Primary producers in aquatic systems

Phytoplankton are a type of microscopic plankton capable of photosynthesis that are found in oceans, seas and freshwater. Carried by currents, they use light energy to make organic matter.

Macrophytes are aquatic plants that are large enough to be visible. They can be emergent from water, submerged or floating. Emergent plants have roots below the water and extend above the surface. Submerged plants grow mainly beneath the surface, while floating plants occur on it.

Both groups are primary producers: organisms that convert light energy into chemical energy stored in biomass. Consumers such as zooplankton feed on phytoplankton, supporting aquatic food webs. Macrophytes may be eaten directly, or they may enter detrital food chains after they die. Biomass from these producers supplies energy to freshwater and marine consumers, including species harvested by people.

Production is highest where producers have enough light and suitable nutrients. Phytoplankton dominate open water. Attached macrophytes, however, are generally limited to water shallow enough for light to reach their leaves or stems.

What people obtain from aquatic ecosystems

People eat aquatic fauna, including fish, molluscs and crustaceans. Aquatic flora such as seaweeds and other edible plants also provide food. These organisms are either gathered from wild populations or produced through aquaculture.

Around Kisumu on Lake Victoria, for example, people catch and eat tilapia. Communities in Southeast Asia grow edible water spinach in nearby wetlands and ponds. On a global scale, skipjack tuna is traded and consumed across many continents, while cultivated nori seaweed is distributed internationally as a food ingredient.

Diet, values and perspectives

Availability alone doesn’t determine what people eat. Cultural traditions shape which species count as food, while religious or ethical beliefs may discourage people from eating particular foods. Income affects what they can afford. Some health perspectives favour seafood as a protein source, and environmental values may lead consumers to avoid threatened species or choose certified products. As a result, the same animal may be seen as food or a source of employment. Others may view it as wildlife with intrinsic value, or as an organism that should not be killed.

Aquatic foods can support food security and livelihoods. However, this benefit remains sustainable only when harvesting doesn’t remove organisms faster than populations and food webs can recover.

Why demand is rising

As the human population grows, so does the number of consumers. Rising incomes and urbanization have also made seafood accessible to more people, helped by improved refrigeration and global trade. Preferences are changing too. Some consumers choose fish for its protein and unsaturated fats; elsewhere, consumption rises as traditional diets become more commercialized.

Useful evidence needs to show change, not simply give one large total. Global per-capita consumption of aquatic animals increased from about $9.9\ \text{kg}$ per person per year in the 1960s to about $20.2\ \text{kg}$ in 2020. In the same year, total fisheries and aquaculture production reached approximately 214 million tonnes. Aquaculture accounted for much of the recent growth.

Demand varies between places. Consumption may be high in wealthy importing states, as well as in coastal or island communities. Elsewhere, affordability, cultural preferences and access can keep consumption lower. Greater demand can support jobs and nutrition, but it puts more pressure on wild stocks and drives the expansion of intensive aquaculture.

Destructive harvesting

Overexploitation is the use of a renewable population at a rate that exceeds its capacity to replace the organisms removed. High market prices and greater demand can push fleets to expand their size and range, while becoming more efficient. When regulation and enforcement fail to keep up, fishing effort may continue despite declining stocks.

Three practices are especially destructive:

• Bottom trawling is a fishing method in which a weighted net is dragged across the seabed. It catches target species, but may also crush benthic habitats, resuspend sediment and capture juvenile or non-target organisms.
• Ghost fishing is the continued capture of organisms by fishing gear that has been lost, abandoned or discarded. Nets, lines and traps can keep killing animals for months or years. Those trapped animals may then attract more scavengers and predators.
• Fishing with poisons or explosives is a harvesting method that stuns or kills organisms indiscriminately. Toxic substances can harm non-target species, while explosions break reef structures used as feeding, nursery and spawning habitat.

Bycatch is the unintended capture of organisms that fishers did not intend or are not permitted to retain. This may include juveniles, turtles, seabirds and marine mammals. Even organisms thrown back may die, so the ecological removal can be far greater than the commercial catch that is recorded.

These techniques are unsustainable because they remove organisms and weaken the habitats and reproductive processes required for future population growth.

What a collapse means

A fishery collapse is the dramatic and lasting decrease in stocks to a point where the commercial fish can no longer recover. It goes beyond a poor fishing season. Breeding biomass and recruitment remain very low, as do catches, even after fishing pressure is reduced.

