| Date | November 2015 | Marks available | 2 | Reference code | 15N.2.SL.TZ0.1 |
| Level | Standard level | Paper | Paper 2 | Time zone | TZ0 |
| Command term | Distinguish | Question number | 1 | Adapted from | N/A |
Question
Sockeye salmon (Oncorhynchus nerka) spend the first years of their lives in the freshwater lakes of Alaska before migrating to marine waters. Their first months in marine waters are spent foraging and growing near the shore line. They then move to offshore regions of the North Pacific Ocean for 2 to 3 years.
The graph shows fork length frequency of juvenile O. nerka caught during their first months in marine waters in autumn 2008 and ocean age one O. nerka caught 15 months later during winter 2009 in the North Pacific Ocean.
Protein content in O. nerka was measured to evaluate possible differences during their first 15 months at sea. The graph shows the relationship between fork length and total protein content per O. nerka caught during autumn 2008 and winter 2009.
Scientists measured mercury levels in different fish. The table shows the results.
Identify the most frequent fork length for O. nerka caught during autumn 2008 and winter 2009.
Autumn 2008:
Winter 2009:
Distinguish between the fork lengths of O. nerka in autumn 2008 and winter 2009.
Suggest a reason for the variation in fork length of ocean age one O. nerka.
Compare the protein content for O. nerka caught during autumn 2008 and winter 2009.
Outline the difficulty in predicting the age of O. nerka from fork length.
Using the data, suggest one reason for the relationship between protein content and fork length.
Compare the results shown in the table for monkfish and shark.
Suggest additional information that would be helpful in evaluating these data.
State which type of fish shows the most variation.
Markscheme
autumn 2008: 175mm or 180mm; (accept either 175mm or 180mm – do not accept in between values)
winter 2009: 250mm or 255mm; (accept either 250mm or 255mm – do not accept in between values) (both needed)
a. shorter salmon in autumn 2008 / longer salmon in winter 2009;
b. wider range of length in fish collected during autumn;
c. higher peaks in winter compared to autumn;
Accept numerical values if clearly stated that one is bigger than the other.
genetics/gender/ food availability/diet/water temperature/predators/age
a. both show direct/positive correlation/linear relationship;
b. values for fish collected in winter 2009 are higher than for autumn 2008;
c. many common values in both sets of data;
d. differences between winter and autumn may not be significant because of the overlapping data;
Award [1 max] if only similarity or difference provided.
Difficult because of overlap in fork length between juvenile and ocean age one O. nerka / total protein depends on fork length/size, not (only) age, so difficult to predict
Growth is a result of incorporating protein / larger fish have more protein/more muscle/more cells.
a. average/mean mercury concentration is higher for shark/lower for monkfish;
b. small number of samples for monkfish (so data less reliable) / large number of samples for shark (so data more reliable);
c. minimum for shark is well below minimum for monkfish / maximum for shark is well above maximum for monkfish;
d. range/standard deviation/variation is greater for sharks;
a. age of fish / details of the method used / chemical form of mercury / part of fish analysed / gender / trophic level of fish;
b. statistical calculations eg: t-test/mode;
c. exact location of sampling as some areas of environment may have more mercury pollution than others;
shark (shows the most deviation/variation)
Examiners report
Most candidates were able to identify the most frequent fork lengths as 175 or 180 and 250 or 255. Weaker candidates lost the mark for stating values in between.
Most were able to spot that the salmon were longer in Winter 2009. Weaker candidates just stated figures without qualification.
Many candidates compared the two ages of salmon, rather than answering the question.
As a compare question, at least one difference and one similarity were needed. Most commented that the values for 2009 were greater than 2008. Surprisingly few noted that they both showed positive correlation. Similarly in the second part the large amount of overlap between the two ages was not well spotted.
As a compare question, at least one difference and one similarity were needed. Most commented that the values for 2009 were greater than 2008. Surprisingly few noted that they both showed positive correlation. Similarly in the second part the large amount of overlap between the two ages was not well spotted.
An answer that implied knowledge that protein was a structural component of the fish was sought, so more protein as part of the fish meant a bigger fish.
Many lost a mark for not stating that shark had a larger mean mercury content. Large numbers commented that there were many more sharks than monkfish in the survey, but did not extend it to say that this meant that the data was more reliable for the shark.
Many lost a mark for not stating that shark had a larger mean mercury content. Large numbers commented that there were many more sharks than monkfish in the survey, but did not extend it to say that this meant that the data was more reliable for the shark.
Most, but certainly not all, were able to state that the shark showed the most variation.
