Life cycles, sustainable fisheries and aquaculture

Life cycles: metamorphosis, larvae and the sessile habit

A life cycle is the sequence of stages an organism passes through from the fertilised egg to an adult that reproduces. In a great many marine animals those stages do not look like each other at all, and the change between them is abrupt rather than gradual. Metamorphosis is that change in body form: one set of structures is broken down and another is built, so that a swimming, feeding larva becomes a juvenile with the adult body plan. It is not the same as growth. A fish that simply gets bigger has not metamorphosed; a crab that turns from a zoea into a megalopa and then into a crab has.

The larval stage is a young stage that lives independently, does not resemble the adult, and is usually small, weakly swimming and planktonic, so it drifts with the currents. That drifting is the point. A larva is a dispersal stage: it carries the species away from the place where it was spawned, which matters enormously for animals that cannot move as adults. Larvae usually feed on phytoplankton or on smaller plankton, and they suffer very high mortality from predators and from being swept into unsuitable water.

Sessile means attached to a surface and not free to move about - adult barnacles, mussels, oysters, tubeworms, soft corals and most seaweeds. Non-sessile means able to move about, either by swimming (fish, whales, squid) or by crawling and drifting (crabs, sea urchins, starfish). The distinction usually describes the adult, and that is exactly why it matters: a sessile adult cannot walk to a new rock, so its only way of colonising new ground is a larva that swims or drifts away, settles on a suitable surface, and metamorphoses into a sessile juvenile. A non-sessile adult has a second option, because it can move itself.

The two habits also mean different things for a fishery or a conservation plan. A stock of sessile animals is fixed in place, so it can be stripped from one shore completely and will only come back if larvae arrive from elsewhere. A stock of mobile animals can move away from a disturbed area, but it can also be followed by the fleet, which is why mobile schooling fish are the easiest stocks to overfish.

Simple and complex life cycles

A simple life cycle (also called direct development) has no larval stage. The embryo develops inside the female or inside an egg, and the young animal that emerges already has the adult body plan; it simply grows and becomes sexually mature. There is no metamorphosis. Marine mammals - whales, dolphins, seals - are the standard example. A whale calf is born looking like a small whale, is fed milk, and stays with its mother while it learns.

A complex life cycle has one or more larval stages, each separated from the next and from the adult by metamorphosis. Crustaceans are the standard example. A shore crab hatches as a zoea, a spiny planktonic larva with a long spine and no resemblance to a crab; it moults into a megalopa, which has begun to look crab-like and settles out of the plankton; that moults into a small crab. Molluscs, barnacles, echinoderms and most reef fish go through similar sequences.

The stages in a complex cycle are not just different shapes - they do different jobs, and they often live in different places. The larva disperses and feeds in the plankton; the settling stage chooses a habitat; the juvenile grows in a nursery such as seagrass or mangrove; the adult reproduces. This is why a complex life cycle creates several points of weakness. A pollutant that kills plankton, a dam that changes an estuary, or a fishery that removes the largest adults can each damage the same stock at a different stage, and protecting only the adult does not protect the population.

Complex life cycles also make stocks harder to assess. The number of adults you count this year tells you little about how many juveniles will arrive in three years, because survival through the larval stages depends on water temperature, food supply and currents. That variability is one reason fisheries managers set cautious limits and review them every year.

Fertilisation, parental investment and the importance of each stage

External fertilisation means the eggs are shed into the water and the male releases sperm over them, so the fusion of gametes happens outside the female's body. It is the normal pattern in most marine invertebrates (barnacles, mussels, crabs, sea urchins, corals) and in most bony fish, including tuna. It only works in water, and it only works if the adults are close together and spawn at the same time, so these species usually gather in large numbers and release gametes in response to a cue such as water temperature, day length or the phase of the moon.

Internal fertilisation means the male deposits sperm inside the female's reproductive tract, so fusion happens inside her body. In this syllabus the two groups to know are mammals (whales, dolphins, seals) and sharks (and rays), which have internal fertilisation even though they live in the sea. It requires courtship and mating, and often competition between males, but it gives each egg a much higher chance of being fertilised and protects the developing embryo.