The Peruvian anchoveta collapse

During the 1950s and 1960s, the Peruvian anchoveta fishery grew rapidly. Industrial fleets and reduction plants supplied a large export market for fishmeal. At the same time, management depended on incomplete knowledge of a population that varies naturally. Very high catches removed a substantial proportion of the breeding stock.

A strong El Niño event in 1972–73 weakened the normally nutrient-rich upwelling off Peru. Primary productivity fell, while anchoveta survival and recruitment declined. Fishing pressure had already made the stock less resilient, so overharvesting and the climatic disturbance acted together. Reported landings dropped from roughly 10 million tonnes in 1971 to under 2 million tonnes in 1973.

Fishing limits and closures followed. Anchoveta abundance later improved, but the collapse led to factory closures, unemployment and a shift towards harvesting other species. The case shows that environmental variation does not excuse overfishing. A heavily exploited population has less capacity to survive an adverse event.

The yield–effort relationship

The maximum sustainable yield (MSY) is the highest possible annual catch that can be sustained over time, so it should be used to set caps on fishing quotas. At this level, population growth should replace what is removed, allowing the stock to replenish itself.

When fishing effort is low, adding boats, nets or fishing days usually increases the catch. Eventually, yield peaks at the MSY. If effort rises beyond this point, breeding stock and recruitment fall. Total yield then declines, despite the extra resources being used.

The downward slope of the curve matters. Once the stock has been depleted, a fleet may work harder and still catch less. Many fisheries have fishing rates far above those associated with MSY. Reaching MSY may therefore require a substantial cut in effort, not just a small adjustment.

Governments can use the estimated MSY as a biological reference point when setting a total allowable catch and dividing it into quotas. However, the estimate doesn't guarantee safety because uncertainty and changing environmental conditions must also be considered.

Warming, acidification and population stress

Ocean acidification is a chemical change in seawater in which absorption of atmospheric carbon dioxide lowers pH. When dissolved carbon dioxide reacts with water, it forms carbonic acid:

$$CO_2 + H_2O \to H_2CO_3$$

This acid dissociates, increasing the concentration of hydrogen ions. The hydrogen ions then react with carbonate ions, so less carbonate remains available for organisms that build calcium-carbonate shells or skeletons. As a result, calcification may slow, shells may weaken and fewer larvae may survive.

Climate change raises water temperatures and changes rainfall and river flow. It also alters oxygen solubility, currents and nutrient supply. Some species can move towards cooler water. Organisms attached to a substrate, or those isolated in lakes, can’t always migrate. Food webs may also be disrupted when the timing of predator and prey activity no longer matches.

Example: the Florida Keys reef ecosystem

Exceptionally warm water during the 2023 marine heatwave caused severe bleaching in reefs of the Florida Keys. Coral bleaching is a stress response in which corals lose or expel their symbiotic algae and become pale. Once bleached, corals receive less photosynthetic energy. If the stress continues, mortality increases. The resulting loss of reef structure reduces feeding and nursery habitat for fish and invertebrates, threatening biodiversity, fisheries and tourism. Acidification creates another pressure because it slows reef calcification and recovery.

Planning an acidification experiment

An investigation could test the effect of pH on empty mollusc shells. Where ethical and practical approval permits, it could instead use a commonly cultured shelled organism.

• Set up several identical containers of artificial seawater across a controlled range of pH values, with an unaltered control. Change the pH gradually using measured additions of dilute acid or controlled carbon-dioxide bubbling.
• Select shells or organisms from the same species with similar initial sizes. Randomly assign equal numbers to each treatment, using several replicate containers at every pH.
• Control temperature, salinity, water volume, light and exposure time. For living organisms, keep feeding and dissolved oxygen constant as well.
• Measure the dependent variable both before and after exposure. Possible measures include change in dry shell mass, shell thickness, visible surface damage or growth rate.
• Calibrate the pH probe and monitor pH throughout the investigation, not just at the start. Calculate a mean for each treatment, then plot mean response against pH to show the pattern.
• Wear eye protection, handle materials carefully and use dilute solutions. Keep stress to living organisms as low as possible. Return them only where this is legal and cannot spread disease or non-native organisms.

Replicates make the results more reliable. However, a short laboratory experiment can’t recreate every feature of a natural ecosystem, including predation, food limitation, fluctuating temperature and long-term acclimatization.

Regulation at several scales

Unsustainable exploitation is harvesting that exceeds a population's capacity to replenish itself. Effective mitigation needs rules and enforcement, backed by consumer choices. One measure on its own is rarely enough.

