2023 Cambridge IGCSE Environmental Management (0680) Practice Paper with Answers

Part (a) requires the precise ecological definition of a population. Part (b) asks for non-living factors that directly affect plant growth. Part (c) tests understanding of trophic levels and energy sources (autotroph vs. herbivore).

(a) [1 mark] Group of organisms of the same species living in the same area at the same time. (b) [2 marks] Any two from: light intensity, water availability/soil moisture, temperature, mineral ion availability, soil pH. (c) [2 marks] 1 mark for producer explanation (autotroph/uses photosynthesis) and 1 mark for primary consumer explanation (herbivore/feeds on plants).

Part (a) focuses on the immediate environmental degradation associated with surface mining. Part (b) tests knowledge of reclamation strategies, including landscaping, soil management, and revegetation.

(a) [2 marks] Any two from: loss of habitat/deforestation, soil erosion, dust/air pollution, noise pollution (from machinery/blasting), water contamination/acid mine drainage. (b) [3 marks] Any three from: backfilling/landfilling the excavated crater, grading/smoothing the land profile, replacing saved topsoil, planting trees/wildflowers/crops (revegetation), bioremediation of toxic soil, or flooding the pit to create a lake/reservoir.

Part (a) asks for other common greenhouse gases. Part (b) requires explaining the link between atmospheric heat retention, ocean warming, thermal expansion, and terrestrial ice melt.

(a) [2 marks] Any two from: methane, water vapour, nitrous oxide, chlorofluorocarbons (CFCs). [Reject: carbon monoxide, nitrogen, oxygen]. (b) [3 marks] 1 mark for greenhouse gases trapping more heat/raising global temperatures; 1 mark for thermal expansion of ocean water (water expands as it warms); 1 mark for melting of land-based glaciers/ice sheets adding water to the oceans.

Part (a) defines the term bycatch. Part (b) identifies indiscriminate fishing practices. Part (c) explores mechanical and procedural mitigation strategies used to make commercial fishing more selective.

(a) [1 mark] Unintentional capture of non-target marine animals (e.g., dolphins, turtles, juvenile fish). (b) [2 marks] Any two from: bottom trawling, drift netting/gill nets, long-line fishing. (c) [2 marks] Any two from: increasing net mesh size, installing turtle excluder devices (TEDs), using acoustic scarers/pingers (to deter marine mammals), using circle hooks (instead of J-hooks), or restricting fishing seasons/areas.

a) Working:
Decrease in catch = \(18,500 - 13,200 = 5,300\) tonnes.
Percentage decrease = \(\frac{5,300}{18,500} \times 100 = 28.648\%\)
Answer: 28.6% (accept 29% if rounded correctly).

b) Alternative strategies:
1. Closed seasons: Banning fishing during spawning periods to allow reproduction.
2. Mesh size regulations: Using larger net meshes to allow juvenile fish to escape and mature.
(Other acceptable answers: net type restrictions, pole-and-line fishing, marine reserves).

c) MPA benefits:
- Protects marine habitats from destructive gear (e.g., bottom trawlers).
- Provides a safe haven for adult fish to breed and larvae to settle undisturbed.
- Leads to a 'spillover effect' where recovered populations migrate into surrounding open-fishing areas.

d) Evaluation:
- Arguments for a complete ban (Agreement): Total protection ensures rapid population recovery; avoids the risk of illegal over-quota fishing; simpler to police than complex quota systems.
- Arguments against a complete ban (Disagreement): Severe economic losses for coastal communities/fishermen; could force fishermen into illegal, unregulated fishing; alternative managed methods (like quotas, size limits, and closed seasons) can achieve sustainability without destroying livelihoods.

a)
1 mark for correct calculation of decrease (5,300 tonnes) or correct formula shown.
1 mark for correct final answer of 28.6% (or 29%).

b)
2 marks for two valid strategies (1 mark each). Accept: closed seasons, net mesh size restrictions, no-take zones, banning drift nets, fish farming.

c)
3 marks for three points explaining how MPAs help (1 mark each):
- Safe breeding/spawning grounds.
- Protects benthic habitats from damaging fishing methods.
- Allows juvenile fish to grow to reproductive age.
- Spillover of adult/larval fish into adjacent fished areas.

