Water and aquatic food production systems and societies
Domain 4 covers the hydrological cycle, water security and aquatic food production. The exam tests storages and flows, scarcity, pollution, fisheries and aquaculture, often with data on water footprints or sustainable yield.
Core rules
- The hydrological cycle moves water through storages (oceans, ice, groundwater, soil, atmosphere) and flows (precipitation, evaporation, transpiration, runoff, infiltration).
- Blue water is liquid surface and groundwater; green water is soil moisture used by plants; grey water is the volume needed to dilute pollution.
- Water scarcity can be physical (shortage) or economic (lack of infrastructure and finance).
- Maximum sustainable yield (MSY) is the largest catch that can be taken without reducing the stock's long-term productivity. In simple logistic models it occurs at half carrying capacity.
- Aquaculture can relieve wild-stock pressure but brings feed, disease, escapee and effluent issues.
Worked example
A fish stock has carrying capacity K = 20,000 t and intrinsic growth r = 0.5 yr⁻¹. Logistic growth is fastest at K/2 = 10,000 t. MSY = rK/4 = 0.5 × 20,000 ÷ 4 = 2,500 t yr⁻¹. Reasoning: below K/2 the stock grows more slowly because there are fewer spawners; above K/2 growth slows through competition. Real MSY is uncertain because r and K vary, and fishing pressure often exceeds the model.
Traps
- Calling evaporation a transfer; it is a transformation from liquid to vapour.
- Assuming MSY is precautionary; it is a theoretical maximum and can collapse stocks if misestimated.
- Treating all aquaculture as sustainable.
- Forgetting that eutrophication is a nutrient problem, not simply a toxic-chemical problem.
- Confusing water scarcity with drought; scarcity can persist when rain falls.