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Egg incubation and larval rearing of navaga (Eleginus navaga Pall.), polar cod (Boreogadus saida lepechin) and arctic flounder (Liopsetta glacialis Pall.) in the laboratory
Aquaculture.. 6 (1975) 233-242 o Elsevier Scientific Publishing
Company,
Amsterdam - Printed in The Netherlands
EGG INCUBATION AND LARVAL REARING OF NAVAGA (ELEGINUS PALL.), POLAR COD (BOREOGADUS SAIDA LEPECHIN) AND ARCTIC FLOUNDER (LIOPSETTA GLACIALIS PALL.) IN THE LABORATORY
NAVAGA
T.M. ARONOVICH, VNIRO,
Moscow
S.I. DOROSHEV,
State
University,
L.V. SPECTOROVA
Moscow
and V.M. MAKHOTIN
(U.S.S.R.)
(Received November 6th, 1974)
ABSTRACT Aronovich, T.M., Doroshev, S.I., Spectorova, L.V. and Makhotin, V.M., 1975. Egg incubation and larval rearing of navaga (Eleginus nnvaga Pall.), polar cod (Boreogadus saida Lepechin) and arctic flounder (Liopsetta glacialis Pall.) in the laboratory. Aquaculture, 6: 233-242. The embryonic and larval development of three White Sea cold-water fish species, rate of yolk sac absorption, age at first feeding and their survival and growth when fed different food organisms, were studied. Eggs were obtained from spawners in the Bay of Kandalaksha, White Sea, and incubated in troughs and aquaria at a mean temperature of 1.5 “C, slightly above that of the sea. The incubation period for polar cod eggs lasted 35 days, for arctic flounder, 42 days and for navaga eggs, 48 days. Emergent larvae were 5.5-6.0 mm long and began feeding at 2-4 “C!, 5-6 days (na.vaga) and 12-14 days (polar cod) after hatching, when their yolk sac was still fairly large. They were fed day-old Artemia nauplii and zooplankton taken from the sea and consisting of Calanus and Pseudocalanus nauplii 400-600 p in length. The period of establishing first feeding is the most critical for larvae.
MATERIALS
AND METHODS
Experiments were conducted in January-April, 1973 at the White Sea biological station, Moscow University. Navaga, polar cod, and arctic flounder spawn in winter (DecemberFebruary) at a temperature of -1.5 “C. Spawners were caught in traps and delivered to the laboratory in isothermal boxes. Fertilized eggs of navaga were obtained from naturally spawning stock in the aquaria, and eggs of polar cod and flounder were fertilized artificially. 5-l jars and 8-10-l glass aquaria were used for incubating flounder and polar cod e,ggs, 3-l polythene bowls placed in troughs with running water were used for navaga eggs. The bowls were connected by polythene tubes to allow water to flow from one bowl into another with a rate of 0.1-0.2 1 per min. Larvae were reared in 60-l polythene troughs and lo- and 30-l glass aquaria.
234
Feeding experiments were carried out mainly on polar cod larvae which seemed best suited for the purpose. Larvae were fed on natural microzooplankton obtained from the sea and consisting of Puracalunus and Pseudocalanus nauplii 250-400 ~1in length and day-old Artemiu salina nauplii 600 cc in length cultivated from eggs in a rearing apparatus as suggested by Shelbourne (1964). The length of larvae was measured from the tip of the lower mandible to the end of the caudal fin; the yolk sac was measured by calculating its area, from the optical section, as that of an ellipsoid. As a rule, samples to be measured consisted of 10-25 larvae which had been anaesthetized in 100 ppm tricane methane sulphonate (MS 222). The relationship between the filling of the swim bladder with air and the ability of larvae to establish feeding was estimated by x ’ test using the Contingency tables 2 X 2 (Baily, 1959). The following equation was used: x2 =
n [ (ad-bc)-0.5n.j
2
(a + b)(c + d)(a + c)(b + d) where: (I and b = number of feeding larvae with their swim bladder filled and not filled with air, respectively. c and d = number of nonfeeding larvae with their swim bladder filled and not filled with air, respectively. n = total number of larvae in the experiment. RESULTS
Embryonic
development
Polar cod and arctic flounder eggs are translucent, the membrane is thin and non-adhesive, the pe~~telline space is very narrow. Their specific gravity is lower than that of water, so they stay near the water surface. Swollen eggs of arctic flounder are smaller than those of polar cod (1.5-1.74 and 1.72-1.90 mm in diameter, respectively). Navaga eggs have a thick membrane and a large perivitelline space. The diameter of the swollen eggs ranges between 1.63 and 1.72 mm, plasma and yolk are a bright orange colour. They sink to the bottom of the aquarium. Arctic flounder and polar cod eggs are oriented with their animal pole downwards, navaga eggs orient theirs upwards. There are no oil globules in the eggs of the three species. As is seen from the Table I, the rate of embryonic development differs in the three species. The first stages (up to the mobile state) occur approximately within the same period, but embryonic organs in navaga and arctic flounder begin to function somewhat earlier (by day 18 or 19) than in polar cod (between days 22 and 26). Hatching times are also different: polar cod larvae hatch between day 26 and 35, navaga hatch between day 28 and 48, and arctic flounder from day 22 to 42.