d)
Up to 4 marks for a balanced evaluation:
- Max 2 marks for arguments supporting a total ban (e.g., zero catch guarantees recovery, prevents accidental bycatch).
- Max 2 marks for arguments opposing a total ban (e.g., job losses, economic collapse, rise in illegal black markets, success of alternative sustainable management).
- 1 mark for a clear, justified concluding stance.

a) Calculation:
\(3,600 / 400 = 9\)
Answer: 9 times.

b) Advantages and Disadvantages:
- Environmental advantage: Geothermal has much lower CO2 emissions (45 g/kWh compared to 820 g/kWh) OR geothermal has a much smaller land footprint (400 m² per MW vs 3,600 m² per MW).
- Economic disadvantage: Geothermal has a significantly higher initial capital cost (4.5 million USD/MW compared to 2.1 million USD/MW).

c) Sustainability explanation:
- Geothermal relies on the constant decay of radioactive elements and residual heat within the Earth, which is an infinite energy supply on human timescales.
- It produces very few greenhouse gases compared to fossil fuels, meaning it does not significantly contribute to global climate change.

d) Electricity generation process:
- Superheated water or steam is pumped up from deep underground reservoirs.
- The high-pressure steam is directed to turn the blades of a turbine.
- The mechanical energy of the spinning turbine drives a generator, which converts the kinetic energy into electrical energy.

e) Reasons for choosing coal:
- Lower initial capital setup cost, making it easier for developing nations to finance.
- Abundance of domestic coal resources which ensures national energy security.
- Coal power plants can be built anywhere, whereas geothermal is restricted to active tectonic margins or volcanic hotspots.
- Well-established existing grid infrastructure and technology for coal power.

a)
1 mark for the correct answer: 9 (times).

b)
1 mark for a correct environmental advantage referencing the data.
1 mark for a correct economic disadvantage referencing the data.

c)
2 marks for two points explaining sustainability (1 mark each):
- Non-depleting/replenished naturally by Earth's core.
- Minimal greenhouse gas emissions/no air pollution.

d)
3 marks for the sequential steps (1 mark each):
- Hot water/steam extracted/pumped from underground.
- Steam turns a turbine.
- Turbine spins a generator to produce electricity.

e)
3 marks for three distinct reasons (1 mark each):
- Coal plants are cheaper to build (lower capital cost).
- Geothermal is location-specific (requires tectonic activity).
- Abundant local supplies of coal/security of supply.
- Existing infrastructure/skills support coal power.

a) Practice D (Agroforestry with terracing) has the lowest value (1.1 t/ha/yr).

b) Calculation:
Reduction = \(45.2 - 12.8 = 32.4\) t/ha/yr.
Percentage reduction = \(\frac{32.4}{45.2} \times 100 = 71.68\%\)
Rounded to one decimal place: 71.7%

c) How terracing works:
- Terracing breaks up a long, steep slope into a series of flat, stepped platforms.
- This design slows down the speed of surface water runoff, reducing its ability to carry away topsoil, and allows more water to sink/infiltrate into the soil.

d) Agroforestry benefits:
- Leaf fall/litter from trees decomposes, adding organic matter/humus to the topsoil.
- Deep tree roots bring nutrients up from deep soil layers to the surface where shallow-rooted crops can access them.
- Nitrogen-fixing trees (e.g., legumes) enrich the soil's nitrogen content naturally.

e) Socio-economic impacts:
- Reduced crop yields: Topsoil loss removes fertile nutrients, causing poor harvests and leading to severe food insecurity/famine.
- Economic loss: Farmers have less surplus crop to sell, decreasing household income.
- High production costs: Farmers are forced to buy expensive artificial fertilizers to replace lost natural fertility.
- Migration: Land becomes completely unproductive, forcing families to abandon farms and migrate to cities (rural-urban migration), leading to social displacement.

a)
1 mark for identifying Practice D (or Agroforestry with terracing).

