KEY PRESENTER 

Dr Stuart J. Rowland [BA (Hons I), PhD]

Senior Research Scientist

Scientist-in-Charge NSW Fisheries Grafton Aquaculture Centre

Dr Stuart J. Rowland [BA(HonsI), PhD]  

NSW Department of Primary Industries

Senior Research Scientist

Scientist-in-Charge, Grafton Aquaculture Centre  (Pictured right)

Stuart has been involved with research into the taxonomy, biology, conservation and aquaculture of Australian native freshwater fishes for 30 years. While based at the Inland Fisheries Research Station, Narrandera between 1978 and 1985, he developed techniques for the hormone-induced spawning and larval rearing of Murray cod (Maccullochella peelii peelii), golden perch (Macquaria ambigua) and silver perch (Bidyanus bidyanus). These techniques form the basis of a hatchery industry (both commercial and Government) that produces up to 8 million fingerlings annually for stock enhancement, conservation and commercial aquaculture.   

Stuart studied aspects of the biology (history, fishery, age and growth, reproduction, diseases) of Australia’s largest and most famous inland fish, the Murray cod. This research lead to a better understanding of the biology of cod and the formulation of new fisheries management regulations. The significant recovery of Murray cod in NSW over the last 10 years has been due, in part to the protection of stocks through these regulations. During this research, a new species, the eastern freshwater cod (Maccullochella ikei) and a new sub-species, the Mary River cod (M. peelii mariensis) were identified and named; both cod species are classified as “Endangered”. Stuart selected a site and designed the Grafton Aquaculture Centre, and ran one of Australia’s first fish conservation projects for eastern freshwater cod during the 1980’s. He is currently supervising a research project on the ecology and conservation of eastern freshwater cod.

 1. Diseases and Health Management of Silver Perch (Bidyanus bidyanus)

Introduction

Infectious diseases are caused by exposure of fish to pathogenic organisms such as protozoans, monogeneans, fungi and bacteria. Diseases are inevitable in semi-intensive and intensive aquaculture and can pose serious threats to the viability of farms and industries. The silver perch aquaculture industry is based mainly on pond culture and produces around 500 tonnes annually, with 300 tonnes in NSW and the remainder in Queensland, Victoria and Western Australia. In 1998 and 1999, there were reports of disease problems causing significant losses on some commercial silver perch farms, including regular outbreaks of a fungal disease during winter, particularly in the cooler, inland areas of south-eastern Australia. A project to address disease and health management issues commenced in 2001 and were funded by NSW Department of Primary Industries (NSW DPI) and the Fisheries Research and Development Corporation.  The Grafton Aquaculture Centre (GAC) and selected commercial silver perch farms in coastal and western regions of NSW were involved in the project. Major objectives were to: (i) identify the cause and predisposing factors of the new fungal disease; (ii) identify other major diseases of silver perch, particularly those prevalent in winter; (iii) develop diagnostic, control and preventative measures for the diseases; (iv) develop a generic health management plan for farms; and (v) development of an overall health management strategy.   

Infectious diseases

The number of known pathogens and infectious diseases of silver perch has increased from 4 in 1983 to around 20 in 2007. Protozoan parasites Ichthyophthirius multifiliis (causes the disease white spot or ichthyophthiriosis), Chilodonella hexasticha (chilodonellosis), Trichodina sp. (trichodinosis) and Ichthyobodo necator (ichthyobodosis), and the monogenean gill fluke, Lepidotrema bidyana, accounted for around 80% of all records. White spot and chilodonellosis are acute diseases that can cause high mortalities if not diagnosed and treated promptly. Silver perch are susceptible to two fungal diseases, winter saprolegniosis (colloquially called winter sap or winter disease) and epizootic ulcerative syndrome (EUS or redspot) caused by Aphanomyces invadans. EUS was initially recorded at GAC in 1991, but has not occurred since 1997 following changes to water management practices. EUS was not recorded on farms in the western region, with one exception following the translocation of infected fingerlings from a farm in a coastal drainage. Bacterial diseases of silver perch include columnaris, tail (or fin) rot, streptococcosis, mycobacteriosis and aeromonad dermatitis, but their incidence was low (< 3%). No viral diseases were recorded, but the epizootic haematopoietic necrosis virus (EHNV) kills silver perch under laboratory conditions and so is a potentially serious pathogen.      

