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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 (1934) 113-11’7 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
113
A TECHNIQUE FOR DELAYING EMBRYOGENESIS OF VENDACE (COREGONUS ALBULA L.) EGGS IN ORDER TO SYNCHRONIZE MASS HATCHING WITH OPTIMAL CONDITIONS FOR LAKE STOCKING M. LUCZYNSKI Institute of Ichthyobiology 1 O-957 Olsztyn-Kortowo, (Accepted
and Fisheries, Academy BE. 37 (Poland)
of Agriculture
and Technology,
13 February 1984)
ABSTRACT Luczynski, M., 1984. 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. To improve the efficiency of stocking lakes with vendace (Coregonus albula) larvae, a technique for delaying egg hatching was developed. This synchronizes mass hatching with the development of suitable thermal conditions and food availability in the lake. Water temperatures were measured in Polish hatcheries and a mathematical model of the dependence of C. albula embryogenesis on incubation temperature was utilized to predict the possibilities of delaying egg hatching. Vendace hatching can be delayed by cooling the circulation water (to 1 or 2°C) during March and April. At the beginning of May, when lake conditions are optimal for stocking, the incubation temperature should be raised (at a rate of 15°C h-‘) to that of the water supply. This ensures mass hatching of strong, normally developed larvae within 2-3 days. This technique facilitates both the delay of vendace hatching at minimal cost and the production of larvae on demand for lake stocking.
INTRODUCTION
Eutrophication of Polish lakes in the last 10 years means that conditions are becoming less favourable for incubation of coregonid eggs (Wilkonska and Zuromska, 1982; Zuromska, 1982), with the result that the continued presence of these species in the lakes will depend on introduced stocks from hatcheries (Szczerbowski, 1977). Coregoninae embryos hatch in spring immediately when the ice cover disappears from lakes. At this time a period of cold weather often occurs, and the larvae stocked into the lakes are faced with low temperatures and a scarcity of food. This causes slow growth and poor survival of larvae (Luczynski, 1984). Incubation of Coregoninae eggs at low temperature (l-2°C) delays hatch-
0044-8486/84/$03.00
0 1984 Elsevier Science Publishers B.V.
114
ing until both rising water temperature and density of zooplankton become conducive to fast growth of stocked larvae (Niimann, 1953). Evidence from Lake Constance suggests that introduction of the “cold method” has produced larger year-classes without any appreciable increase in number of fry stocked (Niimann, 1967,197O). In Polish climatic conditions the first 10 days of May appear to be the right time for stocking lakes with vendace larvae (Luczynski, 1981), and this paper describes a technique for delaying hatching of C. albula eggs until the beginning of May. MATERIALS
AND METHODS
Water temperatures, used in a mathematical model presenting the technique of delaying hatching, were observed in a commercial Coregoninae hatchery in Janowo (Pasym, Poland) during the incubation season of 1978-1979. Water temperature was measured twice a day with an accuracy of O.l”C, and daily temperature was calculated as the mean value of these two measurements. For estimating the progress of vendace embryogenesis to developmental stage DS 14 (10% hatch) I used the mathematical model of the dependence of C. albulu embryogenesis rate on the incubation temperature (Luczynski and Kirklewska, 1984). RESULTS
AND DISCUSSION