b)
1 mark for correct calculation of reduction (32.4 t/ha/yr) or formula shown.
1 mark for 71.7% (must be correct rounding to 1 decimal place).

c)
2 marks for explaining terracing mechanism (1 mark each):
- Steps/platforms slow down the velocity of surface water runoff.
- Increases time for water to infiltrate the soil, reducing transport of soil particles.

d)
2 marks for two distinct fertility benefits (1 mark each):
- Tree leaves decompose to add humus/nutrients to soil.
- Deep roots draw up nutrients from deeper layers.
- Leguminous trees fix nitrogen in the soil.

e)
4 marks for four developed points or two well-explained socio-economic impacts (1 mark per point):
- Decreased crop yields leading to malnutrition/hunger.
- Less income from selling crops at local markets.
- Debt/poverty due to the need to purchase chemical fertilizers.
- Loss of livelihood forcing migration to urban slums (social strain).

a) Calculation:
Total cleared or degraded area = \(250 + 1,850 = 2,100\) hectares.
Percentage of original forest = \(\frac{2,100}{12,500} \times 100 = 16.8\%\)
Answer: 16.8%

b) Road fragmentation process:
- A road acts as a physical gap that divides a large, continuous ecosystem into smaller, separated forest plots.
- Many forest animals cannot or will not cross roads, leading to isolated populations, limited gene flow, and increased 'edge effects' where wind and heat penetrate the forest edges.

c) Microclimate impacts:
- Increased ground temperatures (loss of canopy shade).
- Decreased humidity / drier air (less transpiration from trees).
- Reduced local rainfall.

d) Selective logging:
- Only specific, mature, or high-value tree species are harvested rather than removing the entire canopy.
- Young trees are left to grow, and the forest canopy remains mostly intact, preserving animal habitats, protecting soil from erosion, and allowing rapid natural regeneration.

e) Contribution to climate change:
- Forest trees act as carbon sinks by absorbing carbon dioxide (CO2) from the atmosphere during photosynthesis.
- Deforestation reduces the capacity of the earth to absorb CO2.
- Additionally, when trees are burned or decay after clearing, their stored carbon is released back into the atmosphere as CO2, a major greenhouse gas that traps heat and contributes to global warming.

a)
1 mark for calculating the total affected area (2,100 hectares).
1 mark for the correct percentage calculation (16.8%).

b)
2 marks for describing fragmentation (1 mark each):
- Splits large habitats into smaller, isolated fragments.
- Creates physical barriers that prevent species migration/mating.
- Increases edge effects (microclimate changes at forest boundaries).

c)
2 marks for two distinct microclimate impacts (1 mark each):
- Increased local temperature (loss of shade).
- Decreased transpiration leading to less rainfall/lower humidity.
- Increased wind speeds at ground level.

d)
2 marks for explaining selective logging (1 mark each):
- Only target/mature trees are cut down, leaving the rest of the canopy.
- Allows younger trees to grow/avoids total ecosystem collapse.

e)
3 marks for three points explaining global impact (1 mark each):
- Trees are carbon sinks / deforestation reduces CO2 absorption.
- Burning/decay of cut trees releases stored CO2.
- CO2 is a greenhouse gas that traps heat, enhancing the greenhouse effect.

a) Description of relationships:
- As the distance downstream increases, the nitrate concentration decreases.
- Concurrently, as the distance downstream increases, the dissolved oxygen level increases.

b) Oxygen dip explanation:
- The organic matter discharged at 0 km requires time to mix and flow downstream.
- Aerobic bacteria consume oxygen as they decompose this organic matter.
- It takes time (up to the 2 km mark) for the bacterial population to multiply and reach its peak activity, leading to maximum oxygen depletion (the lowest oxygen levels) slightly downstream of the source.

c) Nitrate effects on ecosystems:
- High nitrates cause rapid growth of algae, leading to algal blooms (eutrophication).
- Algae block sunlight, killing submerged plants. Bacteria decompose the dead plants, further depleting oxygen, leading to anoxia and the death of fish and other aquatic organisms.