The type of fertilisation is tied to the investment in the care of offspring. A tuna releases millions of eggs into open water and then swims away. Nothing is protected: the eggs drift, most are eaten or fail to develop, and the larvae that hatch are eaten by everything larger than them, so only a tiny fraction survive to adulthood. The strategy works because the numbers are enormous and because the adults do not spend energy on care. A shark produces far fewer young: eggs are fertilised internally, the eggs are large and yolky, and in many species the eggs develop inside the mother so that she gives birth to live young. A whale invests most of all - a long gestation of about a year, a single calf, milk fed for months or years, and years of protection while the calf learns - so each calf has a very high chance of surviving.

There is no free option. External fertilisation is cheap for the parent and needs no mate-finding or courtship beyond gathering to spawn, but it wastes enormous numbers of gametes and leaves the young defenceless. Internal fertilisation and parental care are expensive in energy and time and produce few young, but each one is far more likely to live. The number of eggs or young produced is therefore a direct measure of how much care the species gives, and a stock of a high-fecundity broadcast spawner can in principle rebuild faster than a stock of sharks or whales, which is exactly why slow-breeding species are so vulnerable to fishing.

Fertilisation and parental care are only half of the story; the stages around them have to work as well. In a sessile organism every stage has a separate job, and the cycle only works if all of them are possible. The adult is fixed, so it cannot find a mate or escape a predator or move to new ground; it reproduces by releasing gametes into the water and it often feeds by filtering or by catching plankton from the current. The larval stage is the only mobile stage, so it carries the species to new surfaces, and the settling stage - the cypris of a barnacle, the pediveliger of a mussel - selects a surface by texture, light and the presence of adults of its own species, then attaches and metamorphoses. If the shore is covered in sediment, or the adults have been scraped off, or the larvae are killed in the plankton, the cycle breaks at that point and the shore stays bare.

In a non-sessile organism the stages are separated in time and often in space rather than by attachment. The larva disperses; the juvenile grows in a nursery habitat such as seagrass, saltmarsh or mangrove, where there is food and cover; the adult migrates between feeding grounds and spawning grounds and may travel thousands of kilometres. Each of those habitats is essential, and each can be damaged independently. This is why marine protected areas are often designed as networks - a single reserve protects the adults but not the nursery or the spawning route.

Why fisheries need managing, and the technology that makes overfishing possible

Fishing adds a new source of death on top of the natural ones, and a stock is sustainable only while the number removed each year can be replaced by the number that grow into the fishable population. A stock can be pushed past that point: if too many of the large, mature fish are taken, the remaining adults may be too few, too young or too small to produce enough eggs, and the stock can collapse even though fishing continues. The classic example is the Atlantic cod stock on the Grand Banks off Newfoundland, which supported a huge fishery for centuries, was fished with increasingly efficient gear and factory trawlers, and collapsed in the early 1990s, forcing a moratorium that closed the fishery in 1992 and that kept it shut for decades, because the stock has never recovered to its former size.

That is the case for sustainable exploitation: taking fish at a rate the stock can replace, so that the fishery yields a catch every year instead of a large catch once. Sustainability here is not only about the fish. It is also about the habitat the stock depends on, the other species caught with it, and the communities that depend on the fishery for work.

Fishers have always caught fish; what has changed is the catching power of the fleet. Four technologies in the syllabus each remove one of the limits that used to protect a stock - the difficulty of finding fish, the size of the net, the depth of the seabed and the distance to port.

Sonar (and the echosounder and sonar-based fish finder) locates shoals by bouncing sound off them and displaying the echoes. Before sonar, finding a shoal was a matter of experience and luck; now a vessel can steam straight to a school, see its size and depth, and repeat the search all day. The effect on populations is that search time collapses, so fishing mortality rises for the same number of boats, and the deep water and the scattered shoals that once acted as a refuge no longer do. Sonar also makes it easy to find spawning aggregations, which are exactly the gatherings a stock most needs to be left alone.