Determining the time of delayed hatching In Polish Coregoninae hatcheries water temperature decreases gradually in autumn(when vendace spawn) and remains low (1°C or less) during winter. In spring, at the time of the ice melting, water temperature rises quickly and mass hatching of vendace occurs. In different hatcheries and incubation seasons the time of vendace hatching varies from about 20-31 March to lo-20 April. To obtain vendace larvae at the beginning of May, one should cool the hatchery water during spring to the temperature which causes sufficient delay of hatching. It is possible to delay hatching by cooling the circulation water from the time when its temperature exceeds, 1°C until the first 10 days of May (Luczynski, 1984). If hatching of vendace embryos occurs at temperatures of 1-2” C, the percentage of normal hatch is very low, ranging from a few percent to about 50%. Such great losses of eggs can be avoided if the embryos incubated at 1-2” C are acclimated to a higher temperature (that of the water supply) several days before they reach developmental stage DS 14 (10% hatching). This procedure results in mass hatching of viable larvae within the following 2-3 days (Luczynski, 1984). As a consequence, the temperature of cooled water must be selected in
115
order for developing embryos to achieve DS 14 by about 10 May. Acclimation of eggs to a higher temperature one week before the predicted attainment of DS 14 should ensure a high percentage of normal hatch (Luczynski, 1984). As mass hatching occurs about 2-3 days after the acclimation date, this procedure should provide stocking material of vendace on about 5 May. Application of the model of rate of embryogenesis The mathematical model of the dependence of the rate of embryogenesis of C. aEbuZaon the incubation temperature (Luczynski and Kirklewska, 1984) allows computations which predict the time at which vendace embryos will attain DS 14. It is possible to include in the programme the duration of cooling of hatchery water as well as the temperature of cooled water required to produce the desired delay of hatching. Time of attainment of DS 14 by vendace embryos is inversely related to incubation temperature (Luczynski and Kirklewska, 1984). The relation between the mean daily water temperature (x) and the rate of development (DR 14) was described by the equation: DR 14 = (0.005212)
(1.1624”)
(1.OO12XZ).
Table I presents the values for the daily progress of C. albula embryogenesis during the period from egg fertilization (DS 0) to 10% hatching (DS 14). The values expressed as % day-’ were calculated from the DR 14 equation for the mean daily water temperature ranging from 0.1 to 9.9”C. Table II gives an example of the application of the model for predicting the developmental progress towards DS 14 by a hypothetical batch of vendate eggs, fertilized on 25 November 1978, and incubated in Janowo hatchery until the water temperature exceeded 1°C. On the day of egg fertilization, water temperature in the hatchery was 4.0°C. During that day embryos progressed 0.97% of the embryogenesis TABLE
I
Rate of development (% day-‘)zof eggs of C. albula, based on the formula: DR 14 = (0.005212) (1.1624x) (l.OOIZX ). Incubation period from fertilization to time of 10% hatch. The data referring to the temperatures ranging from 0.1 to 0.9”C were extrapolated “C
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
0
-
1 2 3 4 5 6 7 8 9
0.61 0.71 0.83 0.97 1.14 1.34 1.59 1.88 2.23
0.53 0.62 0.72 0.84 0.99 1.16 1.37 1.62 1.91 2.27
0.54 0.63 0.73 0.85 1.00 1.18 1.39 1.64 1.95 2.31
0.55 0.64 0.74 0.87 1.02 1.20 1.41 1.67 1.98 2.35
0.55 0.64 0.75 0.88 1.04 1.22 1.44 1.70 2.01 2.39
0.56 0.66 0.77 0.90 1.05 1.24 1.46 1.73 2.05 2.44
0.57 0.67 0.78 0.91 1.07 1.26 1.49 1.76 2.08 2.48
0.58 0.68 0.79 0.93 1.09 1.28 1.51 1.79 2.12 2.52
0.59 0.69 0.80 0.94 1.10 1.30 1.54 1.82 2.16 2.57
0.60 0.70 0.81 0.96 1.12 1.32 1.56 1.85 2.20 2.61
116 TABLE II Prediction of the developmental progress toward DS 14 (10% hatch) of a hypothetical batch of C. albula eggs. It was assumed that eggs were fertilized on 25 November 1978 (peak of spawning) and then incubated in Janowo hatchery, until the water temperature exceeded 1°C on 22 March 1979 Temperature (“C)
Date
1978 25 26 27 28 29
Nov. Nov. Nov. Nov. Nov.
