Reading Fish Scales Under the Microscope

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

Answer Key

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

On average, a Sun Pond bluegill with 3 winter bands was 55 mm longer than a Mill Pond bluegill with 3 winter bands.

Between 1 and 2 winter bands, the average Sun Pond bluegill grew 55 mm, and the average Mill Pond bluegill grew 35 mm.

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.

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

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 parents
B. Bluegill from Sun Pond parents and bluegill from Mill Pond parents
C. Bluegill from Mill Pond parents and bass from Sun Pond parents
D. Bass from Sun Pond parents and bluegill with 4 bands from Mill Pond

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.

In the same tanks and on the same food, the young bass averaged 76 mm longer than the young bluegill from the same pond.

Sort each difference by what most likely caused it.

In identical tanks, young bass grew to about 118 mm and young bluegill to about 42 mm.Bluegill with 3 winter bands were 160 mm long in Sun Pond and 105 mm in Mill Pond.On the same fish, scale ridges formed far apart in summer and close together in winter.The shortest young bass in the tanks was still about twice as long as the longest young bluegill.Mill Pond bluegill with 2 winter bands were 40 mm shorter than Sun Pond bluegill with 2 bands.

Mostly genes	Mostly the environment
In identical tanks, young bass grew to about 118 mm and young bluegill to about 42 mm. The shortest young bass in the tanks was still about twice as long as the longest young bluegill.	Bluegill with 3 winter bands were 160 mm long in Sun Pond and 105 mm in Mill Pond. On the same fish, scale ridges formed far apart in summer and close together in winter. Mill Pond bluegill with 2 winter bands were 40 mm shorter than Sun Pond bluegill with 2 bands.

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 fish
B. Plant more shade trees around Mill Pond, so its water stays cooler through the summer
C. Bring in bluegill from Sun Pond, since their parents pass on genes for faster growth
D. 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.

Claim: The data do not support this. Mill Pond bluegill are small mainly because of their environment, not their genes. Evidence: In the ponds, bluegill with 3 winter bands averaged 160 mm in Sun Pond but only 105 mm in Mill Pond. But in identical warm tanks with plenty of food, young bluegill from Mill Pond parents averaged 41 mm, almost the same as the 42 mm of young from Sun Pond parents. Reasoning: When the environment was the same, the two groups grew about the same, so their genes for growth are likely similar. Mill Pond is cooler and has many bluegill sharing few insects, so each fish gets less warmth and food for growth. Genes still matter: young bass in the same tanks grew to 118 mm, because bass inherit genes for a larger body.