Purse seine fishing surrounds a shoal with a large wall of net, then closes the bottom of the net like a drawstring purse and hauls it in. It is extremely efficient on pelagic schooling fish - tuna, sardine, anchovy, mackerel - and a single set can take tens of tonnes. It is non-selective by shoal: everything in the school is captured, including juveniles and other species that swim with it, which is how dolphins have been caught in tuna fisheries. Because it takes the whole school, it can remove an entire year class from a spawning ground in a few days.

Benthic trawling drags a heavy net, held open by otter boards and weighted by ground gear, across the seabed to catch demersal fish, scallops and prawns. The habitat effect is the serious one: the gear ploughs the sediment, smashes and uproots structure-forming organisms such as sponges, soft corals, sea pens and maerl beds that take decades to grow, and flattens the three-dimensional structure that juveniles use for cover. Sediment is resuspended and can smother nearby reefs and seagrass. Trawling is also poorly selective: the net takes whatever is in its path, so bycatch of undersized and non-target species can be very high, and much of it is discarded dead.

Factory ships are vessels that catch, process, freeze and pack fish at sea. Because they do not need to return to port, they can stay on a ground for months and can work distant and deep-water stocks that small boats cannot reach. They make a fishery mobile: when a stock is exhausted the ship moves on, so the collapse is exported to the next area rather than being felt and corrected in one place. Their capacity also encourages overcapitalisation - fleets built to catch far more than the stock can supply - and they employ few people per tonne landed, so the economic benefit is concentrated. The combination of sonar, purse seine or trawl, and a factory ship is what makes it possible to reduce a stock below the level at which it can recover.

• Removal against replacement: a stock is sustainable only while the number taken each year is replaced by recruitment, so fishing must leave enough mature adults to spawn.
• Spawner biomass: below a critical mass of mature adults, egg production falls sharply and the stock may not recover even if fishing stops - which is what happened to cod.
• Sonar removes the search limit and makes spawning aggregations findable.
• Purse seine takes whole schools, including juveniles and associated species.
• Benthic trawling destroys seabed structure and takes high bycatch.
• Factory ships let a fleet stay out for months and move on when a stock is exhausted.

The information needed to manage a stock

You cannot set a safe catch without knowing how the stock works. The syllabus limits this to seven pieces of information, and each one answers a different question.

Recruitment is the number of young fish that survive to enter the fishable stock each year. It sets the ceiling on future catch, and it varies hugely from year to year because it depends on larval survival, which depends on temperature, food and currents. Growth is the increase in length and mass with age; it tells managers how quickly a year class reaches a useful size, and therefore what is lost by catching fish young. Natural mortality is the rate of death from predation, disease, starvation and old age - the loss that would happen with no fishing at all. Fishing mortality is the additional death caused by fishing, and it is the only one managers can control directly. A stock declines when fishing mortality plus natural mortality exceeds the production of new biomass.

Age of reproductive maturity is the age (and usually the size) at which an individual first spawns. If fish are caught before they reach it, they never contribute to recruitment, so a minimum size or a mesh size set above the size at maturity protects at least one spawning. Fecundity is the number of eggs a female produces, and in most fish it rises very steeply with body size: one large old female may produce several times as many eggs as several small young ones, and her eggs are often larger and better supplied with yolk. That is why protecting large breeders matters more than protecting numbers alone.

Dependency on particular habitats covers nursery areas, spawning grounds and migration routes. A stock can be fished at a sustainable rate and still collapse if the mangrove creek where its juveniles grow is filled in, or if the reef where it spawns is trawled flat. This is the item students forget, and it is usually the one that earns the last mark.

These figures are not guesses. They come from research trawl surveys repeated on the same grid, from sampling the age structure of the catch, from tagging and recapture, and from otoliths - the ear stones of a fish, which lay down annual rings in the same way as tree rings and give an exact age. Landing records show what was caught; surveys show what is left.

