun Pond	42	35 to 49
Bluegill	Mill Pond	41	34 to 48
Largemouth bass	Sun Pond	118	100 to 135

5.

A classmate wonders whether Mill Pond bluegill stay short because they inherit genes for small size. Which two groups in Table 2 test this idea best?

A. Bluegill from Sun Pond parents and bass from Sun Pond parentsB. Bluegill from Sun Pond parents and bluegill from Mill Pond parentsC. Bluegill from Mill Pond parents and bass from Sun Pond parentsD. Bass from Sun Pond parents and bluegill with 4 bands from Mill Pond

6.

What do the results for those two groups suggest about the short bluegill in Mill Pond?

A. Their slow growth comes mainly from the pond, since their young kept up in warm tanks.B. They inherit genes for slow growth, since their young grew 1 mm less on average.C. Their slow growth comes from the tanks, since young from both ponds stayed under 50 mm.D. They would need many generations of new genes before they grew faster in Sun Pond.

7.

In the same tanks and on the same food, the young bass averaged mm longer than the young bluegill from the same pond.

8.

Sort each difference by what most likely caused it.

9.

The fish room keeps one bluegill in a tank at the same warm temperature all year and feeds it well every day. Two years later, the students look at one of its scales. Which prediction fits paragraph 3 best?

A. Its scale would have ridges crowded close together from the center to the edge.B. Its scale would show two dark bands, one for each year it lived in the tank.C. Its scale would show few or no clear dark bands, since its growth never slowed for winter.D. Its scale would show four dark bands, since a warm fish grows twice as fast.

10.

The nature center wants larger bluegill in Mill Pond. Based on both tables and paragraph 2, which plan would most likely help the most?

A. Move some Mill Pond bluegill elsewhere, so the rest share the insects with fewer fishB. Plant more shade trees around Mill Pond, so its water stays cooler through the summerC. Bring in bluegill from Sun Pond, since their parents pass on genes for faster growthD. Add more bluegill to Mill Pond, so the fish compete harder and grow larger to win food

11.

A visitor says, "The bluegill in Mill Pond are small because they come from a small kind of bluegill." Do the data support this? Write a claim, give evidence from both tables, and explain your reasoning about genetic and environmental factors.