Scientists carried out an investigation into the effects of synthetic oestrogen on a population of fathead minnows living in lakes in Canada. Table 1 summarises the scientis ts’ procedure.
Table 1 The protocol
Experimental lake (A) Control lake (B)
Years 1999 and 2000
(prior to starting treatment)
Years 2001
(treatment)
–2003
No synthetic oestrogen added
Regular addition of synthetic oestrogen over 3 years of treatment
Years 2004 and 2005
(after treatment stopped)
Measurements were collected from fathead minnows over the 7-year period:
 concentration of the egg protein vitellogenin (VTG)
 concentration of vitellogenin mRNA
 numbers and lengths of fish
In the original investigation there were two control lakes, but for simplicity only one is shown here.
Figure 1 summarises the data for the concentration of VTG in whole-body homogenates of male fathead minnows.
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Year
1999
2000
2001
2002
2003
1 Explain the advantage of the break in the y -axis on this graph. (1 mark)
2 Explain what is meant by a whole-body homogenate . (2 marks)
3 Explain the difference in the results for lakes A and B. (3 marks)
Table 2 shows the population of fathead minnows in lakes A and B for each of the 7 years of the investigation.
Table 2
Mean population index
Control lake (A)
490.3
Experimental lake (B)
180.0
19.1
56.1
9.6
20.6
50.3
117.7
0.7
2.6
Addition of synthetic oestrogen
2004
2005
51.9
355.9
0.1
0.1
4 The figures were obtained by counting the number of minnows caught in traps and calculating a population index. Explain why the figures were not given as the total population. (1 mark)
5 Describe the results shown in the table. (3 marks)
Figure 2 shows two ways of measuring the length of a fathead minnow.
Figure 2
6 Fork length gives a more reliable measurement of the length of the fish than total length.
Suggest why. (1 mark)
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The histograms in Figure 3 show the lengths of samples of fish taken from the experimental lake (A) and from the control lake (B) in 2000, 2002 and 2004.
Figure 3 Lengths of fish
7 (a) Few fathead minnows live beyond 2 years. In any given year there are young fish that hatched that year and mature fish that hatched in the previous year. Explain how the histograms support this information. (1 mark)
(b) Use the data from the table to explain the results for 2002 in lake A. (3 marks)
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1 Allows a wide range of values/high and low values to be plotted/read accurately; (1)
2 In homogenate the cells have been mechanically disrupted;
In this case the whole fish/all tissues homogenised; (2)
3 Lake B and lake A in 2000 and 2001 show natural variation/small differences may not be significant;
Synthetic oestrogen added to lake A in 2002;
Stimulates secretion of VTG by liver;
Carried by blood to all organs (so homogenate concentration increases); (max 3)
4 Idea of difficulty of obtaining accurate/reliable count; (1)
5 Variable population in lake B and in lake A before treatment;
In 2002/year after treatment large drop in population of lake A;
Population in lake A after treatment stays low/does not recover; (3)
6 The ends of the tail may be worn/two lobes of tail fin may be different lengths; (1)
7 (a) The histogram is bimodal/contains two peaks, suggesting two classes of fish; (1)
Better students might point out that this is not the only explanation for the bimodal distribution. It is possible that all the fish are in a single year group with the difference attributable to sexual dimorphism.
7 (b) The table shows the large decrease in population the year after the addition of synthetic oestrogen;
Histogram shows no young fish hatch in 2002;
Small number in histogram the result of few fish hatching in 2001; (3)
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