the potential for replacement of live feeds in larval culture

JOURNAL OF THE WORLD AQUACULTURE SOCIETY

Vol. 24, No. 2 June, 1993

The Potential for Replacement of Live Feeds in Larval Culture

DAVID A. JONES,' MOHD. SALLEH KAMARUDIN AND LEWIS LE VAY School of Ocean Sciences, University College of North Wales, Bangor, Menai Bridge,

Gwynedd, North Wales, LL59 5E Y, United Kingdom

The cultivation and management of live feeds to support hatchery production of bivalves, crustaceans and fish remains costly, unpredictable in many instances, and often provides sub-optimum nutrition. In an ide- al world, larval culture would simply consist of adding appropriate amounts of pre-prepared dried feeds to the larval culture, and after a suitable period, harvesting seed for ongrow. Progress toward this goal is sum- marized in Tables 1-3, which detail recent achievements in the substitution of artificial diets for conventional live feeds in bivalve, crustacean and fish larval culture.

Bivalve Culture Although metamorphosis has been

achieved for the larvae of Crassostrea virginica fed on microencapsulated diets (Chu et al. 1987), these feeds are likely to remain at the research level due to cost and diffi- culty in producing capsules of the correct small size on a large scale. Hidu and Ukeles (1 962) demonstrated that freeze dried algae supported growth to metamorphosis for Mercenaria larvae, and recently Laing et al. ( 1 990) obtained growth equal to that achieved on live algae when Tapes philippinarum larvae were fed on spray dried Tet- raselmis and Nannochloris. Oyster larvae showed poor growth on these diets, and even the clam larvae grew better on an optimal mixture of live algae. Nevertheless, by development of heterotrophic algal culture methods it may be possible to improve the biochemical composition and extend the range of spray dried algae available. Also further research is required on culture con-

' Corresponding author.

ditions for phototropic species to optimize their nutritional quality.

Spat, juvenile oysters and clams accept a wider range of particle sizes, thus live replacement feeds have proven more successful (Table 1). Microgel particles and microcapsules (Langdon and Siegfried 1984; Laing 1987) have been shown to support up to 70% of the growth obtained on live algal diets, and when used as partial replacements (50:50), may compensate for nutritional de- ficiencies present in algal cultures. How- ever, growth similar to that on live Tetra- selmis was achieved for five oyster and clam species when they were fed this alga in a spray dried form (Laing and Verdugo 199 1). Once again, growth was less than that achieved on live Chaetoceros or on algal mixtures, but addition of 20% of a reference daily ration of live Chaetoceros to spray dried algae improved growth rates.

On further area where artificial diets have proved effective has been in the conditioning of broodstock for spawning. Lane (1 989) demonstrated that feeding a 5050 (w/w) diet of Zsochrysis together with microcapsules high in HUFA to Ostrea edulis produced significantly more larvae than live algal fed controls. To date, similar results have not been attained using spray dried algae, but potential exists if biochemical modification of algal species grown heterotrophically or autotrophically becomes a reality.

Crustacean Culture The past decade has seen a rapid increase

in the utilization of artificial feeds to replace live feeds in penaeid culture (Table 2), and a wide range of microparticulate and encapsulated feeds are now marketed for rou-

0 Copyright by the World Aquaculture Society 1993

199

200 JONES ET AL.

TABLE 1. Replacement of live feeds for bivalve molluscs.

Species Feed replacement Result Author

Larvae Crassostrea virginica

Mercenaria rnercenaria

Tapes philippinarum

Spatljuveniles Crassostrea virginica

Crassostrea gigas C. virginica Ostrea edulis Tapes sernidecussata Mercenaria rnercenaria

Crassostrea gigas Ostrea edulis Tapes philippinarum T. decussata Mecenaria rnercenaria

Broodstock conditioning Ostrea edulis

Tapes philippinarum

gelatin acacia capsules lipid walled capsules

freeze-dried Dunaliella Isochrysis

heat-dried Scenedesmus spray-dried Nannochloris

Tetraselmis

microgel particles + microcapsules

microencapsulated diet

spray-dried Tetraselmis

microencapsulated diet + Pavlova 5050

spray-dried Tetraselmis Cyclotella

low survival but SUC- Chu et al. (1987) cessful metamorphosis

phosis ( 1962) survival to metamor- Hidu and Ukeles

growth equal to same live alga, but less than mixture of live algae

Laing et al. (1 990)

variable growth, best Langdon and Siegfried 73% of algal fed control

growth 5 4 4 4 % of that achieved on live algal diets. Addition 1540% algae (w/w) gave similar growth to control

growth similar to live Tetraselmis but less (1991) than for Chaetoceros or mixed algal diets