235
TABLE I Rate of embryogenesis in the fishes studied (days). Incubation temperature for arctic flounder ranged from -0.5 to 3.1 “C (the mean temperature being 1.0 “C), for navaga from -0.4 to 3.8 “C (with a mean temperature of 1.39 “C); for polar cod from -0.3 to 2.5 “C (the mean temperature 0 “C) Stages
Arctic flounder
Navaga
Polar cod
> 0.5
> 0.5
> 0.5
II Cleavage (Ei-10 h between mitoses at a given temperature; from two blastomeres stage to beginning of epiboly)
0.5-5
0.5-5
0.5-7
III Epiboly an’d beginning of gsstrulation
5-9
5-8
7-10
IV Organogenasis and continued epiboly (formation of axial- organs)
Q-19
8-18
10-22
I
Formation
of blastodisc
V Embryo in a mobile state (onset of functionai activity of embryonic organs: tail section gets longer, somites grow in number, development and enlargement of separate organs)
19-22
18-28
22-26
VX Hatching (intensive eye and body pigmentation, formation of hatching glands)
22-42
28-48
26-35
Larval growth and yolk sac absorption
Emergent larvae of arctic flounder are 5.56 + 0.04 mm in length (CV = not very active, and coneentkate along the sides of the troughs in a suspended state. The intestines look like a narrow tube, the stomach has not yet developed and the mouth and intestines are not different’iated. Larval growth of White Sea fishes in this period (Fig. 1) and yolk sac absorption (Fig. 2) are slow. The process is ~rnpe~t~e dependent and lasts about a month. Newly hat.ched navaga larvae are somewhat larger, 6.0 + 0.05 (CV = 0.83%) Positive phot,otaxis is well pronounced in them. Like flounder larvae, during the first several days they concentrate along the sides of the aquaria in the corners close to the water surface. In contrast, polar cod larvae are distributed throughout the whole water column. They do not react to light during the first days of life. Polar cod larvae measure on hatching 5.54 _+0.07 mm (CV = 1.26%). The rate of yolk sac absorption changes after feeding is established; Thus in non0.72%). They are translucent,
age, days
age. daw
Fig. 1. Growth rate of larval navaga, polar cod and arctic flounder .- _- arctic flounder,---*-snavaga. W
2. Yolk SBCabsorption, -
p&r
cod, - - -
arctic flounder, -*-e-1
polar cod, navam.
feeding larvae it was 0.148 mm2/day, after feeding was established it dropped sharply to 0,065 mm*/day. This may be attributed to the fact that with the food being taken from outside, utilization of the yolk sac for nutrition decreased. Yolk sac absorption in starving larvae proceeds more quickly than in feeding larvae. As the larvae grow, their yolk sac is gradually depleted, and by 18-20 days after hatching it has disappeared. In navaga complete yolk sac absorption occurs later, 22-24 days after hatching. Larvae were regularly offered food prior to complete yolk sac absorption when they were able to take and digest it, which in polar cod larvae occurred on day 12 and in navaga, day 5 or 6. Only a small number of larvae could feed at this age (5%). The percentage of feeding larvae later increased, gradually from 25% in 14-day old larvae to 65% in 26-day old larvae (Fig. 3). The maximum number of larvae with full guts could be observed 5-8 h after the food had been offered. The number of food organisms in the stomach increased with age. Thus, while in the stomach of 12-14-day old larvae there was only one food organism, the stomach content of 23-26-day old larvae was 6-7 artemia nauplii at a time. Newly-hatched larvae of navaga can survive without food beyond days 5 and 6, and polar cod larvae to even day 14-16, without affecting the percentage of larvae capable of establishing feeding, however, a starvation period of 8-10 days following hatching (navaga) or 20 days after hatching (polar cod) proved critical (Fig. 4). Larvae which were given food for the first time in this period were unable to swallow it. This resulted in degeneration of the liver and gastro-
237
5.. a ,
. 5--6
2-1
4
how
7
t
9
-.
