Diseases of the Australian Freshwater Fish Silver Perch (Bidyanus bidyanus) - Part 1

Introduction

Silver perch (Bidyanus bidyanus) is an Australian native freshwater fish endemic to the inland Murray‑Darling River System (Fig. 1). Hatchery techniques were developed at the Narrandera Fisheries Centre (formerly the Inland Fisheries Research Station) in the late 1970’s, and transferred to a new commercial industry in 1982. The routine production of fingerlings by government and private hatcheries over the past 27 years has provided a solid foundation for the development of a grow-out industry. 

Research at the Grafton Aquaculture Centre in the early 1990’s demonstrated that silver perch is an excellent species for pond culture with high survival, fast growth and high production rates. An industry has been developing since the mid 1990’s and is based primarily on aerated, earthen ponds. There is some production in recirculating aquaculture systems (RAS) and cages, and these intensive systems have potential for significant production in the future. 

Some silver perch farms have achieved high survival and production under commercial conditions; however, for success, farms must have a good supply of high quality water, a reliable power supply for aeration and pumps, be geographically located in a region that enables fast growth and good health, and be well managed.

Diseases of silver perch under culture conditions have been previously described by Ashburner (1983), Rowland (1983), Rowland and Ingram (1991) and Callinan and Rowland (1995). The expansion of the silver perch grow-out industry since the mid 1990’s has seen a corresponding increase in the incidence of diseases, plus several new diseases and pathogens including a winter fungal disease, gill flukes, and various syndromes and conditions. The common diseases of silver perch are caused by protozoans, monogeneans, fungi and bacteria. Diseases have had a significant impact on commercial production through induced stress on fish, loss of growth and production, death of stock and high costs of treatments. 

In 2001 – 2005, a project entitled ‘Development of a Health Management Strategy for the Silver Perch Aquaculture Industry’ was undertaken to study the diseases and health management of silver perch. Major outcomes were: identification of new and important diseases; development of control and preventative methods for most diseases; and a health management strategy for the industry based on three separate publications, ‘Health Management Plan for Silver Perch Culture’, ‘Hatchery Quality Assurance Program for Murray Cod (Maccullochella peelii peelii), Golden Perch (Macquaria ambigua) and Silver Perch (Bidyanus bidyanus)’ and the manual ‘Diagnosis, Treatment and Prevention of the Diseases of the Australian Freshwater Fish Silver 

Perch (Bidyanus bidyanus)’. This manual has been prepared as an easy-to-use publication with numerous photographs of diseased fish and pathogens that will facilitate the prompt diagnosis and appropriate treatment of silver perch diseases. Its use in conjunction with the health management plan, should reduce the incidence and severity of disease outbreaks, leading to improved survival and performance of fish, and increased production, efficiency and profitability of silver perch farms. 

Fig. 1

ANATOMY of SILVER PERCH

Silver perch (Bidyanus bidyanus) belongs to the Class Osteichthyes or bony fishes. Teleosts are the highest bony fishes in evolutionary terms, and constitute the largest group within Osteichthyes. 

Teleosts can loosely be divided into two main groups, the soft‑rayed and spiny‑rayed fish. The latter group, which includes silver perch, is more advanced and typically possess bony spines in some fins, ctenoid scales and a swim bladder which lacks a connection with the oesophagus. Most of the organ systems (heart, liver, kidney, etc) of teleosts are similar to those found in mammals, with differences reflecting adaptation to an aquatic life. The cardiovascular system consists of a single circuit, with blood being pumped by the heart to the gills from where it passes under low pressure to the body and then back to the heart. 

Silver perch also have lymphatic, nervous, reproductive and endocrine systems. Some organs, like the pancreas are difficult to locate and are sited in  Gross anatomy of the viscera of silver perch.

Source: Patrick Tully and Phil Read mesenteric fat between the pyloric caecae (Figs. 2 and 3).

Fig. 2

Fig. 3

Silver perch are poikilothermic (cold blooded) animals, i.e. their body temperature is determined by the environmental temperature. Fins are median (dorsal, caudal and anal) or paired (pectoral and pelvic). The skeleton (Fig. 4) is made up of the skull, vertebral column and the fin skeleton. The skull has a complex arrangement of bony plates (neurocranium) which still allow flexibility and surrounds the olfactory, optic and otic areas. The lower section of the skull (branchocranium) consists of bones associated with the jaw, operculum and gill arches.

Fig. 4

Opercula – bony cover which serves to protect the gills and assist with respiration.

Lateral line – pressure sensory organ having epidermal pores; runs bilaterally from head to caudal fin. 

Fins – bony/spiny and soft‑rayed; assist with locomotion, positioning and aggressive behaviour. 

Diagrammatic representation of a section through fish skin.

Source: Phil Read (adapted from Storer et al. 1972) Epidermis – cellular‑epidermal material, sloughed cells, and mucus secreted to the surface; immuno‑active properties; assist in osmoregulation and swimming; protects against abrasion; primary protection against the environment; sensory receptors; excretory, some respiratory functions; dermis contains many pigment cells containing melanin (Fig. 5).

Fig. 5

Scales – calcified, flexible plates with growth rings; covered by an epidermal layer of cells; assists in osmotic control; physical protection (Fig. 6).

Fig. 6

Respiration

Gills – main respiratory organ; large surface area; uptake of oxygen and excretion of nitrogenous wastes and carbon dioxide; maintenance of osmotic and ionic balance (absorption of water, excretion of salts) (Fig. 7).

Fig. 7

Internal Features

Swim bladder (gas bladder) – thin‑walled sac filled with gases; detects water pressure changes; assists with body buoyancy and positioning; sound production and perception.

Kidney – excretion of water to maintain blood osmolarity balance; protein and ion resorption; removal of nitrogenous wastes from the blood; anterior and posterior kidney.

Heart – two chambered (atrium and ventricle); posterior to gills in separate thoracic cavity; distributes blood via arteries to the gills, organs and body.

Spleen – flat, strap‑like, dark, red‑coloured organ located near the stomach within the abdominal fat; circulatory system filter; capable of generating new blood cells.

Liver – large, pale tan to red in colour; produces enzymes to assist digestion; stores carbohydrates and fats; processes nutrient and toxins absorbed from gut; blood cell destruction and regulation; nitrogen excretion. 

Gall bladder – dark, mottled green; bile production to assist in lipid digestion. Stomach – firm, sac‑like organ at termination of oesophagus; digestion; secretes mucus, enzymes and acid. Pyloric caecae – finger‑like pouches; digestion. 

Blood – hyper‑osmotic; transfer of nutrients, gases and wastes; immune response.

Intestine – digestion; osmoregulatory control; lipid and protein regulation. 

Gonads – sex organs; paired; suspended from the dorsal abdominal wall; testes white/cream coloured, flattened/angular; ovaries pink/cream coloured, rounded.

Musculoskeletal system – skeleton of true bone, skull, vertebral column, ribs pectoral girdle, accessory bones; red and white muscle utilised for aerobic activity and short anaerobic bursts of power.