Marine Fish Culture in Britain VII. Plaice {Pleuronectes platessa L.) Post-larval Feeding. and the Effects of Varying Feeding Levels

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1 204 Marine Fish Culture in Britain VII. Plaice {Pleuronectes platessa L.) Post-larval Feeding on Artemia salina L. Nauplii and the Effects of Varying Feeding Levels By John D. Riley Fisheries Laboratory, Lowestoft Plaice produced by hatcheries have an unusually large size range and exhibit pigmentation peculiarities. Small size is known to be correlated with pigment and certain structural abnormalities. The experiments demonstrated the quantities of food eaten in the larval and post-larval stages and the role of food availability in determining the percentage survival, overall size and normality of pigmentation of survivors. The number of Artemia nauplii consumed per fish per day varied from ten during stage I and early stage II to 240 in late stage III. Midway through metamorphosis stages IV and V food consumption fell to fifty-eight nauplii per day but recovered to on the completion of metamorphosis. Peak mortalities per day during stage I to II ranged from 6-4% in excess feeding conditions to 18-5% in tanks receiving one-eighth of the food the fish would normally take. Similarly, modal total lengths of survivors were reduced from 15 mm to 11 mm. Increased pigmentation deficiency within millimetre size groups was also found in the poorly fed fish although fish in surplus feeding conditions still showed appreciable abnormality. The fundamental cause was probably physiological disturbance induced by other features of the hatchery environment. Metamorphosis was delayed by only an average of sixty-three hours by low rations or 6-4% of the time since first feeding. The experiments were part of a series of rearing experiments during which production of Artemia nauplii as food reached up to seven million per day. This was made possible by a multiple hatching unit and by the use of electronic counting methods. Introduction ROLLEFSEN (1939) discovered that Artemia nauplii were an acceptable food for post-larval plaice, flounder and cod. In subsequent years Artemia has been used widely as a food in fish-culture systems, particularly for plaice. Quantitative work on the ingestion rate and food requirements of plaice larvae under hatchery conditions has previously of necessity been rather crude, as no accurate and rapid method of counting the nauplii was available. MITSON J. Cons. perm. int. Explor. Mer 30 No Copenhague, June 1966

2 Marine Fish Culture in Britain, VII 205 Figure 1. The experimental tanks. Marine Biological Station, Port Erin. (1963) designed an electronic counting device which allowed known numbers of Artemia nauplii to be fed to plaice post-larvae in a series of tanks. In this way the result of varying feeding levels at different stages of development could be evaluated and the consequence of such treatment on the physical characteristics of the resultant 0-group plaice could be studied. Since the start of fish-culture work in Britain the plaice reared through metamorphosis have included highly variable proportions of both pigment-deficient and stunted fish (SHELBOURNE, RILEY and THACKER, 1963) and the influence on these defects, if any, of varying food supply needed to be determined. In addition, as larval food production is an important element in the cost of rearing plaice from the egg to 0-group stage in hatcheries, the most economic feeding regime had to be found to allow the greatest economic viability of the scheme as a whole. An experiment was therefore made for these purposes in March-June Material and Methods The plaice eggs and larvae were kept throughout the 84 days of the experiment in two series of six glass tanks, each 60 X 30 X 30 cm, in the plaice hatchery building of the Marine Biological Station, Port Erin. The tanks (Figure 1) were supported on "Dexion" frames and had a common insulated seawater supply manifold, and each tank supply had a visual drip flow controlled by a clip. The seawater outflow was protected by nylon mesh on a polythene