’
*
*
10 11 12 'f 14
fwm the stat
-
4
uf leedirty
Fig. 3. Larval feeding depending on number of days after hatching.
v.
1
so 80 70 60
navaga
polar cod
C__-__.-
/
l”li\. /
SQ 40
1’
3Q 20 10 0
/---------
/’
5
7
9
11
/I
I’
13 15 1 19 21 23 25 27 age,dw
Fig. 4. Relationship between starvation period and first feeding . --control!.
starvation period,
238
intestinal tract. Based on the data obtained, it is recommended that the feeding of navaga larvae should not start later than 8-10 days after hatching, and polar cod larvae no% later than 17-20 days, at temperatures of 3-4 “C. Filling of the swim bladder with air The swim bladder filling with air is one of most critical phases in larval development. Among 6.4-6.5 mm long larvae of polar cod about l-296 had their swim bladder filled, of larvae which were 7.1-7.2 mm in length, those with the swim bladder filled with air constituted 65%. At this stage, larvae were motile and moved freely in the water column. To see if there was any relationship between the filling of the swim bladder with air and the ability of larvae to take food, 995 polar cod larvae were analyzed 16-26 days following hatching. There were specimens with their swim bladder filled and some not filled with air; feeding and nonfeeding larvae. The relationship between these two characteristics was evaluated with the help of a ~2 test. Distribution of larvae by the four groups is illustrated in Table II.
TABLE
II
Relationship between the ability of larvae to establish feeding and the filling of swim bladder with air Larvae
Feeding Nonfeeding Total number
Swim bladder filled
not filled
170 178 348
207 440 647
Total number
X2
- X’ n
377 618 995
26.61
0.02
P
< 0.001
As follows from the table, at P < 0.001 there is a definite relationship between the ability of larvae to begin feeding and their swim bladder: those with the swim bladder filled with air feed more actively. In view of the fact that the above relationship may change with the age of larvae, five age groups (16,18,20, 23 and 26 days from hatching) were analyzed. This enabled the determination of the above relationship in each age group. In 16-l&day old larvae, which have just started feeding (less than 25% of the total number), this relationship was found at a P < 0.05significance level. In larger larvae, it was established with greater confidence (P < 0.001). Thus, the filling of swim bladders with air, increased the number of feeding larvae.
239
Abnormalities
in larval development owing to inadequate conditions
During albsorption, the yolk sac acquires a different shape as it sepa rates from the walls of the abdominal cavity. The space formed herewith is sometimes filled. with exudate causing “dropsy” of the yolk sac, pericardiu .m or
Fig. 5. Polar cod larva with normal yolk sac (food seen in the stomach).
Fig. 6. Polar cod larva with “dropsy”
of pericardium and yolk sac (food seen in the stomach).
240
Fig. 7. Navaga larva with abnormal yolk
sac.
abdomen (Figs 5,6 and 7). It appeared important to trace the effect of “dropsy” on the ability of larvae to establish feeding. A total of 1191 polar cod larvae, 14-26 days after hatching, were analyzed for the purpose. They included specimens with signs of “dropsy” and those not affected by it, among them there were feeding and nonfeeding larvae. The relationship between this disease and larval feeding was also assessed with the help of a x2 test. The specimens analyzed were distributed as shown on Table III.
