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

Phytoplankton (microscopic floating producers) and macrophytes (large aquatic plants and algae, such as kelp and pondweed) capture light energy by photosynthesis and provide the energy for freshwater and marine food webs. Zooplankton, fish and top predators depend on them. Humans consume organisms from many levels: fish, molluscs, crustaceans and seaweed.

Demand for seafood is increasing because of population growth and changes in dietary preferences (for example fish seen as a healthy protein). To meet demand, unsustainable harvesting practices such as industrial trawling and overfishing have been used. Overexploitation means harvesting faster than the stock can replace itself. It can lead to the collapse of fisheries, such as the Grand Banks cod off Newfoundland, closed in 1992 with tens of thousands of jobs lost.

The maximum sustainable yield (MSY) is the highest possible annual catch that can be sustained over time. At the MSY the catch equals the rate at which the stock replaces itself. MSY should be used to set caps on fishing quotas: catching more than the MSY reduces the number of breeding adults, so the stock declines.

Climate change warms water, changes currents and shifts where species live. Ocean acidification (more CO2 dissolving in sea water) lowers pH, making it harder for shell-forming organisms such as molluscs and corals to build shells and skeletons. These pressures affect ecosystems and may cause the collapse of some populations in freshwater or marine ecosystems, so past MSY values may not stay valid.

Unsustainable exploitation can be reduced by:

• Policy legislation: quotas, licences, gear restrictions (minimum mesh size), closed seasons and limits on boat numbers.
• Changes in consumer behaviour: buying ecolabelled fish (for example MSC), choosing less-threatened species, eating less seafood.
• Marine protected areas (MPAs): areas where fishing is limited or banned, protecting spawning and nursery grounds, supporting aquatic food chains and helping maintain sustainable yields through fish spilling over into nearby waters.

Both the strengths and the limits (enforcement, cost, mobile fish) should be discussed in evaluation.

Aquaculture is the farming of aquatic organisms, including fish, molluscs, crustaceans and aquatic plants. It is expanding to increase food supplies and to support economic development, but has environmental impacts: waste and uneaten feed pollute the water and seabed, escaped fish can interbreed with wild populations, parasites such as sea lice and disease can spread to wild fish, wild fish are used as feed, and habitats such as mangroves can be cleared for ponds.