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Improvement in the efficiency of stocking lakes with larvae of Coregonus albula L. by delaying hatching
Aquacutture, 41 (1984) 99-111 Elsevier Science Publishers B.V., Amsterdam -Printed
99 in The Netherlands
IMPROVEMENT IN THE EFFICIENCY OF STOCKING LAKES WITH LARVAE OF C~~G~~U~ ALBULA L. BY DELAYING HATCHING
M. LUCZYNSKI Institute of Ichthyobiology and Fisheries, Academy 1 O-957 Olsztyn-Kortowo, Bl. 37 (Poland) (Accepted
13 February
of Agriculture
and Technology,
1984)
ABSTRACT Luczynski, M., 1984. Improvement in the efficiency of stocking lakes with larvae of Coregonus albula L. by delaying hatching. Aquaculture, 41: 99-111. Low survival of vendace (Coregonus afbulu) eggs incubating on natural spawning grounds (due to silting, oxygen deficits and action of predators) is discussed as one of the reasons for the great ~uctuation in numbers of commercial fish. Incubation of eggs in hatcheries where the survival rate averages 60% could compensate for the great numerical disparity between the eggs spawned naturally and those taken for hatcheries. In the absence of the effects of mass stocking of lakes with vendace larvae, one can conclude that apparent mortality during the larval stage also determines the strength of the successive year-class. Vendace larvae are usually stocked immediately after the ice cover recedes from lakes. At this time they often face a period of cold weather when water temperature is low and food organisms are scarce. Poor environmental conditions cause slow growth of larvae. This increases the period of highest ~lnerability of the larvae to predation. It is possible to accelerate the growth of larvae by synchronizing the time of stocking with the development of favourable thermal conditions and food availability in lakes. In Polish climatic conditions the first 10 days of May seem to be a suitable time for stocking lakes. A technique for delaying vendace egg hatching by cooling the hatchery water has been developed. It enables hatching of vendace to be delayed from the beginning of April until the beginning of Nlay, thereby inducing mass hatching of larvae when lake conditions are optimal for stocking. INTRODUCTION
Vendace (Coregonus albula L.) is a common autochthonous species of considerable commercial value in Poland (Leopold et al,, 1970). For this reason it has been introduced more or less successfully into lakes of various limnological type (Bernatowicz, 1953,196O). In lakes where vendacepopulations are commercially exploited, the catches show great fluctuations (Dabrowski and Eichler, 1972). This is also commonly observed with other Coregoninae (Lapworth, 1956; Klein, 1980). Low survival of Coregoninae eggs developing in natural spawning grounds 0044-8486/84/$03.00
Q 1984 Elsevier Science Publishers B.V.
100
is one of the possible reasons for the appearance of weak year-classes. Adult whitefish were observed devouring eggs being dropped by other spawners (Fabricius and Lindroth, 1954). During the long incubation season great losses of eggs are caused by silting and low oxygen concentration as well as by fish (Eltsova, 1976) and invertebrate predators (Niimann and QUOSS, 1972; Shemeikka et al., 1978). During the past decade eutrophication of the Polish lakes which are natural spawning grounds for Coregoninae has made the environment for egg incubation much less favourable. The survival of C. albula eggs, formerly higher than 4%, now ranges from 0.0 to 2.6% (Zuromska, 1982a), and survival of C. Zauaretus dropped to nil in most lakes that were investigated (Wilkonska and Zuromska, 1982). Survival of Coregoninae eggs incubated in hatcheries averages over 60% (Liiffler and Deufel, 1980; Zuromska, 1982a). It was assumed that such high survival could compensate substantially for the initial great numerical disparity between the eggs spawned naturally and those taken for hatcheries. But contrary to expectations, mass stocking of lakes with Coregoninae larvae was not followed by the strong year-classes predicted (Lapworth, 1956; Christie, 1963; Lawler, 1965; Bernatowicz et al., 1975). It became evident that losses occurring after hatching were also important for the determination of the year-class strength (Einsele, 1965; Salojarvi, 1982). Coregoninae embryos usually hatch when water temperatures rise rapidly after the ice cover recedes from lakes (Faber, 1970; Colby and Brooke, 1973; Bidgood, 1974; Salojti, 1982). At this time a period of cold weather often occurs, and Coregoninae larvae face cold water and a scarcity of food. The newly hatched larvae are extremely susceptible to predation, both by fish (Grim, 1951; Yocom and Edsall, 1974; Eltsova, 1976; Selgeby et al., 1978) and by invertebrates (Fritzsche and Taege, 1979). Suitable thermal and nutritional conditions, which enable Coregoninae larvae to grow fast enough to escape the effect of predators (Lindstrijm, 1962), appear to determine larval survival and the subsequent development of a strong year-class. This was confirmed by the observation that an appropriate succession of suitable climatic conditions, i.e. cold winters followed by late and warm springs, during spawning, incubation and hatching, allowed high survival of -embryos and larvae and in consequence was correlated with the occurrence of strong year-classes (Christie, 1963; Lawler, 1965; Hartmann, 1980). Such beneficial climatic conditions occur only rarely in nature. In some whitefish hatcheries, eggs are incubated in water cooled to 1°C in order to delay hatching of embryos. Lakes are then stocked with larvae when water temperature and food concentration become optimal for their growth and survival (Niimann, 1953). Stocking Lake Constance with C. wartmanni larvae of delayed hatching has produced larger year-classes without an appreciable increase in number of fry stocked (Ntimann, 1967, 1970). This paper describes the possibilities of delaying hatching of vendace eggs when incubated in commercial hatcheries under Polish (or similar) climatic conditions.
101 MATERIALS
AND METHODS
Studies in 1977-l
978
Ripe spawners of C. albuZa gillnetted in Lake Kosno were stripped and eggs were fertilized on 2 December 1977. Surface water temperature was 2.5”C. Water-hardened eggs were transported to the laboratory of the Inland Fisheries Institute in Olsztyn, and incubate in 7-l glass incubation jars at constant temperatures of 1 and 2°C. When hatching began, samples of eggs were taken and placed into the incubation cylinders. The bottom and cover of the cylinders were made of ‘“Steelon” screen, and the cylinders were suspended vertically in glass incubation jars. Eggs in incubation cylinders were examined daily, and dead eggs, normally developed .and abnormal hatched larvae were removed and counted. Studies in 1978-1979 Freshly fertilized and water-hardened eggs of vendace from Lake Kobylocha were taken on 21 November 1978 and transported to Janowo hatchery. Samples of eggs were placed in incubation cylinders within incubation jars. A sample of eggs was transported to the laboratory and incubated in several incubation cylinders at l.l*C. On 30 November 1978, eggs from approximately 50 females from Lake Kosno were fertilized with sperm taken from about 10 males. Lake water temperature was 5.3”C. Two hours after fertilization a sample of eggs was transported to the laboratory and incubated in several incubation cylinders at l.l”C. Another sample of eggs was transported to Janowo hatchery and placed in incubation cylinders within incubation jars.