TABLE III “Dropsy” in polar cod larvae and its effect on their feeding Larvae
With dropsy
Not affected by dropsy
Total number
X2
x’ n
P
Feeding Nonfeeding Total number
56 262 307
324 560 884
3 812 1191
36.01
0.03
< 0.001
x2 calculated from data of Table III, exceeds the table value at P < 0.001, which testifies to the relationship between “dropsy” in larvae and their ability to feed: the majority of larvae affected by “dropsy” were nonfeeding. Naturally, this relationship could vary with their age. x2 calculated for each
241
of 6 larval groups (14,16,18,20, 23 and 26 days) showed that this relationship was most pronounced in 16-l&day old larvae, whereas in 20, 23 and 26-day old larvae it disappeared. DISCUSSION
This experiment with winter-spawning fish has shown that they differ essentially from spring-spawning species whose embryogenesis occurs at higher water temperatures. In the present study, egg incubation at a mean temperature of 1.5 “C took 35-40 days, depending on the species. The incubation period of species ecologically and taxonomically related to those studied, is much shorter. Plaice eggs at 7 “C reach the hatching stage within 14 clays (Shelbourne, 1964; Nordeng and Bratland, 1971) and cod at 2-6 “C, within 16-29 days (Westernhagen, 1970). All biological processes in the spec:ies studied proceed very slowly. This is illustrated by the rate of embryogenesis, the time of established feeding, the rate of yolksac absorption; the latter at 3-4 “C lasting 18-20 days (polar cod and arctic flounder) and 22-24 days (nav~a). Consequently, work with cold-water fish species requires a different approach from that with warm-water fish. This should be taken into consideration when developing rearing techniques and in particular, when determining the permissible starvation period and the amount of food required. The experiments have shown that White Sea fish can survive without food until day 20 (polar cod), whereas Sakhalin herring larvae can do without food only until day 10 (Kryzhanovsky, 1956), round goby up to day 12 and Black Sea turbot (Scoph thalmus maeo ticus maeoticus) until day 8. Food digestion in White Sea fish proceeds at a slow rate too (from 6 to 12 h in the first days of established feeding), which is largely due to the low temperatures. Unfo~unately, we could trace no data on the rate of food digestion in cod or plaice. In larval Black Sea horse-mackerel, food digestion takes 1.5-3 h at a temperature of 24 “C (Dooka, 1961). The maximum number of larvae with food in their stomach could be observed 5-8 hours after they had been fed. This low feeding activity testifies, in all probability, to low metabolism typical of cold-water North Sea species, which is associated with low ambient temperatures. The fact that not all of the larvae had fully digested the food taken (50% of them had food remnants in the guts ‘or stomach) is also indicative of this low rate of feeding. In view of the slow digestion process in White Sea fish, the amount of food required prsoved to be very small. In the present experiments, the initial density of food organisms in the rearing containers did not exceed one organism per ml. Plaice larvae need much more food when establishing feeding, up to 10 nauplii a day (Riley, 1966), whereas the number of food organisms in the early stages of herring larvae should be 20-100 times as great as their daily
242
diet which is four to eight organisms per ml (Rosental and Hempel, 1971). The experiments have shown that polar cod proved most viable and least difficult to feed. The feeding technique was simple and larvae took any suitable organisms, including wild plankton and organisms cultured in the laboratory. SUMMARY
(1) The incubation period for polar cod eggs lasts 35 days, that for arctic flounder is 41-42 days, and for navaga, 48 days. Emergent larvae average 5.54 mm (polar cod), 6.0 mm (navaga) and 5.56 mm (arctic flounder). (2) The rate of yolk sac absorption is higher in starving larvae than in feeding ones. In polar cod and arctic flounder the yolk sac is fully utilized by day 18-20 after hatching. The longest absorption time was found for larval navaga (22-24 days). (3) Starvation beyond 8-10 days after hatching is critical for navaga, and’ such a critical period for polar cod larvae occurs on day 20. It is advisable to begin feeding navaga larvae not later than between days 8 and 10 following hatching, and polar cod larvae about 17-20 days at temperatures of 3-4 “C. (4) Artemia nauplii of about 600 pm in length and natural plankton consisting of Puracalanus and Pseudocalanus nauplii may be used as an initial food for navaga and polar cod at a density of one organism per ml. (5) Feeding efficiency of larvae at the time when they first establish feeding is rather low. Even among late-stage larvae, those with food in the stomach constitute only 60-70%. Food digestion in older larvae proceeds faster than in the early stages of development. The maximum number of larvae with food in the stomach within a period of a day has been observed between 5-8 h after the introduction of food.
REFERENCES Bailey, N., 1959. Statistical Methods in Biology. English Universities Press, London, 250 pp. Dooka, L., 1961. Feeding of Black Sea anchovy larvae. Trudy Sevastopolskoy Biologicheskoy Stantsii, 14: 242-256. Kryzhanovsky, S., 1956. Materials on development of Clupeidae. Moskva, Vyp., 17: 251. Nordeng, B. and Bratland, P., 1971. Feeding of plaice Pleuronectessplatessa and cod Godus morhua L. larvae. J. Cons. Perm. Int. Explor. Mer, 34 (1): 51-57. Rosental, H. and Hempel, G., 1971. Experimental estimates of minimum food density for herring larvae. Rapp. Comm. Int. Mer Medit., V: ,160. Riley, J., 1966. Marine fish culture in Britain. VII Plaice (Pleuronectessplatessa L.). Postlarval feeding on Artemia salina L. nauplii and the effects of varying feeding levels. J. Cons. Perm. Int. Explor. Mer, 30 (2): 204-221. Shelbourne, J., 1964. The artificial propagation of marine fish. Adv. Mar. Biol., 2: l-83. Westernhagen, H., 1970. Erbrutung der Eier von Dorsch, Flunder und Scholle. Helgol. Wiss. Meeresunters., 21 (l-2): 2-102.