Winter saprolegniosis

There were 22 outbreaks of winter saprolegniosis on five of the study farms. A survey of 20 farms found that 25% had outbreaks in the winter of 2001, with the locality of farms ranging from southern Queensland (Stanthorpe area) to the Riverina. Experimental work on this disease was limited because of the absence of winter saprolegniosis at GAC, and the lack of opportunities for replication and control treatments on commercial farms. The causative agent of winter saprolegniosis was identified as the pathogenic fungus, Saprolegnia parasitica. Detection of the disease in the initial stages is difficult in characteristically turbid ponds, where fish are difficult to see and feeding activity is reduced in winter. Heavily-infected fish often swim slowly near the surface and edges of ponds, with patches of fungus clearly visible on de-pigmented areas of the skin and gills. Large silver perch (> 180 g) are more susceptible than smaller fish, and if untreated, winter saprolegniosis can cause 100% mortality. Most outbreaks commenced at water temperatures below 16°C, and rapid decreases in temperature (e.g. 4° - 5°C in 5 – 7 days) to below 10°C following cold changes during winter were associated with onset and severity suggesting that suppression of the immune system is involved in the initiation of winter saprolegniosis. Pre-disposing factors include: infestations of ecto-parasites; damage to fish from handling during partial harvesting of ponds; high stocking densities and biomasses (> 20,000 fish/ha and 10 tonnes/ha); high organic loads; and poor water quality. Recommended management actions to assist in the prevention and control of winter saprolegniosis are: reduce fish biomass to < 6 tonnes/ha; ensure fish are free of ecto-parasites; do not over-feed; maintain good water quality; avoid partial harvest; and treat harvested fish in tanks with a continuous bath of salt (2 g/L) and formalin (30 mg/L). 

White spot

White spot is caused by the ciliate protozoan Ichthyophthirius multifiliis. It is a common and acute disease of freshwater fish through-out the world, and causes significant mortalities and economic losses in many industries. Silver perch are susceptible to white spot, and outbreaks may occur at any temperature; at GAC the disease in prevalent in winter (< 15ºC) and spring. White spot is a difficult disease to treat, particularly at low water temperatures because of the complex life cycle of I. multifiliis. Experiments were carried out at GAC to determine: (i) the efficacy of copper and formalin in treating white spot; and (ii) the depletion rates of these chemicals. Copper (as copper sulfate) at concentrations of 0.1 – 0.2 mg/L controlled white spot, but higher concentrations of 0.25 – 1.0 mg/L were toxic to silver perch, and 0.05 mg/L was ineffective. Formalin at a concentration of 30 mg/L controlled white spot, but 20 and 10 mg/L did not completely controlling the disease. In earthen ponds containing silver perch, copper (0.2 mg/L) and formalin (30 mg/L) were depleted to or below therapeutic concentrations within 24 and 48 h post-treatment respectively. Using these data, new treatment regimes for copper and formalin involving applications daily or on alternate days were developed and validated. Outbreaks were successfully controlled using either chemical at costs of $486.00/ha/day for formalin and $68.34/ha/day for copper (as copper sulfate). A continuous salt (NaCl) bath of 2 g/L was effective in controlling white spot and preventing saprolegniosis. This treatment is recommended in aquaria, tanks and re-circulating aquaculture systems, but not in ponds because of the large quantities of salt required, potential for accumulation and detrimental environmental effects of saline water.