1979 20 March 21 March 22 March
Percentage progress towards development On the given day
Cumulative
0.97 0.97
0.97
4.0 4.0 4.0 4.0 3.6
0.97 0.91
1.94 2.91 3.88 4.79
0.5 0.8 1.2
0.56 0.59 0.63
67.64 68.23 68.86
0.97
period toward the stage of 10% hatch (DS 14) (Table I). During the next day, 26 November 1978, the mean daily incubation temperature was also 4.O”C. Hence, during that day, the embryos progressed another 0.97% of the embryogenesis period DS 0-DS 14. The sum of these values of developmental progress (25 and 26 November) was 1.94%. This was the part of the embryogenesis period DS 0-DS 14 reached during the first 2 days of incubation. Taking into consideration the values chosen from Table I in accordance with mean temperatures of the successive days of incubation, the computation was continued until 22 March 1979 when the water temperature in the hatchery exceeded l.O”C. On that day the progress of embryonic development toward DS 14 reached 68.86%. To attain the stage of 10% hatch the embryos should pass another: 100% - 68.86% = 31.14% of the embryogenesis period DS 0-DS 14. As the date of DS 14 was planned for 10 May 1979, the embryos should be incubated in cooled water for 49 days, i.e. from 22 March until 10 May. Assuming that they would be incubated at constant temperature, daily progress of the development of embryos should equal: 31.14%/49 days =.0.64% day-l. According to data in Table I, the rate of C. aZbuZudevelopment equal to 0.64% day-l occurs at a water temperature of 1.3”C. Therefore, to obtain mass hatching on 5 May 1979, water should be cooled to 1.3”C from 22 March until 3 May 1979. Following acclimation of vendace eggs to the observed ‘temperature of the water supply on 3 May 1979, mass hatching of viable larvae should occur on about 5 May 1979.
117 CONCLUSIONS
Cooling of hatchery water in spring appeared to be sufficient to produce the desired delay in hatching of vendace larvae. Consequently it seems to be unnecessary to cool the water in autumn. This is an advantage because the difficult acclimation of vendace eggs spawned in autumn can thereby be eliminated. The comparatively short time of water cooling makes this technique cheaper than the similar one utilized to delay hatching of C. wartmanni at Lake Constance and Lake Starnberg. These lakes are not usually covered with ice, and the water temperature remains at about 4°C during the whole winter. In these conditions sufficient delay of C. wartmanni hatching is possible due to cooling of water (to about 1°C) during the whole incubation season (Niimann, 1970; J. Fliichter, Bavarian State Fisheries Institute, personal communication, 1981). Induction of vendace mass hatching by means of,acclimation of batches of eggs from l-2°C to higher temperatures provides the possibility of obtaining the desired number of larvae on the chosen day on which stocking of a given lake with the scheduled number of larvae was planned. ACKNOWLEDGEMENTS
The investigation was financed by the Inland Fisheries Institute in Olsztyn, Poland. I thank Dr. K. Dabrowski, Institute of Ichthyobiology and Fisheries in Olsztyn, for critically reading the manuscript. REFERENCES 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., 1984. Improvement in the efficiency of stocking lakes with larvae of Coregonus albula L. by delaying hatching. Aquaculture, 41: 99-111. Luczynski, M. and Kirklewska, A., 1984. Dependence of Coregonus albula TX, embryogenesis rate on the incubation temperature. Aquaculture (in press). Niimann, W., 1953. Felchenerbriitung mit kiinstlich abgekiihltem Wasser im Umwllzverfahren. 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. 531-552. Szczerbowski, J., 1977. Effectiveness of stocking lakes with lake whitefish. Rocz. Nauk Roln., 98(2): 117-133 (in Polish). Wilkonska, H. and Zuromska, H., 1982. Effect of environmental factors on egg quality and the mortality of spawn in Coregonus albula (L!) and Coregonus lauaretus (L.). Pol. Arch. Hydrobiol., 29: 123-157. Zuromska, H., 1982. Egg mortality and its causes in Coregonus albula (L.) and Coregonus lauaretus (L.) in two Mazurian lakes. PoI. Arch. Hydrobiol., 29: 29-69.