The temperature of water supplying the hatchery was measured, with an accuracy of 0.1% twice a day, and daily temperatures’were calculated as the mean value of these measurements. Embryonic development of eggs incubated in the hatchery was examined twice a week when dead eggs and hatching embryos were removed and counted. Course of em b~oge~es~ The course of C. albufu embryogenesis was determined by using the 16 developmental stages described by Luczynski and Kirklewska (1984). Stages DS 14, DS 15 and DS 16 were described as thehatching of 10, 50, and 90% of embryos, respectively. Stages DS l-DS 13 were determined under a binocular microscope. A group of eggs ‘was considered to have reached a
102
particular developmental stage when 50% or more of the embryos showed the characteristics of that stage. Laboratory experiments In the laboratory, water supplied to jars was re-circulated and sterilized by U.V. radiation (Luczynski, 1981). The water temperature was measured three times a day to an accuracy of 0,l”C and daily temperatu~ was calculated as the mean of these measurements. Mortalities were recorded and dead eggs were removed from all incubation cylinders daily. Eggs were considered dead when they were completely opaque. Incubation in c&bed water On 8 December 1978, when the water temperature in the hatchery decreased to l.O”C, samples of eggs were transported in vacuum bottles from the laboratory to the hatchery and were placed in incubation cylinders. In spring, when the water temperature in Janowo hatchery exceeded 1°C (22 March 1979), samples of eggs were transported in vacuum bottles from the hatchery to the laboratory and were incubated in incubation cylinders at 1.1%.
During both incubation seasons, while the eggs incubated at 1 (or 1.1) and 2°C were hatching, samples of eggs were taken from jars every few days and acclimated at a rate of 1.5”C h-l to the temperature of the lake water supplying the hatchery. The samples were then placed in incubation cylinders supplied with lake water. Hatching embryos were removed daily. Some of them were abno~~y developed (scoliosis, kyphosis, and lordosis - Colby and Brooke, 1970), but when none of these malformations could be seen with the naked eye, larvae were considered to be normally developed. Length of taPvaeand yolk sac dime~io~s Samples of n = 30 freshly hatched larvae were anaesthetized with MS 222 (1 : 10 000). Total length (1.t. ) of larvae and length (2) and height (h) of their yolk sac were measured under an ocular micrometer with an accuracy of 0.01 mm. Yolk sac volume (v) was calculated from the formula for a prolate spheroid: V = 0.5236
I h2
(Blaxter and Hempel, 1963).
103 RESULTS
Delaying of hatching In Polish Coregoninae hatcheries, water temperature decreases gradually in autumn when vendace spawn (Fig. 1A). During the winter (when lakes are covered with ice) the water remains at a constant low temperat~e (O.l0.2”C was observed in Janowo hatchery). In spring, when the ice melts, lake water temperature rises quickly and mass hatching of vendace occurs. The hatching of vendace embryos can be delayed by cooling the water at times when the hatchery water temperature is higher than l-2”C, i.e. during autumn and spring, There are three possibilities for water cooling: (a) in autumn, (b) in spring, and (c) in both autumn and spring. Fig. IB, C and D show the course of embryogenesis of vendace from Lake Kobylocha and Lake Kosno, when these three possibilities for water cooling were applied,’ For comparison Fig. 1A shows the course of embryogenesis of samples of these eggs incubated in the usual way in Janowo hatchery. There were no si~~i~~t differences in hatching time between batches of eggs incubated normally and those incubated in autumn in water cooled to l.l”C (Fig. 1A and 1B). However, cooling the water during spring significantly delayed hatching (Fig. 1C). There were no significant differences in hatching time between the eggs incubated in cooled water in spring and those incubated at 1,l”C in autumn and spring (Fig. 1C and 1D). batching
success
At temperatures ranging from 1 to 2°C hatching of C. atbuta embryos began late and lasted for a long time (DS 14-DS 16 on Fig. 1). Unfortunately, at these temperatures the percentage of normal hatch was very low and ranged from a few percent to about 50% (Fig. 2). Such great losses of eggs were avoided when the incubation temperature was raised (at a rate of 15°C h-‘) during the final period of incubation. This procedure caused immediate mass hatching of embryos, and the percentage of normal hatch usually exceeded 90% (Fig. 2). As the delay in hatching increased there was a trend toward a decrease in the percentage of normal larvae hatch~g from the batches of eggs successively acclimated to higher temperatures (Fig. 2). The combined effect of these two causes of egg mortality should be taken into consideration when planning the delay of hatching. Table I contains some examples of egg mortality observed during incubation and hatching when batches of eggs incubated in a traditional hatchery were compared with those subjected to a delay in hatching. In every case there was some “cost” of hatching delay, expressed in egg mortality. When hatching was delayed excessively, the third potential cause of egg mortality appeared. This was when the temperature of the lake water supplying the
16
4 2 0
0
16
8
12
6
8
4
4
2 0
0 NOV
DEC1979
JAN
FE8
MAR
APR
MAY
JUNErm
Fig. 1. Mean daily water temperatures and course of embryogenesis of C. albula eggs incubated in the normal way in Janowo hatchery (A), and in water cooled to l.l”C during autumn (B), spring (C), and autumn and spring (R). Egg fertilization is described as stage DS 0, whereas stages DS 14, DS 15, and DS 16 are 10, 50 and 90% of hatching, respec-
tively .
hatchery became too high for vendace embryos (13.5-16.O’C) and the acclimation caused additional mortality of eggs. As a result, the general survival of eggs from sample A3 (Table I) was only 42% compared with 76% (Al) in a traditional hatchery. On the other hand, a similar survival of 41% of eggs from Lake Kobylocha (B2 in Table I) was rather a good result when compared to 54% survival of eggs incubated traditionally (Bl in Table I).
105
Days of incubation Fig. 2. Course of hatching and survival of C. albula embryos incubated at constant temperatures of 2°C (A) and 1°C (B). Batches of eggs were successively acclimated to higher temperature (arrows show the day of acclimation). Temperature range during hatching is shown for every batch of eggs, as well as the percentage of normal hatch (in brackets).
