ADRON, BLAIR, and COWEY: REARING OF PLAICE LARVAE 



pared from swine skin 300 bloom) and 

 a -tocopherol (500 mg a -tocopherol per g) were 

 obtained from the Sigma Chemical Co., Ltd. Sun- 

 set yellow was a gift from Imperial Chemical In- 

 dustries; it was included in the diet to simulate the 

 color of Artemia nauplii. Furanace, a broad spec- 

 trum antibiotic developed specifically for fish 

 (Shimizu and Takase, 1967) was obtained from 

 Dainippon Pharmaceutical Co., Ltd. Half of the 

 vitamin supplement was encapsulated in hy- 

 drogenated coconut oil to prevent the leaching out 

 of the vitamins into the sea water. Lest this proce- 

 dure rendered vitamins unavailable to the larvae, 

 the other half of the supplement was added with- 

 out further treatment. To encapsulate the vitamin 

 mixture 28 g were homogenized in 100 ml of 

 di-ethyl ether in which 5 g of hydrogenated 

 coconut oil MP 32-34°C (Loders and Nucoline Ltd., 

 London) had been dissolved. The homogenate was 

 dried in a Bucchi Rotary evaporator. 



Freeze-dried cod muscle, freeze-dried whole 

 egg, and shrimp meal were finely ground together 

 in a hammer mill. To these ground components 

 were added the vitamin mixture, mineral mix- 

 ture, and glucose. The cod liver oil and 

 Q-tocopherol were mixed together and then 

 thoroughly mixed with the dry components, mix- 

 ing being carried out in a Hobart food mixer. The 

 furanace and sunset yellow were dissolved in % of 

 the allotted water and mixed with the dry compo- 

 nents. The gelatin was dissolved in the remaining 

 water at 50°C before being mixed with the other 

 dietary components. While still warm the moist 

 diet was pressed into slabs 5 mm thick and cooled 

 to room temperature. The slabs of diet were then 

 dried in a bulk freeze-drier, ground with a pestle 

 and mortar and graded with sieves, to give sizes of 

 250 u-355 Id and 180 iJ-250 fi. For the first 2 days of 

 feeding the larvae were given only diet of 180 

 /U-250 u ; for the next 8 days increasing quantities 

 of the 250 jU-355 /j'size were mixed with the 180 

 /u-250 u size until only the 250 a/ -355 ju size was 

 offered. By analysis the diet contained protein (N 

 X 6.25) 70.7%, lipid 9.7%, and ash 7.9%. 



Two hundred newly hatched larvae were put 

 into each of three tanks on 2 April 1973. These 

 larvae were obtained from eggs kindly supplied by 

 White Fish Authority, Hunterston; the eggs had 

 been artificially fertilized on 12 March. Food was 

 introduced into one of the tanks on 5 April; 

 Artemia nauplii were added to the second tank, 

 while the third tank was kept as an unfed control 

 mainly because unfiltered seawater was being 



used, and it may have contained enough natural 

 food to maintain a number of the larvae. 



For the first few days of the experiment rela- 

 tively few mortalities occurred in any of the tanks 

 but then 10-12 days after hatching, a rapid mor- 

 tality occurred in the unfed control, together with 

 relatively high mortalities in the tanks fed 

 artificial and natural {Artemia nauplii) diets. By 

 15 April no larvae remained alive in the unfed 

 control tank, while the numbers surviving in the 

 two tanks receiving food were about 70 in the tank 

 receiving the compounded artificial diet and about 

 100 in the tank receiving Artemia nauplii. It 

 seems possible that this high mortality corre- 

 sponds with the complete utilization of the yolk 

 and that the fish surviving in the tanks receiving 

 food correspond to Shelbourne's "established feed- 

 ers." 



After about 15 April mortality rates fell to a low 

 level in both the remaining tanks; some of the 

 deaths in the tank receiving the artificial food 

 were a direct consequence of tank-cleaning opera- 

 tions. Fish began to metamorphose in both these 

 tanks as early as 6 May and by 28 May some 35 

 metamorphosed plaice from the tank receiving the 

 artificial compounded diet were transferred to a 

 conventional tank in our recirculated system 

 where they continued to eat the same powdered 

 diet. This represents a 17.5% survival of 

 metamorphosed fish from hatched larvae. Of the 

 larvae which were reared on Artemia some 76 

 survived to metamorphosis (last day of May), rep- 

 resenting 38% of the original newly-hatched lar- 

 vae. No abnormalities of pigmentation were dis- 

 cernible in the larvae, possibly because of the rela- 

 tively uncrowded conditions in which they were 

 reared. 



The survival rates with the artificial food were 

 much lower than with larvae fed Artemia; this 

 may be due to the greater acceptability of the live 

 moving diet as compared to the inert artificial 

 food; the higher number of "established feeders" 

 obtained when feeding Ar^emm perhaps supports 

 this view. Both our survival rates are considerably 

 lower than those achieved by Shelbourne (1963) 

 using Artemia, his most successful regime giving 

 about 67% survival to metamorphosis (including 

 mortalities between fertilization and hatching). 

 However, with a temperature regime somewhat 

 similar to that used by us (his water bath 4) Shel- 

 bourne obtained survival rates not greatly differ- 

 ent from our "Artemia" tank, i.e. 55% survival 

 (when egg incubation was carried out in the pres- 



355 