113
A TECHNIQUE FOR DELAYING EMBRYOGENESIS OF VENDACE (COREGONUS ALBULA L.) EGGS IN ORDER TO SYNCHRONIZE MASS HATCHING WITH OPTIMAL CONDITIONS FOR LAKE STOCKING M. LUCZYNSKI Institute of Ichthyobiology 1 O-957 Olsztyn-Kortowo, (Accepted
and Fisheries, Academy BE. 37 (Poland)
of Agriculture
and Technology,
13 February 1984)
ABSTRACT Luczynski, M., 1984. 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. To improve the efficiency of stocking lakes with vendace (Coregonus albula) larvae, a technique for delaying egg hatching was developed. This synchronizes mass hatching with the development of suitable thermal conditions and food availability in the lake. Water temperatures were measured in Polish hatcheries and a mathematical model of the dependence of C. albula embryogenesis on incubation temperature was utilized to predict the possibilities of delaying egg hatching. Vendace hatching can be delayed by cooling the circulation water (to 1 or 2°C) during March and April. At the beginning of May, when lake conditions are optimal for stocking, the incubation temperature should be raised (at a rate of 15°C h-‘) to that of the water supply. This ensures mass hatching of strong, normally developed larvae within 2-3 days. This technique facilitates both the delay of vendace hatching at minimal cost and the production of larvae on demand for lake stocking.
INTRODUCTION
Eutrophication of Polish lakes in the last 10 years means that conditions are becoming less favourable for incubation of coregonid eggs (Wilkonska and Zuromska, 1982; Zuromska, 1982), with the result that the continued presence of these species in the lakes will depend on introduced stocks from hatcheries (Szczerbowski, 1977). Coregoninae embryos hatch in spring immediately when the ice cover disappears from lakes. At this time a period of cold weather often occurs, and the larvae stocked into the lakes are faced with low temperatures and a scarcity of food. This causes slow growth and poor survival of larvae (Luczynski, 1984). Incubation of Coregoninae eggs at low temperature (l-2°C) delays hatch-
0044-8486/84/$03.00
0 1984 Elsevier Science Publishers B.V.
114
ing until both rising water temperature and density of zooplankton become conducive to fast growth of stocked larvae (Niimann, 1953). Evidence from Lake Constance suggests that introduction of the “cold method” has produced larger year-classes without any appreciable increase in number of fry stocked (Niimann, 1967,197O). In Polish climatic conditions the first 10 days of May appear to be the right time for stocking lakes with vendace larvae (Luczynski, 1981), and this paper describes a technique for delaying hatching of C. albula eggs until the beginning of May. MATERIALS
AND METHODS
Water temperatures, used in a mathematical model presenting the technique of delaying hatching, were observed in a commercial Coregoninae hatchery in Janowo (Pasym, Poland) during the incubation season of 1978-1979. Water temperature was measured twice a day with an accuracy of O.l”C, and daily temperature was calculated as the mean value of these two measurements. For estimating the progress of vendace embryogenesis to developmental stage DS 14 (10% hatch) I used the mathematical model of the dependence of C. albulu embryogenesis rate on the incubation temperature (Luczynski and Kirklewska, 1984). RESULTS
AND DISCUSSION