3 206 J. D. RlLEY 15 (J (_ E c D t'l'li o O 60 7O 8O Days after start of experiment Figure 2. Mean temperatures and temperature ranges, fish-rearing tanks, Port Erin frame, similar to those used previously (SHELBOURNE, et al, 1963), except that all polythene parts were black and the folds in the top of the nylon bag were covered by a polythene disc held in place by a rubber band, and so were screened from the fish which had previously frequently become trapped in them, often with fatal results. The rate of new seawater flow through the tanks was between 2 and 3 I/hour throughout the experiment, except during cleaning of the tanks of debris and counting of the plaice larvae, when the flow was temporarily suspended. Lighting was made uniform over the whole twelve tanks. The windows of the building to the west and north were screened with white Venetian blinds kept permanently closed; there were no windows to the south or east. Above each series of six tanks were suspended two 120 cm fluorescent "warm white" tube lights, end to end, partially screened with black polythene strips to give light intensity readings of 655 to 665 lux over the perspex lids of each fish tank, and 480 to 500 lux at the water surface. By the end of the experiment the light values above the tank lids had drifted lower (to 610 to 585 lux) due to the "ageing" of the tubes. The lights were controlled by a time switch to give twelve hours light and twelve hours darkness, coinciding approximately with day and night. The tanks were covered with clear perspex lids only, but the sides and bottom were screened with black polythene film. The temperatures of the tanks were not controlled and depended on the i

4 Marine Fish Culture in Britain, VII 207 temperature of the water flowing in from the main header tank system and on the ambient air temperature in the hatchery. The temperature of the water at the surface of the tanks frequently varied by as much as O-2 -O-6 C on any one day (Figure 2). In addition there was always a temperature gradient between the surface and bottom water, usually of 0-2 C, but occasionally during very warm weather towards the end of the experiment it was 0-6 C. The relative temperatures, tank to tank, seemed to depend on small changes in the rate of flow through the tanks, which could be put right on a day to day basis, rather than on any positional effect of the tank in the hatchery. The nylon outflow screens tended to become blocked, especially in the well-stocked tanks, and were changed when any backing up of the water occurred. Tank stocking and feeding procedure When the plaice eggs were put into the tanks on 22. March (Day 1) they had developed to late blastulae three to four days old. They had been skimmed off the spawning pond (temperature 4-8 C) with a hand-net, sorted and counted in hundreds, and put randomly into the twelve tanks (temperature C) to give 1,000 per tank. Hatching started on Day 11, with peak hatching on Day 14 and 100% hatched on Day 16. On Day 18 the six tanks in each series were paired off in a random fashion and feeding with newly hatched Artemia salina nauplii started, with the following numbers of nauplii per tank per day: 16,000, feeding level A, tanks C 5 and D 3 ; 8,000, feeding level B, tanks C 4 and Di (controls); 6,000, feeding level C, tanks C 2 and D 5 ; 4,000, feeding level D, tanks C 3 and D 2 ; 2,000, feeding level E, tanks C 6 and D 4 ; 1,000, feeding level F, tanks Ci and D 6. These ration ratios of 2-0, 1-0 (the controls), 0-75, 0-5, 0-25 and were maintained throughout the experiment but in tanks C 5 and D 3 (feeding level A), where a large excess of food accumulated, the full "ration" was not always fed but only that amount which maintained a reasonably large excess of nauplii at all times. On Day 21 the numbers of swimming larvae in all tanks were equalized at, and the experiment proper started. This was made necessary because larvae had died accidentally, more in some tanks than in others, due to factors other than food availability. By Day 23, of the larvae in tanks C 4 and Dj (feeding level B) 50% were feeding and only a slight residue of the daily ration of 8,000 nauplii remained after twenty-four hours. This pair of tanks acted as experimental controls and the feeding level in them was raised or lowered to maintain the slight surplus. At this time (Day 23) the tanks receiving less than 8,000 nauplii per day were completely devoid of food after twenty-four hours, but those receiving 16,000 nauplii were accumulating a large surplus. By Day 30 it was necessary to raise the rations, since the controls were virtually grazed out. All the other tank pairs also had their rations raised to maintain the same ration proportions, i.e. 2-0, 1-0 (the controls), 0-75, 0-5, 0-25 and