Chilodonellosis

This is an acute disease of silver perch caused by the ciliate protozoan Chilodonella hexasticha. It can cause high mortalities within several days unless diagnosed and treated promptly. Fortunately, it is relatively easy to treat using formalin (30 mg/L) in ponds, or salt (10 g/L for 1 h) or formalin in tanks.      

Gill flukes

Monogenean gill flukes are common fish parasites with strict host-specificity and a non-pathogenic nature which reflects a highly developed relationship to their hosts. The fluke infesting silver perch is Lepidotrema bidyana. Mortalities from infestations of gill flukes were generally low (< 5%), and light infestations did not cause problems. However, heavy infestations can cause gill damage, impair respiration, reduce appetite and growth, and predispose fish to bacterial and fungal infections. Gill flukes were not recorded at GAC or on commercial farms in the early 1990’s, but have become common over the last 10 years, probably due to the introduction of wild broodfish, the movement of fish between farms, and the difficulty in eradicating the parasites from farms. Infestations can be controlled in ponds and cages by formalin (30 mg/L) or trichlorfon (0.5 mg/L; Lepidex®), but eggs are resistant to formalin and trichlorfon, and three consecutive treatments 7 – 21 days apart are needed to control infestations. In tanks, 0.25 mg/L trichlorfon (Lepidex® 500) is sufficient to control flukes.

Chemicals

Chemicals play an important role in aquaculture, by facilitating many operations such as handling fish, control of reproduction, management of water quality, promotion of plankton blooms, sterilisation of facilities and equipment, and control of plants and diseases. Chemical therapeutants are essential in controlling some infectious diseases, particularly acute diseases that can cause high mortalities if not treated promptly. The Australian Pesticides and Veterinary Medicines Authority (APVMA) regulates chemical use in aquaculture, and only chemicals that are registered or permitted can be used legally. The current status of chemicals is outlined and includes the following: registered – Aqui-s and HCG; permitted – formalin, benzocaine, hydrogen peroxide; applications to the APVMA for minor use permits – salt, copper sulfate, trichlorfon, oxytetracycline; exempt – fertilisers, lime, zeolite, aluminium sulphate. Chemicals that must not be used are malachite green, chloramphenicol and nitrofurans. The use or detection of illegal chemicals will jeopardise the future of an individual farm, a species, an industry, and Australian aquaculture in general. Care should be taken with the use of chemicals, and directions and recommendations must be followed (see key references).       

Health management

Health management is a concept of dealing with fish health by providing general environmental and culture conditions that reduce the incidence and severity of diseases, leading to efficient and economic production of fish. The old saying “An ounce of prevention is worth a pound of cure” is very applicable to silver perch farming. Health management commences with the selection of a good site and encompasses all aspects of the production cycle by combining a well-designed farm with Good Aquaculture Practices to minimise stress and health problems, and to optimise survival, growth, food conversion, reproduction and production. To aid on-farm disease diagnosis and health management, two publications, a Disease Diagnostic Manual and a generic Health Management Plan (HMP), have been published. The manual is an easy-to-use, desk-top publication that will facilitate the accurate and rapid diagnosis of silver perch diseases. The HMP provides guidelines for the monitoring, treatment and prevention of diseases, as well as for site selection, design and operation of farms, water quality monitoring and management, husbandry and production techniques, and nutrition and feeding. All these aquacultural practices form the basis of health management. In addition, the NSW Hatchery Quality Assurance Program (HQAP) addresses concerns about diseases as well as genetics and trash fish in the native fish hatchery industry. Use of these publications and implementation of the HQAP should significantly improve health management on silver perch hatcheries and farms.  

During the project, extension and disease diagnostic support was provided to participating farmers. This support greatly increased the farmer’s knowledge of silver perch diseases, and their skills in using microscopes and diagnosing and treating diseases. The need for regular monitoring of fish for disease and for improved health management and aquaculture practices was demonstrated on participating farms.     