d) Management strategies:
- Constructing manure storage tanks or anaerobic digesters to safely contain and process waste before disposal.
- Creating buffer zones (planting wide strips of vegetation/trees) along the riverbanks to absorb agricultural runoff before it reaches the water.
- Restricting the application of organic fertilizers/slurry on fields close to the river during rainy periods.

e) Other abiotic factors:
- pH (acidity of the water).
- Water temperature.
- Turbidity / suspended solids (clearness of the water).
- Phosphate concentration.

a)
1 mark for stating that dissolved oxygen increases downstream.
1 mark for stating that nitrate concentration decreases downstream.

b)
3 marks for explanation of oxygen dip (1 mark each):
- Organic waste takes time to mix/travel downstream.
- Aerobic bacteria decompose the organic matter.
- Bacterial populations/activity peak slightly downstream, consuming the dissolved oxygen faster than it can dissolve back into the water.

c)
2 marks for two distinct effects (1 mark each):
- Promotes rapid growth of algae/algal blooms (eutrophication).
- Blocks sunlight from reaching bottom-dwelling plants.
- Causes death of fish/aquatic animals due to anoxia/lack of oxygen.

d)
2 marks for two agricultural management strategies (1 mark each):
- Slurry storage lagoons/tanks.
- Buffer zones/riparian strips of vegetation.
- Proper crop/fertilizer management (timing applications).

e)
2 marks for two correct abiotic factors (1 mark each). Accept: pH, temperature, turbidity, phosphate levels, heavy metal levels. Reject: biotic index/macroinvertebrates (these are biotic).

I agree with the statement to some extent, as organic farming is highly effective at protecting ecosystems. It avoids synthetic fertilizers, which prevents nutrient leaching and eutrophication in nearby water bodies. Furthermore, by not using chemical pesticides, it prevents the bioaccumulation of toxins in food chains and protects vital pollinators like bees. It also uses crop rotation and natural compost, improving soil quality and reducing erosion. However, I disagree that it is the only way or even a fully viable way to ensure global food security. Organic farming generally has much lower crop yields than intensive farming. To feed the rising global population entirely on organic food would require a massive expansion of agricultural land, leading to widespread deforestation and habitat loss, which actually harms ecosystems. Additionally, organic food is more expensive, which could worsen food insecurity for low-income populations. In conclusion, a hybrid approach is better. Combining organic practices with sustainable intensive techniques, such as integrated pest management (IPM) and precision farming, can maximize yields to secure food while minimizing environmental harm.

Level 3 [5–6 marks]: A coherent response is given that develops and supports the candidate’s conclusion using relevant details and examples. Indicative content and subject-specific vocabulary are used precisely and accurately. Good responses present a balanced evaluation of the statement (both benefits and limitations of organic farming). Level 2 [3–4 marks]: Development and support of the conclusion is evident, though the response may lack some coherence and/or detail. Indicative content and subject-specific vocabulary are used but may lack precision and/or accuracy. Contains evaluation of the statement, but this may not be balanced. Level 1 [1–2 marks]: The response may be limited in development and/or support. Contradictions or irrelevant detail may be present. Indicative content is limited or absent. Level 0 [0 marks]: No response or no creditable response. Indicative content: Benefits of organic farming: no synthetic fertilizers reduces eutrophication, no chemical pesticides protects non-target species/pollinators, use of compost improves soil structure/water retention, crop rotation maintains soil fertility. Limitations of organic farming: lower crop yields per hectare, requires more land (could lead to deforestation), higher labor costs and expensive food prices. Alternative/hybrid solutions: Integrated Pest Management (IPM), precision agriculture, selective use of GM crops to reduce chemical dependency.

(a) (i) Use a transect line along the 100 m stretch; place a kick-sampling net at fixed intervals (e.g., every 10 m); disturb the riverbed upstream of the net for a set period of time (e.g., 30 seconds) to standardize the sample collection.

(ii) Dissolved Oxygen (DO): decreases as decomposers use up oxygen during aerobic respiration while breaking down organic matter. Temperature: may increase if the industrial effluent is warm, lowering oxygen solubility.

(b) (i) Total population at Site B = \(2 + 45 + 80 = 127\).
Percentage of Bloodworms = \(\frac{45}{127} \times 100 = 35.4\%\).