Company,
Amsterdam - Printed in The Netherlands
EGG INCUBATION AND LARVAL REARING OF NAVAGA (ELEGINUS PALL.), POLAR COD (BOREOGADUS SAIDA LEPECHIN) AND ARCTIC FLOUNDER (LIOPSETTA GLACIALIS PALL.) IN THE LABORATORY
NAVAGA
T.M. ARONOVICH, VNIRO,
Moscow
S.I. DOROSHEV,
State
University,
L.V. SPECTOROVA
Moscow
and V.M. MAKHOTIN
(U.S.S.R.)
(Received November 6th, 1974)
ABSTRACT Aronovich, T.M., Doroshev, S.I., Spectorova, L.V. and Makhotin, V.M., 1975. Egg incubation and larval rearing of navaga (Eleginus nnvaga Pall.), polar cod (Boreogadus saida Lepechin) and arctic flounder (Liopsetta glacialis Pall.) in the laboratory. Aquaculture, 6: 233-242. The embryonic and larval development of three White Sea cold-water fish species, rate of yolk sac absorption, age at first feeding and their survival and growth when fed different food organisms, were studied. Eggs were obtained from spawners in the Bay of Kandalaksha, White Sea, and incubated in troughs and aquaria at a mean temperature of 1.5 “C, slightly above that of the sea. The incubation period for polar cod eggs lasted 35 days, for arctic flounder, 42 days and for navaga eggs, 48 days. Emergent larvae were 5.5-6.0 mm long and began feeding at 2-4 “C!, 5-6 days (na.vaga) and 12-14 days (polar cod) after hatching, when their yolk sac was still fairly large. They were fed day-old Artemia nauplii and zooplankton taken from the sea and consisting of Calanus and Pseudocalanus nauplii 400-600 p in length. The period of establishing first feeding is the most critical for larvae.
MATERIALS
AND METHODS
Experiments were conducted in January-April, 1973 at the White Sea biological station, Moscow University. Navaga, polar cod, and arctic flounder spawn in winter (DecemberFebruary) at a temperature of -1.5 “C. Spawners were caught in traps and delivered to the laboratory in isothermal boxes. Fertilized eggs of navaga were obtained from naturally spawning stock in the aquaria, and eggs of polar cod and flounder were fertilized artificially. 5-l jars and 8-10-l glass aquaria were used for incubating flounder and polar cod e,ggs, 3-l polythene bowls placed in troughs with running water were used for navaga eggs. The bowls were connected by polythene tubes to allow water to flow from one bowl into another with a rate of 0.1-0.2 1 per min. Larvae were reared in 60-l polythene troughs and lo- and 30-l glass aquaria.
234
Feeding experiments were carried out mainly on polar cod larvae which seemed best suited for the purpose. Larvae were fed on natural microzooplankton obtained from the sea and consisting of Puracalunus and Pseudocalanus nauplii 250-400 ~1in length and day-old Artemiu salina nauplii 600 cc in length cultivated from eggs in a rearing apparatus as suggested by Shelbourne (1964). The length of larvae was measured from the tip of the lower mandible to the end of the caudal fin; the yolk sac was measured by calculating its area, from the optical section, as that of an ellipsoid. As a rule, samples to be measured consisted of 10-25 larvae which had been anaesthetized in 100 ppm tricane methane sulphonate (MS 222). The relationship between the filling of the swim bladder with air and the ability of larvae to establish feeding was estimated by x ’ test using the Contingency tables 2 X 2 (Baily, 1959). The following equation was used: x2 =
n [ (ad-bc)-0.5n.j
2
(a + b)(c + d)(a + c)(b + d) where: (I and b = number of feeding larvae with their swim bladder filled and not filled with air, respectively. c and d = number of nonfeeding larvae with their swim bladder filled and not filled with air, respectively. n = total number of larvae in the experiment. RESULTS
Embryonic
development
Polar cod and arctic flounder eggs are translucent, the membrane is thin and non-adhesive, the pe~~telline space is very narrow. Their specific gravity is lower than that of water, so they stay near the water surface. Swollen eggs of arctic flounder are smaller than those of polar cod (1.5-1.74 and 1.72-1.90 mm in diameter, respectively). Navaga eggs have a thick membrane and a large perivitelline space. The diameter of the swollen eggs ranges between 1.63 and 1.72 mm, plasma and yolk are a bright orange colour. They sink to the bottom of the aquarium. Arctic flounder and polar cod eggs are oriented with their animal pole downwards, navaga eggs orient theirs upwards. There are no oil globules in the eggs of the three species. As is seen from the Table I, the rate of embryonic development differs in the three species. The first stages (up to the mobile state) occur approximately within the same period, but embryonic organs in navaga and arctic flounder begin to function somewhat earlier (by day 18 or 19) than in polar cod (between days 22 and 26). Hatching times are also different: polar cod larvae hatch between day 26 and 35, navaga hatch between day 28 and 48, and arctic flounder from day 22 to 42.
