Aquaculture Production Cycles
In order to have a comprehensive understanding of aquaculture, it is important to understand standard production cycles, from seeds and fry to grow-out stages. Broodstock for aquatic animals and seaweed are usually collected from the wild and used to produce juveniles or used as seed to stock cages or lines. For finfish, once adult animals produce a viable spawn, the eggs are collected and incubated until the larvae hatch. These larvae are fed a combination of live feeds (rotifers, artemia, copepods, microalgae, diatoms) until they are big enough to be moved into the nursery. Once in a nursery, finfish are fed artificial feed until they reach the appropriate size for transport. Other animals, such as sea cucumbers, may not need to be fed as they can scavenge feed from the seafloor of a pen or pond. Seaweed seedlings are generally produced one of two ways: 1) small seedlings or healthy new growth are taken from parent stock (wild or farmed) and transferred directly to new lines; or 2) seaweed spores are gathered from the wild or grown in a hatchery and then grown onto seaweed lines in a hatchery until large enough to be outplanted on a farm.
Hatchery juvenile production tends to be a bottleneck for new species, as the demand is greater than the supply. For example, in Palau in 2019, the amount of rabbitfish fry that the farmers demanded was greater than what the National Aquaculture Center could supply to them, forcing farmers to stock less fish than they wanted.
Finfish Farming & Culture Methods
There are many different types of farming methods used for finfish aquaculture, including land-based recirculation, flow-through tanks, nearshore cages, coastal ponds, and offshore cages. The focus of this section is on cages and pens since this method is often used for finfish aquaculture in reef areas.
Cage Culture Methods
Finfish cage culture can be used for a variety of species and generally has a smaller environmental cost than coastal ponds. Ponds, being on land rather than in a marine environment, directly compete for space and have historically been a driver of negative environmental impacts on sensitive nearshore coastal habitats, such as mangroves and estuaries.
Cage and pen cultures are types of enclosures where the animals being farmed are enclosed in an area by a structure, usually a net or cage. Cages have netting on all sides, in some cases, even the top to prevent predators from entering the cage. Pens can use the seafloor as the bottom of the pen and they only have nets on the sides.
Cages and pens are built from different types of materials. In Asia, small to mid-scale farms have used materials such as bamboo and wood for decades; switching to new materials such as nylon, plastic, polyethylene, and steel mesh, which although have a higher cost, present a much longer life span and allow better water exchange.
In coastal areas the design that is most often utilized is a cage that is constructed with locally sourced materials (wood or bamboo) that floats due to buoyant materials (drums filled with air or styrofoam blocks). A synthetic fiber net is then hung from the wood platform to become the enclosure that holds the fish.
Cage and net pen shapes are generally square, rectangular, or circular and can be used in relatively shallow or deep environments.
Shellfish & Seaweed Farming & Culture Methods
Seaweed and shellfish are sometimes referred to as extractive species as they help remove and filter out waste and nutrients from the water column. Shellfish are filter feeders and extract organic matter from water while seaweed absorbs dissolved nutrients as part of photosynthesis. When these species are harvested, this waste and nutrients are removed from the water column. For this reason, co-culture of these species with fed species like finfish is often encouraged. In 2018, extractive species production accounted for 57.4% of total world aquaculture production. ref
Seaweed and shellfish aquaculture are profitable industries with diverse market applications:
- Seaweed is a massive and diverse industry worldwide, with farmed seaweed alone worth $11.7 billion annually. ref It is also an important industry globally for many women, rural populations, and Indigenous people. In addition to seaweed like nori and wakame that is gathered and raised for direct consumption, Eucheuma and Kappaphycus seaweeds are grown in tropical regions around the globe for use as a thickening agent for food and cosmetics, among other uses.
In 2018, shelled mollusks accounted for 56.3% (17.3 million tons) of the global production of marine and coastal aquaculture. Oyster and mussel shells can be turned into calcium carbonate or calcium oxide, two chemicals with diverse industrial applications. Shellfish shells are also used in cosmetics, traditional medicines, calcium supplements in animal feed, handicrafts, and jewelry. ref
Seaweed and shellfish can both be farmed using raft or longline systems and grown on the seafloor:
- Raft systems: In raft systems, seaweed or shellfish infrastructure is hung from lines or nets suspended from a floating wooden framework (a raft). These raft systems are securely anchored to prevent drift with currents or strong waves.
- Longline systems: In a longline system, a length of rope is suspended in the water column and anchored at both ends with flotation devices attached to the rope. Seaweed seed or shellfish culture systems are hung on the rope. Longline systems are preferred in high exposure areas. ref
- Intertidal systems: Shellfish can be grown in intertidal habitats where they are either buried into sediment or grown on stakes, racks, or intertidal longlines. In many cases, intertidal shellfish plots are protected from predators with overlaid mesh that is secured into place. ref
- Off-bottom method: Wooden stakes are driven into the seafloor about 20-25 cm from each other in straight rows. A rope is attached firmly stretched between the two stakes and seaweed seed is tied to the rope.
- Bottom stocking: Seaweed can also be placed at the bottom of a pond and not fixed to the sediment in any way. In more open waters, seaweed can be planted into the sediment or held in place by weights to a sandy bottom. ref
Sea Cucumber Farming & Culture Methods
Sustained demand for sea cucumber products in Asian markets, especially in China, have provided a valuable source of income for coastal communities in the tropics. Sea cucumbers have primarily been harvested for a luxury dried food item known as beche-de-mar. ref However, recent declines of wild stock due to overfishing have led to an increased interest in sea cucumber farming to help meet demand, increase income security, and diversify livelihoods.
