Causes of mortality of fish eggs and spawn and their treatment

INTRODUCTION Some of the causes of mortality of fish eggs and spawn are: Poor water quality: Lot of major carp eggs perish during incubation as a result of heavy siltation (particularly so during the first few weeks after the onset of monsoon), oxygen depletion, high hydrogen sulfide level, etc More serious if reservoir water is taken directly to hatching tank High density of eggs and spawn - high mortality, particularly in Chinese type of carp hatchery where a large quantity of unfiltered water is used Use of filtered water will greatly help reduce mortality of fish eggs and spawn

Poor pond hygiene: Mortality of seed – due to prevalence of protozoan parasites specially in improperly dried ponds Pond drying followed by liming -considerably reduce mortality and improve seed quality Presence of Pests: Fairly shrimp (Streptocephalus sp) in carp nurseries - known to hamper the growth and survival by competing with fry for food, space and oxygen Presence of weed fishes in nursery pond - stunted fingerlings

Poor management of broodfish and seed: Unhealthy competition among fish seed producers - poor management of broodfish and negatively affect seed quality Substandard quality seed – due to high stocking density in nurseries Quality is important than quantity Transportation stress: During transportation, fish seed - confined environment, higher metabolic load, stress, strain and exhaustion - seed becomes susceptible to diseases and parasites

Conditioning of fish seed : Conditioning - acclimatizing seed to a restricted environment prior to packing and transportation Here, the seed is stocked at a very high stocking density in a hapa or a pond with running water, but without provision for food so that the weak seed dies and only the healthy fry survives The seed that survives the `stress test’ only is selected for transport to a required destination

Mortality of eggs and hatchlings: Heavy mortality of eggs and hatchlings in West Bengal - Due to immature bursting of egg shells and release of premature hatchlings before the anticipated period of hatchling Solution- fish farmers of the State have been using a solution which is a mixture of extract of catechu (Acacia catechu) and Myrobalan (Myrobolus indica) The plant extract enhances hatching period and prevents hatchlings from premature release due to the presence of tannin which helps toughen the egg membrane (chorion)

Diseases and parasites These high density systems offer the ideal environment for disease outbreak due to stressed host and virulent pathogen Results in mass mortality of the affected population in a short time, produce protracted small scale mortality, reduce growth, make the larvae unsuitable for stocking Need for adopting suitable health management measures to reduce the loss due to diseases

Common disease problems in carp hatcheries and early rearing systems The nursery and rearing systems - an ideal environment for many of the pathogens Important problems in hatcheries and early rearing systems are caused by some of the following pathogens (Mohan, 1998) : 1.Protozoan ciliates like Ichthyophthirius multifilis (white spot disease), Trichodina complex and ectodermal ciliates like Epistylis, Vorticella- cause mass mortality of younger stages of carp; the situation becomes worst in waters with low oxygen and high organic matter disease by Ichthyophthirius multifilis Disease by Epistylis

2. Myxosporidians are a serious threat. These sporozoan spores present in the pond soil are normally ingested by the developing fry. Once inside the target tissue the sporozoans cause massive destruction of their target tissue and produce large scale mortality 3. Worm parasites like Dactylogyrus (gill fluke) and Gyrodactylus (skin fluke) with their well developed attachment haptor and feeding apparatus can cause mortality in early developmental stages of carps. DactylogyrusGyrodactylus

4. Opportunistic secondary bacteria (Aeromonas) and fungi (Saprolegnia) can become serious problem in fish larvae which are heavily parasitized In carp rearing ponds major problems are because of larger ectoparasites, secondary invaders, systemic bacterial pathogens and viruses Ectoparasites like Dactylogyrus and Argulus (fish lice) and endoparasites like sporozoans are very important Bacterial problems like surface ulcerative conditions and acute systemic diseases are common in carp rearing systems

Many a times mortalities seen in carp culture systems are a result of ectoparasitic and systemic bacterial diseases External fungal problems are normally associated with fish which are poorly handled The possibility of viral diseases causing mortalities in carp nurseries cannot be ruled out

Bacterial and fungal diseases in carp rearing systems Bacteria - either secondary invaders or primary pathogens Bacterial diseases are classsified as surface ulcerative, acute systemic and chronic granulomatous type Surface ulcerative type: characterized by haemorrhagic surface ulcers and are normally caused by Aeromonas, Pseudomanas, Vibrios, Flexibacteria, Myxobacteria, etc. Surface ulcerative disease conditions develop to acute systemic disease

These are characterized by the presence and proliferation of bacteria in internal organs like kidney, heart, spleen, blood and other visceral organs Produce significant necrotic changes in all the affected organs and can cause mortality in a short time scale Bacterial haemorrhagic septicaemia caused by numerous serotypes of Aeromonas hydrophila is a major problem Reports of viral diseases in freshwater carp rearing systems of India are very few- may be due to the lack of facilities and personnel for viral disease investigations

Disease agentChemicalMethodConcentration/time 1. Ectoparasitic protozoans a. Ichthyophthirius sp.FormalinShort bath ppm for 30 minutes b. Trichodina sp.FormalinLong bath20-30 ppm c. Epistylis & VorticellaFormalinDip ppm for 1 minute d. Ichthyobodos sp.FormalinDip20-30 pm for 30 minutes 2. Monogenetic worms a. DactylogyrusOrganophoshporus pesticides Dip10 ppm for < 1 minute b. GyrodactylusNuvan, DipterexLong bath0.5 ppm for 24 hours 3. Crustaceans a. LernaeaNuvan, DipterexLong bath0.5 ppm for 24 hours b. ArgulusNuvan, DipterexLong bath0.5 ppm for 24 hours

4. Endoparasites a. SporozoansNuvan, DipterexLong bath0.5 ppm for 24 hours b. Cercaria and metacercaria of digenetic trematodes Nuvan, DipterexLong bath0.5 ppm for 24 hours 5. External Mycosis – SaprolegniaMalachite green Formalin Dip 60 ppm for < 1 minute 1-2 ppm for 1 hr ppm for 30 minutes ppm for 1-3 minutes 6. Surface bacterial diseasesProflavine Oxytetracycline & Furnace Short bath 20 ppm 1-5 ppm 7. Systemic bacterial diseasesFurazolidone Oxonilic acid Chloramphenicol In feed 50mg /kg fish/day 10mg/kg fish/day 50 mg/kg fish/day

Gas disease: Heavy mortality of fish seed due to `gas disease’ has been observed on several occasions The symptoms of the disease - the presence of gas bubbles in gill filaments, heart and blood vessels and also in the gut The air bladder gets highly distended The `gas disease’ also referred to as ‘gas embolism’ occurs due to supersaturation of water with either oxygen or nitrogen and is generally encountered in fish nurseries