5 208 J. D. RlLEY This procedure was followed daily until Day 82, 48 hours before the survivors were killed. The two days without food allowed the fish to empty their guts so that meaningful individual weights of the larvae could be obtained. Dead larvae were removed and the tanks cleaned three times a week. Records of the origin, stage and "quality" of each dead larva were made. The bottoms of the tanks were sanded 0-25 cm deep with washed local beach sand between Days 45 and 49, just after the first stage-iv larvae were seen (onset of metamorphosis). On Day 84 all surviving 0-group plaice were killed and preserved in 5% filtered neutral formalin. They were then examined individually, and total length, weight, degree of pigmentation and incidence of bitten fins were recorded. Artemia hatching and ration allocation The experiment here described was only one of many carried out at Port Erin in 1962 by the author and three colleagues. The total food requirement at the period of maximum plaice post-larval feeding (late stage III) was about six million Artemia salina nauplii per day. The Artemia hatching and ration allocation was performed in an insulated wooden hut 3 m X 5 m in which the air was heated by a 3-Kw fan heater, and thermostatically controlled at 22 C. This gave a seawater temperature in the reservoir and hatching trays of 21 C. At this temperature the hatching and separation process took about forty-two hours and, as food was required daily, two banks of hatching trays were used alternately. The hatching trays (Figure 3) measured 92 cm X 45 cm; they were constructed of moulded black fibreglass and were hung by their rims from a "Dexion" angle-iron frame. The tray bottoms sloped to two corners at one side where there were outlet holes, one on each side of afibreglasspartition across the tray approximately one third of the distance from one end. A series of holes 0-4 cm diam., 1 cm apart, were drilled in a horizontal line starting 0-5 cm from the bottom of the tray on one side, and 2 cm from the bottom at the deeper side. A black fibreglass sheet covered the larger division of the tray to exclude the light. Fluorescent tubes situated near the roof of the building gave good illumination in the smaller compartment. Six trays were required to hatch one day's supply, so twelve trays were required in all. Each bank of six was constructed with the trays in three pairs, one above the other. Every tray had an individual outlet on the "dark" side, to allow draining, and an outlet on the "light" side to allow the hatched Artemia to flow into a concentrator. Each outlet was controlled by a clip. The procedure was as follows. After the Artemia nauplii had been drained off, the "light" outlet was closed, and the old egg capsules and bacterial slime were rinsed off the walls of the trays with a small hosepipe, and out of the outlet on the "dark" side to waste. A wipe round with a cloth and a final rinse completed the cleaning process. All outlets were then closed and warm seawater run in through a second hose from a reservoir within the building until the water was 1 cm from the rim of the trays. Twenty-five ml of good quality Artemia eggs were put on the surface of the water on the "dark" side of the tray and the lightproof lid replaced. As the Artemia hatched they were attracted to the light shining through the holes in the partition and swam through them into the "light" side where they accumulated. A sintered glass

6 Marine Fish Culture in Britain, VII 209 light V///////////X Figure 3. Flow diagram of Anemia salina incubation and dispensing as post-larval plaice food. aerator in this side, supplied by air pumps, maintained acceptable oxygen and carbon dioxide levels. After forty-two hours the outlet on the light side was opened and the hatched Artemia were run into a concentrator. Any pelagic nauplii in the "dark" side were likely to be carried through the holes in the partition during this process. The old eggshells and other detritus were either floating or on the bottom on the dark side and were not carried through, since when the level fell to that of the row of holes in the partition, the meniscus at the holes remained complete and prevented further flow. The concentrator (Figure 3), constructed of perspex and nylon throughout, was a perspex cylinder of internal diameter 30 cm and height 30 cm. A nylon mesh screen held in place by two perspex rings and nylon screws was fastened 5 cm from the bottom of the vessel, and an overflow system, constructed on the side, of perspex tube 2-5 cm bore, allowed surplus water to run to waste. The part of the concentrator where the nauplii accumulated had an outlet, controlled by a clip, and an aerator which kept the nauplii at uniform density. The concentrator was lifted by a polythene handle at the top. The density of Artemia in the concentrator was kept below 175,000/1 by visual estimation, as this was the greatest density which was accurately determinable by the electronic counting device. The concentrator, which for draining the hatching trays had been on the floor of the building, was placed on a shelf above the Artemia counter to allow a gravity feed through the device. The operation of the counter has been described previously (MITSON, 1963). The nauplii, after being counted were further concentrated by running the flow from the counter into 10 cm lengths of 2-5 cm bore polythene pipe, over 15'