( I 984)

Laing (1987)

Laing and Verdugo

significant increase in Lane (1989) larval production over either diet alone

conditioned stock but slower than live al- municaton) gae and lower fecun- dity

Laing (personal com-

tine use by hatcheries. In most cases, these feeds are still fed only as partial replacements (50-709'0) for live feeds at the hatchery level, although total replacement using microencapsulated feeds has been demonstrated both at the laboratory (Jones et al. 1989) and hatchery level (Jones et al. 1987). Recently Ottogalli (1 99 1) reported the total replacement of cultured algae with microcapsules for the culture of several penaeid species in hatcheries in New Caledonia, with similar growth and survival to controls on live feeds (Table 2). While total replacement of algae is also possible with microparticu-

late feeds (Kanazawa 1990), growth may be significantly lower than live feed controls (Galgani and Aquacop 1988). Several authors have demonstrated successful partial replacement of live feeds, with microparticulate feeds (Table 2), and Taiwanese penaeid larval culture systems rely on a wide range of different compounded diets. How- ever, it has been demonstrated (Liao et al. 1988; Jones et al. 1989) that the lack of water stability in most microparticulate diets leads to rapid leaching, bacterial build- up and water pollution.

Under laboratory conditions the culture

REPLACEMENT OF LIVE LARVAL FEEDS 20 1

of P. monodon larvae solely on microencapsulated diets has been possible for several years, but size at metamorphosis is usually less than that achieved on live diets. Jones et al. (1 989) found that the addition of only 10 algal cells/pl to the culture water containing microcapsules during protozoeal stages was sufficient to produce growth similar to live feed controls. Algal homogenates and extracts taken from frozen algae added at an equivalent concentration (1 0 cells/jtl) have also produced this growth improvement.

In contrast, there is as yet no successful total replacement for live feeds in caridean prawn or homarid lobster larval culture (Ta- ble 2). Macrobrachium rosenbergii hatchery culture relies on partial (50:50 w/w) Artemia nauplii substitution from the second week of life using either commercial microcapsules or microparticles (Ling 1969; Deru 1990).

A wide range of microparticulate and encapsulated diets have been used in an at- tempt to replace live or natural (frozen my- sids) diets for lobster (Homarus gammarus) larvae (Kurmaly et al. 1990). Despite the range of dietary materials presented, larvae did not molt beyond stage I11 on artificial diets, and lack of success is attributed to poor diet digestibility together with long gut retention time.

Fish Culture Few species have been successfully reared

from first feeding exclusively on artificial diets, and in most cases success at the experimental level has yet to be reproduced on a commercial scale. In general, freshwater larvae are fairly large at hatching and can adapt to dry feeds relatively easily. This is true particularly for the salmonids (12- 25 mm at hatching) which possess a functional stomach at first feeding and do not require live prey at this stage. Among other freshwater species, the most promising results have been achieved with coregonid larvae, which in many experimental studies have been reared exclusively on dry diets

(Champigneuille 1988). It is now possible to mass rear Coregonus lavaretus larvae from first feeding on a yeast based dry diet, with good growth and survival (Champigneuille 1988). Other freshwater species which have been reared on artificial diet include Cypri- nus carpi0 (Charlon and Bergot 1984; Char- lon et al. 1986), Plecoglossw altivelus (Ka- nazawa et al. 1985) and Micropterus dolomieui (Ehrlich et al. 1989).

For marine larvae, many of which hatch with small yolk reserves and feed at an early stage (2-3 mm length), success with total replacement of conventional live feeds has been extremely limited (Table 3). Complete weaning onto artificial diets can be achieved earlier than is currently practiced for Di- centrarchus labrax (Person Le Ruyet 199 1) and potentially for Solea solea (Appelbaum 1985).

Early larvae can be successfully fed artificial diets as a partial replacement for, or supplement to, live feeds. In some cases, co- feeding live and artificial diets can produce growth and survival in early larvae superior to that achieved with either live feeds or artificial diets alone. This has been found for Micropterus dolomieui (Ehrlich et al. 1989), Chanos chanos (Marte and Duray 199 l), Lutes calcarifer (Walford and Lam 199 1) and Sciaenops ocellatus (Holt 199 1). This form of early weaning may allow a reduced dependence on live feeds, and up to 90% substitution with artificial diets has been achieved for Pagrus major and Par- alichthys olivaceus with 10 day old larvae (Kanazawa et al. 1989). Sparus aurata larvae have been reared successfully from first feeding with 50% live feed substitution (Vergara Martin et al. 1990), and up to 80% substitution with microdiets is possible without impairing growth (Tandler and Kolkoski 199 1).

