V. Pruuki and M. Pursiainen
Evo Inland Fisheries and Aquaculture Research Station
Finnish Game and Fisheries Research Institute
Over a period of several years the native whitefish (Coregonus muksun (Pallas)) and the introduced Siberian whitefish (C. peled (Gmelin)), were stocked in two small forest lakes in Evo in southern Finland. The stocking density was circa 100 individuals/ha and stocking size was 6–14 cm (one-summer old). In both lakes, natural reproduction of whitefish had not been observed. The evolution of the stocked whitefish populations were followed using catch statistics, trial fishings and marking-recapture.
Coregonus peled grew faster than C. muksun up to three-four years of age. The growth of whitefish stocked after the first stocking deteriorated year by year. The stockings created such a dense whitefish population that the amount of food became a limiting factor for growth.
In both lakes the instantaneous rate of natural mortality (M) was lower for one-four year old whitefish than for other age groups. The considerably higher instantaneous rate of natural mortality (M) for Lake Rahtijarvi (M = 0.5 – 0.8 for one-four year old whitefish) compared to Lake Valkea-Mustajarvi (M = 0.1 – 0.4 for one-four year old whitefish) caused primarily by the large number of predators in Lake Rahtijarvi, especially pike (Esox lucius L.).
After five years the instantaneous rate of natural mortality (M) for whitefish in both lakes was about 1. The rise was evidently due to the attainment of sexual maturity, a too dense population, and a lack of food.
The maximum biomass of the year-class of C. peled stocked in Lake Rahtijarvi was 37 kg/1 000 stocked individuals. The respective maximum biomass for C. muksun was only 16 kg/1 000 stocked individuals, due to the slower growth rate.
The maximum biomass in Lake Valkea-Mustajarvi obtained for C. peled was 307 kg/1 000 stocked individuals and for C. muksun 231 kg/1 000 stocked individuals. These maximum biomasses, in comparison to those in Lake Rahtijarvi, were many times higher, due to better growth and the lower instantaneous rate of natural mortality (M) during the first years.
It is obvious that the value of small lakes in terms of fisheries can be considerably raised by stocking them with whitefish that utilize zooplankton production. Of the whitefish species available, C. peled from the U.S.S.R. provides a serious alternative to the Finnish whitefish species because of its good growth.
Pendant plusieurs années, on a utilisé Coregonus muksun, poisson blanc indigène planctophage, et C. peled, poisson blanc sibérien introduit, pour le repeuplement de deux petits lacs forestiers du sud de la Finlande sans rien changer aux sources naturelles de nourriture. La densité du repeuplement était de l'ordre de 100 individus à l'hectare et la taille des poissons de 6 à 13 cm (poissons d'un été). On n'avait observé de reproduction naturelle du poisson blanc dans aucun de ces deux lacs. Les statistiques des captures ainsi que des essais de pêche et des opérations de marquage/recapture ont permis de suivre l'évolution des poissons mis à l'eau.
Dans le lac de Rahtijarvi le poisson blanc sibérien s'est d'abord développé plus vite que le poisson blanc indigène mais on a constaté au bout de cinq ans que ce dernier l'avait dépassé car le taux de croissance du premier cité avait nettement diminué la troisième année. Dans le lac de Valkea-Mustajarvi, au bout de cinq ans le taux annuel de croissance de C. muksun était supérieur à celui de C. peled, mais ce dernier avait pris une telle avance au cours des toutes premières années qu'il était impossible pour C. muksun de combler son retard.
Le taux de mortalité naturelle instantanée (M) a été assez élevé (environ 0,5) au cours des premières années dans le lac de Rahtijarvi où il existait une forte population de brochets alors qu'il est resté faible (environ 0,1) dans le lac de Valkea-Mustajarvi où il y avait très peu de prédateurs. Au bout de cinq ans, la mortalité des poissons blancs dans les deux lacs était voisine de 1.
Le ralentissement de la croissance et le taux élevé de mortalité naturelle des groupes d'âge les plus vieux indiquent que, dans le lac de Rahtijarvi, la biomasse maximale d'un groupe d'âge a été atteints dès l'âge de 2–3 ans pour C. peled et à l'âge de 3–4 ans pour C. muksun. A ce moment, les possibilités de captures étaient au maximum de 30–450 kg/1 000 unités de repeuplement pour C. peled et de 10–20 1 kg/1 000 unités de repeuplement pour C. muksun. Dans le lac de Valkea-Mustajarvi, pour les deux espèces, un groupe d'âge atteignait sa biomasse maximale à 4–5 ans. A ce moment, les possibilités de captures étaient au maximum de 275–325 kg/1 000 unités de repeuplement pour C. peled et de 200–250 kg/1 000 unités pour C. muksun. Ces résultats sont nettement supérieurs à ceux obtenus dans le premier lac, ce qui tient au fait que le taux de mortalité a été faible les premières années et le taux de croissance plus élevé.