7 210 J. D. RlLEY in C30O u D. a c 20O E c 100 c o <D 2 I 4, O O Time in days Egg- St I- St n- -st nr st n-*«st i»- -Yolk sac- Metamorphosis-" Figure 4. Plaice food consumption from yolksac stage I to O-group fish. Feeding level A, food offered Feeding level B, food eaten - control Feeding levels C-F, food eaten. the lower end of which had been fused nylon mesh, which retained the nauplii but allowed the excess water to escape. The Anemia were kept suspended in seawater, but just prior to being fed to the fish they were drained of the old seawater in which they had hatched and were suspended in new seawater at the temperature of the fish tank. When the Artemia were actually put into the fish tanks they were drained again and washed out of the holding tubes with the water already in the fish tanks. The ration of nauplii was then distributed uniformly throughout the tank by very gentle stirring. N. Results 1. Food consumption during development Feeding level B (the controls) gives a measure of the maximum number of nauplii eaten by the plaice post-larvae. At feeding level A, where a large surplus

8 Marine Fish Culture in Britain, VII 211 Series D tonks o Series C lanks Day 36 0 _ 200 o n 100 _J I I l_ i _l b 2-0 F E DC B A (doy 64) A(Oay36) Ration level index Figure 5. Effects of feeding level on survival. of nauplii was maintained, a proportion of them died after a few days and were removed when the tanks were cleaned. The number of nauplii fed to the controls divided by the number of fish fed (the survivors on that day) gives a mean ingestion rate per fish (Figure 4). During stages II and III the number of nauplii eaten per day increased from 10 to 240. With the start of metamorphosis (Stage IV) the number fell to 58 but increased during stage V, and on completion of metamorphosis the number of nauplii taken per day was between 200 and 250. These figures are based on nauplii taken by post-larvae of different developmental stages (although of the same age) and the changes in feeding rate are actually even more abrupt than Figure 4 shows; fish have been observed to stop feeding, lying quite still on the sand, during the later part of stage IV and early stage V. Plaice post-larvae in the wild also seem to behave in a similar way, as fish at this stage (late IV to early V) frequently have empty stomachs or are feeding very lightly. PETERSEN (1906) reports that during this time the size of the plaice becomes less, as in fact it does in many other flatfishes; this reduction in size is probably due mainly to a cessation of feeding. The record of mean stage of development in the controls (see Figure 4) gives the duration of the stages, at the temperatures shown in Figure 2, of: eggs, 18 days; stage I, 10 days; stage II, 16 days; stage III, 16 days; stage IV, 9 days; stage V, 7 days; total, 76 days. The mean number of nauplii taken per fish at each feeding level remained in the order A (the largest) to F (the smallest) throughout, in spite of variable

9 212 J. D. RlLEY mortalities, although these changed the ratios of tanks A and C to F relative to B (the controls). The ratios can be calculated for particular days from the values plotted in Figure Effect of feeding levels on survival The numbers of plaice surviving in all tanks were recorded at intervals of two or three days and are shown in Table 1; final survivors are in the last column. The numbers of plaice surviving in each tank on Days 36 and 84 have been plotted in Figure 5. On Day 36 the survivors are larvae which have established feeding and have started to grow (stage II); by this time those which failed to start feeding have used up all yolk reserves and died. The numbers surviving to this stage are increased by higher rations. High food density is advantageous, as daily increments in excess of what the fish can consume give higher survival (feeding level A) than just what is consumed (feeding level B). Progressively reduced rations below B give corresponding reductions in post-larval survival. On Day 84 the greatest survival of metamorphosed plaice was at feeding level C. Food offered in excess of this, at level B and level A (now only 1-25 x B), gave reduced survival due to badly fouled tanks with high bacterial and ciliate concentrations leading directly or indirectly to mortality. A white ciliate was observed to congregate round an injured fish and was found apparently feeding within the wound and moving about among frayed tissues. When the distribution of mortality is plotted (Figure 6) the difference in the degree of "non-feeding" and subsequent mortality is shown. The peak mortalities in all tank pairs occurred on Day 28, 14 days after peak hatch, when the percentage mortality/day ranged from 6-4% at feeding level A to 18-5% at feeding level F. At level F, and to a lesser extent at level E, the persistence of relatively high mortalities up to Day 44 was due to starvation, as the nauplii were grazed down completely within an hour or two each day. Plaice larvae attempting to feed for the first time took several seconds to adopt the S-flexing of the tail which always precedes the feeding snap; if a Feeding level A B C D E F Finally 1.25 Feeding Index 2* Tank Series c D C D C DC D C D C D Table 1 Numbers of plaice surviving in tank Day

