4.3 Aquatic food production systemsIB Environmental Systems and Societies HL: Revision notes

Phytoplankton (in marine and freshwater systems) and macrophytes (large aquatic plants and algae, e.g. seagrass and kelp) are the producers that provide energy for aquatic food webs. Humans consume organisms from freshwater and marine environments, and demand for seafood is increasing because of population growth and changes in dietary preferences, e.g. a shift towards fish as a healthy protein.

Rising demand has encouraged unsustainable harvesting practices, such as large trawlers, bottom trawling, driftnets and bycatch, and overexploitation: harvesting faster than the population can replace itself. Fewer breeding adults means fewer young, and the stock can collapse, as happened to the northern cod fishery off Newfoundland (moratorium in 1992). Collapse harms ecosystems and the people who depend on fishing.

The maximum sustainable yield (MSY) is the highest possible annual catch that can be sustained over time. It is used to set caps on fishing quotas. A stock is largest when unfished, and growth is fastest at a medium population size, so harvesting a stock down to about that size gives the most fish each year.

Climate change warms water and shifts species' ranges; ocean acidification (CO₂ dissolving to form carbonic acid) makes it harder for shellfish, corals and some plankton to form calcium carbonate shells. These can cause the collapse of some populations in freshwater or marine ecosystems.

Unsustainable exploitation can be mitigated by policy and legislation (quotas, gear restrictions, minimum mesh sizes, closed seasons, bans on illegal fishing) and by changes in consumer behaviour (eco-labels, choosing sustainable species). Marine protected areas (MPAs) protect habitats and spawning grounds, support aquatic food chains and help maintain sustainable yields; for example, the Great Barrier Reef Marine Park has large no-take zones.

Aquaculture is the farming of aquatic organisms, including fish, molluscs, crustaceans and aquatic plants. The industry is expanding to increase food supplies and support economic development.

Environmental impacts include nutrient and waste pollution (eutrophication) under cages, disease and parasites (e.g. sea lice) spreading to wild fish, escapes that breed with wild fish, antibiotic and chemical use, loss of habitats such as mangroves cleared for shrimp ponds, and the use of wild fish as feed.

(HL) Productivity, thermal stratification, nutrient mixing and nutrient loading are interconnected. In warm water the surface forms a warm layer above colder, denser water (thermal stratification), which prevents nutrients from the deep from mixing upwards, so surface productivity can be limited. Mixing (e.g. upwelling, storms or seasonal turnover) brings nutrients to the surface and raises productivity and fish catches. Nutrient loading from run-off or sewage can raise productivity too, but too much can cause eutrophication.

(HL) Accurate assessment of fish stocks and monitoring of harvest rates (research surveys, catch records, observers) are needed for conservation and sustainable use. There are risks in harvesting at MSY: stock estimates are uncertain, MSY ignores species interactions, and environmental change can lower the stock, so the rate must be managed carefully (e.g. a precautionary quota below MSY).

(HL) Overexploited species may recover with multi-stakeholder cooperation (governments, industry, scientists, conservation groups), as the Atlantic bluefin tuna has begun to under ICCAT.

(HL) Under the UN Convention on the Law of the Sea (UNCLOS), coastal states have an exclusive economic zone (EEZ) stretching 370 km out to sea, in which they have the rights to the resources and a responsibility to manage them. Beyond it lie the high seas, where cooperation between states is needed.

(HL) The harvesting of seals, whales and dolphins raises ethical issues relating to animal rights: whether intelligent animals that can suffer should be killed, balanced against the livelihoods and cultural traditions of some coastal and indigenous communities.