235
TABLE I Rate of embryogenesis in the fishes studied (days). Incubation temperature for arctic flounder ranged from -0.5 to 3.1 “C (the mean temperature being 1.0 “C), for navaga from -0.4 to 3.8 “C (with a mean temperature of 1.39 “C); for polar cod from -0.3 to 2.5 “C (the mean temperature 0 “C) Stages
Arctic flounder
Navaga
Polar cod
> 0.5
> 0.5
> 0.5
II Cleavage (Ei-10 h between mitoses at a given temperature; from two blastomeres stage to beginning of epiboly)
0.5-5
0.5-5
0.5-7
III Epiboly an’d beginning of gsstrulation
5-9
5-8
7-10
IV Organogenasis and continued epiboly (formation of axial- organs)
Q-19
8-18
10-22
I
Formation
of blastodisc
V Embryo in a mobile state (onset of functionai activity of embryonic organs: tail section gets longer, somites grow in number, development and enlargement of separate organs)
19-22
18-28
22-26
VX Hatching (intensive eye and body pigmentation, formation of hatching glands)
22-42
28-48
26-35
Larval growth and yolk sac absorption
Emergent larvae of arctic flounder are 5.56 + 0.04 mm in length (CV = not very active, and coneentkate along the sides of the troughs in a suspended state. The intestines look like a narrow tube, the stomach has not yet developed and the mouth and intestines are not different’iated. Larval growth of White Sea fishes in this period (Fig. 1) and yolk sac absorption (Fig. 2) are slow. The process is ~rnpe~t~e dependent and lasts about a month. Newly hat.ched navaga larvae are somewhat larger, 6.0 + 0.05 (CV = 0.83%) Positive phot,otaxis is well pronounced in them. Like flounder larvae, during the first several days they concentrate along the sides of the aquaria in the corners close to the water surface. In contrast, polar cod larvae are distributed throughout the whole water column. They do not react to light during the first days of life. Polar cod larvae measure on hatching 5.54 _+0.07 mm (CV = 1.26%). The rate of yolk sac absorption changes after feeding is established; Thus in non0.72%). They are translucent,
age, days
age. daw
Fig. 1. Growth rate of larval navaga, polar cod and arctic flounder .- _- arctic flounder,---*-snavaga. W
2. Yolk SBCabsorption, -
p&r
cod, - - -
arctic flounder, -*-e-1
polar cod, navam.
feeding larvae it was 0.148 mm2/day, after feeding was established it dropped sharply to 0,065 mm*/day. This may be attributed to the fact that with the food being taken from outside, utilization of the yolk sac for nutrition decreased. Yolk sac absorption in starving larvae proceeds more quickly than in feeding larvae. As the larvae grow, their yolk sac is gradually depleted, and by 18-20 days after hatching it has disappeared. In navaga complete yolk sac absorption occurs later, 22-24 days after hatching. Larvae were regularly offered food prior to complete yolk sac absorption when they were able to take and digest it, which in polar cod larvae occurred on day 12 and in navaga, day 5 or 6. Only a small number of larvae could feed at this age (5%). The percentage of feeding larvae later increased, gradually from 25% in 14-day old larvae to 65% in 26-day old larvae (Fig. 3). The maximum number of larvae with full guts could be observed 5-8 h after the food had been offered. The number of food organisms in the stomach increased with age. Thus, while in the stomach of 12-14-day old larvae there was only one food organism, the stomach content of 23-26-day old larvae was 6-7 artemia nauplii at a time. Newly-hatched larvae of navaga can survive without food beyond days 5 and 6, and polar cod larvae to even day 14-16, without affecting the percentage of larvae capable of establishing feeding, however, a starvation period of 8-10 days following hatching (navaga) or 20 days after hatching (polar cod) proved critical (Fig. 4). Larvae which were given food for the first time in this period were unable to swallow it. This resulted in degeneration of the liver and gastro-
237
5.. a ,
. 5--6
2-1
4
how
7
t
9
-.
