David Lit­tle re­ports

Dr David Lit­tle, sec­onded to the Asian In­sti­tute of Tech­nol­ogy by the Over­seas De­vel­op­ment Ad­min­is­tra­tion UK, re­ports

Fish Farmer - - Contents -

RE­SEARCH, over the past eight years, at the Asian In­sti­tute of Tech­nol­ogy (AIT), near Bangkok, Thai­land, has re­fined tech­niques for pro­duc­ing pre­dictable quan­ti­ties of Nile tilapia seed that are suit­able for hor­monal sex re­ver­sal. The work built on ear­lier in­ter­est and stud­ies in both the USA (Auburn) and the UK (Stir­ling) and is now pro­duc­ing re­sults in the field with com­mer­cial hatch­eries us­ing the meth­ods and stim­u­lat­ing the wide­spread adop­tion of sex re­versed tilapia by farm­ers.

The Nile tilapia is a huge, and largely un­sung, suc­cess in Thai­land. Govern­ment sta­tis­tics show that af­ter its in­tro­duc­tion in 1965 the Thai tilapia, known as the Chi­tral­ada strain, now ranks as the most im­por­tant cul­tured fresh­wa­ter fish in the coun­try. The ev­i­dence is also on the streets and in the mar­kets, with this low-cost fish present in large amounts and sold at a cheap price. Tilapia ap­pears to be fill­ing an im­por­tant niche in pro­vid­ing low-cost an­i­mal pro­tein to poorer ur­ban and ru­ral peo­ple.

Qual­ity is a prob­lem, how­ever, with grow­ers re­port­ing in­con­sis­tent per­for­mance and sea­sonal short­ages of seed fish.

A re­cent sem­i­nar or­gan­ised by the Thai De­part­ment of Fish­eries and Charoen Pokhap­hand com­pany in­di­cated the great in­ter­est in ex­port of tilapia and a gen­eral up­grad­ing in cul­ture tech­niques.

The sex re­versed fish has es­tab­lished it­self over the last two years among com­mer­cial farm­ers, par­tic­u­larly in Bangkok’s ‘tilapia belt’ – the prov­inces sur­round­ing the cap­i­tal.

Two large hatch­eries, both pro­duc­ing 2-5 mil­lion fry/month, have de­vel­oped the mar­ket for the fry us­ing AIT’s tech­ni­cal as­sis­tance.

A stan­dard hor­monal sex re­ver­sal tech­nique is used, in which 17 al­pha methyl testos­terone is in­cor­po­rated into a good qual­ity, fine feed and fry are raised in ha­pas (orig­i­nally sim­ply in­verted mosquito nets) over the first few weeks of life. Af­ter such treat­ment, fewer than 1 per cent of the fish are func­tional fe­males, which ef­fec­tively con­trols breed­ing in the pond as the fish ma­ture. The whole­sale price of MT-treated seed is ap­prox­i­mately five times the price of un­treated fish of the same size (US$0.8/100 fry com­pared to US$0.16/100 fry).

The large num­bers of first-feed­ing fry re­quired to make this process eco­nomic are pro­duced by the fre­quent har­vest of seed from in­cu­bat­ing fe­male brood­stock af­ter nat­u­ral spawn­ing in large (120m2) ny­lon ha­pas. The un­de­vel­oped egg and yolk-sac fry are then in­cu­bated un­der hatch­ery con­di­tions un­til the first feed­ing stage, when hor­mone treat­ment be­gins.

This is dif­fer­ent to the way most tilapia seed are cur­rently pro­duced in Thai­land, whereby vast num­bers of large fry are seined from shal-

low earthen ponds in the bor­der area be­tween Chon­buri and Cha­co­engsao Prov­inces in Eastern Thai­land and sold via fish seed trad­ing net­works all over the coun­try. These fish are of vari­able age and size, and un­suit­able for sex re­ver­sal. Earthen ponds can be used to pro­duce seed suit­able for treat­ment, if first feed­ing fry are skimmed very reg­u­larly from the edges of ponds, but out­put/unit area is low and man­age­ment needs to be in­ten­sive to main­tain reg­u­lar out­put and qual­ity.

There are many re­ports of pro­duc­ing tilapia seed in more in­ten­sive tank sys­tems, and such meth­ods have been com­pared with hapa-in­pond and pond meth­ods at AIT. Greater con­trol over wa­ter qual­ity al­lows the con­di­tion of brood­fish and in­ten­sity of breed­ing to be main­tained for longer pe­ri­ods, but start-up costs are much higher than the other two meth­ods. In ar­eas con­strained by a lack of land and wa­ter, tank sys­tems may well be vi­able, though.