At the international level, states can set catch limits for shared or migratory stocks. They can also control trade in threatened species and cooperate to tackle illegal fishing. National governments may issue permits, limiting who can fish, and set quotas for how much may be landed. Seasonal closures protect breeding aggregations. Minimum landing sizes and larger net mesh sizes let more juveniles escape and reproduce.

Spatial zones may separate fishing methods, close nursery areas or prevent harvesting in sensitive habitat. These rules need monitoring through vessel tracking, observers, patrols and port inspections. Without enforcement, a rule is mostly decoration.

Local communities can establish customary closures, protect spawning sites and report illegal fishing. Individual consumers can reduce demand for threatened species by choosing lower-impact alternatives or using credible food labels and certification schemes. Retailers and restaurants may publish sourcing policies and refuse seafood without traceable origins.

Food labelling can shift demand and reward better-managed fisheries, but only when standards are reliable, audits are independent and consumers have access to the labels. Certified seafood may cost more. Complex supply chains can also make mislabelling hard to detect. Policy and behaviour work best together: legislation changes what producers may do, while purchasing choices change what markets reward.

Protection inside and benefits outside

A marine protected area (MPA) is a defined marine zone in which human activities are restricted to conserve species, habitats or ecological processes. Protection can involve controls on particular activities or a no-take area where extraction is prohibited.

In a well-enforced MPA, organisms have a better chance of surviving to larger sizes and reproducing repeatedly. Protected seabeds offer shelter and feeding habitat. Reefs, mangroves or seagrass beds may also act as nursery and spawning grounds. Together, these effects strengthen the target stock and its food-web relationships.

The benefits may spread beyond the MPA boundary. Adult or juvenile organisms can move into neighbouring fishing grounds through a process called spillover. Currents may also carry eggs and larvae into wider areas. As a result, fishers may gain higher catches around a successful reserve even though fishing inside it is restricted.

For an MPA to succeed, it needs adequate size and suitable placement, along with ecological representation and connections to other protected areas. Long-term monitoring and real enforcement are also necessary. Local support matters too. Excluding users without consultation can shift costs onto fishing communities and encourage non-compliance. An MPA cannot protect migratory species across their entire range. On its own, it also cannot stop warming, acidification or pollution arriving from outside.

Expansion and benefits

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, but there are associated environmental impacts. It can provide a predictable food supply, create jobs and ease pressure on some wild stocks. Whether it is sustainable depends heavily on the species and feed used, as well as the location and production system.

Example: salmon farming in southern Chile

Open-net salmon farms in Chile supply export markets. They also support jobs in processing and transport. However, the industry brings several environmental risks:

• cages and other coastal infrastructure can disturb or replace habitat;
• faeces and uneaten feed add nutrients and organic matter, which lowers oxygen beneath cages;
• copper-based anti-fouling agents used on nets can contaminate sediment;
• antibiotics and other medicines may enter the surrounding water, selecting for resistant microorganisms;
• parasites and diseases spread quickly through dense stocks and may reach wild fish;
• salmon that escape can prey on or compete with native organisms and may establish non-native populations;
• where genetically modified stocks are used in aquaculture, escapees could transfer modified genes or traits, although their use differs among countries and species.

Feed may cause impacts elsewhere when farmed carnivorous fish rely on fishmeal and fish oil produced from wild catches. By contrast, farming filter-feeding molluscs or seaweeds usually needs little or no external feed, so resource demand can be lower.

Good management can limit these effects. Farms can operate below the site's ecological carrying capacity and be placed away from sensitive habitat. Lower stocking densities, fallow periods, vaccination and disease surveillance help control disease. Enclosed or land-based recirculating systems reduce waste release. Secure cages reduce escapes, as does the use of sterile farm stock, while captured waste can be treated instead of discharged. Such measures raise costs and reduce impacts, but no system is necessarily impact-free.

The judgement is conditional. Aquaculture can make a substantial contribution to food security and economic development, but it is sustainable only when inputs, pollution, disease, habitat change and escapes are managed effectively.

Light, nutrients and water layers

Productivity is the rate at which biomass is generated in an ecosystem per unit area or volume per unit time. Aquatic primary productivity depends on both light and nutrients. It is highest where the two occur together.

Thermal stratification is the layering of a water body caused by temperature-related differences in water density. Warm surface water is less dense, so it can stay above the colder, denser water below. This difference in density resists vertical mixing.