Management tools, and how well each one works

Every tool in the syllabus works by removing one of the things a fisher needs - time, a place, a method, a size of fish, a boat, a market - or by making the rule worth obeying. The skill is in matching the tool to the biology of the stock and in seeing its weakness.

Input controls limit the effort that goes in: closed seasons, licences, restrictions on the number and size of boats and on the amount of gear, and rules on method such as minimum mesh size or compulsory rod-and-line. They are relatively cheap to administer and they let managers slow the race, but effort tends to leak: if boats are limited, each boat gets bigger; if mesh size rises, fishers tow for longer; if one area is closed, effort moves next door. Output controls limit what comes out, chiefly the quota, or total allowable catch, divided into shares. A quota is directly related to the biology and easy to monitor at the landing point, but it depends on accurate reporting: fish caught above the quota may be discarded at sea, and high-grading - keeping only the most valuable fish - wastes the rest.

Spatial controls are the strongest tool for habitat and for non-target species. A refuge zone protects part of the stock; a no-take zone bans all extraction; a network of these inside a marine protected area (MPA) protects the habitat as well as the fish. Inside a well-enforced no-take zone, fish grow larger and older, produce far more eggs, and the surplus spills over into the fished ground beside it, so catches nearby can rise. The weakness is that a reserve must be big enough, well placed and actually enforced; a small 'paper park' with no patrols protects nothing, and closing an area simply displaces effort elsewhere unless the total is also controlled.

Rules about size and method protect the future of the stock. A minimum landing size or mesh size lets juveniles escape and spawn at least once; a maximum size can protect the big breeders. The limitation is that escaped fish may still be damaged, and a minimum size creates an incentive to discard dead undersized fish. Compulsory rod-and-line is highly selective and creates local employment and tourism value, but it catches far less, so it only suits small, high-value fisheries.

Enforcement decides whether any of this is real. Monitoring by air and sea patrols, satellite tracking of vessel position (VMS), and inspection of catch and gear at port make detection likely; penalties - fines, confiscation of boats and gear, imprisonment - make the risk expensive. Neither works alone: patrols are expensive and the sea is large, so satellite monitoring of licensed vessels is far more cost-effective than hunting unlicensed ones, and penalties that are never imposed are no deterrent. Consumer tools - certification and labelling, publicity campaigns and price tariffs - work through the market, rewarding compliant fishers with a better price. They are slow and depend on shoppers caring, and certification can favour large exporters who can afford the audit, but they are the only tools that reach fishing that no national patrol can police.

No single tool is sufficient. A quota without mesh regulation takes juveniles; a no-take zone without effort control just moves the fleet; a label without enforcement certifies nothing. Effective management is a package matched to the stock's life cycle, and its credibility depends on the catch being counted.

Who pays for restrictions, and rebuilding a depleted stock

Restrictions are painful in the short term and beneficial in the long term, and examiners want both. Immediately, a closed season, a smaller quota or a no-take zone cuts the catch, so income falls for crews, processors, chandlers and ports, and unemployment rises in communities that may have no alternative work. The costs land on the people who can least absorb them, and the benefits - a rebuilt stock - arrive years later and are shared by everyone, including those who left the fishery. Quota trading can concentrate fishing rights in the hands of a few large operators and companies, which is efficient but hollows out the small-boat fleet and the towns that depended on it. In the long term, though, rebuilt stocks give higher catch rates, less fuel burned per tonne, steadier incomes, cheaper fish for consumers, and food security. A well-enforced MPA can also create work in tourism, diving and research.

Unrestricted fishing has the mirror image: in the short term, high catches, high profit, plenty of employment and cheap fish; in the long term, a collapsed stock, lost jobs and skills that cannot be replaced, government subsidies and buy-outs, higher prices, and a community that loses its identity with the fishery. Once a stock has collapsed, the cost of waiting is paid by the next generation, which is the argument for acting before the collapse rather than after it.