(ii) Bar chart plotted with: x-axis labeled as 'Sampling Site' with bars for A, B, C, D of equal width; y-axis labeled as 'Abundance of Tubifex worms' with a linear scale from 0 to 120; bars correctly plotted at 110, 80, 20, 2.

(c) (i) Site A. It has the highest abundance of Tubifex worms (110) and zero Mayfly nymphs. Tubifex worms are highly tolerant of very low oxygen levels (anoxic conditions), whereas Mayfly nymphs require highly oxygenated water to survive.

(ii) Install an on-site wastewater treatment plant to filter organic solids and neutralize chemicals; recycle and reuse cooling water to prevent thermal pollution; switch to biodegradable dyes and non-toxic processing chemicals.

(a) (i) Max 3 marks:
- Use of systematic sampling / transect line / fixed intervals [1];
- Use of kick-sampling net / standardized collection method [1];
- Standardized sampling duration / area of riverbed disturbed [1].

(ii) Max 4 marks:
- Parameter 1: Dissolved oxygen [1];
- Explanation 1: Decreases because decomposers/bacteria multiply and use up oxygen during aerobic respiration of organic waste [1];
- Parameter 2: Temperature / pH [1];
- Explanation 2: Industrial waste is often warmer than river water, reducing dissolved oxygen solubility OR chemicals alter pH, harming sensitive organisms [1].

(b) (i) Max 2 marks:
- Correct working: \(45 / (2 + 45 + 80) \times 100\) or \(45 / 127\) [1];
- Correct answer: 35.4% (accept 35% or 35.43%) [1].

(ii) Max 4 marks:
- Both axes fully labeled with units where appropriate [1];
- Suitable linear scale on y-axis taking up at least half the grid [1];
- All four bars plotted accurately with equal width and spacing [1];
- Clear identification of the bars matching the sites [1].

(c) (i) Max 3 marks:
- Site A identified [1];
- Explanation: Highest concentration of Tubifex worms which are highly tolerant of organic pollution/low oxygen [1];
- Supporting detail: Complete absence of sensitive Mayfly nymphs indicates oxygen is too depleted for them [1].

(ii) Max 4 marks (any two fully explained points or four stated points):
- Treat wastewater/effluent before discharge [1] to remove organic compounds/dyes [1];
- Use cooling ponds/towers [1] to prevent thermal pollution [1];
- Implement cleaner production techniques [1] such as water recycling or non-toxic chemical substitutes [1].

(a) Create a grid system across the 50 m by 50 m field (e.g., using 1 m intervals); use a random number generator to select five pairs of coordinates; locate these coordinate points in the field using measuring tapes to place the quadrats/sampling pits.

(b) (i) Field 2 mean: \((18+22+15+20+25) / 5 = 100 / 5 = 20.0\).
Field 3 mean: \((12+14+11+16+12) / 5 = 65 / 5 = 13.0\).

(ii) Field 2 has a much higher earthworm population (mean of 20) compared to Field 1 (mean of 3). Intensive tillage in Field 1 mechanically destroys earthworm burrows and kills earthworms directly, while synthetic fertilizers degrade soil organic matter. In contrast, no-till in Field 2 preserves soil structure and compost provides abundant organic food sources for the worms.

(c) (i) Mass of water retained = Wet mass - Dry mass.
Field 1: \(124\text{ g} - 100\text{ g} = 24\text{ g}\). Water retained % = \(\frac{24}{100} \times 100 = 24.0\%\).
Field 2: \(156\text{ g} - 100\text{ g} = 56\text{ g}\). Water retained % = \(\frac{56}{100} \times 100 = 56.0\%\).

(ii) Earthworms digest organic matter, creating humus which acts as a sponge to hold water; earthworm burrowing activity creates macro-pores in the soil which allows water to infiltrate and be retained, rather than running off the surface.

(d) High initial cost of purchasing specialized no-till seed drills; slower soil warming in spring due to surface mulch/residue delaying seed germination; potential increase in herbicide dependency in the first few years to control weeds without plowing; organic compost is bulky and labor-intensive to transport and apply.