’
*
*
10 11 12 'f 14
fwm the stat
-
4
uf leedirty
Fig. 3. Larval feeding depending on number of days after hatching.
v.
1
so 80 70 60
navaga
polar cod
C__-__.-
/
l”li\. /
SQ 40
1’
3Q 20 10 0
/---------
/’
5
7
9
11
/I
I’
13 15 1 19 21 23 25 27 age,dw
Fig. 4. Relationship between starvation period and first feeding . --control!.
starvation period,
238
intestinal tract. Based on the data obtained, it is recommended that the feeding of navaga larvae should not start later than 8-10 days after hatching, and polar cod larvae no% later than 17-20 days, at temperatures of 3-4 “C. Filling of the swim bladder with air The swim bladder filling with air is one of most critical phases in larval development. Among 6.4-6.5 mm long larvae of polar cod about l-296 had their swim bladder filled, of larvae which were 7.1-7.2 mm in length, those with the swim bladder filled with air constituted 65%. At this stage, larvae were motile and moved freely in the water column. To see if there was any relationship between the filling of the swim bladder with air and the ability of larvae to take food, 995 polar cod larvae were analyzed 16-26 days following hatching. There were specimens with their swim bladder filled and some not filled with air; feeding and nonfeeding larvae. The relationship between these two characteristics was evaluated with the help of a ~2 test. Distribution of larvae by the four groups is illustrated in Table II.
TABLE
II
Relationship between the ability of larvae to establish feeding and the filling of swim bladder with air Larvae
Feeding Nonfeeding Total number
Swim bladder filled
not filled
170 178 348
207 440 647
Total number
X2
- X’ n
377 618 995
26.61
0.02
P
< 0.001
As follows from the table, at P < 0.001 there is a definite relationship between the ability of larvae to begin feeding and their swim bladder: those with the swim bladder filled with air feed more actively. In view of the fact that the above relationship may change with the age of larvae, five age groups (16,18,20, 23 and 26 days from hatching) were analyzed. This enabled the determination of the above relationship in each age group. In 16-l&day old larvae, which have just started feeding (less than 25% of the total number), this relationship was found at a P < 0.05significance level. In larger larvae, it was established with greater confidence (P < 0.001). Thus, the filling of swim bladders with air, increased the number of feeding larvae.
239
Abnormalities
in larval development owing to inadequate conditions
During albsorption, the yolk sac acquires a different shape as it sepa rates from the walls of the abdominal cavity. The space formed herewith is sometimes filled. with exudate causing “dropsy” of the yolk sac, pericardiu .m or
Fig. 5. Polar cod larva with normal yolk sac (food seen in the stomach).
Fig. 6. Polar cod larva with “dropsy”
of pericardium and yolk sac (food seen in the stomach).
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Fig. 7. Navaga larva with abnormal yolk
sac.
abdomen (Figs 5,6 and 7). It appeared important to trace the effect of “dropsy” on the ability of larvae to establish feeding. A total of 1191 polar cod larvae, 14-26 days after hatching, were analyzed for the purpose. They included specimens with signs of “dropsy” and those not affected by it, among them there were feeding and nonfeeding larvae. The relationship between this disease and larval feeding was also assessed with the help of a x2 test. The specimens analyzed were distributed as shown on Table III.