The main methods for culturing sea cucumber include pond farming, pen culture, sea ranching, and tank culture. Stocking larger juveniles in pond farming, pen culture, and sea ranching usually results in a higher survival rate but this also means the cost of production at the nursery level will be more expensive. Generally, juveniles weighing over 20 g are no longer vulnerable to predators such as crabs. Once juveniles are placed in any of these farming methods, with the exception of tank culture, they do not usually require feeding as they can graze on algae and aquatic invertebrates on the seafloor. This grazing on substrate may also help improve water conditions. For all methods, stocking density needs to be less than 200 g/m². When density reaches more than about 200-250 g/m2, growth slows down or stops altogether.
Ponds are usually located near to the shore to facilitate water exchange (intertidal zone). Tides bring in fresh seawater and water flow is controlled by opening and closing sluice gates. ref Sea cucumber ponds can be former shrimp or crab ponds or newly built earthen ponds with sandy-muddy bottoms or coral-sand substrates. Sea cucumber can also be grown in rotation with shrimp, improving pond conditions by ingesting substrate and removing organic detritus.
For tropical species, pond water depths are usually around 0.8 to 1.5 m. Sea cucumber can also be produced within multitrophic polyculture ponds (recycling of feeds and waste products). For example, in southern China Holothuria scabra are grown with pearl oysters and groupers in earthen ponds of several hundred hectares.
For temperate sea cucumber species, the ideal conditions for pond culture include: ref
- Be near to the low tide mark so that seawater can be fed into the pond by gravity
- Be in an area not affected by pollution
- Ocean salinity in the 28-31 range
- Have a sandy or sandy-muddy bottom
- Be at 2 m depth or more
- Have a pond size between 1 to 4 ha
- Have shelters to protect the cultured organisms against typhoons or strong wave attack
Preparing ponds for sea cucumber farming includes: drying the pond (using sluices and pumps), removing unwanted predators such as crabs, tilling the sediment to disturb the mud layer (at least 5 cm burial layer), building a net pen at the sluice-gates to exclude predators and prevent sea cucumber from escaping or aggregating in this area, applying lime (agriculture or hydrated) at a rate of 0.5-1.0 t/ha and filling the pond with seawater one week before stocking with juvenile sea cucumbers.
Some challenges with pond culture include heavy rain during the wet season as sea cucumber do not tolerate freshwater and intense heat during the dry season. Challenges can arise due to the relatively long duration of culture which can increase the expense of renting ponds and labor costs due to the high risk over the wet season.
In pen culture, sea cucumbers can be hung in cages under wooden rafts or placed in pens on the seafloor. ref Pens are often constructed from locally available materials, including nylon fishing nets, wooden stakes, and rope. The nets are held in place by iron rebar which is buried into the sediment. ref Nets or mesh are used to delimit the enclosures and allow water exchange but also to prevent the sea cucumbers from getting out of and to reduce attacks from predators.
Some optimal conditions for pen culture include:
- Placing pens in a sheltered area protected from high energy waves
- Ensuring pens always have water coverage even at low water springs
- Being accessible by foot during spring low tide
- Having significant tidal inflow of settleable solids
- Having sediment that consists of fine sand
- Being at least 15 cm of sediment above the rock below
- Placing pens away from areas with a high density human population
- Placing pens in areas that have been agreed for use by the area’s local resource users
- Being in an area easily patrolled by security guards at night.
Pens can be either circular or square. Circular pens are more resistant to currents and require less material than square pens to construct, making them a cheaper option. Square pens are better for optimizing space and facilitating spatial planning. ref Small internal covered pens are used as nurseries at sites which suffer from high levels of predation. Pens also require regular maintenance, such as the removal of predators, including crabs, and removing seaweed or mud from the nets to ensure good water exchange.
In Madagascar, a buffer zone marked by buoys where fishing is prohibited surrounds sea cucumber pens. To ensure the safety of the stock, watchtowers are built at strategic points on the farm where people approaching the farm can be seen, but also where there is a good view of the larger part of the farm. The watchtowers are also equipped with solar-powered torchlights to ensure the safety of the stocks during the night.
Sea ranching involves releasing hatchery-produced juveniles into unenclosed marine environments and allowing them to grow naturally in these areas for later harvesting when they reach a marketable size. Some site selection criteria for determining good sea ranching locations include:
- Location: Sites that are not vulnerable to wind, waves, and currents; Have some form of security against natural predators and human thieves; Are accessible and allow for easy transportation to both the hatchery and market.
- Bio-physical: Sites that are within the correct temperature and salinity range; Have good ecosystem health including having existing wild sea cucumbers and seagrass; Have substrate that is the right quality for sea cucumbers (sandy, muddy, coral rubble); Water quality is high.
- Social and governance: Have rights to the site as well as community and government support.
Different zones are delimited for sea ranching release, including the core zone, where the juveniles are released, and a buffer zone around it where fishing is prohibited. These zones are differentiated by the use of buoys. This type of culture requires the least amount of labor for upkeep, but stock survival will be considerably lower than pen or pond culture. Additionally, property rights of sea cucumber in sea ranching is not well defined, so poaching is a key concern. ref
The section on Sea Cucumber Farming & Culture Methods was developed collaboratively with Hery Lova Razafimamonjiraibe, Blue Ventures.