(a) Max 3 marks:
- Lay out two perpendicular tape measures along the edges of the field to form axes [1];
- Use a random number generator/table to generate pairs of coordinate numbers [1];
- Locate the selected coordinates in the field to dig the sampling pits [1].

(b) (i) Max 2 marks:
- Correct mean for Field 2: 20 [1];
- Correct mean for Field 3: 13 [1].

(ii) Max 4 marks:
- Identification of difference: Field 2 has significantly more earthworms than Field 1 [1];
- Tillage effect: Plowing physically damages worms/burrows [1];
- Synthetic fertilizer effect: Chemical inputs reduce natural organic matter / acidify soil, driving worms away [1];
- Compost/no-till effect: No-till preserves habitats and compost provides food/nutrients for earthworms [1].

(c) (i) Max 3 marks:
- Correct formula/method of subtraction (Wet mass - 100 g) [1];
- Field 1 value: 24.0% [1];
- Field 2 value: 56.0% [1].

(ii) Max 4 marks:
- Worms create burrows/tunnels which improve soil pore structure/macropores [1];
- Tunnels increase rainwater infiltration / reduce surface run-off [1];
- Earthworms increase organic humus content in soil [1];
- Humus particles have high water-binding capacity / retain water molecules [1].

(d) Max 4 marks (any two points explained or four points stated):
- Higher initial equipment costs (e.g., no-till direct drills) [1];
- Compost is expensive/difficult to source and apply on a large scale [1];
- Transition period may see temporary drop in crop yields [1];
- Weeds can build up quickly without plowing, requiring alternative management [1].

(a) (i) Stretch a 100 m tape measure perpendicular to the highway edge into the forest to act as the transect line; place a 1 m² frame quadrat at fixed, systematic intervals along the tape (e.g., every 10 m); estimate or count the percentage cover of native shrubs and invasive weeds within each quadrat; repeat this process along multiple parallel transects to gain reliable, representative data.

(ii) The environmental conditions (light intensity, wind exposure, soil compaction) change continuously along a gradient from the edge of the road to the interior forest. Systematic sampling allows the changes across this gradient to be accurately mapped and correlated with distance.

(b) (i) Graph drawn with: x-axis labeled as 'Distance from highway edge / m' scaled from 0 to 60; y-axis labeled as 'Average cover / %' scaled from 0 to 100; both sets of data plotted with clear, distinct points connected by straight lines; clear key identifying native forest shrubs and invasive weeds.

(ii) As distance from the highway edge increases, the cover of native forest shrubs increases from 8% to 82%; conversely, the cover of invasive weeds decreases significantly from 85% to 2% over the same distance. The rate of change is greatest between 15 m and 35 m for both species.

(c) (i) Habitat fragmentation splits large populations into smaller, isolated groups; this reduces the gene pool and increases the risk of inbreeding depression; small isolated populations are more vulnerable to localized extinction from natural disasters; barriers like roads prevent migration to find food, water, or mates, leading to starvation or roadkill.

(ii) Construct wildlife underpasses or overpasses (green bridges) across the highway to link habitats; plant native vegetation corridors along fence lines to guide animals; establish buffer zones of dense native trees near the road edge to reduce noise and light pollution.

(a) (i) Max 4 marks:
- Lay out a tape measure perpendicular to the highway edge [1];
- Place quadrats at regular/predetermined intervals (e.g., every 5 m or 10 m) [1];
- Estimate the percentage cover of both native shrubs and invasive weeds inside each quadrat [1];
- Repeat with parallel transects to ensure reliability [1].

(ii) Max 2 marks:
- There is an environmental gradient / conditions change with distance from the highway [1];
- Systematic sampling allows changes along a clear physical gradient to be mapped [1].

(b) (i) Max 5 marks:
- Correctly labeled axes with units (x-axis: Distance / m, y-axis: Average cover / %) [1];
- Appropriate linear scales using more than half of the grid [1];
- Accurate plotting of all points for both datasets [1];
- Points joined with neat, thin lines [1];
- Clear key or labeled lines to distinguish the two species [1].