Si l'objectif n'est pas de maximiser les captures mais, plutôt, d'obtenir des poissons d'une taille supérieure à la moyenne, C. muksun donne plus ou moins les mêmes résultats que C. peled.
In Finland, the productive capacity and utilization of small waters for fisheries have been studied for a long time (Levander, 1906; Valle, 1924; Toivonen, 1964; Tuunainen, 1970). As Ryhänen (1972) has observed, there are good reasons to assume that in fisheries in dystrophic lakes, those species that feed on zooplankton are to be preferred, which would also be practical from the point of view of energy economics. However, because the fish stock in small Finnish waters generally lacks those species that utilize zooplankton, in recent years we have begun large-scale trials in small waters stocked with whitefish which feed on zooplankton. In the Evo Inland Fisheries and Aquaculture Research Station (located near Lammi in southern Finland) comparative studies have been carried out since 1973 between the native plankton-feeding Coregonus muksun (Pallas) and Coregonus peled (Gmelin), which was imported to Finland from Siberia, to see which species was better suited for management in small waters. In general, neither of these species has been observed to reproduce naturally in small Finnish lakes.
Coregonus peled was first imported to Finland from the U.S.S.R. in 1965 (Toivonen, 1965) where it had been found to survive in waters with little oxygen and low pH. For this reason, it was felt that it might also be suited for many Finnish waters.
In Finland in 1980, 5.2 million one-summer old C. peled and 8.0 million one-summer old C. muksun were stocked (Eskelinen and Sumari, 1981).
The present study follows and compares the growth and estimated production, and factors which affect them, of C. peled and C. muksun stocked in two small forest lakes, Lake Rahtijärvi and Lake Valkea-Mustajärvi, located near the Evo Research Station.
This paper is based on a study by Pruuki (1982) in which the subject has been treated in greater detail.
For the stocking trials, two similar sized lakes were chosen near the Evo Inland Fisheries and Aquaculture Research Station, located near Lammi in southern Finland (25°5'E; 61°12'N) (Fig. 1). The surface area of Lake Rahtijärvi (shown in Fig. 2 and hereinafter referred to as Lake R) is 13.2 ha, with a maximum depth of 15 m. The water of the lake has a very high humus content, as shown by its average colour values: 107 mg/Pt 1 at the surface, and 153 mg/Pt 1 at the bottom. The oxygen content of the hypolimnion is poor both at the end of the winter and at the end of the summer. The major species found naturally in Lake R are European perch (Perca fluviatilis (L.)), pike (Esox lucius L.), European bleak (Alburnus alburnus (L.)), and roach (Rutilus rutilus (L.)). The lake has several inlets and one outlet.
The surface area of Lake Valkea-Mustajärvi (shown in Fig. 2 and hereinafter referred to as Lake V-M) is 13.9 ha, with a maximum depth of 10 m. The average colour values for the water were 25 mg/Pt 1 at the surface and 50 mg/Pt 1 at the bottom. The oxygen content of the hypolimnion was poor at the end of winter and the end of summer. The major species occurring naturally in Lake V-M is perch (Perca fluviatillis (L.)). The lake has neither inlets nor outlets.
Fishing in the lakes, except for that in connexion with the experiment and stocking trials, is forbidden.
The whitefish populations in the study came from one-summer old stocked whitefish raised with natural feed, which had been stocked in Lake R since 1974 and in Lake V-M since 1973 (Table 1). The average size of the stocked fish in different years was 6–14 cm. There were no observations of natural reproduction of whitefish.
Monitoring of the whitefish stocking was carried out on the basis of catch records, trial fishing carried out with nets, and marking-recapture trials in 1980 and 1981. The total whitefish catch in Lake R and Lake V-M, on which the results of this study are primarily based, is shown in Table 2.
The age of the whitefish was determined by scale samples taken from between the abdominal fins (Einsele, 1943; Bagenal and Tesch, 1978). Part of the growth data was obtained from back calculation of lengths. The length-weight relationships for whitefish and von Bertalanffy's growth equations (von Bertalanffy, 1938; ref. Ricker, 1975) were calculated using Abramson's (1971) computer programme. The size of the whitefish populations in 1980 and 1981 was determined through marking-recapture using the so-called Petersen method (see e.g., Ricker, 1975).
