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Problems of Diseases onand Prawn Farming Marinein India Fish

P. Nammalwar & T. ThangarajDepartment of Zoology. University of Madras. Madras-5 .

The problems of diseases are ofgreat concern to the fish cultivatorsand research workers. The disease insevere cases inflicts heavy losses onthe fish crop by reducing the yield.Information regarding its identificationnature, extent of damage and controimeasures etc, is being briefly summarisedin this article .

The marine fish and prawn diseasesconstitute one of the important factorSin regulating tha populations in naturalenvironment and in impoundm ents.Symptoms of several diseases in fishesand prawns differ according to it snature and behavioural pattern. Generallyfishes have very great resistance todi seases when compared to prawns solong as they are weakened by unfavourable food. lack of oxygen. extremehigh temparature and salini ty or otherecological influences. The term Oisease.is used here in its broadest possiblesense to include any. departure fromnormal structure or function of theorganism encompassing those statesthat result from activities of infectiousagents, parasite invasion and geneticor environmentally induced abnormalities.Fish and prawn dissases may becaused by different groups of agentssuch as micro - organisms (Virus, bacte-December, 1980

ria, fungi and certain protozoans),parasites (ectoparasites like monogenes,.copepods . isopops, leeches and endoparasitic helminths), toxic substances.(pesticides and heavy metal;), harmful'temperature and other pathogenic conditions.

Fishes and prawns are particularly 'susceptible to infection only in enclosed,artitical surroundings or with unnatura'l>feeding and crowding. The need ' formuch more investigations on diseases:of fishes and prawns is stressed . herebecause of increasing trend of aquacultural practices through the coutry .The following fishes and prawns arebeing cultivated in our coastal, estuarine.brackish and backwater areas.HERBIVORUS FISHES:

Milk fish. Chanos chanos, Mullets,.Mugil cephalus. Liza maerolepis, Liz8cparsia and Liza tade; Pearl spol. Etroplus'suratensis. Carnivorus fishes: Lates,calearifer, threadfin, Eleutheronema tetrs ..daclylum and the ox eyed herring Megalops cyprinoides.' Eels: Anguilla bieolar .PRAWNS:

Penaeus mondon, Penaeus indicus,.Penacus semisulcatus Penaeus, mergui-ens s. Metapenaeus monoceros, Me/apen-9

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B8US dobsoni. Metapenaeus e!finis andMetapena eus brevicornis.M any biological factors pose hazards

to the successful rearing of healthymarine animal stocks. Among th esepathogens such as protozoans bacter ia.worms, crustacea, fungi and virusesare large ly responsible for fish mortality .Other causes of debilitation and mortality include dietary defic iencies. woundspoisons and environmental factorssuch as tempreature and salinity. Theapproach to fi sh disease necessarilym ust bear such simil arity to th e pr acticeof human medicine. More existing dataon mar ine fi sh pathology have be enaccumulated through the study ofmarine aquaria although similar diseasesa,e showing up as problems amongthe limited numbe r of marine speciesso far undergoing cultivation. Grantedthat th e infant body o f knowledge ofmarine animal pathology is largery limited"\0 mar in e fi shes held in captivity Orreared under hatchery conditions andthat it may therefore prove to have.limited pertinence to fish reared in the.somewhat diss im i lar environment ofopen sea mariculture and coastalaquaculture.

Many diseases are ch aracteristic in

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rium. Many of the bacteria normallypiesent in sea water or polluted waterr on the surface of fish, can invade

d Cause pathological effects of fish,are injured or subjected to oth er severeenvironmental stresses. Reports offungus pathogens cf marine fishes arescarce in the scientific literature.Brackish water f ishes may be parasitisedby the saprolegniaceae.

Protozoans are the most se riouspathogens of marine fishes and prawns.Among protozoans. heavy infection ofMyxosporidea may cause enlargement,discoloration and disruption of thefunction of the organ. Gill invadingmyxosporidea are not usually of seriousconsequence to marine fishes andprawns. An epizotic of Myxobolusexigums in the gills of mullet Mugilcephalus caused heavy mortaiity. Mechanical disruption of gill function, causedheavy infection with myxosporideancysts, was considered the cause ofd ea th. No abnormalities were notedin the viscera. Several species ofMyxosporidea invade the cranial cartilages of fishes. Erosion and in somein stances hypertrophy of head cartilageresults several serious diseases of marinecultivable fishes and prawnS result frommicrosppridean invasion. Heavy infec:tions have been reported to preventreproduction by m echanical acclusionof the vent and le sser infecticn causedeleterious ellects to the digestiveorgans and . gonad an d impair othermetabolic functions .

Among parasites, the invasivediseases iuclud e those caused by thelarger parasites those that are nonmultiplicative in the fish host onceinvasion has occuted. Of primaryconcern in marine fishes are the variousDecember, 1980

parasitic worms and the tissue invadingcopepods, para sitic cirripedia and Isopoda,and lampreys which although of occasional concern probably do not after>exert serious el fectson marine fish.populatIOns.