Current Design of Artijkial Larval Feeds

The processes used to produce artificial larval diets were reviewed by Langdon et al. (1 985) and include microbound particles

202 JONES ET AL.

TABLE 2. Replacement of live feeds in commercial larval crustacean culture.


Penaeid shrimp P. vannamei

P. vannamei

P. stylirostris

P. monodon

P. monodon

P. monodon

P. monodon

P. stylirostris

P. japonicus

P. japonicus

P. japonicus

P. monodon P. vannamei P. japonicus

P. monodon

P. vannamei

P. monodon

P. indicus

P. vannamei

microcapsules + algae + Artemia 3-5 ml-'

microcapsules + algae no Artemia



microcapsules only

microscapules only

microcapsules + 10 cell pl-I algae

microcapsules + Artemia

microparticulate diet

microparticulate

microparticulate

microparticulate diet + Artemia

microparticulate + algae + Artemia

spray dried algae +

microparticulate Artemia

microparticulate

microparticulate

90% survival to

80% survival to

65% survival to

9 4 7 % survival to PL7 (20 t tank) settlement diet

3-29% survival to PL7 (1.2 t tank) settlement diet

5 144% survival sig. less than live fed controls (2 L flasks) diet suspended

76% survival, growth same as live fed control (2 L flasks)

growth and survival comparable with live feeds (commercial scale)

90% survival, but growth less than live feed controls

90% survival, growth same as live feeds (lab scale)

75% survival (1 5 t tank)

post-larvae fed en- riched Artemia survive better than those fed non-en- riched Artemia

survival and growth less than live feed controls

84% survival on 66% replacement algae

85% survival to mysis I

Up to 62% survival to mysis I. Growth less than algae

42% survival to mysis I. Growth less than

PL5-7 (2 t tank)

PL5-7 (25 t tank)

PL5-7 (25 t tank)

algae

Jones et al. (1987)

Jones et al. (1987)

Jones et al. (1987)

Jones et al. (1987)

Jones et al. (1987)

Kurmaly et al. (1989a)

Jones et al. (1989)

Ottogali (1 99 1)

Kanazawa et al. (1982a)

Kanazawa (1985)

Kanazawa (1 990)

Tackaert et al. (1989)

Liao et al. (1988)

Biedenbach et al. (1990)

Galgani and Aquacop (1 988)



REPLACEMENT OF LIVE LARVAL FEEDS 203

TABLE 2 . Continued.

Svecies Feed replacement Result Author ~

Caridean shrimp Macrobarchium egg custard, fish, clam, 77% survival to PL Ang and Cheah (1986)

rosenbergii shrimp partial replacement Artemia

M . rosenbergii freeze dried catfish 1 1 % survival to PL Sick and Ekaty (1975)

M . rosenbergii Microcapsules from 84% survival similar Deru ( I 990) Artemia stage I-V

stage VI to control, growth 1 day slower

Lobster Homarus gammarus microcapsules, micro- no survival to post Kurmaly et al. ( 1 990)

particulate larva

using carrageenan, zein, gelatin, agars and alginates, encapsulation using coacervation and interfacial polymerization, and lipid wall capsules. Spray drying and freeze drying techniques can now be added. Each process must meet certain criteria: 1) Accept- ability-Particles must be of the correct size for ingestion, available in the water at a similar density to live feed and ingested at a similar rate; 2) Stability-Formulated diets must remain stable with minimal leach loss and breakdown until ingested; 3) Digest- ibility-Diets must be as digestible as live prey and readily assimilated; 4) Nutritional content-Similar to that of natural prey or- ganisms; and 5) Storage-Suitable for long term (1 2 month) storage. Results (Tables 1- 3) indicate that as yet no process has proven totally satisfactory and that, in contrast to juvenile crustaceans and fish, weaning larvae onto artificial diets is difficult due to the differences in gut physiology.