TABLE III “Dropsy” in polar cod larvae and its effect on their feeding Larvae
With dropsy
Not affected by dropsy
Total number
X2
x’ n
P
Feeding Nonfeeding Total number
56 262 307
324 560 884
3 812 1191
36.01
0.03
< 0.001
x2 calculated from data of Table III, exceeds the table value at P < 0.001, which testifies to the relationship between “dropsy” in larvae and their ability to feed: the majority of larvae affected by “dropsy” were nonfeeding. Naturally, this relationship could vary with their age. x2 calculated for each
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of 6 larval groups (14,16,18,20, 23 and 26 days) showed that this relationship was most pronounced in 16-l&day old larvae, whereas in 20, 23 and 26-day old larvae it disappeared. DISCUSSION
This experiment with winter-spawning fish has shown that they differ essentially from spring-spawning species whose embryogenesis occurs at higher water temperatures. In the present study, egg incubation at a mean temperature of 1.5 “C took 35-40 days, depending on the species. The incubation period of species ecologically and taxonomically related to those studied, is much shorter. Plaice eggs at 7 “C reach the hatching stage within 14 clays (Shelbourne, 1964; Nordeng and Bratland, 1971) and cod at 2-6 “C, within 16-29 days (Westernhagen, 1970). All biological processes in the spec:ies studied proceed very slowly. This is illustrated by the rate of embryogenesis, the time of established feeding, the rate of yolksac absorption; the latter at 3-4 “C lasting 18-20 days (polar cod and arctic flounder) and 22-24 days (nav~a). Consequently, work with cold-water fish species requires a different approach from that with warm-water fish. This should be taken into consideration when developing rearing techniques and in particular, when determining the permissible starvation period and the amount of food required. The experiments have shown that White Sea fish can survive without food until day 20 (polar cod), whereas Sakhalin herring larvae can do without food only until day 10 (Kryzhanovsky, 1956), round goby up to day 12 and Black Sea turbot (Scoph thalmus maeo ticus maeoticus) until day 8. Food digestion in White Sea fish proceeds at a slow rate too (from 6 to 12 h in the first days of established feeding), which is largely due to the low temperatures. Unfo~unately, we could trace no data on the rate of food digestion in cod or plaice. In larval Black Sea horse-mackerel, food digestion takes 1.5-3 h at a temperature of 24 “C (Dooka, 1961). The maximum number of larvae with food in their stomach could be observed 5-8 hours after they had been fed. This low feeding activity testifies, in all probability, to low metabolism typical of cold-water North Sea species, which is associated with low ambient temperatures. The fact that not all of the larvae had fully digested the food taken (50% of them had food remnants in the guts ‘or stomach) is also indicative of this low rate of feeding. In view of the slow digestion process in White Sea fish, the amount of food required prsoved to be very small. In the present experiments, the initial density of food organisms in the rearing containers did not exceed one organism per ml. Plaice larvae need much more food when establishing feeding, up to 10 nauplii a day (Riley, 1966), whereas the number of food organisms in the early stages of herring larvae should be 20-100 times as great as their daily
242
diet which is four to eight organisms per ml (Rosental and Hempel, 1971). The experiments have shown that polar cod proved most viable and least difficult to feed. The feeding technique was simple and larvae took any suitable organisms, including wild plankton and organisms cultured in the laboratory. SUMMARY
(1) The incubation period for polar cod eggs lasts 35 days, that for arctic flounder is 41-42 days, and for navaga, 48 days. Emergent larvae average 5.54 mm (polar cod), 6.0 mm (navaga) and 5.56 mm (arctic flounder). (2) The rate of yolk sac absorption is higher in starving larvae than in feeding ones. In polar cod and arctic flounder the yolk sac is fully utilized by day 18-20 after hatching. The longest absorption time was found for larval navaga (22-24 days). (3) Starvation beyond 8-10 days after hatching is critical for navaga, and’ such a critical period for polar cod larvae occurs on day 20. It is advisable to begin feeding navaga larvae not later than between days 8 and 10 following hatching, and polar cod larvae about 17-20 days at temperatures of 3-4 “C. (4) Artemia nauplii of about 600 pm in length and natural plankton consisting of Puracalanus and Pseudocalanus nauplii may be used as an initial food for navaga and polar cod at a density of one organism per ml. (5) Feeding efficiency of larvae at the time when they first establish feeding is rather low. Even among late-stage larvae, those with food in the stomach constitute only 60-70%. Food digestion in older larvae proceeds faster than in the early stages of development. The maximum number of larvae with food in the stomach within a period of a day has been observed between 5-8 h after the introduction of food.
REFERENCES Bailey, N., 1959. Statistical Methods in Biology. English Universities Press, London, 250 pp. Dooka, L., 1961. Feeding of Black Sea anchovy larvae. Trudy Sevastopolskoy Biologicheskoy Stantsii, 14: 242-256. Kryzhanovsky, S., 1956. Materials on development of Clupeidae. Moskva, Vyp., 17: 251. Nordeng, B. and Bratland, P., 1971. Feeding of plaice Pleuronectessplatessa and cod Godus morhua L. larvae. J. Cons. Perm. Int. Explor. Mer, 34 (1): 51-57. Rosental, H. and Hempel, G., 1971. Experimental estimates of minimum food density for herring larvae. Rapp. Comm. Int. Mer Medit., V: ,160. Riley, J., 1966. Marine fish culture in Britain. VII Plaice (Pleuronectessplatessa L.). Postlarval feeding on Artemia salina L. nauplii and the effects of varying feeding levels. J. Cons. Perm. Int. Explor. Mer, 30 (2): 204-221. Shelbourne, J., 1964. The artificial propagation of marine fish. Adv. Mar. Biol., 2: l-83. Westernhagen, H., 1970. Erbrutung der Eier von Dorsch, Flunder und Scholle. Helgol. Wiss. Meeresunters., 21 (l-2): 2-102.