10 Marine Fish Culture in Britain, VII Days after start of experiment Figure 6. Survival curve log plot. Each line is a mean of two tank populations. larva failed to take a nauplius at the first attempt it would snap three or four times at about five-second intervals, but if it failed each time the snapping action was not repeated for some time, often over half an hour, by which series C and D, Port Erin ! Day

11 214 J. D. RlLEV GO 200 Weight / 00 O ISO B o. 100 o 0) jq r> 50 c c o (P Survivors Survivors ^N normally pigmented-^ 1 1 I 1 1 o D /. 0 E 30 o i_ 20 Z "5 HO HI 1-25 " F E D C B A Ration level index Figure 7. The effect of different feeding levels on survivor numbers and quality (mean weight in grammes, pigmentation and incidence offinbiting) at day 84. time, at least at feeding levels F and E, the food density would already have been appreciably grazed down by those fish which had already established feeding. The effect on mortality of surplus food (levels A and B) is also shown in Figure 6. Tank pair A was the only one to have a percentage mortality/day greater than 2% after Day 52 (late stage III). C o 3. Other effects of varying feeding levels The numbers of survivors reared at the varying feeding levels differed, as did the mean total weight, the mean number of normally pigmentedfishand the number of fish without bitten fins (Figure 7). Feeding level B gave the largest numbers of fish normally pigmented, the greatest total weight of fish and the largest number of unbitten fish, but feeding level C gave the greatest overall survival (21 %).

12 Marine Fish Culture in Britain, VII o (U -t-> c a n E / / \ - A I / I I / I I I V. E "rt ; -A Total length in millimetres Figure 8. Survivor size distributions at feeding levels A-F (Day 84). Discussion The size distribution of the survivors (Figure 8) shows, as expected, an increase in mean size at higher feeding levels, but the size distributions at feeding levels A and B show an extremely wide range, going as low as those at the lowest feeding level F, i.e. between 9 and 27 mm total length. The local 'wild' plaice populations in Port Erin Bay only rarely complete metamorphosis when as small as 11 mm, and only 4 % do so when under 13 mm. Stage V post-larvae are regularly taken during May and early June at lengths of between 14 and 18 mm, suggesting that this is the usual size range within which metamorphosis is completed in the sea. At all experimental feeding levels the plaice of a final size below 13 mm had a higher percentage pigment deficiency (Figure 9), and frequently had abnormal mandibular and opercular bone development and showed incomplete migration of the eye. Abnormalities were expected in the fish at the very low feeding levels E and F, but at feeding level B and even at "feeding level" A over 20% of the survivors were less than 13 mm long and must be considered

13 216 J. D. RlLEY sz ^ 60 r> V 50 c (U E 40 O) a / Starved fish o(6 fish) Controls (2 fish) Total length in millimetres Figure 9. The effect of low rations on frequency of pigment normality. Controls, demand feeding conditions. Feeding level B. O O Starved fish, eating about half ration of controls. Feeding levels E and F. as "runts" and relatively unsatisfactory for any growing-on programme, be it release into the sea or some other more controlled environment. The wide length range at feeding levels A and B developed largely during the last fourteen days of the experiment. With a food intake per fish no higher than that of the free-swimming late stage III post-larvae, the sedentary habit of the metamorphosed plaice allowed length increments of 0-25 mm/day to be maintained. The mean feeding level per fish during this last period of the experiment was the important factor in determining the mean weight of survivors (Figure 10). The only feeding level to give mean weights deviating from a straight line relationship was feeding level A, in which the ration offered was not all eaten and where a late mortality around metamorphosis upset the pattern. The wide range of ration levels affected the time of onset of metamorphosis; on Day 59 the numbers of surviving post-larvae at each ration level were counted and their developmental stages ascertained (SHELBOURNE et al., 1963). The percentages in each stage in each tank pair have been calculated and plotted on Figure 11. The percentage of post-larvae in stage IV at the different ration levels varied in a random way between 60% and 67%, with a mean of 64%. The midline of stage IV separates the post-larvae in stage III and the earlier stage IV from those later in stage IV and in stage V. If the two lowest ration levels E and F are considered together and compared with the two highest levels A