Determining the time of delayed hatching In Polish Coregoninae hatcheries water temperature decreases gradually in autumn(when vendace spawn) and remains low (1°C or less) during winter. In spring, at the time of the ice melting, water temperature rises quickly and mass hatching of vendace occurs. In different hatcheries and incubation seasons the time of vendace hatching varies from about 20-31 March to lo-20 April. To obtain vendace larvae at the beginning of May, one should cool the hatchery water during spring to the temperature which causes sufficient delay of hatching. It is possible to delay hatching by cooling the circulation water from the time when its temperature exceeds, 1°C until the first 10 days of May (Luczynski, 1984). If hatching of vendace embryos occurs at temperatures of 1-2” C, the percentage of normal hatch is very low, ranging from a few percent to about 50%. Such great losses of eggs can be avoided if the embryos incubated at 1-2” C are acclimated to a higher temperature (that of the water supply) several days before they reach developmental stage DS 14 (10% hatching). This procedure results in mass hatching of viable larvae within the following 2-3 days (Luczynski, 1984). As a consequence, the temperature of cooled water must be selected in
115
order for developing embryos to achieve DS 14 by about 10 May. Acclimation of eggs to a higher temperature one week before the predicted attainment of DS 14 should ensure a high percentage of normal hatch (Luczynski, 1984). As mass hatching occurs about 2-3 days after the acclimation date, this procedure should provide stocking material of vendace on about 5 May. Application of the model of rate of embryogenesis The mathematical model of the dependence of the rate of embryogenesis of C. aEbuZaon the incubation temperature (Luczynski and Kirklewska, 1984) allows computations which predict the time at which vendace embryos will attain DS 14. It is possible to include in the programme the duration of cooling of hatchery water as well as the temperature of cooled water required to produce the desired delay of hatching. Time of attainment of DS 14 by vendace embryos is inversely related to incubation temperature (Luczynski and Kirklewska, 1984). The relation between the mean daily water temperature (x) and the rate of development (DR 14) was described by the equation: DR 14 = (0.005212)
(1.1624”)
(1.OO12XZ).
Table I presents the values for the daily progress of C. albula embryogenesis during the period from egg fertilization (DS 0) to 10% hatching (DS 14). The values expressed as % day-’ were calculated from the DR 14 equation for the mean daily water temperature ranging from 0.1 to 9.9”C. Table II gives an example of the application of the model for predicting the developmental progress towards DS 14 by a hypothetical batch of vendate eggs, fertilized on 25 November 1978, and incubated in Janowo hatchery until the water temperature exceeded 1°C. On the day of egg fertilization, water temperature in the hatchery was 4.0°C. During that day embryos progressed 0.97% of the embryogenesis TABLE
I
Rate of development (% day-‘)zof eggs of C. albula, based on the formula: DR 14 = (0.005212) (1.1624x) (l.OOIZX ). Incubation period from fertilization to time of 10% hatch. The data referring to the temperatures ranging from 0.1 to 0.9”C were extrapolated “C
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
0
-
1 2 3 4 5 6 7 8 9
0.61 0.71 0.83 0.97 1.14 1.34 1.59 1.88 2.23
0.53 0.62 0.72 0.84 0.99 1.16 1.37 1.62 1.91 2.27
0.54 0.63 0.73 0.85 1.00 1.18 1.39 1.64 1.95 2.31
0.55 0.64 0.74 0.87 1.02 1.20 1.41 1.67 1.98 2.35
0.55 0.64 0.75 0.88 1.04 1.22 1.44 1.70 2.01 2.39
0.56 0.66 0.77 0.90 1.05 1.24 1.46 1.73 2.05 2.44
0.57 0.67 0.78 0.91 1.07 1.26 1.49 1.76 2.08 2.48
0.58 0.68 0.79 0.93 1.09 1.28 1.51 1.79 2.12 2.52
0.59 0.69 0.80 0.94 1.10 1.30 1.54 1.82 2.16 2.57
0.60 0.70 0.81 0.96 1.12 1.32 1.56 1.85 2.20 2.61
116 TABLE II Prediction of the developmental progress toward DS 14 (10% hatch) of a hypothetical batch of C. albula eggs. It was assumed that eggs were fertilized on 25 November 1978 (peak of spawning) and then incubated in Janowo hatchery, until the water temperature exceeded 1°C on 22 March 1979 Temperature (“C)
Date
1978 25 26 27 28 29
Nov. Nov. Nov. Nov. Nov.
