| Date | November 2015 | Marks available | 1 | Reference code | 15N.2.HL.TZ0.1 |
| Level | Higher level | Paper | Paper 2 | Time zone | TZ0 |
| Command term | Describe | 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.
Lipid in O. nerka was measured to evaluate possible differences in energy status during their first 15 months at sea. The graph shows the relationship between fork length and lipid content for O. nerka caught during autumn 2008 and winter 2009.
Persistent organic pollutants, such as polychlorinated biphenyls (PCBs), have been shown to reach unpolluted arctic areas by air currents. Another method of transport of these pollutants into these ecosystems is provided by migrating O. nerka.
Pollutant transport was studied in a population of O. nerka in the Copper River (Alaska). The graph shows concentration of PCBs in muscle lipids of O. nerka in relation to the distance of upstream migration.
Identify the total number of O. nerka with fork length from 240 to 245 mm caught in autumn 2008 and winter 2009.
Compare the data in the graph for autumn 2008 and winter 2009.
Suggest two factors that could affect the distribution of O. nerka in the North Pacific Ocean.
State the range of lipid content measured in O. nerka caught during autumn 2008.
.......g
Outline any correlation between total lipid content and fork length in autumn 2008 and in winter 2009.
Autumn 2008:
Winter 2009:
Suggest reasons for the differences in lipid content.
Describe the relationship between the distance of upstream migration and the concentration of PCBs in O. nerka.
State the concentration of PCBs in muscle lipids at 125 km from the ocean estimated by the correlation line.
As the O. nerka migrate upstream they no longer feed. Suggest a reason for the relationship of distance of upstream migration and concentration of PCBs in muscle lipids.
Markscheme
59 or 50 + 9
higher average/mean/mode/median length in winter;
higher minimum length in winter / none below 225 mm in winter;
higher maximum length in winter / none above 260 mm in autumn;
smaller range (of length) in winter;
higher peaks in winter;
overlap between lengths of 260 and 225 mm;
more data/more caught in winter than in autumn;
Allow the converse for any of the mark points.
Allow size instead of length and allow the two groups to be referred to either by season (autumn/winter) or year (2008/09).
migration;
ocean currents;
food availability/distribution;
predation / fishing;
temperature;
location of rivers (flowing into the ocean);
depth of (sea)water;
Reject "pollution" and mark only the first two responses.
11.7 g; (accept answers in the range of 11.4 to 13.0 g)
1.8 to 13.5 g; (accept answers in the range of 1.6 to 2.0 g and 13.4 to 13.6 g)
autumn 2008:
positive correlation / fork length increases as lipid content increases;
winter 2009:
no correlation / no overall trend; (reject "constant" or "almost same")
lipid stores accumulated during summer/in freshwater/before moving out to sea;
food resources/availability may decline rapidly during autumn/winter;
more food close to the shore (than in offshore water);
lipid stores used up during winter / more active/migrating in winter;
Do not accept answers relating to the size of O. nerka because O. nerka with similar sizes showed different lipid content values.
direct/positive correlation / higher PCB concentration further up the river
1000 ng g–1 (accept answers in the range 950 ng g–1 to 1050 ng g–1)
Do not award the mark if the units are missing or incorrect.
Allow units shown as ng g-1.
lipids used up so same quantity of PCB in smaller amount of lipid;
no excretion of lipids so same quantity of PCB in smaller amount of lipid;
PCB absorption through gills;
Examiners report
Most candidates read off the two numbers from the bar chart and added them together correctly, though some misread the intervals on the x-axis or read off the values on the y axis incorrectly.
The essential skill in questions such as this is to pick out significant features in the data. The answer should tell the reader the overall differences between the two populations and not simply quote values. There was some overlap between the two populations in fork length but the winter 2009 population had a larger sample size, larger average length and larger maximum and minimum fork lengths. It also had a smaller range of fork lengths than the autumn 2008 population. In future exams the command term ‘compare’ will mean only similarities, not similarities and/or differences.
Most candidates suggested two acceptable factors that could affect distribution.
Candidates found it hard to read off values from this scatter graph, perhaps because of the large size of the squares and diamonds used for the data points. Candidates should be reminded that it is the centre of any data point that indicates the precise value, not the upper or lower edges. Because of the difficulties, a relatively wide range of answers was accepted, but even so only about half of candidates gave an acceptable range.
This was generally well answered with candidates stating that there was a positive correlation in Autumn 2008 and no correlation in Winter 2009. The commonest mistake was to state that there was no variation in total lipid content in Winter 2009: there was, but no clear trend lipid content according to fork length.
Candidates gave a wide range of reasons for the differences in lipid content. The best answers were from candidates who had read all the information in the question about the biology of Sockeye salmon and had used it to deduce the reason for depletion of lipids in the salmon in winter.
There was confusion in some candidates’ minds between cause and effect, with statements such as that the sea water is colder in winter so the salmon are less active, need less energy and therefore have smaller stores of lipid. If we applied this sort of logic to humans, the reason for obesity would be a need for more stored energy reserves in obese people because they are more active. Some candidates assumed erroneously that the physiology of fish is the same as that of mammals, with constant high body temperature and subcutaneous storage of lipids for heat insulation.
Most candidates correctly stated that there is a positive correlation between PCB concentration and distance upstream.
This proved to be quite a discriminating question, with the strongest candidates working out that if lipids are used up, but PCBs cannot be removed from the body, then the concentration of PCBs in lipids will increase.