Ha­pas have been used most in the Philip­pines for spawn­ing and hold­ing tilapia fry. Nor­mal prac­tice is to re­move fry af­ter nat­u­ral spawn­ing and in­cu­ba­tion. This lim­its the pro­duc­tiv­ity of in­di­vid­ual fe­male fish and in­creases early losses, par­tic­u­larly through can­ni­bal­ism. It has long been ap­pre­ci­ated that early ‘rob­bing’ of eggs from in­cu­bat­ing fe­males re­duces the in­ter­val be­tween spawn­ing, but this prac­tice has only re­cently been scaled up to com­mer­cial pro­por­tions at AIT. The low fe­male fe­cun­dity of tilapia means that any boost to in­di­vid­ual pro­duc­tiv­ity through im­proved brood­fish man­age­ment can have a great im­pact on sys­tem out­put. Large num­bers of brood­fish are, nev­er­the­less, re­quired for mass pro­duc­tion and this in­flu­ences the size of spawn­ing units and their man­age­ment.

Tra­di­tional ha­pas are small and use con­sid­er­ably more hapa ma­te­rial and har­vest­ing labour than the long and nar­row ‘jumbo’ ha­pas used at AIT. The shape and size fa­cil­i­tates both the con­cen­tra­tion of brood­fish and re­moval of seed, as well as re­duc­ing costs. The lo­cally made ny­lon ma­te­rial is cheap and durable and costs around US$1/m2 of hapa area.

A ma­jor con­straint on the com­mer­cial­i­sa­tion of such sys­tems has been poor suc­cess dur­ing ar­ti­fi­cial in­cu­ba­tion of Ore­ochromis eggs. Ear­lier re­search at the Uni­ver­sity of Stir­ling had shown the eggs to be very sen­si­tive to in­fec­tion, par­tic­u­larly by bac­te­ria com­mon in hatch­ery sys­tems, and that this was ex­ac­er­bated by poorly de­signed in­cu­ba­tors. The UV light ster­il­i­sa­tion units and in­verted, round bot­tomed (soft drink) bot­tles de­vel­oped to solve these prob­lems have since been scaled up at AIT to han­dle the large num­bers of eggs pro­duced un­der com­mer­cial con­di­tions.

Poor in­cu­ba­tion can be re­lated to vari­able and of­ten low egg fer­til­i­sa­tion dur­ing nat­u­ral spawn­ing (usu­ally around 60 per cent). The in­fer­tile eggs quickly en­cour­age a bac­te­rial broth in any in­cu­ba­tor with a poor wa­ter ex­change and/or poor fil­tra­tion. Ore­ochromis eggs are heavy and yolky and suf­fer me­chan­i­cal trauma if move­ment in the in­cu­ba­tor is too tur­bu­lent, par­tic­u­larly close to hatch­ing. High egg den­si­ties in suit­ably de­signed in­cu­ba­tors are de­sir­able, as the egg to egg con­tact pro­motes phys­i­cal clean­ing, rather like peb­bles on a beach, and is more akin to the in­cu­ba­tion process in the mother’s mouth.

Twenty litre plas­tic drink­ing wa­ter bot­tles have been mod­i­fied into in­cu­ba­tion ves­sels and an im­proved ver­sion is un­der de­vel­op­ment. Re­cir­cu­lated wa­ter for the in­cu­ba­tors is passed through slow sand fil­tra­tion to main­tain wa­ter of high bac­te­ri­o­log­i­cal qual­ity.

Af­ter hatch­ing, the fry are held un­til yolk-sac ab­sorp­tion in shal­low, alu­minium trays for first feed­ing. The nat­u­ral re­pro­duc­tive bi­ol­ogy again favours in­ten­si­fi­ca­tion, since fry are crowded dur­ing nat­u­ral oral in­cu­ba­tion at this stage of the life cy­cle. Fry den­sity and flow rates are reg­u­lated to main­tain gen­tle move­ment and the shal­low tray de­sign pro­motes rapid wa­ter ex­change and op­ti­mal wa­ter qual­ity.

These tech­niques pro­duce fry of a proven qual­ity for sex re­ver­sal, as well as high seed pro­duc­tiv­ity for a given pond area and num­ber of brood­fish. Losses through can­ni­bal­ism and in­com­plete har­vest­ing are min­imised, as brood­fish are han­dled in­ten­sively. In ad­di­tion to care­ful man­age­ment of egg and lar­val in­cu­ba­tion, the sus­tained high out­put of fry re­quires the use of good qual­ity brood­fish feeds and the main­te­nance of good wa­ter qual­ity.

im­por­tant niche in pro­vid­ing low-cost an­i­mal pro­tein to poorer ur­ban and peo­ple” ru­ral

Be­low: for seed har­vest. Op­po­site (top): be­ing sold. More than 50 tonnes are han­dled daily at Bangkok’s Sa­pan Bla (be­low): from the mouths of

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