The helminths-trematodes, cestodes ,nematodes and acanthocephala are common parasites of marine fishes. Adults.Occur in the digestive tract. but larvaeare usually found in the flesh or liver'in the viscera. The effects of larve onthe host include growth retardation .tissue disruption , metabolic disturbances .and even heavy infestations.

Adult digenetic trematodes arecommon in the digestive tracts or marinefishes and particularly reported from eels. Among monogenic trematodeparasitic on gills and body surraces o fmarine fishes, a number become seriousparasites in aquaria where conditionsfor reinfestation are optimum. Onlyrarely and under unusual conditions ,nave members of this group beer>demonstrated to be pathogenic to fishin natural habitats. Adult cestodes are-common and oecassionally h a r m f udiges tive tract parasites of fishesLa rval tape worms occur frequently inthe viscera and flesh of marine andestuarine tishes. Adult cestodes may 'occur in significant numbers in thedeg estive tract of fishes although their'prevalance in marine teleosts is lowin com parisian with that of otherhelminths . The nematode p a r ~ s i t e s o fmarine fi shes have received attentionof primarily because certain larvaethat infest the flesh and viscera reducethe commercial value of the fish .Adult nematod es inhabitants of th e-digestive tract and gonads occur if>many species of marine fish. Acanthocephala, the spiny headed wormS

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are represented as adults and as larvaein marine fishes. Larval acanthocephalaare common in the viscera of manymarine teleosts.Marine fishes are. parasitized by avariety of copepodes of the genusCaligus and Lerneopoda. Parasiticcopepods other than tissue invadingiorms may occassionally damage marineiishes. Surface abre ss ions and le sionscaused by these parasites can be ofserious consequenc e to the fishes suchas grey mullets directly or as a routeof entry for secondary invaders.In addition to abnormalities whichcan be associated with particular diseasecausing organisms. marine fishes offernumerous examples of physiologicalor structural defects or conditionswhich may have genetic or other causes.

A number of inherited abnormalities of'i ishes have been observed including-defective spinal columns. pigmented"tumors. catracts. kidney tumors andassociation of certain pigment producing~ e n e s with physiological disturbances.,Physiologicol abnormalities in the larvae,are less easily identical except as theyare reflected in lack of growth and.death of individuals. Abnormal condi"tions in larval fishes are undoubtedlygenetic and some due to environmentalvariations.

Increasing levels of pollution in the;marine. estuarine and brackish waterenvironment in addition to killing fish.and destroying habitats may produce.abnormalities. On an interesting com-.parison of marine fishes taken in polluted.and unpolluted areas. there are changesin consistency of flesh reduced weig ht..external lesion s. exopthalmia and papil-lomas as characteristic of fi shes fromcrossly polluted waters. External lesionson Mullets were produced experimentallyby introducing between effluent dilutedwith sea water.

Diseases may cause continuous:substraction of individu als by weaken ingand disorienting infected fi sh reducing"their ability to escape predators andsurvive variations in the physicalenvironment; by blinding fish; by making12

of infected fish more c.onspicuous orby altering behaviour In ways thatrender fish more vulnerable to predation.Protozoans. helminth and copepodeparasites of the cultivable marine fishesand prawns although not often directcause of death can act to weaken Orslow the host and or also of greateconomic significance.

Economic effects of diseases incultivable marine fishes and prawnsmay be catergorized as follows: reduc-tion in numbers of food fish availableto harvestable fisheries. weight loss ofindividuals. rejection by consumers andsub,squent loss of interest in fisheryproducts as good and indirect effortseither favourable or unfavourable onsurvival of other species in a foodchain . The re st istance to fish diseaseinvolves a complex of interacting factors.including individual variability. speciescharacteristics. seasonal Influences and nutritional effects.

Knowledge is still rudimentary inthe field of disease control for captivemarine fishes ond prawns. Methodsdeveloped for freshwater purposes andespecially for hatcheries are not directlytransferable to sea water where thepH and salt content may change thecharacteristics of antibiotics and otherchemicals used in disease therapy.However. there is a growing interestin developing control measures fo rmariculture . Bacteria. protozoa andmonogenetic trem atodes comprise themost serious menaces to captive andcultured species and outbrakes of eathhave caused .evere mortalities. Otherpathogens and viruses in particularmay be important but their roles havenot yet been d etermined.

Extrapolating from present know-ledge. several potential means ofcontrolling disease in open sea mari-culture crops and these means mayapply to any form of coastal aquaculture.Obviously. none of these methods hasyet been attempted in the o p ~ n seaor coas)al regions. They all representavenues for further research and deve-lopment in fish and prawn farming.Seafood Export Journal