Where a stock has already been depleted, three rehabilitation strategies are in the syllabus. Replanting mangroves restores nursery habitat for fish, shrimps and crabs, and at the same time stabilises the shoreline, filters runoff and stores carbon; it is slow, needs the right species and the right tidal level, and fails if the seedlings are grazed, buried or cut for firewood. Artificial reefs - concrete modules, rock, or purpose-built structures placed on the seabed - add hard surface and complexity where there was only sand, which attracts fish and can rebuild a local population, and they can be designed as dive sites that earn money; but they can simply concentrate fish that were already there and make them easier to catch, they cost money to build and place, they can leach pollutants if the wrong material is used, and they are no substitute for protecting the stock.

Introducing cultivated stock to the wild - releasing hatchery-reared juveniles - can jump-start a recovery, and it works best where the habitat is intact, the species stays in one place and the cause of the decline has been removed. The risks are real: released fish may carry disease or parasites into the wild population, they may be genetically adapted to captivity and dilute the wild gene pool, they may have poor survival skills and be eaten quickly, and the operation is expensive per fish that survives. Restocking without habitat protection and without controls on fishing just feeds the fleet. Rehabilitation is always slower and more expensive than prevention, which is why the exam answer should end there.

• Short term costs of restriction: lower catch, lower income, unemployment, gear that must be replaced, effort displaced to other areas or other species, and quota concentrated into fewer hands.
• Long term benefits of restriction: larger and older fish, higher egg production, higher catch rates, lower fuel cost per tonne, stable income, food security and, around MPAs, tourism income.
• Short term benefits of no restriction: high catch and profit, employment, cheap fish. Long term costs: collapse, lost jobs and skills, subsidies, higher prices, loss of a way of life.
• Mangrove replanting: restores nursery habitat and protects the coast; slow, and fails without protection from cutting and grazing.
• Artificial reefs: add structure and can rebuild local stocks and create dive tourism; may simply concentrate fish for the fleet, cost money and do nothing for the wider stock.
• Restocking with cultivated stock: can accelerate recovery, but risks disease introduction, genetic dilution, poor survival and very high cost per surviving fish.

Marine aquaculture: intensive and extensive systems, and how the three crops are grown

Aquaculture is the rearing of aquatic organisms under some degree of human control, as an alternative to catching them in the wild. The syllabus divides systems into extensive and intensive, and the difference is the density of stock and how much of the food and conditions you supply.

An extensive system has low stocking density and relies on natural food already in the water, with little or no added feed, no artificial aeration and low capital input. Yield per unit area is low, but so is the cost and so is the pollution. Examples are blue mussels grown on ropes and longlines, where the mussels filter naturally occurring phytoplankton, oysters on intertidal beds, and traditional shrimp ponds in mangrove areas that are flooded by the tide and stocked at low density.

An intensive system has high stocking density and controls the environment: formulated feed is added, water is pumped and aerated, oxygen and temperature may be managed, and disease and predators are actively controlled. Yield per unit volume is high, but capital, feed, labour and energy costs are high and so is the waste produced. Examples are Atlantic salmon in floating sea cages and shrimp in heavily stocked, aerated ponds. Between the two lies semi-intensive practice, common in shrimp farming, with some feed and some aeration at moderate density.

Atlantic salmon are farmed in a sequence. Broodstock are stripped of eggs and milt in a freshwater hatchery, and the eggs hatch into alevins that live on their yolk sac. The young salmon - now fry, then parr - are reared in freshwater tanks or cages until they smoltify, the physiological change that lets them tolerate salt water. Smolts are transferred to floating sea cages in a sheltered loch or fjord, where they are fed pelleted feed containing fishmeal and fish oil for roughly 18 months to two years until they reach market size, then harvested and slaughtered. The cycle is entirely controlled, which is why the industry can plan production - and why sea lice, escapes and waste concentrate at the cage.