TABLE I Thermal conditions of incubation and the survival of eggs of C. albula from Lake Rosno (A) and Lake Kobylocha (B) incubated in normal hatchery water and in artificially cooled water (incubation season 1978-1979) Date of 50% hatch Normal
Delayed
Temperature during hatching (“C)
Percentage of dead eggs and ab- Percentage of normal normal hatch appeared during hatch Incubation Hatching in hatchery
Al A2 A3
23 April -
-
Bl B2
12 April -
-
9 May 18 May 1 May
5.0- 7.3 10.0-11.0 13.5-16.0 3.59.0-
7.3 9.5
at 1S”C
19.6 17.0 17.0
3.0 5.5
4.5 9.0 35.5
76 71 42
41.0 40.5
13.5
5.0 5.0
54 41
106
Dimensions of ‘delayed ” larvae Fig. 3 shows the regression line describing the relationship between the yolk sac volume and the total length of freshly hatched vendace larvae from Lake Kosno, when incubated at constant temperatures ranging from 1.1 to 9.9”C (Luczynski et al., 1984). Points Al, A2 and A3 represent the respective data for vendace from Lake Kosno when incubated in a traditional hatchery (Al) and those delayed in hatching for 16 (A2) and 25 days (A3) (see Table I for identification of Al-A3). All points lie within 95% (Al and A2) and 99% (A3) tolerance limits of the regression line. 1.4
1.2
_
1.0 . -2 5 ’ 0.8 _
~
E 2 s 0.6 . : u) -Y g 0.4
y =-0.4743L+ .
5.1886
r q-0.98954
Total length of eleutheroembryos
\\ \ \ ‘\ ‘\
L (mm)
Fig. 3. Relationship between the yolk sac valume and total length of C. albulu eleutheroembryos incubated in the usual way (Al) and when hatching was delayed for 16 (A2) and 25 (A3) days (see Table I for Al-3 incubation conditions). Regression line with 95% and 99% tolerance limits was derived from the data obtained from the incubation of C. albula eggs at various constant temperatures (Luczynski et al., 1934).
DISCUSSION
Incubation of Coregoninae eggs in hatcheries allows for the possibility of controlling the environmental conditions related to embryogenesis in order to minimize egg mortality. It would be interesting to compare the influence of factors affecting survival of C. albula eggs developing on natural spawning grounds with their effect on eggs incubated in hatcheries.
107
Spawning of vendace usually begins around 10-20 November, with the culmination around 20-30 November when autumn homoiothermy occurs in Polish lakes (Zuromska, 1982b). Optimal conditions for successful spawning of Coregoninae occur when autumn temperatures steadily decrease (Lawler, 1965). Pokrovskii (1961) stated that the course of water temperature at the time of spawning exercised an influence on the quantity of fertile eggs of C. albula which varied from year to year between 100~-80% and 50-35%. (In one year it was only lo%.) Low fertility was attributed to a long drawn-out autumn in which the males leave the spawning grounds before the optimum spawning temperature for females is reached. By employing the dry method of vendace egg fertilization it is always possible to obtain a sufficient quantity of sperm to fertilize all the stripped eggs. It can be expected that the losses of hatchery-incubated eggs due to lack of fertilization should be very low. Eggs developing in a hatchery are protected against predators, silting, oxygen deficits, etc. In spring time the water temperature in the hatchery rises similarly to that in lakes when the ice melts, and thereby creates suitable conditions for hatching of normally developed, viable larvae (Luczynski, 1984a). Therefore, by incubating Coregoninae eggs in hatcheries one avoids the majority of harmful environmental factors which cause mass egg mortality on natural spawning grounds. In fact, the survival of vendace eggs incubated in hatcheries equals 54 and 76% (this study) or from 62 to 94% (Zuromska, 1982a) which is many times higher than egg survival observed on natural spawning grounds. However, we still face the mortality occurring during the larval and fry stages of the life of Coregoninae. Mortality rates during the early life stages are usually the highest and most variable from year to year. Changes in the mortality rate during these life stages have a major effect on the final numbers of commercial fish. Very often Coregoninae larvae are stocked into lakes during a period of cold and windy weather, when water temperature remains low and food organisms are scarce. Larvae have sufficient yolk material to tide them over temporary periods of food shortage (Hoagman, 1973; Dabrowski, 1976; Jezierska et al., 1978, 1979; Grudniewski, 1980), but in poor thermal and food conditions their growth rate is low. This substantially lengthens the period when larvae are most vulnerable to predation and other unfavourable environmental factors (Lindstrijm, 1962; May, ,1974; Volkova, 1976). Niimann (1970) confirmed the significance of a high growth rate during the first year of life of Coregoninae for the subsequent development of large year-classes. He stated that small C. wartmanni year-classes grew slowly in the first year of life and that large classes grew quickly during their first year. The technique for delaying C. albula hatching synchronizes stocking of larvae with favourable thermal and food conditions in lakes. Experiments with larvae of different species showed that higher temperatures after
108
hatching increase the feeding activity of larvae and they begin external feeding earlier in ontogeny (Braum, 1967; Korovina et al., 1968; Ishibashi, 1974). At higher temperatures the utilization of food for growth also begins earlier (Heming et al., 1982) and the instantaneous growth rate of larvae markedly increases (McCormick et al., 1971). At higher food concentrations gross growth efficiency is higher (Laurence, 1977) and fish larvae grow faster (Korovina et al., 1975; Houde, 1978). Larval survival increases at higher food concentrations even more significantly than does the growth rate (Laurence, 1977). This suggests that stocking of lakes at a time when the water gets warmer and the concentration of food organisms becomes optimal should result in fast growth and high survival of vendace larvae. In Polish climatic conditions the first 10 days of May are believed to be the optimal time for stocking lakes with vendace larvae (Luczynski, 1981). The details of the technique for delaying vendace mass hatching until the beginning of May are described in a separate paper (Luczynski, 1984b). Delay of vendace hatching due to prolonged incubation at low temperature decreased the mean incubation temperature of eggs, but did not change the efficiency of yolk conversion into the tissues of developing embryos (Fig. 3). Yolk conversion efficiency remained similar to that observed in vendace embryos incubated at different constant temperatures (Luczynski et al., 1984). Correlated with the lowering of the incubation temperature is an increase in the length of the larvae which are hatching (Braum, 1967; Luczynski et al., 1984). Longer larvae are expected to be stronger, better swimmers (Braum, 1967; Hoagman, 1974), less susceptible to damage and less susceptible to predation (Blaxter, 1969). Hatching of vendace embryos should be induced on the earliest possible data, thus securing suitable environmental conditions for stocked larvae. Excessive delay of hatching would shorten the first growing season of fish. In addition, as hatching is further delayed, the lake water temperature rises quickly. Too high a temperature of the hatchery water supply increases the mortality of acclimated embryos during hatching (sample A3 in Table I). In stocked lakes a period of high temperatures (exceeding 17-20°C) can also occur (Luczynski, 1981) and this too could cause larval mortality (McCormick et al., 1971), because during the first weeks of life Coregoninae larvae tend to remain close to the water surface, even when temperatures are unfavourably high (Hoagman, 1974). ACKNOWLEDGEMENTS
The investigation was financed by the Inland Fisheries Institute in Olsztyn, Poland. I am grateful to the State Fishery Farms in Olsztyn and Pasym for assistance in acquiring vendace spawners, and to the staff of Janowo hatchery for their kind assistance during the field observations. Thanks also go to Dr
