Aquatic food webs and human diets

· Phytoplankton = microscopic photosynthetic plankton in oceans, seas and freshwater; they are key primary producers at the base of many aquatic food webs.
· Macrophytes = visible aquatic plants; may be emergent, submerged or floating.
· Aquatic ecosystems support human diets through consumption of freshwater and marine flora and fauna.
· Examples to know: global seafood such as tuna, cod, salmon, shrimp or seaweed; local examples should be chosen from the student’s own region.
· Exam link: aquatic food production is strongly linked to trophic levels, energy transfer, sustainability and environmental values/perspectives.

Rising demand for aquatic foods

· Demand for foods from freshwater and marine environments is increasing because of human population growth and changing dietary preferences.
· Rising demand can increase pressure on fish stocks, shellfish, crustaceans, aquatic plants and wider aquatic ecosystems.
· Diet choices are influenced by values and perspectives, including health, culture, affordability, tradition, animal welfare and sustainability.
· High exam performance: link demand to overexploitation, unsustainable harvesting, aquaculture expansion and consumer behaviour.

Unsustainable harvesting and overexploitation

· Overexploitation occurs when harvesting exceeds the ability of a population to replace itself.
· Increasing seafood demand has encouraged unsustainable harvesting practices.
· Bottom trawling = dragging weighted nets across the seabed; damages benthic habitats and causes bycatch.
· Ghost fishing = lost or abandoned fishing gear continues to trap and kill organisms.
· Poisons and explosives are destructive harvesting methods that damage habitats and kill non-target species.
· Overexploitation can cause fishery collapse = a dramatic, lasting fall in stock size so the commercial fishery can no longer recover.
· Case study to know: Grand Banks Newfoundland cod fishery collapse; use it to show how intense fishing pressure can push stocks below recovery levels.

This diagram shows why bottom trawling is environmentally damaging: fishing gear directly contacts and disturbs the seabed. It is useful for linking unsustainable harvesting to habitat destruction, bycatch and reduced ecosystem resilience. Source

Maximum sustainable yield (MSY) and fishing quotas

· Maximum sustainable yield (MSY) = the highest possible annual catch that can be sustained over time.
· MSY should be used to set caps on fishing quotas so harvest does not exceed stock replacement.
· MSY is shown on a yield/fishing effort graph: yield rises with fishing effort until a maximum point, then falls as overfishing reduces stock size.
· Harvesting at MSY usually requires much lower fishing rates than many fisheries actually use.
· Strong exam point: MSY is an estimate, so using it without caution can still risk overharvesting.
· Management should apply the precautionary principle by setting quotas below uncertain MSY estimates.

This graph is useful for explaining that MSY occurs before fishing effort becomes excessive. It helps students interpret why quotas should limit catch before stock decline becomes self-reinforcing. Source

Climate change and ocean acidification impacts

· Climate change and ocean acidification can stress aquatic ecosystems and cause population collapse.
· Warming water can shift species distributions, reduce dissolved oxygen, alter food webs and increase coral bleaching risk.
· Ocean acidification reduces carbonate availability, making it harder for shelled organisms and corals to build calcium carbonate structures.
· Case study example: Great Barrier Reef coral bleaching, where heat stress disrupts the coral–algae relationship and can reduce reef biodiversity.
· Application of skills: plan an experiment investigating how acidification affects shelled organisms.
· Good experimental design should include independent variable = pH/acidity, dependent variable = shell mass/change/condition, and controls such as temperature, species, exposure time and sample size.

Mitigation: policy, legislation and consumer behaviour

· Unsustainable exploitation can be reduced through actions at international, national, local and individual levels.
· Policy tools include permits, quotas, closed seasons, mesh-size regulations, fishing zones and food labelling.
· Permits/licences limit who can fish and how much effort is allowed.
· Quotas limit total catch to reduce overharvesting.
· Closed seasons protect breeding periods and allow population recovery.
· Mesh-size rules reduce capture of juveniles, supporting future recruitment.
· Food labelling/certification helps consumers choose seafood from more sustainable sources.
· Individual choices include reducing consumption of overexploited species, choosing certified seafood and supporting sustainable fisheries.

Marine protected areas (MPAs)

· Marine protected areas (MPAs) protect habitats, breeding areas and food chains.
· MPAs can support sustainable yields by protecting spawning grounds and allowing stocks to rebuild.
· Benefits can extend beyond the protected area through spillover, where adults or larvae move into surrounding fishing grounds.
· MPAs work best when they are well enforced, ecologically connected and designed around species’ breeding and movement patterns.
· Limitation: poorly enforced or badly located MPAs may provide little real protection.

Aquaculture: benefits, impacts and management

· Aquaculture = farming of aquatic organisms, including fish, molluscs, crustaceans and aquatic plants.
· Aquaculture is expanding to increase food supplies and support economic development.
· Potential benefits: reduces pressure on wild stocks, produces protein efficiently and can support livelihoods.
· Negative impacts include habitat loss, feed pollution, anti-fouling agents, antibiotics/medicines, disease spread and escapees.
· Escaped farmed organisms may compete with wild populations, spread disease or reduce genetic integrity; this is especially concerning if escapees are genetically modified.
· Management techniques include careful site selection, lower stocking densities, disease monitoring, waste management, fallowing, closed-containment systems and integrated multi-trophic aquaculture (IMTA).
· IMTA farms species from different trophic levels so wastes from one species can be used by another, reducing nutrient pollution.

Exam command term reminders

· Describe: give key features of a practice such as bottom trawling or aquaculture.
· Explain: link cause and effect, such as rising demand → overexploitation → fishery collapse.
· Evaluate: give strengths and limitations, such as MPAs protecting spawning grounds but requiring enforcement.
· Discuss: include contrasting perspectives, especially for seafood diets, indigenous hunting and governance of the high seas.
· Apply: use a named case study, graph or data set to support the point.