14 Marine Fish Culture in Britain, VH 217 Series O tanks - o Series C tanks c 003 ID Feeding level index Survivors Figure 10. The effect of feeding level after metamorphosis on the mean weight of survivors. and B, the difference in the position of the midline is a measure of the variation in mean developmental stage and can be translated into time if the mean duration of stage IV is known. This difference was 18-5 % (Figure 11). At these temperatures (Figure 2) the fish in the controls (feeding level B) spent about nine days in stage IV. This 18-5% difference within stage IV suggests that the starved fish are about 63 hours behind the well fed ones. This is not an absolute measure of the different rates of development, but proof that the difference is of this order is provided by the observation that after Day 63 no normal stage-ill post-larvae were seen in any tanks; all tanks contained 3-5% deformed stage-ill post-larvae which attempted metamorphosis during the remainder of the experiment but which were so abnormal that they can be disregarded. By Day 59 the fish had been fed for 41 days, and the very low ration at feeding levels E and F had caused a delay in reaching mid-stage IV of only 63 hrs (or 6-4%) since first feeding, while the delay since fertilisation was only 4-2%. This small delay was surprising in view of the unnaturally small size of the poorly fed fish (Figure 8), and it suggest that the onset of metamorphosis is not controlled to any large degree by size but by a mechanism dependent mainly on temperature and time. The percentage of normally pigmented survivors was influenced by the feeding level. In Figure 9 the percentage of normally pigmented fish in each size group in the controls (feeding level B) has been compared with that of the starved fish (feeding levels E and F). There is demonstrated an effect additional to that previously known (that pigment deficiency is correlated with small size): viz, that nutritional deficiency and the resultant physiological

15 218 J. D. RlLEY Stage 21 E DC Ration ol nouplii per A O fish day 58 Figure II. Effect of varying Anemia food rations on the speed of plaice post-larval development. Tank pairs, plaice larval stages expressed as %. (Day 59.) stress can cause increased pigment abnormality. However, Figure 9 also shows that food availability is only a part of the cause of pigment deficiency, since the controls (feeding level B) had food always available and still showed appreciable abnormality, varying from 67% normal in the larger fish to 30% normal in the smaller. It therefore seems likely that pigment deficiency and the other abnormalities already mentioned, which occur in other experiments and in mass-rearing pilot plants with good feeding levels, result from factors other than food availability, which may be behavioural, chemical or physical. A high proportion of the survivors at all feeding levels had fins bitten by other fish. As in previous experiments, size was by far the most important factor. When the percentages not bitten in well-fed tanks (controls B) and starved tanks (E and F) are plotted in size groups (Figure 12) the effect of food shortage appears to be slight; however, it is very likely that the incidence of fin biting in the starved tanks is held down by the small size of even the largest fish in these tanks. The chances of having bitten fins are reduced slightly at all feeding levels in normally pigmented fish, presumably since they are less conspicuous to other fish, due either directly to the advantage of camouflage or to some indirect behavioural factor, e.g. hyperactivity of pigment-deficient fish. The number of nauplii required to produce one metamorphosed plaice (irrespective offish size) is plotted for each tank in Figure 13, and ranges from 3,600 per survivor in one tank at feeding level F to 26,000 in one of the surplus-fed tanks at feeding level A. This quantity of food was supplemented at all feeding levels by small quantities of food material which developed in the tanks in spite of the water being filtered before entering. Slime on the