109
K. Dabrowski, Institute of Ichthyobiology critically reading the manuscript.
and
Fisheries,
Olsztyn,
for
REFERENCES Bernatowicz, S., 1953. Distribution and growth of the small whitefish (Coregonus albula L.) in Mazurian lakes on the background of the environmental conditions. Rocz. Nauk Roln., 67: l-17 (in Polish). Bernatowicz, S., 1960. Observations on stocking lakes with a high hydrogen sulfide ceiling with vendace (Coregonus albulu L.). Rocz. Nauk Roln., 75: 272-280 (in Polish). Bernatowicz, S., Dembinski, W. and Radziej, J., 1975. Vendace. PWRiL Warszawa, 284 pp. (in Polish). Bidgood, B.F., 1974. Reproductive potential of two lake whitefish (Coregonus clupeaformis) populations. J. Fish. Res. Board Can., 31: 1631-1639. Blaxter, J.H.S., 1969, Development: eggs and larvae. In: W.S. Hoar and D.J. Randall (Editors), Fish Physiology, Volume III: Reproduction and Growth. Bioluminescence, Pigments, and Poisons. Academic Press, New York and London, pp. 177-252. Blaxter, J.H.S. and Hempel, G., 1963. The influence of egg size on herring larvae. J. Cons. Int. Explor. Mer, 228: 211-240. Braum, E., 1967. The survival of fish larvae with reference to their feeding behaviour and the food supply. In: S.D. Gerking (Editor), The Biological Basis of Freshwater Fish Production, Blackwell Scientific Publications, Oxford, pp. 113-131. Christie, W.J., 1963. Effects of artificial propagation and the weather on recruitment in the Lake Ontario whitefish fishery. J. Fish. Res. Board Can., 20: 597-646. Colby, P.J. and Brooke, L.T., 1970. Survival and development of lake herring (Coregonus artedii) eggs at various incubation temperatures, In: C.C. Lindsey and C.S. Woods (Editors), Biology of Coregonid Fishes. University of Manitoba Press, Winnipeg, Man., pp. 417-428. Colby, P.J. and Brooke, L.T., 1973. Effects of temperature on embryonic development of lake herring (Coregonus artedii). J. Fish. Res. Board Can., 30: 799-810. Dabrowski, B. and Eichler, M., 1972. The effects of the course of temperatures on fluctuations of Coregonus albula L. production. Rocz. Nauk Roln., 94 (2): 59-74 (in Polish). Dabrowski, K., 1976. An attempt to determine the survival time for starving fish larvae. Aquaculture, 8: 189-193. Einsele, W., 1965. Problems of fish-larvae survival in nature and the rearing of economically important middle European freshwater fishes. Calif. Coop. Oceanic Fish. Invest. Rep., 10: 24-30. Eltsova, V.N., 1976. Mortality of hatchery-reared larval omul Coregonus autumnalis migratorius (Georgi) in Lake Baikal due to predation by common fish species. Vopr. Ikhtiol., 16: 1069-1074 (in Russian). Faber, D.J., 1970. Ecological observations on newly hatched lake whitefish in South Bay, Lake Huron. In: C.C. Lindsey and C.S. Woods (Editors), Biology of Coregonid Fishes. University of Manitoba Press, Winnipeg, Man., pp. 481-500. Fabricius, E. and Lindroth, A., 1954. Experimental observations on the spawning of whitefish, Coregonus lavaretus L., in the stream aquarium of the Hijlle laboratory at River Indalsalven. Rep. Inst. Freshwater Res. Drottningholm, 35: 105-112. Fritzsche, S. and Taege, M., 1979. Schiidigungen bei der Aufzucht von Fischbrut (Hypophthalmichthys molitrix, Cyprinus carpio und Coregonus albula) in industriem%sigen Produktionsanlagen durch starkes Vorkommen von Ruderfusskrebsen (Copepoda). Z. Binnenfischerei DDR, 26: 304-308. Grim, J., 1951. Kannibalismus bei Blaufelchen und seine moglichen Folgen. Csterr. Fisch., 4: 165-171.
110 Grudniewski, C., 1980. Effect of starvation on hatch survivability of vendace (Coregonus albula L.), whitefish (Coregonus lavaretus L.) and Coregonuspeled Gmel. Zesz. Nauk. ART Olsztyn, 10: 1-38 (in Polish). Hartmann, J., 1980. Zur Jahrgangs- und Bestandsstarke von Felchen und Barsch des Bodensees. Fischwirt, 30: 3-4. Heming, T.A., McInerney, J.E. and Alderdice, D.F., 1982. Effect of temperature on initial feeding in alevins of chinook salmon (Oncorhynchus tshawytscha). Can. J. Fish. Aquat. Sci., 39: 1554-1562. Hoagman, W.J., 1973. The hatching, distribution, abundance, growth, and food of the larval whitefish (Coregonus c2upeaformis Mitchill) of Central Green Bay, Lake Michigan. Rep. Inst. Freshwater Res. Drottningholm, 53: l-20. Hoagman, W.J., 1974. Vital activity parameters as related to the early life history of larval and postlarval lake whitefish (Coregonus clupeaformis). In: J.H.S. Blaxter (Editor), The Early Life History of Fish. Springer-Verlag, Berlin, pp. 547-558. Houde, E.D., 1978. Critical food concentrations for larvae of three species of subtropical marine fishes. Bull. Mar. Sci., 28 (3): 395-411. Ishibashi, N., 1974. Feeding, starvation and weight changes of early fish larvae. In: J.H.S. Blaxter (Editor), The Early Life History of Fish Springer-Verlag, Berlin, pp. 339--344. Jezierska, B., Korwin-Kossakowski, M. and Jowko, G., 1978. Influence of starvation on whitefish (Coregonus lavaretus L.) larvae. Verh. Int. Verein. Limnol., 20: 2134-2136. Jezierska, B., Korwin-Kossakowski, M. and Jowko, G., 1979. The effect of starvation and temperature conditions on vendace (Coregonus albula L.) larvae. Pol. Arch. Hydrobiol., 26: 387-395. Klein, M., 1980. Die Situation der Renkenfischerei am Starnberger See. Fischer