Nutritional Composition

For herbivorous larvae, mixed algal diets inevitably produce better growth than single species; for carnivores, a diet of each other, or failing this, rotifers and Artemia provide the best alternatives. Recent reviews on the nutrition of bivalves (Webb and Chu 1983), crustaceans (Kanazawa 1984,1990), and fish

(Cowey et al. 1985) provide limited infor- mation on larval nutritional requirements so that, apart from a general HUFA requirement for marine larvae, little is known of specific dietary requirements, although the importance of free amino acids which are present in high concentrations in zooplankton and readily absorbed by the larval fish gut has been stressed recently (Fyhn 1989). Only when a water stable formulated artificial diet is accepted, ingested, digested and assimilated at rates comparable to live feeds will it be possible to investigate specific nutritional requirements. Currently most larval diets are formulated from fish or shellfish meals, cod roe or other natural products and have a broad composition similar to that of phyto or zooplankton. However, due to differences in acceptability and stability, these support widely differing levels of survival and growth (Jones et al. 1989).

For the few artificial diets which meet these criteria of acceptability and digestibility, it is possible to draw some conclu- sions as to the nutritional requirements of the larvae to which they are fed. Thus Laing et al. (1 990) have shown that the larvae of Tapes philippinarum do not have an essential requirement for long chain (20 and 22C) PUFA’s, but synthesize these from short chain (18C) fatty acids present in the diet.

204 JONES ET AL.

TABLE 3 . Complete replacement of live feeds for first-feeding firh larvae.


Fresh water Cyprinus carpio

Cyprinus carpio

C. carpio

Coregonus lavaretus

Micropterus dolomieni

Morone saxatilis Plecoglossus altivelis

Plecoglossus altivelus

Marine Pleurnoectes platessa

Solea solea

Solea solea

Dicentrarchus labrax

Sparus aurata

Lates calcarger Gadus morhua

Clupea harrengus

Pagrus major



microparticulate

microparticulate (yeast based)

commercial dry diet

microparticulate microcapsules, zein-

zein microbound coated, microbound

microparticulate

microparticulate/microencapsulated

zein coated particles

zein coated particcles

micro diet witwwith- out exogenous enzymes

microcapsules microencapsulated roe

encapsulated cod roe

microcapsules, zein- coated. microbound

90% survival, growth comparable with live feeds

87% survival, growth similar to live feeds

90% survival, growth 2 5 4 0 % of live feeds

85-95% survival mass rearing, good growth

2 6 4 5 % survival, growth better than Artemia control

no survival at 20 d little survival and

growth

Charlon and Bergot (1 984)

Charlon et al. (1986)

Slaminska and Pryzybyl (1 986)

Champigneuille (1988) (and see references therein)

Herlich et al. (1 989)

Tuncer et al. (1990) Kanazawa et al. (1982b)

good growth and sur- Kanazawa et al. ( 198 5) viva1

survival 50% of control on live feeds

lower survival and growth than live fed controls



best survival and growth with enzymes still less than live feeds

no survival after 10 d poor growth and sur-

poor growth and sur-

little survival and

vival

vival

growth

Adron et al. (1 974)

Appelbaum (1985)

Gatesoupe et al. (1977)

Gatesoupe et al. (1 977)

Kolkovsky et al. (1990)

Walford and Lam (I 99 I ) Garatun-Tjeldsoto et al.

( I 989) Fox ( 1990)

Kanazawa et al. (1982b)

Similarly, Jones et al. (1 979) were able to demonstrate, by specifically excluding them from formulated larval diets, that the same fatty acids are essential in diets for penaeid larvae.

Acceptability

For bivalve larvae, small particle size is a rigid criterion which has been successfully met by the process of spray drying algae of


.--

Larval Stager PZ- M3 PL

Larval Stager

FIGURE 1 A. Total trypsin activity of Penaeus monodon larvae fed on live food (solid line) and microcapsules (dashed line). Error bars indicate standard error (N = 3 or 4). FIGURE IB. Total trypsin activity of Macrobrachium rosenbergii larvae fed on live food. Solid line is trypsin activity, dashed line is hepatopancreas volume/dry weight. Error bars indicate standard error (N = 3 or 4).

the appropriate size, although larvae small- er than 150 pm are unable to ingest spray dried Tetraselmis (Laing et al. 1990). For bivalve spat and adult conditioning, particle size is less of a barrier, and a range of dietary delivery systems can be employed (Laing 1 987; Lane 1989; Laing and Verdugo 199 1). Penaeid larvae are also predominantly algal feeders and have been shown to accept a wide range of inert particles even through the mysis stages, providing these are of the correct size and are not noxious (Kurmaly et al. 1989a). Larval stages of Mucrobru- chium and Homants display similar en- counter feeding and accept inert particles, although ingestion usually only occurs if

particles are nutritious. However, Kurmaly et al. (1990) have shown recently that lobster larvae use dietary conditioning behavior to reject artificial diets unless attractants are regularly alternated.

While Table 3 demonstrates that most fish larvae accept inert particles, there is considerable literature on their feeding behavior which suggests that at least some species (Clupea harengus) display visual prey selection triggered by prey movement (Ro- senthal and Hempel 1970; Fox 1990). In addition, taste and texture of particles once taken into the mouth may elicit rejection (Jones et al. 1984).

Digestibility As spray dried algae support growth equal

to their live algal counterparts for clam larvae and juveniles (Table l), it must be as- sumed that these processed diets are equally digestible. Bivalve molluscs can also digest nylon-protein capsules (Langdon et al. 1985). Kurmaly et al. (1 989b) report similar assimilation efficiencies by P. monodon larvae fed on live algal feeds or encapsulated diets. The suggestion that the large anterior midgut diverticulae found in P. monodon protozoea are an adaptation to produce sufficient enzymes for the digestion of phytoplankton (Jones and Kurmaly 1987) has now been confirmed for this species (Abubakr and Jones, in press), and P. setiferus (Lovett and Felder 1989, 1990a, 1990b). Fig. 1A plots trypsin activity (enzyme activity x

IU/pg dry wt) for each larval stage of P. monodon fed on microcapsules, with controls fed on mixed algae (Tetruselmis, Rhodomonas) until M 1 and Artemiu (Ocean Star) nauplii from M1-PL1. It appears that herbivorous larval stages, adapted to digest prey that cannot contribute enzymes towards autolysis, are equally capable of digesting artificial diets. Furthermore, the continued production of trypsin at high levels seen in later stage larvae fed artificial diets (Fig. 1A) suggests that, once activated, high levels of secretion are maintained. In contrast, when algae-fed larvae transfer to

206 JONES ET AL.

zooplankton feeding, trypsin levels fall possibly due either to the prey (Arternia) contribution of exoenzymes towards digestion or to the differing protein composition in Arternia. Recent work (Galgani and Benya- min 1985; Lovett and Felder 1990a, 1990b) confirms this pattern of enzyme activity in other penaeid larvae (P. japonicus, P. seti- fern) . For fish larvae with little or no endogenous gut enzymes Munilla-Moran et al. (1 990) suggest that exogenous enzymes in zooplankton prey such as Arternia and co- peepods contribute significantly towards their digestion once ingested.

Table 4 shows the trypsin activity levels for P. rnonodon protozoea 1 fed on live mixed algae, microcapsules and microcapsules with live mixed algae at 10 cells/pl. It appears that small quantities of algae supplied with the artificial diet stimulate trypsin production, possibly explaining the in- creased growth obtained on this diet combination. Crustacean larvae such as Macrobrachiurn and Hornarus, which first feed at the next trophic level on zooplankton, appear to rely on prey autolysis for digestion. Kurmaly et al. (1990) obtained high assimilation efficiencies for Hornarus larvae only when they were fed live diets, and con- cluded that low enzyme secretion levels together with a long gut retention time pre- cludes total replacement of live diets at present. Fig. 1B shows that carnivorous Macrobrachiurn larvae also have low levels of trypsin activity at the first feeding larval stage 11, and hence may rely upon exogenous prey enzymes to meet their energetic requirements. Levels of trypsin increase at larval stages V-VI when the hepatopancreas undergoes a dramatic increase in volume. Replacement of all live feeds is possible from this stage in development (Deru 1990).

As most larval fish are carnivorous at first feeding, it might be expected that they will display similar difficulties in digesting artificial feeds to those seen in carnivorous crustacean larvae. Salmonids develop a functional stomach before changing from endogenous to external feeds, cyprinids re-

TABLE 4. Trypsin activity in Penaeus monodon protozoea I fed diyerent diets, expressed per organism and per pg dry weight (mean values & SD. N = 3 for each treatment).

Trypsin activity x 10-5

x 1 0 - 4 IUlpg dry Treatment IU/organism weight

Unfed (OH) 5.54 k 0.52 9.93 k 0.93 Live mixed algae

(24W 9.80 f 1.84 11.65 f 2.18 Microcapsules

(24W 9.85 & 1.22 13.42 f 1.67 Microcapsules

and live mixed algae (24H) 12.60 k 0.09 15.77 f 0.11

main stomachless throughout life, and most other fish have no functional stomach at first feeding, but develop digestive organs during larval life (Dabrowski 1984). The first group survive and grow on artificial diets, the second adapt at an early stage as the ontogeny of tryptic enzyme activity is rapid, the third group show poor adaptation to artificial diets during early larval life. The suggestion that prey enzymes of prey contribute a large proportion of the proteolytic activity measured in fish larvae (Dabrowski and Glogowski 1977a, 1977b) has been supported by Lauf and Hofer (1 984) who found lower enzyme levels in larvae fed zooplankton processed to denature contained enzymes. Further support is supplied by Mu- nilla-Moran et al. (1 990) who estimate that prey zooplankton make a very significant contribution to the digestive activity in first feeding turbot larvae. These authors also demonstrate that pre-feeding prey organ- isms (Arternia and copepods) elevates enzyme activity levels within the prey, thus enhancing the exogenous contribution made by the prey to fish larval digestion.

In contrast, Hjelmeland et al. (1 988,1990) argue that an exogenous prey enzyme requirement for larval fish is not proven, and cite as evidence experiments in which her- ring larvae secreted trypsin in response to feeding with inert polystyrene beads. These and many others such as Pedersen et al.


(1 987), Pedersen (1 990), Munilla-Moran and Stark (1989) and Rosch and Segner (1 990) provide evidence for de novo enzyme production by fish larvae in response to both live and artificial diets. Generally, larvae can partially control their enzyme secretion according to food composition and ingestion rate.

Several attempts have been made to sup- ply enzymes in artificial diets to fish larvae to improve digestion. Dabrowski and Glo- gowski (1977a, 1977b) and Dabrowska et al. (1 979), working with carp larvae, demonstrated elevated levels of gut enzyme activity but no improvement in growth. Kol- kovsky et al. (1990) report a 100% improvement in growth of Sparus aurata larvae fed microdiets containing pancreatin in comparison with a control diet without an enzyme supplement. Tandler and Kol- kovski (1 99 1) have further demonstrated an enhancement of protein assimilation in Sparus aurata larvae fed a pancreatin sup- plemented microdiet.

The herbivorous larval feeding strategy employing a short gut retention time and low assimilation efficiency relies on a fast turnover to balance metabolic requirements and hence is well adapted to utilize artificial diets which are relatively indigestible. In contrast, carnivorous larvae invest considerable energy in hunting and are adapted with a long gut retention time to extract maximum energy from ingested prey. One further area of investigation for such species is the use of cholinergic drugs to influence gut evacuation time and increase daily throughput of feed (Fange and Grove 1979).

Economics The real cost of algal production to sup-

port bivalve or prawn larval hatcheries is notoriously difficult to estimate, as culture conditions vary from country to country. Langdon et al. (1985) give values of $18- 200/kg dry wt for live algal production while Lee and Wickens (1 992) estimate feed costs to be 12% of total production costs. How- ever, if failure occurs at a critical stage, costs

in lost seed production can be much higher. The potential for the use of spray dried algae or algal substitutes in bivalve hatcheries is high, providing production costs are eco- nomical and additional algal species can be processed. For prawn hatcheries which em- ploy both algal and Artemia culture, considerable savings are already possible by partially replacing these with artificial feeds.

If, as seems likely (Ottogali 1991), total replacement of algae and even Artemia becomes routine for penaeid larval culture, rearing costs will be greatly reduced. Fish larval culture is currently supported by both rotifers and Artemia for most small mouthed marine species. Although partial replacement of live feeds is possible for early larvae of many species, total replacement from first feeding remains a long-term objective. However, it has now been demonstrated for Dicentrarchus labrax that savings of up to 80% in Artemia usage, with consequent sub- stantial reduction in juvenile production cost, may be achieved through use of suitable diets for early weaning at 15-20 days post-hatch, as opposed to current practice of weaning after 4 0 4 5 days (Person Le Ruyet 199 1). Similarly, Ehrlich et al. (1 989) estimate that early weaning of Micropterus dolomieui can reduce juvenile production costs by 45%.

Future Research Priorities Bivalves- Cost effective production of a

range of spray dried algal species for hatcheries and capsule improvements.

Penaeids-Optimization of water stable microfeeds to provide total replacement of live feeds in hatcheries; incorporation of feeding and gut enzyme stimulants; modification of hatchery systems to optimize use of inert feeds.

Fish (and carnivorous crustacean larvae)- Improvement in acceptability and digestibility of artificial diets; investigation into gut enzyme stimulants; addition of enzymes to diets; incorporation of free amino acids into diets; modification of gut retention times.

208

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