Marine mussels need no feed at all. In rope culture, ropes are suspended from rafts or longlines in the water column, and mussel larvae settle on them naturally as spat (or hatchery spat is used); the mussels grow by filtering phytoplankton for 12 to 24 months and are then harvested, either by hand or by a mechanised harvester that strips the ropes. There is no feed input and no antibiotic use, so the environmental footprint per tonne is small, but the site must have clean water and enough current to bring food, and the mussels can accumulate toxins from harmful algal blooms, which is why harvesting areas are monitored and closed when toxin levels rise.

Shrimp farming begins in a hatchery, where broodstock spawn and the larvae pass through several stages before becoming postlarvae. Postlarvae are stocked into coastal ponds, usually at higher density than a traditional pond would support, and are fed compound feed, with paddlewheel aerators keeping oxygen levels up. Harvest comes after roughly three to six months. Because the ponds need brackish water and flat coastal land, shrimp farms have often been cut into mangrove forest, and because densities are high, disease can sweep through a pond in days.

Aquaculture: what it needs to succeed, and what it costs the environment and society

A project that ignores any one of the ten requirements in the syllabus will fail, however good the site looks. Availability of stock means a reliable supply of eggs, juveniles, postlarvae or spat, from hatcheries or from wild settlement; if you depend on wild seed you are fishing the very population you are trying to farm. Availability of clean water means the right temperature, salinity, oxygen and pH, free of sewage, industrial waste and harmful algae - polluted water gives slow growth, disease and contaminated product. Availability of feed and the efficiency with which feed is used are the commercial heart of carnivorous culture: salmon and shrimp need feed containing fishmeal and fish oil made from wild fish, and a poor feed conversion ratio means both higher cost and more waste. Labour must be available, affordable and skilled enough to notice disease and handle stock. Disease management - vaccines, biosecurity, separating year classes, fallowing sites - is what keeps an intensive system from losing a whole crop. Location must offer shelter, depth, current, and room to expand without conflicting with shipping, fishing or tourism. Market demand and access to market decide whether a harvested crop can be sold at a profit, which needs roads, ice, cold storage and transport. Return on investment ties it together: the capital is large and the crop takes months or years to sell, so the project must survive low prices and bad years.

The impacts are equally wide-ranging, and the syllabus lists them. Habitat destruction is direct and often permanent: mangrove forest cleared for shrimp ponds, seagrass uprooted, and the seabed beneath cages smothered by faeces and waste feed. Overexploitation of feedstocks is indirect and easily missed: farming carnivores consumes wild fish - anchoveta, sardine, blue whiting - caught to make fishmeal and oil, so aquaculture can add pressure to wild stocks rather than relieve it. Pollution includes dissolved nutrients that cause algal blooms, uneaten feed and faeces fouling the bottom, medicines and antibiotics, and copper-based antifoulants on nets.

Escape of cultured stock matters because farmed fish are selected for fast growth and are not adapted to the wild: they can compete with wild fish for food and spawning sites, interbreed and reduce the fitness of the wild population, and carry diseases and parasites with them. Introduction of potentially invasive species happens when a species is farmed outside its natural range and escapes or is released; the Pacific oyster and various seaweeds have spread this way. Spread of disease is a two-way problem: high densities make outbreaks likely inside the farm, and water flow carries lice, viruses and bacteria out to wild populations - sea lice from salmon farms are the standard example.

Competition for resources covers space in a sheltered loch or bay, clean water, and wild seed or spat. Reduction in the exploitation of native stocks is the positive environmental case: farmed salmon, mussels and shrimp substitute for wild-caught fish, so pressure on wild stocks can fall if the feed problem is managed. Social impacts include local employment and a new source of protein and income, but also the displacement of traditional fishers and farmers from the shoreline, disputes over common water, and jobs that are seasonal and low paid. Economic impacts include export earnings and foreign exchange, diversification for coastal communities and a more reliable supply - set against high capital and feed costs, disease losses, price competition from low-cost producers, and profits that often leave the local area.