Teichwirt, 10. Korovina, V.M., Maximova, L.P., Lebedeva, L.I. and Burmakina, T.N., 1968. The dependende of growth and development of Coregonus lauaretus baeri Kessler larvae in terms of their transition to active feeding under various temperature conditions. Izv. Gos. NIORCh, 67: 136-164 (in Russian). Korovina, V.M., Lebedeva, L.I., Maximova, L.P. and Privodina, V.P., 1975. Feeding, growth and development of the chir larvae under different environmental conditions. Izv. Gos. NIORCh, 104: 162-179 (in Russian). Lapworth,.E.D., 1966. The effect of fry plantings on whitefish production in Eastern Lake Ontario. J. Fish. Res. Board Can., 13: 547-668. Laurence, G.C., 1977. A bioenergetic model for the analysis of feeding and survival pot tential of winter flounder, Pseudopleuronectes americanus, larvae during the period from hatching to metamorphosis. Fish. Bull., 76: 629-646. Lawler, G.H., 1965. Fluctuations in the success of year-classes of whitefish populations with special reference to Lake Erie. J. Fish. Res. Board Can., 22: 1197-1227. Leopold, M., Marciak, Z. and Backiel, T., 1970. Coregonid fishery in Poland. In: C.C. Lindsey and C.S. Woods (Editors), Biology of Coregonid Fishes. University of Manitoba Press, Winnipeg, Man., pp. 563-560. Lindstrom, T., 1962. Life history of whitefish young (Coregonus) in two lake reservoirs. Rep. Inst. Freshwater Res. Drottningholm, 44: 113-144. Loffler, H. and Deufel, J., 1980. Erfahrungen und Probleme bei der Felchenerbrtitung am Bodensee. Fischwirt, 30: 57-59. Luczynski, M., 1981. The influence of the incubation temperature of Coregonus albula L. eggs on the embryogenesis rate and on the survival and morphology of eleutheroembryos and larvae. Ph.D. Thesis, Inland Fisheries Institute, Olsztyn, Poland (in Polish). Luczynski, M., 1984a. Survival of Coregonus albula L. embryos incubated at different thermal conditions. Hydrobiologia (in press). Luczynski, M., 198413. A technique for delaying embryogenesis of vendace (Coregonus albula L.) eggs in order to synchronize mass hatching with optimal conditions for lake stocking. Aquaculture, 41: 113-117.
111 Luczynski, M. and Kirklewska, A., 1984. Dependence of Coregonus albula L. embryogenesis rate on the incubation temperature. Aquaculture (in press). Luczynski, M., Dlugosz, M., Szutkiewicz, B. and Kirklewska, A., 1984. The influence of the incubation temperature on the body length and the yolk sac volume of Coregonus albula L. eleutheroembryos. Acta Hydrochim. Hydrobiol. (in press). May, R.C., 1974. Larval mortality in marine fishes and the critical period concept. In: J.H.S. Blaxter (Editor), The Early Life History of Fish. Springer-Verlag, Berlin, pp. 3-19. McCormick, J.H., Jones, B.R. and Syrett, R.F., 1971. Temperature requirements for growth and survival of larval ciscos (Coregonus artedii). J. Fish. Res. Board Can., 28: 924-927. Niimann, W., 1953. Felchenerbriitung mit kiinstlich abgekiihltem Wasser im Umwiilzverfahren. Z. Fisch. (N.F.), 2: 83-92. Niimann, W., 1967. Ungewollte und gezielte Eingriffe in die Populationsdynamik der Blaufelchen. Arch. Fisch., 18: 12-24. Niimann, W., 1970. The “Blaufelchen” of Lake Constance (Coregonus wartmanni) under negative and positive influence of man. In: C.C. Lindsey and C.S. Woods (Editors), Biology of Coregonid Fishes. University of Manitoba Press, Winnipeg, Man., pp. 531552. Niimann, W. and QUOSS, H., 1972. Strudelwiirmer dezimieren den Felchenleich - Ursachen fiir die unterschiedlichen Fangertdge in der Blaufelchen-Fischerei des Bodensees. Fischwirt, 22: 25-27. Pokrovskii, V.V., 1961. Basic environmental factors determining the abundance of the whitefish. Tr. Soveshch. Ikhtiol. Kom. Acad. Nauk SSSR, 13: 228-234 (in Russian). Salojgrvi, K., 1982. Spawning ecology, larval food supplies and causes of larval mortality in the whitefish (Coregonus lavaretus L.). Pol. Arch. Hydrobiol., 29 (1): 159-178. Selgeby, J.H., MacCallum, W.R. and Swedberg, D.V., 1978. Predation by rainbow smelt (Osmerus mordax) on lake herring (Coregonus artedii) in Western Lake Superior. J. Fish. Res. Board Can., 35: 1457-1463. Shemeikka, P., Oksman, H. and Mikkola, H., 1978. On some factors affecting the survival of vendace (Coregonus albula) and whitefish (Coregonus lavaretus oxyrhynchus) eggs. Savon Luonto, 10: 23-39 (in Finnish with English summary). Volkova, L.A., 1976. The role of the school in the formation of defence reflexes in juvenile Coregonus autumnalis migratorius from Lake Baikal. Vopr. Ikhtiol., 16: 540-645 (in Russian). Wilkonska, H. and Zuromska, H., 1982. Effect of environmental factors on egg quality and the mortality of spawn in Coregonus albula (L.) and Coregonus lavaretus. Pol. Arch. Hydrobiol., 29: 123-157. Yocom, T.G. and Edsall, T.A., 1974. Effect of acclimation temperature and heat shock on vulnerability of fry of lake whitefish (Coregonus clupeaformis) to predation. J. Fish. Res. Board Can., 31: 1503-1506. Zuromska, H., 1982a. Egg mortality and its causes in Coregonus albula (L.) and Coregonus Zavaretus (L.) in two Mazurian lakes. Pol. Arch. Hydrobiol., 29: 29-69. Zuromska, H., 198215. Conditions of natural reproduction of Coregonus albula (I,.) and Coregonus lavaretus (L.). Pol. Arch. Hydrobiol., 29: l-28.
99 in The Netherlands
IMPROVEMENT IN THE EFFICIENCY OF STOCKING LAKES WITH LARVAE OF C~~G~~U~ ALBULA L. BY DELAYING HATCHING
M. LUCZYNSKI Institute of Ichthyobiology and Fisheries, Academy 1 O-957 Olsztyn-Kortowo, Bl. 37 (Poland) (Accepted
13 February
of Agriculture
and Technology,
1984)
ABSTRACT Luczynski, M., 1984. Improvement in the efficiency of stocking lakes with larvae of Coregonus albula L. by delaying hatching. Aquaculture, 41: 99-111. Low survival of vendace (Coregonus afbulu) eggs incubating on natural spawning grounds (due to silting, oxygen deficits and action of predators) is discussed as one of the reasons for the great ~uctuation in numbers of commercial fish. Incubation of eggs in hatcheries where the survival rate averages 60% could compensate for the great numerical disparity between the eggs spawned naturally and those taken for hatcheries. In the absence of the effects of mass stocking of lakes with vendace larvae, one can conclude that apparent mortality during the larval stage also determines the strength of the successive year-class. Vendace larvae are usually stocked immediately after the ice cover recedes from lakes. At this time they often face a period of cold weather when water temperature is low and food organisms are scarce. Poor environmental conditions cause slow growth of larvae. This increases the period of highest ~lnerability of the larvae to predation. It is possible to accelerate the growth of larvae by synchronizing the time of stocking with the development of favourable thermal conditions and food availability in lakes. In Polish climatic conditions the first 10 days of May seem to be a suitable time for stocking lakes. A technique for delaying vendace egg hatching by cooling the hatchery water has been developed. It enables hatching of vendace to be delayed from the beginning of April until the beginning of Nlay, thereby inducing mass hatching of larvae when lake conditions are optimal for stocking. INTRODUCTION
Vendace (Coregonus albula L.) is a common autochthonous species of considerable commercial value in Poland (Leopold et al,, 1970). For this reason it has been introduced more or less successfully into lakes of various limnological type (Bernatowicz, 1953,196O). In lakes where vendacepopulations are commercially exploited, the catches show great fluctuations (Dabrowski and Eichler, 1972). This is also commonly observed with other Coregoninae (Lapworth, 1956; Klein, 1980). Low survival of Coregoninae eggs developing in natural spawning grounds 0044-8486/84/$03.00
Q 1984 Elsevier Science Publishers B.V.
100
is one of the possible reasons for the appearance of weak year-classes. Adult whitefish were observed devouring eggs being dropped by other spawners (Fabricius and Lindroth, 1954). During the long incubation season great losses of eggs are caused by silting and low oxygen concentration as well as by fish (Eltsova, 1976) and invertebrate predators (Niimann and QUOSS, 1972; Shemeikka et al., 1978). During the past decade eutrophication of the Polish lakes which are natural spawning grounds for Coregoninae has made the environment for egg incubation much less favourable. The survival of C. albula eggs, formerly higher than 4%, now ranges from 0.0 to 2.6% (Zuromska, 1982a), and survival of C. Zauaretus dropped to nil in most lakes that were investigated (Wilkonska and Zuromska, 1982). Survival of Coregoninae eggs incubated in hatcheries averages over 60% (Liiffler and Deufel, 1980; Zuromska, 1982a). It was assumed that such high survival could compensate substantially for the initial great numerical disparity between the eggs spawned naturally and those taken for hatcheries. But contrary to expectations, mass stocking of lakes with Coregoninae larvae was not followed by the strong year-classes predicted (Lapworth, 1956; Christie, 1963; Lawler, 1965; Bernatowicz et al., 1975). It became evident that losses occurring after hatching were also important for the determination of the year-class strength (Einsele, 1965; Salojarvi, 1982). Coregoninae embryos usually hatch when water temperatures rise rapidly after the ice cover recedes from lakes (Faber, 1970; Colby and Brooke, 1973; Bidgood, 1974; Salojti, 1982). At this time a period of cold weather often occurs, and Coregoninae larvae face cold water and a scarcity of food. The newly hatched larvae are extremely susceptible to predation, both by fish (Grim, 1951; Yocom and Edsall, 1974; Eltsova, 1976; Selgeby et al., 1978) and by invertebrates (Fritzsche and Taege, 1979). Suitable thermal and nutritional conditions, which enable Coregoninae larvae to grow fast enough to escape the effect of predators (Lindstrijm, 1962), appear to determine larval survival and the subsequent development of a strong year-class. This was confirmed by the observation that an appropriate succession of suitable climatic conditions, i.e. cold winters followed by late and warm springs, during spawning, incubation and hatching, allowed high survival of -embryos and larvae and in consequence was correlated with the occurrence of strong year-classes (Christie, 1963; Lawler, 1965; Hartmann, 1980). Such beneficial climatic conditions occur only rarely in nature. In some whitefish hatcheries, eggs are incubated in water cooled to 1°C in order to delay hatching of embryos. Lakes are then stocked with larvae when water temperature and food concentration become optimal for their growth and survival (Niimann, 1953). Stocking Lake Constance with C. wartmanni larvae of delayed hatching has produced larger year-classes without an appreciable increase in number of fry stocked (Ntimann, 1967, 1970). This paper describes the possibilities of delaying hatching of vendace eggs when incubated in commercial hatcheries under Polish (or similar) climatic conditions.
101 MATERIALS
AND METHODS
Studies in 1977-l
978
Ripe spawners of C. albuZa gillnetted in Lake Kosno were stripped and eggs were fertilized on 2 December 1977. Surface water temperature was 2.5”C. Water-hardened eggs were transported to the laboratory of the Inland Fisheries Institute in Olsztyn, and incubate in 7-l glass incubation jars at constant temperatures of 1 and 2°C. When hatching began, samples of eggs were taken and placed into the incubation cylinders. The bottom and cover of the cylinders were made of ‘“Steelon” screen, and the cylinders were suspended vertically in glass incubation jars. Eggs in incubation cylinders were examined daily, and dead eggs, normally developed .and abnormal hatched larvae were removed and counted. Studies in 1978-1979 Freshly fertilized and water-hardened eggs of vendace from Lake Kobylocha were taken on 21 November 1978 and transported to Janowo hatchery. Samples of eggs were placed in incubation cylinders within incubation jars. A sample of eggs was transported to the laboratory and incubated in several incubation cylinders at l.l*C. On 30 November 1978, eggs from approximately 50 females from Lake Kosno were fertilized with sperm taken from about 10 males. Lake water temperature was 5.3”C. Two hours after fertilization a sample of eggs was transported to the laboratory and incubated in several incubation cylinders at l.l”C. Another sample of eggs was transported to Janowo hatchery and placed in incubation cylinders within incubation jars.
The temperature of water supplying the hatchery was measured, with an accuracy of 0.1% twice a day, and daily temperatures’were calculated as the mean value of these measurements. Embryonic development of eggs incubated in the hatchery was examined twice a week when dead eggs and hatching embryos were removed and counted. Course of em b~oge~es~ The course of C. albufu embryogenesis was determined by using the 16 developmental stages described by Luczynski and Kirklewska (1984). Stages DS 14, DS 15 and DS 16 were described as thehatching of 10, 50, and 90% of embryos, respectively. Stages DS l-DS 13 were determined under a binocular microscope. A group of eggs ‘was considered to have reached a
102
particular developmental stage when 50% or more of the embryos showed the characteristics of that stage. Laboratory experiments In the laboratory, water supplied to jars was re-circulated and sterilized by U.V. radiation (Luczynski, 1981). The water temperature was measured three times a day to an accuracy of 0,l”C and daily temperatu~ was calculated as the mean of these measurements. Mortalities were recorded and dead eggs were removed from all incubation cylinders daily. Eggs were considered dead when they were completely opaque. Incubation in c&bed water On 8 December 1978, when the water temperature in the hatchery decreased to l.O”C, samples of eggs were transported in vacuum bottles from the laboratory to the hatchery and were placed in incubation cylinders. In spring, when the water temperature in Janowo hatchery exceeded 1°C (22 March 1979), samples of eggs were transported in vacuum bottles from the hatchery to the laboratory and were incubated in incubation cylinders at 1.1%.
During both incubation seasons, while the eggs incubated at 1 (or 1.1) and 2°C were hatching, samples of eggs were taken from jars every few days and acclimated at a rate of 1.5”C h-l to the temperature of the lake water supplying the hatchery. The samples were then placed in incubation cylinders supplied with lake water. Hatching embryos were removed daily. Some of them were abno~~y developed (scoliosis, kyphosis, and lordosis - Colby and Brooke, 1970), but when none of these malformations could be seen with the naked eye, larvae were considered to be normally developed. Length of taPvaeand yolk sac dime~io~s Samples of n = 30 freshly hatched larvae were anaesthetized with MS 222 (1 : 10 000). Total length (1.t. ) of larvae and length (2) and height (h) of their yolk sac were measured under an ocular micrometer with an accuracy of 0.01 mm. Yolk sac volume (v) was calculated from the formula for a prolate spheroid: V = 0.5236
I h2
(Blaxter and Hempel, 1963).
103 RESULTS
Delaying of hatching In Polish Coregoninae hatcheries, water temperature decreases gradually in autumn when vendace spawn (Fig. 1A). During the winter (when lakes are covered with ice) the water remains at a constant low temperat~e (O.l0.2”C was observed in Janowo hatchery). In spring, when the ice melts, lake water temperature rises quickly and mass hatching of vendace occurs. The hatching of vendace embryos can be delayed by cooling the water at times when the hatchery water temperature is higher than l-2”C, i.e. during autumn and spring, There are three possibilities for water cooling: (a) in autumn, (b) in spring, and (c) in both autumn and spring. Fig. IB, C and D show the course of embryogenesis of vendace from Lake Kobylocha and Lake Kosno, when these three possibilities for water cooling were applied,’ For comparison Fig. 1A shows the course of embryogenesis of samples of these eggs incubated in the usual way in Janowo hatchery. There were no si~~i~~t differences in hatching time between batches of eggs incubated normally and those incubated in autumn in water cooled to l.l”C (Fig. 1A and 1B). However, cooling the water during spring significantly delayed hatching (Fig. 1C). There were no significant differences in hatching time between the eggs incubated in cooled water in spring and those incubated at 1,l”C in autumn and spring (Fig. 1C and 1D). batching
success
At temperatures ranging from 1 to 2°C hatching of C. atbuta embryos began late and lasted for a long time (DS 14-DS 16 on Fig. 1). Unfortunately, at these temperatures the percentage of normal hatch was very low and ranged from a few percent to about 50% (Fig. 2). Such great losses of eggs were avoided when the incubation temperature was raised (at a rate of 15°C h-‘) during the final period of incubation. This procedure caused immediate mass hatching of embryos, and the percentage of normal hatch usually exceeded 90% (Fig. 2). As the delay in hatching increased there was a trend toward a decrease in the percentage of normal larvae hatch~g from the batches of eggs successively acclimated to higher temperatures (Fig. 2). The combined effect of these two causes of egg mortality should be taken into consideration when planning the delay of hatching. Table I contains some examples of egg mortality observed during incubation and hatching when batches of eggs incubated in a traditional hatchery were compared with those subjected to a delay in hatching. In every case there was some “cost” of hatching delay, expressed in egg mortality. When hatching was delayed excessively, the third potential cause of egg mortality appeared. This was when the temperature of the lake water supplying the
16
4 2 0
0
16
8
12
6
8
4
4
2 0
0 NOV
DEC1979
JAN
FE8
MAR
APR
MAY
JUNErm
Fig. 1. Mean daily water temperatures and course of embryogenesis of C. albula eggs incubated in the normal way in Janowo hatchery (A), and in water cooled to l.l”C during autumn (B), spring (C), and autumn and spring (R). Egg fertilization is described as stage DS 0, whereas stages DS 14, DS 15, and DS 16 are 10, 50 and 90% of hatching, respec-
tively .
hatchery became too high for vendace embryos (13.5-16.O’C) and the acclimation caused additional mortality of eggs. As a result, the general survival of eggs from sample A3 (Table I) was only 42% compared with 76% (Al) in a traditional hatchery. On the other hand, a similar survival of 41% of eggs from Lake Kobylocha (B2 in Table I) was rather a good result when compared to 54% survival of eggs incubated traditionally (Bl in Table I).
105
Days of incubation Fig. 2. Course of hatching and survival of C. albula embryos incubated at constant temperatures of 2°C (A) and 1°C (B). Batches of eggs were successively acclimated to higher temperature (arrows show the day of acclimation). Temperature range during hatching is shown for every batch of eggs, as well as the percentage of normal hatch (in brackets).
TABLE I Thermal conditions of incubation and the survival of eggs of C. albula from Lake Rosno (A) and Lake Kobylocha (B) incubated in normal hatchery water and in artificially cooled water (incubation season 1978-1979) Date of 50% hatch Normal
Delayed
Temperature during hatching (“C)
Percentage of dead eggs and ab- Percentage of normal normal hatch appeared during hatch Incubation Hatching in hatchery
Al A2 A3
23 April -
-
Bl B2
12 April -
-
9 May 18 May 1 May
5.0- 7.3 10.0-11.0 13.5-16.0 3.59.0-
7.3 9.5
at 1S”C
19.6 17.0 17.0
3.0 5.5
4.5 9.0 35.5
76 71 42
41.0 40.5
13.5
5.0 5.0
54 41
106
Dimensions of ‘delayed ” larvae Fig. 3 shows the regression line describing the relationship between the yolk sac volume and the total length of freshly hatched vendace larvae from Lake Kosno, when incubated at constant temperatures ranging from 1.1 to 9.9”C (Luczynski et al., 1984). Points Al, A2 and A3 represent the respective data for vendace from Lake Kosno when incubated in a traditional hatchery (Al) and those delayed in hatching for 16 (A2) and 25 days (A3) (see Table I for identification of Al-A3). All points lie within 95% (Al and A2) and 99% (A3) tolerance limits of the regression line. 1.4
1.2
_
1.0 . -2 5 ’ 0.8 _
~
E 2 s 0.6 . : u) -Y g 0.4
y =-0.4743L+ .