Deep water then presents a familiar problem. The illuminated surface layer may run short of nutrients, while decomposition releases nutrients at depths where too little light is available for photosynthesis. As dead material sinks, it continually carries nutrients away from the surface. Strong stratification often limits surface productivity as a result.

Nutrient mixing is the vertical transport of dissolved nutrients between water layers. Wind-driven mixing, seasonal turnover and upwelling can carry deep nutrients back to the illuminated surface. If light and other conditions are suitable, phytoplankton grow rapidly.

Nutrient loading is the input of nutrients into a water body from an external source. Coastal or shallow water may be enriched by river discharge, sediment runoff, sewage and fertilizers. Moderate enrichment can raise productivity. Excessive loading, however, can cause algal blooms followed by decomposition, oxygen depletion and the loss of aquatic animals.

High productivity is common near coastlines and in shallow seas, estuaries and upwelling zones. Shallow water mixes more easily and may receive nutrients from land. Coastal upwelling also carries nutrient-rich deep water into the photic zone. By contrast, permanently stratified open-ocean water can be clear yet nutrient-poor.

Measuring the stock

A fish stock is a population or group of populations of a fish species managed as one harvesting unit. Managers estimate its abundance, biomass, age structure, distribution and reproductive condition. Catch totals can't provide all this information. A low catch might reflect low fishing effort or bad weather rather than a depleted stock.

One way to estimate abundance is a tag–release–recapture survey. Researchers catch an initial sample, mark the fish harmlessly, then release them. Once the fish have had time to mix, a second sample is taken. A high proportion of marked fish in this sample suggests a relatively small population; a low proportion points to a larger one. For an accurate estimate, the marks must stay attached, marking must not change survival or catchability, the fish must mix fully and the population should remain approximately closed during sampling.

Fishery-independent scientific surveys provide another check. Each year, vessels can sample standardized routes using the same gear and effort. Sonar or underwater cameras can also estimate where schools are found and how densely they are distributed. What matters is repeated, standardized sampling, since this separates changes in stock abundance from changes in fishing behaviour.

Monitoring harvest rates

Landing records and vessel logbooks can be used to monitor harvest. They record catch mass, species, location, date, fishing effort and gear. Reports can be checked through electronic vessel monitoring, onboard observers and port inspections, which can also detect activity in closed zones.

No method is complete. Landing records may leave out discarded bycatch, subsistence catches and illegal or unreported landings. Fishers might misidentify species or locations, while catch mass gives no information about the number, age or sex of individual fish. Scientific surveys sample only part of a large, mobile population. The strongest assessment combines independent surveys with recapture evidence and verified harvest records instead of relying on a single source.

Why an MSY estimate is uncertain

MSY is an estimate, not a fixed property that can be measured exactly. The models rely on incomplete stock data, along with assumptions about population growth, carrying capacity, age structure and recruitment. Conditions also shift from year to year: temperature, salinity, acidification, prey availability, disease, parasites and invasive species may all change.

Fishing at the estimated MSY leaves almost no room for error. The estimate may be too high, catches may be underreported or environmental conditions may reduce recruitment. In any of these cases, actual removals can exceed population growth. A quota that appeared sustainable on paper may then lower the number of mature breeders.

Positive feedback and rapid decline

When breeding biomass falls, fewer offspring may be produced. Poor recruitment leads to a smaller future breeding population, which then produces even fewer offspring. This is positive feedback, a process in which an initial change triggers effects that amplify that change. Catch per unit effort can stay temporarily high when fish gather in predictable places, so fleets may fail to notice the decline until the stock is already severely depleted.

Managers can reduce the risk by setting quotas below the estimated MSY and applying precautionary buffers. They can also protect spawning areas and mature breeders, update estimates frequently and cut quotas automatically when monitoring detects poor recruitment. A flexible, ecosystem-based approach is safer than treating MSY as a precise annual target that must be taken.

Stakeholders and their priorities

A stakeholder is a person or group that affects, or is affected by, a decision or environmental issue. In fish-stock recovery, stakeholders’ interests often overlap, but they aren’t the same.

• Governments aim to protect food security and employment, collect tax income and safeguard ecosystems. At the same time, they face pressure from voters and international agreements.
• Fishing businesses depend on income, so they may oppose sudden restrictions. However, their long-term future also relies on viable fish stocks.
• Consumers generally want seafood that is affordable and readily available. Some are prepared to pay more for products that are traceable and sustainably harvested.
• Environmental NGOs tend to prioritize biodiversity, precautionary quotas and transparent monitoring.
• Wholesale fish markets rely on steady volumes, consistent quality and predictable prices.
• Local supermarkets react to prices and customer demand. Through purchasing standards and clear labels, they can also influence suppliers.