The NSW DPI continues to provide aquaculture extension and disease diagnostic support to silver perch and other aquaculture and fisheries industries. There are also disease diagnostic services provided by commercial companies with highly competent veterinarians who have extensive fish health management experience. Health management is an essential component of fish farms, and the success of any farm is dependent on the implementation of a health management plan.  

Key references 

Ingram, B.A., Gavine, F. and Lawson, P. (2005). Fish Health Management Guidelines for Farmed Murray Cod. Fisheries Victoria, Research Report Series No. 32.   

Read, P. Landos, M., Rowland, S.J. and Mifsud, C. (2007). Diagnosis, Treatment and Prevention of the Diseases of the Australian Freshwater Fish Silver Perch (Bidyanus bidyanus). NSW Department of Primary Industries.

Rowland, S.J., Landos, M., Callinan, R.B., Allan, G.L., Read, P., Mifsud, C., Nixon, M. and Tully, P. (2007). Development of a Health Management Strategy for the Silver Perch Aquaculture Industry. Final Report to the Fisheries Research and Development Corporation for Project Nos 2000/267 and 2004/089. NSW Department of Primary Industries – Fisheries Final Report Series.   

Rowland, S.J. and Tully, P. (2004). Hatchery Quality Assurance Program for Murray Cod (Maccullochella peelii peelii), Golden Perch (Macquaria ambigua) and Silver Perch (Bidyanus bidyanus). NSW Department of Primary Industries.  

Silver Perch Aquaculture Research and Development 

Silver perch (Bidyanus bidyanus) is an Australian native freshwater fish that is endemic to the Murray-Darling River System. It is a high quality, white-fleshed finfish with high levels of omega oils that has long been recognised as having potential for aquaculture. Hatchery techniques based on hormone-induced spawning and extensive larval rearing were developed at the Narrandera Fisheries Centre in the early 1980’s. Fingerlings were used for stock enhancement in impoundments, and sold for stocking farm dams; in 1982 the sale of native fish was transferred to commercial hatcheries. Research into grow-out commenced at the Grafton Aquaculture Centre in 1990, and demonstrated that silver perch is an excellent species for culture in earthen ponds with high survival rates (> 90%), fast growth rates (2-5 g/fish/day) at high stocking densities (20,000/ha) leading to high production rates (10 tonnes/ha/year). Between 1996 and 2005, there were research projects into nutrition and diet development, feeding strategies, broodfish domestication and management, evaluation of genetic strains, tank and cage culture, and production in tank-based re-circulating aquaculture systems. Current projects include diseases and health management, genetic improvement and cage culture. Technology has been transferred to industry through major conferences (1995, 2003), workshops, field days, extension, seminars and numerous scientific and technical publications. Over the years, funding has been provided by the Fisheries Research and Development Corporation, Meat and Livestock Australia, and the Cotton Catchment Communities Co-operative Research Centre (Cotton CRC).  

The high quality of silver perch, its excellent culture attributes and relatively low feeding costs suggest that the species has the potential to form a large industry (> 5,000 tonnes/year) based on high-volume, low-cost production. Although some commercial farms have achieved high survival and production rates, the industry has not realised its potential to become one of Australia’s largest fisheries. Contributing factors have been: poor site selection and design, the use of poor husbandry and production strategies, low survival due to bird predation and disease, difficulties with pond management, high costs of electricity and labour, fragmentation of the industry, limited marketing and promotion, and no large-scale investment. Recent research has found that silver perch perform well in cages [high survival (> 90%), good growth (1.7 – 3.5 g/fish/day), high production rates (50 – 90 kg/m³)] and cage culture has advantages such as ease of management and prevention of bird predation that may help overcome some of the problems associated with pond production. Cage culture and genetically-improved silver perch offer opportunities for increased production and efficiencies, and further development of the industry. In addition, current research is determining the feasibility of integrating silver perch culture with cotton and other irrigation industries. The potential of silver perch and other native freshwater finfish for commercial aquaculture remains very high.