5.1886
r q-0.98954
Total length of eleutheroembryos
\\ \ \ ‘\ ‘\
L (mm)
Fig. 3. Relationship between the yolk sac valume and total length of C. albulu eleutheroembryos incubated in the usual way (Al) and when hatching was delayed for 16 (A2) and 25 (A3) days (see Table I for Al-3 incubation conditions). Regression line with 95% and 99% tolerance limits was derived from the data obtained from the incubation of C. albula eggs at various constant temperatures (Luczynski et al., 1934).
DISCUSSION
Incubation of Coregoninae eggs in hatcheries allows for the possibility of controlling the environmental conditions related to embryogenesis in order to minimize egg mortality. It would be interesting to compare the influence of factors affecting survival of C. albula eggs developing on natural spawning grounds with their effect on eggs incubated in hatcheries.
107
Spawning of vendace usually begins around 10-20 November, with the culmination around 20-30 November when autumn homoiothermy occurs in Polish lakes (Zuromska, 1982b). Optimal conditions for successful spawning of Coregoninae occur when autumn temperatures steadily decrease (Lawler, 1965). Pokrovskii (1961) stated that the course of water temperature at the time of spawning exercised an influence on the quantity of fertile eggs of C. albula which varied from year to year between 100~-80% and 50-35%. (In one year it was only lo%.) Low fertility was attributed to a long drawn-out autumn in which the males leave the spawning grounds before the optimum spawning temperature for females is reached. By employing the dry method of vendace egg fertilization it is always possible to obtain a sufficient quantity of sperm to fertilize all the stripped eggs. It can be expected that the losses of hatchery-incubated eggs due to lack of fertilization should be very low. Eggs developing in a hatchery are protected against predators, silting, oxygen deficits, etc. In spring time the water temperature in the hatchery rises similarly to that in lakes when the ice melts, and thereby creates suitable conditions for hatching of normally developed, viable larvae (Luczynski, 1984a). Therefore, by incubating Coregoninae eggs in hatcheries one avoids the majority of harmful environmental factors which cause mass egg mortality on natural spawning grounds. In fact, the survival of vendace eggs incubated in hatcheries equals 54 and 76% (this study) or from 62 to 94% (Zuromska, 1982a) which is many times higher than egg survival observed on natural spawning grounds. However, we still face the mortality occurring during the larval and fry stages of the life of Coregoninae. Mortality rates during the early life stages are usually the highest and most variable from year to year. Changes in the mortality rate during these life stages have a major effect on the final numbers of commercial fish. Very often Coregoninae larvae are stocked into lakes during a period of cold and windy weather, when water temperature remains low and food organisms are scarce. Larvae have sufficient yolk material to tide them over temporary periods of food shortage (Hoagman, 1973; Dabrowski, 1976; Jezierska et al., 1978, 1979; Grudniewski, 1980), but in poor thermal and food conditions their growth rate is low. This substantially lengthens the period when larvae are most vulnerable to predation and other unfavourable environmental factors (Lindstrijm, 1962; May, ,1974; Volkova, 1976). Niimann (1970) confirmed the significance of a high growth rate during the first year of life of Coregoninae for the subsequent development of large year-classes. He stated that small C. wartmanni year-classes grew slowly in the first year of life and that large classes grew quickly during their first year. The technique for delaying C. albula hatching synchronizes stocking of larvae with favourable thermal and food conditions in lakes. Experiments with larvae of different species showed that higher temperatures after
108
hatching increase the feeding activity of larvae and they begin external feeding earlier in ontogeny (Braum, 1967; Korovina et al., 1968; Ishibashi, 1974). At higher temperatures the utilization of food for growth also begins earlier (Heming et al., 1982) and the instantaneous growth rate of larvae markedly increases (McCormick et al., 1971). At higher food concentrations gross growth efficiency is higher (Laurence, 1977) and fish larvae grow faster (Korovina et al., 1975; Houde, 1978). Larval survival increases at higher food concentrations even more significantly than does the growth rate (Laurence, 1977). This suggests that stocking of lakes at a time when the water gets warmer and the concentration of food organisms becomes optimal should result in fast growth and high survival of vendace larvae. In Polish climatic conditions the first 10 days of May are believed to be the optimal time for stocking lakes with vendace larvae (Luczynski, 1981). The details of the technique for delaying vendace mass hatching until the beginning of May are described in a separate paper (Luczynski, 1984b). Delay of vendace hatching due to prolonged incubation at low temperature decreased the mean incubation temperature of eggs, but did not change the efficiency of yolk conversion into the tissues of developing embryos (Fig. 3). Yolk conversion efficiency remained similar to that observed in vendace embryos incubated at different constant temperatures (Luczynski et al., 1984). Correlated with the lowering of the incubation temperature is an increase in the length of the larvae which are hatching (Braum, 1967; Luczynski et al., 1984). Longer larvae are expected to be stronger, better swimmers (Braum, 1967; Hoagman, 1974), less susceptible to damage and less susceptible to predation (Blaxter, 1969). Hatching of vendace embryos should be induced on the earliest possible data, thus securing suitable environmental conditions for stocked larvae. Excessive delay of hatching would shorten the first growing season of fish. In addition, as hatching is further delayed, the lake water temperature rises quickly. Too high a temperature of the hatchery water supply increases the mortality of acclimated embryos during hatching (sample A3 in Table I). In stocked lakes a period of high temperatures (exceeding 17-20°C) can also occur (Luczynski, 1981) and this too could cause larval mortality (McCormick et al., 1971), because during the first weeks of life Coregoninae larvae tend to remain close to the water surface, even when temperatures are unfavourably high (Hoagman, 1974). ACKNOWLEDGEMENTS
The investigation was financed by the Inland Fisheries Institute in Olsztyn, Poland. I am grateful to the State Fishery Farms in Olsztyn and Pasym for assistance in acquiring vendace spawners, and to the staff of Janowo hatchery for their kind assistance during the field observations. Thanks also go to Dr
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K. Dabrowski, Institute of Ichthyobiology critically reading the manuscript.
and
Fisheries,
Olsztyn,
for
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