Aquavetplan enterprise Manual Version 0, 2015

B2 Semi-open systems

B2.1 Overview of semi-open systems

Semi-open systems, generally used for culture of finfish and molluscs, are typified by net-pen systems in which the stock is contained or controlled in a relatively uncontrolled environment. Movement and control of stock is possible, but there is no control over the movement of water in, through and around the culture system.

B2.1.1 Finfish industries

In Australia, the main finfish industries using semi-open systems are:

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  the southern bluefin tuna industry in South Australia (see Section B2.2)
  
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  the salmonid industry that grows Atlantic salmon and rainbow trout in Tasmania (see Section B2.3)
  
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  barramundi farming in the tropics and other regions where warm water is available (see Section B2.4)
  
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  the yellowtail kingfish industry in South Australia (see Section B2.5).
  

Other finfish species such as snapper, cobia and mulloway are also being introduced to semi-open systems, and it is possible that these species will become commercially important in future.

The aquatic environment of semi-open systems is not modified for culture purposes; instead, the aquatic environment is chosen for its suitability for the culture system. The fish are reared in net-pens moored in marine, estuarine or freshwater environments, usually in sheltered situations. Because of the stocking density in semi-open systems, adequate circulation of water through the net-pens is essential for the wellbeing of the fish. There is virtually no control over the water in which the net-pens are located. Impermeable liners can be placed around net-pens, and are used to apply immersion treatments. They prevent movement of water, but also prevent oxygen-rich water reaching the fish and waste products (e.g. ammonia) being diluted.

In a semi-open system, excess feed falls through the net-pen to the bottom of the sea or estuary. Good management minimises such losses, to reduce waste and pollution. Some excess feed is eaten by cohabiting fish species or the benthic community that gather under the net-pens. Faecal material, depending on water movement, is either carried away from the site or deposited on the sea floor nearby. Good management incorporates seasonal fallowing of used net-pen sites within a farm lease to minimise the accumulation of fish waste on the substrate below, and to allow recovery of the sea floor and benthic biodiversity.

Premises in semi-open systems are usually land based, but much equipment needs to be suitable for use in open water, and able to withstand the harsh conditions of an open-water site. Important features of premises and equipment in semi-open systems are as follows:

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  Boats are the main form of transport.
  
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  Equipment used to house and contain the stock needs to be robust and, in many cases, transportable by towing or in boats.
  
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  Premises can range from substantial buildings that contain machinery and feed, to sheds, offices, mess rooms, laboratories and simple sheds designed to protect workers from the weather.
  
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  Large, open, on-land work areas are often available for maintenance and storage of nets and other equipment.
  
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  Designated areas on land are usually available for disposal of dead stock, and other wet waste from harvesting and processing.
  

Transport of fish is usually tailored to the fish species and the particular culture techniques used for that species. Wild-caught tuna are caught in a purse-seine net and then transferred to a tow-cage that is towed by boat to the aquaculture site. Salmonids and kingfish are usually transported by road from the hatchery to the grow-out site. Once at the grow-out site, they may be transferred directly to the net-pens or transported to the site by well boats, helicopter drop or fish pumps. The barramundi industry is more widespread than other aquaculture industries, and the larger distances between hatcheries and grow-out sites often require barramundi fry to be transported by air, then road and boat in foam boxes or aerated bulk bins. Once at the grow-out site, the fry can be released directly into net-pens. Empty or stocked net-pens can be towed between different culture sites.

B2.1.2 Mollusc industries

Semi-open farming systems for molluscs usually have the shellfish suspended from longlines, held in baskets from lines or housed in racks. Young shellfish may be harvested from wild ‘spat-fall’ or cultured in tanks in sophisticated hatcheries from wild-caught or domesticated broodstock. Shellfish hatcheries supplying farms are usually semi-closed or closed systems, but some sectors rely on natural spat settlement—for example, mussel farming in some states and some Sydney rock oyster production.

Algae and diatom cultures are used to feed larvae during the hatchery and nursery phases. The molluscs are not actively fed during the grow-out period but rely on natural food floating through the lease area.

In northern and north-western Australia, pearl oysters are the basis of a large semi-open mollusc farming industry. The pearl industry is based on a sustainable wild-harvest pearl oyster fishery, using unique seeding and husbandry processes to cultivate pearls. The industry has also developed the capacity for pearl oyster production in hatcheries to ensure a secondary source of pearl oyster supply (see Section B2.6). Spasmodic mortality episodes from oyster oedema disease in hatchery spat have resulted in a dedicated research programme to determine the cause. No causative agent has yet been identified. Industry is managing operations to minimise the impacts of oyster oedema disease.

The commercial production of molluscs for human consumption is based on oysters (mainly Pacific oysters, Sydney rock oysters and flat oysters), mussels, abalone and scallops (see Section B2.7).

B2.1.3 Husbandry practices and disease control

In semi-open systems, the issues that are most important for disease control include:

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  location of the culture site
  
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  location of the stock source
  
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  transport and movement of stock
  
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  housing and husbandry of stock
  
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  net cleaning schedule
  
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  type, origin and method of application of stock feed
  
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  handling during the semi-open production phase
  
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  protection of stock from predation and disease (e.g. vaccination)
  
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  fish health monitoring of stock feeding behaviour, mortality and disease status
  
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  application of treatments
  
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  harvesting method
  
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  method and schedule for removing dead stock from culture facilities.
  

B2.1.4 Interactions between semi-open systems and the environment

There is a two-way interaction between the animals farmed in semi-open systems and the environment. There are no barriers between the farm area and the environment, and waterborne agents can move freely into and out of the net-pens. Animals in these systems interact directly with outside populations, as wild animals are attracted to the food source on, under and within the net-pens or other structures. Large structures, such as racks and nets, quickly become substrates for complex biofouling communities of sea plants and animals. Interaction can also occur if animals from semi-open systems escape into the wild.