The mortality and population size development were calculated using virtual population analysis (VPA) (Gulland, 1965), followed by the FPROG 4 programme (Salojärvi et al., 1978) based on a computer simulation population analysis. The instantaneous rates of mortality, population sizes, and recruitment in the simulation were adjusted to correspond with the known total catches, the results of the marking-recapture and the numbers of juveniles stocked. On the basis of the catch data, the rate of natural mortality calculated for both C. peled and C. muksun in the same lake was estimated to be the same (Pruuki, 1982).
The development of the biomass of a year-class as a function of its age was calculated using the FPROG 6 programme (Salojärvi et al., 1978) which was based on population analysis, assuming that mortality due to fishing did not occur. The value of the instantaneous rate of natural mortality (M) used was the average value described above, which was obtained with a computer simulation for the years 1973–81. The net production of a year-class in this method is equal to the peak biomass; and the most efficient catching age is that in which the maximum biomass is achieved. The calculations and evaluations have been made by the senior author of this paper.
Table 3 shows von Bertalanffy's growth parameters for the whitefish populations being studied. Figs. 3 and 4 show the growth curves drawn on the basis of the growth equations, and Fig. 5 shows the annual incremental growth for those year-classes in which both C. peled and C. muksun were stocked in the same lake.
The average instantaneous rates of mortality for the years 1973–81 are shown in Table 4. The development of population size and the results of the mark-recapture experiment are shown in Table 5. Figs. 6 and 7 show the development of the size of the whitefish population in the experimental lakes, as a function of the age of the year-classes, assuming mortality due to fishing does not occur.
It can be seen from Table 3 that the asymptotic length (Loo) in von Bertalanffy's equation is on average greater for C. muksun than for C. peled and the Brody growth coefficient (K) is on average greater for C. peled than for C. muksun. In the growth curves (Figs. 3–5) this can be seen in the more rapid growth of C. peled up to 3–4 years of age. The values for the 1975 Rahtijärvi C. muksun, which deviate from those of other year-classes, will evidently approach average values when additional samples are taken in following years. It is evident that C. peled in these conditions grow faster than C. muksun in the first years. C. peled is sexually mature at 2–3 years, while C. muksun reach maturity only at 4–5 years; which may be the reason that the growth of C. peled slows earlier than that of C. muksun.
In both lakes, that year-class which was stocked first in the lake had the best growth (Figs. 3 and 4). The growth of whitefish stocked after that first stocking deterioriated year by year. From this it may be concluded that stocking created such a dense whitefish population that the amount of food became a limiting factor of growth. A corresponding deterioration of growth has been observed, e.g., in the U.S.S.R., in the 67.5-ha Lake Krivogo (Rudenko, 1975). The reasons for this have been given as inter alia increased population density and the large number of other fish species in the lake (Tikhomirova, 1975). The size of the population and growth have been shown to be connected also in larger lakes (Healey, 1975, 1975a and 1980; Kenyon, 1978).
The parameters in the von Bertalanffy's equations for the whitefish populations of Lake R and Lake V-M are on the order of those for the plankton whitefish populations of large lakes in the Oulujoki river system, calculated by Salojärvi (1982). Thus, on the basis of growth, the whitefish species being studied appear to be suitable for stocking also in small lakes.
In both lakes, the instantaneous rates of natural mortality (M) for the autumn of the stocking year was higher than that for the next years (Table 4). This higher mortality rate was evidently due to damage caused during transportation and stocking, the change of the living environment, and predation.
The considerably higher instantaneous rate of natural mortality (M) for Lake R compared to Lake V-M was evidently caused by the large number of predators in Lake R, especially pike (Esox lucius L.). In addition, there were in Lake R along with whitefish, large numbers of cyprinoid fish which competed with whitefish for food and living space, and which were not present at all in Lake V-M. It should also be noted that the quality of the water in the comparatively clear. Lake V-M is evidently more suitable for whitefish than the water in Lake R.
Usually the same instantaneous rate of natural mortality (M) has been shown for all age groups, but in Lake R and Lake V-M at least the instantaneous rate of natural mortality (M) was lower for 1–4 year old whitefish than for other age groups. Spangler (1970) has also observed a rise in natural mortality for older age groups.
In both lakes, the rise in the instantaneous rate of natural mortality (M) at the age of 4–5 years was evidently due to the attainment of sexual maturity, a too dense population, and a lack of food. In both lakes, the fishing mortality was particularly low and the whitefish populations in question were only slightly exploited (Pruuki, 1982).