1979 20 March 21 March 22 March
Percentage progress towards development On the given day
Cumulative
0.97 0.97
0.97
4.0 4.0 4.0 4.0 3.6
0.97 0.91
1.94 2.91 3.88 4.79
0.5 0.8 1.2
0.56 0.59 0.63
67.64 68.23 68.86
0.97
period toward the stage of 10% hatch (DS 14) (Table I). During the next day, 26 November 1978, the mean daily incubation temperature was also 4.O”C. Hence, during that day, the embryos progressed another 0.97% of the embryogenesis period DS 0-DS 14. The sum of these values of developmental progress (25 and 26 November) was 1.94%. This was the part of the embryogenesis period DS 0-DS 14 reached during the first 2 days of incubation. Taking into consideration the values chosen from Table I in accordance with mean temperatures of the successive days of incubation, the computation was continued until 22 March 1979 when the water temperature in the hatchery exceeded l.O”C. On that day the progress of embryonic development toward DS 14 reached 68.86%. To attain the stage of 10% hatch the embryos should pass another: 100% - 68.86% = 31.14% of the embryogenesis period DS 0-DS 14. As the date of DS 14 was planned for 10 May 1979, the embryos should be incubated in cooled water for 49 days, i.e. from 22 March until 10 May. Assuming that they would be incubated at constant temperature, daily progress of the development of embryos should equal: 31.14%/49 days =.0.64% day-l. According to data in Table I, the rate of C. aZbuZudevelopment equal to 0.64% day-l occurs at a water temperature of 1.3”C. Therefore, to obtain mass hatching on 5 May 1979, water should be cooled to 1.3”C from 22 March until 3 May 1979. Following acclimation of vendace eggs to the observed ‘temperature of the water supply on 3 May 1979, mass hatching of viable larvae should occur on about 5 May 1979.
117 CONCLUSIONS
Cooling of hatchery water in spring appeared to be sufficient to produce the desired delay in hatching of vendace larvae. Consequently it seems to be unnecessary to cool the water in autumn. This is an advantage because the difficult acclimation of vendace eggs spawned in autumn can thereby be eliminated. The comparatively short time of water cooling makes this technique cheaper than the similar one utilized to delay hatching of C. wartmanni at Lake Constance and Lake Starnberg. These lakes are not usually covered with ice, and the water temperature remains at about 4°C during the whole winter. In these conditions sufficient delay of C. wartmanni hatching is possible due to cooling of water (to about 1°C) during the whole incubation season (Niimann, 1970; J. Fliichter, Bavarian State Fisheries Institute, personal communication, 1981). Induction of vendace mass hatching by means of,acclimation of batches of eggs from l-2°C to higher temperatures provides the possibility of obtaining the desired number of larvae on the chosen day on which stocking of a given lake with the scheduled number of larvae was planned. ACKNOWLEDGEMENTS
The investigation was financed by the Inland Fisheries Institute in Olsztyn, Poland. I thank Dr. K. Dabrowski, Institute of Ichthyobiology and Fisheries in Olsztyn, for critically reading the manuscript. REFERENCES 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., 1984. Improvement in the efficiency of stocking lakes with larvae of Coregonus albula L. by delaying hatching. Aquaculture, 41: 99-111. Luczynski, M. and Kirklewska, A., 1984. Dependence of Coregonus albula TX, embryogenesis rate on the incubation temperature. Aquaculture (in press). Niimann, W., 1953. Felchenerbriitung mit kiinstlich abgekiihltem Wasser im Umwllzverfahren. 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. 531-552. Szczerbowski, J., 1977. Effectiveness of stocking lakes with lake whitefish. Rocz. Nauk Roln., 98(2): 117-133 (in Polish). Wilkonska, H. and Zuromska, H., 1982. Effect of environmental factors on egg quality and the mortality of spawn in Coregonus albula (L!) and Coregonus lauaretus (L.). Pol. Arch. Hydrobiol., 29: 123-157. Zuromska, H., 1982. Egg mortality and its causes in Coregonus albula (L.) and Coregonus lauaretus (L.) in two Mazurian lakes. PoI. Arch. Hydrobiol., 29: 29-69.