Resolving differences

Recovery plans are more effective when stakeholders share stock data, set measurable rebuilding targets and take part in decision-making. Short-term bans may be easier for fishing communities to accept if temporary financial support, retraining or alternative livelihoods are available. Trust can also grow through independent monitoring and scheduled policy reviews.

Possible measures include temporary fishing bans, fewer fishing licences and lower quotas. Juveniles and spawning grounds can be protected, while different fishing gear can reduce bycatch. Markets and supermarkets can give consumers information to help them avoid species that are being harvested unsustainably.

The eastern Atlantic and Mediterranean bluefin tuna stock shows how cooperation can support recovery. International catch controls, tighter monitoring and minimum-size rules reduced pressure, alongside action against illegal fishing. Improved stock assessment then guided later quota decisions. Biomass increased, although continued monitoring remains necessary because the stock’s market value still creates a strong incentive to overfish.

Cooperation doesn’t mean that every stakeholder gets everything they want. Instead, stakeholders negotiate a package where ecological limits are non-negotiable, evidence is shared and the short-term social costs of recovery are distributed more fairly.

Coastal jurisdiction

An exclusive economic zone (EEZ) is a marine area extending up to $370\ \text{km}$ from a coastal state's baseline in which that state has rights to explore, use, conserve and manage marine resources. Under the UN Convention on the Law of the Sea, the coastal government may regulate fishing within this zone. Other states still retain certain navigation rights.

Almost 60% of the ocean lies beyond national EEZs. Marine areas outside national jurisdiction are known as the high seas. Migratory stocks cross boundaries, while enforcement is expensive and no single government has direct responsibility. As a result, intergovernmental regulation has historically been limited and vulnerable to free-rider behaviour.

The UN has developed and signed an international agreement on biodiversity beyond national jurisdiction, often called the High Seas Treaty. The agreement provides a framework for protected areas and environmental assessment, as well as cooperation over marine genetic resources. Whether it works depends on ratification, implementation, finance and enforcement by states.

Equity and justice

A government may earn revenue by selling foreign fleets access to its EEZ. This can make economic sense, but it raises an equity issue when industrial vessels remove stocks used by local small-scale fishers. Central government or foreign companies may receive the benefits, while coastal communities face lower catches, food insecurity and the loss of livelihoods.

A fairer agreement would use transparent stock assessments and reserve access for local food needs. It would also limit total effort, require monitoring and direct an agreed share of revenue towards affected communities. The central question goes beyond who legally owns the access: who receives the benefits, and who bears the ecological and social costs?

A conflict between legitimate values

Harvesting seals, whales and dolphins raises difficult questions. These concern animal welfare, species conservation, cultural rights, food security and political power. People may agree on the facts yet reach different conclusions because they hold different environmental value systems.

Take the Makah people's proposed hunting of eastern North Pacific gray whales in the United States. Whaling has cultural and treaty significance for the Makah, and their relationship with it began before modern commercial exploitation.

From an indigenous-rights and anthropocentric perspective, a strictly limited hunt can be defended through treaty rights, cultural identity and community self-determination. Supporters draw a line between small-scale ceremonial or subsistence harvesting and industrial commercial whaling. They argue that outsiders shouldn’t erase a living tradition, especially when the wider whale population can sustain the permitted removal.

The animal-rights perspective reaches a different conclusion. Whales are sentient, socially complex animals that can suffer, so deliberately killing an individual is ethically unacceptable even when the population isn’t threatened. Some opponents also doubt that any killing method can guarantee a rapid death. They argue that whale watching could bring cultural or economic benefits without lethal harvesting.

Conservationists ask whether the hunt will damage the population or ecosystem. They may accept a small, monitored quota if independent evidence shows that the stock is healthy. Vulnerable subpopulations would still need protection, and the hunt would have to be suspended if abundance declined.

One reasoned resolution could combine indigenous participation in governance with a very small science-based quota. It could also require humane-method standards, independent monitoring, protection of vulnerable groups and regular review. This addresses the conservation question more easily than the animal-rights question. Someone who believes that each whale has an inviolable right not to be killed will still reject the hunt.

Diet and harvesting therefore can’t be understood through ecology alone. What people consider acceptable is also shaped by cultural history, lived experience, legal rights and beliefs about the moral standing of animals.