(ii) Max 3 marks:
- Native forest shrubs increase as distance from the highway increases [1];
- Invasive weeds decrease as distance from the highway increases [1];
- Use of comparative paired data from the table to support the trend (e.g., at 5 m weeds are 85% and shrubs 8%, whereas at 55 m weeds are 2% and shrubs 82%) [1].

(c) (i) Max 4 marks:
- Division of habitat isolates populations into smaller, non-viable groups [1];
- Loss of genetic diversity / increased risk of inbreeding [1];
- Restricts movement/migration to find seasonal food/water resources [1];
- Increases mortality due to traffic collisions / edge effects (predation/invasive species) [1].

(ii) Max 2 marks:
- Install eco-ducts / green bridges / wildlife tunnels [1];
- Maintain/restore ecological corridors of native vegetation [1];
- Erect animal-proof fencing to direct wildlife toward safe crossings [1].

(a) (i) Capture a sample of cod using standard, non-lethal fishing methods (e.g., traps or nets); count and tag each captured fish with a harmless marker that does not affect swimming or survival; release the tagged fish back into the bay and allow them sufficient time to mix evenly with the rest of the population; capture a second sample of cod after a set period, count the total caught, and record how many are tagged.

(ii) No births or deaths occur within the population during the study period; no immigration or emigration occurs; the tags do not fall off or make the fish more vulnerable to predators.

(b) (i) \(N = \frac{300 \times 250}{40} = \frac{75000}{40} = 1875\) adult cod.

(ii) Change in population = \(1875 - 4500 = -2625\).
Percentage change = \(\frac{-2625}{4500} \times 100 = -58.33\%\).
This represents a decrease of 58.3% (or 58.33%).

(c) (i) A larger mesh size allows juvenile/small fish to escape through the holes in the net; this ensures they survive to reach reproductive maturity and spawn at least once, replenishing the future population.

(ii) Large, remote geographic areas are difficult to patrol continuously; high cost of fuel, boats, and satellite tracking systems for enforcement; flags of convenience allow foreign vessels to bypass local laws; illegal, unreported, and unregulated (IUU) fishing vessels can operate at night or in bad weather.

(iii) Implementing closed seasons during spawning periods; establishing marine protected areas (MPAs) or no-take zones where fishing is completely banned.

(a) (i) Max 4 marks:
- Catch a sample of cod safely, record numbers, and mark/tag them harmlessly [1];
- Release them back into the same water body [1];
- Allow sufficient time for marked fish to mix randomly/thoroughly with unmarked fish [1];
- Catch a second sample, count the total caught, and record the number of recaptured tagged fish [1].

(ii) Max 3 marks:
- Marking does not affect survival rates / susceptibility to predators [1];
- Marked fish mix completely and randomly with the rest of the population [1];
- No significant migration (immigration/emigration) or births/deaths during the study period [1];
- Tags/marks are not lost or misidentified [1].

(b) (i) Max 2 marks:
- Correct working shown: \((300 \times 250) / 40\) [1];
- Correct final answer: 1,875 [1].

(ii) Max 3 marks:
- Calculate difference: 2,625 [1];
- Correct percentage calculation: \((2625 / 4500) \times 100 = 58.33\%\) (accept 58.3% or 58%) [1];
- State direction: Decrease [1].

(c) (i) Max 3 marks:
- Small/young/juvenile fish escape through the larger gaps [1];
- Allows young fish to grow to adult size and reproduce/spawn [1];
- Prevents the collapse of the breeding stock / maintains recruitment [1].

(ii) Max 3 marks:
- Large areas of ocean make monitoring and surveillance physically difficult/expensive [1];
- Lack of international cooperation/jurisdiction in international waters [1];
- High costs of patrol boats, radar, and satellite tracking [1];
- Corrupt practices or falsifying logbooks by vessel captains [1].

(iii) Max 2 marks (any two from):
- Closed fishing seasons (during breeding/spawning) [1];
- Marine Protected Areas (MPAs) / marine reserves / no-take zones [1];
- Banning destructive fishing methods (e.g., drift nets, bottom trawling) [1];
- Promoting sustainable fish farming / aquaculture [1].