16 Marine Fish Culture in Britain, VII 219 n o a Total length in Normal pigmentation Pigment deficient» Starved (E «. F) Controls (B) millimetres 23 25«day -84 Figure 12. The incidence of bitten fins with size, feeding level and pigmentation. fish-tank walls was browsed off by the pelagic larvae, and at very low feeding levels larvae which had brown gut contents, and had been feeding in this way, were regularly seen. Normally, the gut contents when feeding on newly hatched Artemia are bright pink. This slime accumulated quickly and at the higher temperature had to be removed by scraping every time the tanks were cleaned. In addition, in some tanks free-living marine nematodes were occasionally found living in and feeding on the detritus on the tank bottoms; these would be taken occasionally by the post-larvae. These nematodes were also removed during tank cleaning. Figure 13 also shows diagramatically the relative performance of the twelve tanks in producing metamorphosed plaice of acceptable size and number. Ration levels giving high survivals are to the right, and high rations/metamorphosed fish to the top of the figure. As expected the two points for each tank, representing food offered and mean size, tend to go together. The horizontal broken lines delimit the range of suitable mean weights; they enclose one tank at each of the ration levels A, B and C. The other tanks at A and B gave fish of unnecessarily high mean weights, and at C unacceptably low mean weight. To produce fish of a suitable size, rations at or slightly below (say 85%) those in demand-feeding conditions are the lowest which can be offered after the establishment of feeding. Surplus feeding before this increases the percentage which do establish feeding. Summary 1. An improved method of hatching and preparing Artemia nauplii for fish-feeding in large quantities is described.

17 220 J. D. RlLEY A S, 5 D (/) O i_ 5 i 50 Number of 100 plaice C I ri *! " 150 surviving 200 day Figure 13. A comparison of the performance of the tank populations in the production of numbers of metamorphosed plaice of a desired mean weight. O, mean weight;, food offered. 2. The development of a counting device for Artemia nauplii allowed the feeding of plaice post-larvae at different ration levels. 3. The percentage of plaice post-larvae surviving beyond first feeding is increased if a surplus of Artemia nauplii is maintained up to that time. After feeding is established surplus feeding creates problems of tank hygiene leading to increased mortality. 4. Growth rates are not seriously affected until rations fall below threequarters of feeding capacity. 5. Lower feeding levels gave progressively higher percentages of pigmentdeficient and deformed larvae (the abnormalities only becoming obvious at metamorphosis), and a slightly lower speed of development up to metamorphosis. 6. The mean numbers of Artemia nauplii taken under demand-feeding conditions by plaice larval and post-larval stages, and the duration of the stages in days and approximate water temperature, were: - Stage Egg Stage I Stage JI Stage III Stage IV Stage V O-group Duration (days) No. nauplii range _ > < Temperature ( C)

18 Marine Fish Culture in Britain, VII 221 References MITSON, R. B., "Marine fish culture in Britain. V. An electronic device for counting the nauplii of Anemia salina L." J. Cons. perm. int. Explor. Mer, 28: PETERSEN, C. G. J., "On the larval and post-larval stages of some Pleuronectidae (Pleuronectes, Zeugopterus)". Meddr Kommn Havunders., serie Fiskeri, 2: (1) 9 pp. RILEY, J. D., & THACKER, G. T., "Marine fish culture in Britain. III. Plaice (Pleuronectes platessa (L)) rearing in closed circulation at Lowestoft, 1961". J. Cons. perm, int. Explor. Mer, 28: ROLLEFSEN, G., "Artificial rearing of fry of sea water fish. Preliminary communication". Rapp. P.-v. Reun. Cons. perm. int. Explor. Mer, 109: (3) 133. SHELBOURNE, J. E., "Marine fish culture in Britain. II. A plaice rearing experiment at Port Erin, Isle of Man, during 1960, in open sea water circulation". J. Cons. perm, int. Explor. Mer, 28: -79. SHELBOURNE, J. E., RILEY, J. D., & THACKER, G. T., "Marine fish culture in Britain. I. Plaice rearing in closed circulation at Lowestoft, ". J. Cons. perm. int. Explor. Mer, 28: