4.3 Aquatic food production systems

Phytoplankton are drifting microscopic producers; macrophytes are visible aquatic plants, and both convert light into biomass.

Phytoplankton drift in open water and support consumers as suspended primary production. Macrophytes are visible aquatic plants that may be emergent above the surface, submerged below it or floating; rooted forms also create shelter and breeding structure.

A microscopic cyanobacterium floating in a lake is phytoplankton; reeds rooted at the shore are macrophytes.

Reeds are emergent macrophytes: their visible rooted structure differs from the suspended microscopic biomass of phytoplankton.

Phytoplankton is a functional group, not one plant species; cyanobacteria can be included.

A useful aquatic-food example names the organism, freshwater or marine setting, production method and scale of use.

Separate flora (plants/algae) from fauna (fish, molluscs, crustaceans), and wild capture from farming. This makes the ecological and social pathway testable.

Hand-harvested dulse is regional marine flora food; farmed freshwater tilapia is fauna produced and traded widely.

A complete food example states whether the organism is flora or fauna, freshwater or marine, wild-caught or farmed, and local or globally traded.

A named species is not enough; include how and where it enters the food system.

Aquatic-food demand rises through two separate levers: number of consumers and amount consumed per person.

Population, income, urbanization, nutrition preferences and cold-chain trade can change the two levers differently. Consumption is not the same graph as wild catch because aquaculture may fill demand.

Local textbook evidence shows sustained global growth in fish and seafood consumption from 1961 to 2018. Per-person consumption rose by about 34–44% in China, Indonesia, Malaysia, Cambodia and South Korea and by nearly 74% in the Maldives; population growth then multiplies those per-person changes into still larger total demand.

Farmed production can expand while capture fisheries remain constrained.

Do not infer wild-stock recovery from a flat catch graph; supply may have shifted to aquaculture.

Different fishing practices damage different parts of the system: habitat, future recruitment, toxicity exposure or selectivity.

Bottom trawls disturb seabed; lost gear causes ghost fishing; poisons harm reef organisms; explosives kill indiscriminately and shatter habitat. Link practice to mechanism before judging impact.

A lost gillnet continues catching animals without a vessel present, so removal of lost gear addresses continuing mortality.

Bottom trawling and explosives directly damage physical habitat, whereas ghost gear continues capturing organisms after fishing equipment has been lost or abandoned.

‘Destructive’ is not one mechanism; specify what is damaged and how recovery is reduced.

Grand Banks cod collapse followed excessive industrial removal and weak management; a moratorium alone could not instantly restore a depleted, altered system.

Trace catch and habitat damage → low breeding stock → weak recruitment, bycatch and food-web change → slow recovery. Renewed pressure can reinforce the decline.

After the 1992 moratorium, few mature spawners meant even reduced catch produced too few recruits for rapid recovery.

Recovery also depends on age structure, habitat, food web, bycatch and enforcement.

A moratorium changes one pressure; it does not reverse every ecological legacy immediately.

Maximum sustainable yield is at the peak of the repeatable yield curve; effort beyond it can lower both stock and future catch.

As effort rises, catch first increases, then declines when breeding stock and recruitment are damaged. A real quota should normally sit below the theoretical peak because estimates are uncertain.

If modelled yield peaks at 800 tonnes at effort 60, a fleet catching 900 at effort 90 is not more successful—it is above the sustainable peak.

Over-effort reduces replenishment, so each extra boat shrinks the future stock.

The largest historical catch is not MSY; MSY is a repeatable rate under stated conditions.

Heat stress can bleach corals by disrupting symbiosis; acidification changes carbonate chemistry and can reduce calcification. They are distinct pressures.

Warm water stresses coral physiology and may cause algal expulsion. Dissolved CO₂ lowers pH and carbonate availability. Storms, pollution and overfishing can add further stress.

On the Great Barrier Reef, marine heatwaves can trigger coral bleaching by disrupting the coral–algal symbiosis. Ocean acidification is a separate CO₂-driven pressure that reduces carbonate availability and can slow reef calcification; repeated stress can degrade habitat and food webs used by aquatic populations.

Temperature/bleaching data for heat stress; pH/carbonate and calcification data for acidification.

Both involve climate change, but one does not explain every observed coral impact.

Effective fisheries combine controls that limit who fishes, how much is removed, when and where, and whether rules are obeyed.

International agreements can coordinate shared or migratory stocks; national permits and quotas cap participation and removal; local closed seasons, mesh sizes and no-take zones protect spawning, juveniles and habitat. Food labels and traceability let consumers avoid unsustainably harvested species, while monitoring and enforcement make every level credible.

A quota without a spawning closure can still remove breeding adults at the most sensitive time.

Each rule targets a different failure mode; combined controls reduce loopholes and protect recruitment.

A label or rule on paper is not management success without monitoring and credible enforcement.

A well-designed marine protected area can protect habitat and breeding stock, then support nearby fisheries through larval export or adult spillover.

The chain is protection → survival/reproductive size → dispersal or movement → adjacent benefit. It requires ecologically important boundaries, duration and enforcement.

A no-take nursery lets larger fish reproduce; larvae drift into fished water, but only if currents connect the areas.

Without compliance and habitat fit, extraction continues and spillover is only a claim.

Spillover is not guaranteed outside every boundary; track movement, recruitment and fishing effort.

Aquaculture can improve food supply and income, but sustainability depends on habitat, feed, disease, escape and waste pathways.

In integrated multi-trophic aquaculture, fish are farmed with organisms such as mussels and algae. Filter feeders remove suspended particles and algae take up dissolved nutrients, which can reduce organic waste, oxygen demand and eutrophication compared with fish-only production. Risks still include habitat loss, excess feed, anti-fouling chemicals, antibiotics or other medicines, disease transfer and escapees.

Management must match each pathway: lower stocking density and biosecurity reduce disease, settling or biological filters treat effluent, secure enclosures limit escapes, and protecting mangroves avoids replacing nursery habitat.

Measured feed efficiency, effluent, disease/escape control, habitat and social outcomes—not the label alone.

Farmed does not automatically mean low-impact or high-impact; inspect the production system.