Healey (1975) has compiled a table of the values of the instantaneous rates of natural mortality (M) for whitefish populations which had not been fished. They varied from 0.2 to 1.2 with an average value of 0.69. Salojärvi (unpublished) has obtained momentary death rates of 0.1–0.2 in a whitefish population stocked in the Oulujoki river system. The population in question has been thoroughly fished.
When comparing with the results presented above the instantaneous rates of natural mortality (M) were common in Lake R and Lake V-M.
Lake V-M is able to maintain a much larger whitefish population than is Lake R (see Table 5). This is influenced by the structure of the fish species, the production of feed (zooplankton) and the quality of the water. The year-class of C. peled stocked in Lake R reached its biomass peak of 37 kg/1 000 stocked individuals, i.e., a net production of 4 kg/ha/year according to growth, in 1975. The respective maximum biomass for C. muksun was only 16 kg/1 000 stocked individuals, i.e., a net production of 2 kg/ha/year, due to the slower growth rate.
The maximum biomass in Lake V-M were many times higher than those in Lake R, due to better growth and the lower instantaneous rate of natural mortality (M) during the first years. The maximum biomass obtained for C. peled was 307 kg/1 000 stocked individuals, which would correspond to an annual net production of 31 kg/ha. The poorer growth of C. muksun during their first few years resulted in a maximum biomass of 231 kg/1 000 stocked individuals, i.e., a net production of 23 kg/ha/year.
Hakkari et al. (1983) have estimated the zooplankton production utilizable for fish and predatory invertebrates to be more than 400 kg/ha/year in Lake V-M. This estimation corresponds well with the whitefish net production in Lake V-M presented above.
According to the catch statistics complied at the beginning of the century by Brofeldt (1920), the lakes with the Evo Inland Fisheries and Aquaculture Research Station have produced a whitefish catch of circa 20 kg/1 000 stocked one-summer old individuals (Salojärvl, 1980). According to Salojärvi (1980 and unpublished) the average yield obtained from some small Finnish lakes has been 95 kg/1000 one-summer old stocked whitefish, when fishing was intensive.
According to the statistics published by Bell Hanford and Dietz (1977), the whitefish yield from 13 lakes in Alberta with a surface area of less than 1 000 ha has been on average 5.7 kg/ha/year. Titova (1978, ref. Reshetnikov, 1979) has presented data according to which the yield obtained from C. peled stockings has been 24 kg/ha/year in certain waters in the U.S.S.R. In comparison to the above mentioned results, the results from the whitefish stockings in Lake R are fair, and those from Lake V-M good.
The results from Lake R and V-M are, however, only theoretical because the fishing mortalities were low and the whitefish populations slightly exploited. In any case it is obvious that the value of small lakes in terms of fisheries can be considerably raised by stocking them with whitefish that utilize zooplankton production, as Ryhanen (1972) assumed. Of the whitefish species available, C. peled from the U.S.S.R. provides a serious alternative to the Finnish whitefish species because of its good growth.
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Table 1 Stocking of one-summer old C. peled and C. muksun in Lake Rahtijarvi and Lake Valkea-Mustajarvi between 1973–81
|Year||Lake Rahtijarvi (13.2 ha)||Lake Valkea-Mustajarvi (13.9 ha)|
|C. peled||C. muksun||C. peled||C. muksun|
|No. of ind.||No. of ind.||No. of ind.||No. of ind.|
|1978||1 300||-||-||1 400|
|1979||1 300||-||-||1 400|
|1980||1 320||-||-||1 400|
|1981||1 000||-||-||1 400|
Table 2 Total whitefish catch in Lake Rahtijarvi and Lake Valkea-Mustajarvi from 1974 to 1981
|Lake Rahtijarvi||Lake Valkea-Mustajarvi|
|C. peled||C. muksun||C. peled||C. muksun|
Table 3 The von Bertalanffy growth parameters according to year-class for C. peled and C. muksun stocked in Lake Rahtijarvi and Lake Valkes-Mustajarvi.
|Lake||Year class||Species of whitefish||von Bertalanffy's growth parameters|
|1975||C. muksun||69.9||3 363||0.1078||-0.8947|
Loo = asymptotic length
Woo = asymptotic weight
K = Brody growth coefficient
To = hypothetical moment at which the fish length is 0
Table 4 The average instantantaneous rates of natural mortality for C. peled in Lake Rahijarvi and for C. muksun in Lake Valkea-Mustajarvi for the period 1974–1981 based on population analysis
|Age||Lake Rahtijarvi||Lake Valkea-Mustajarvi|
M the instantaneous rate of natural mortality
F the instantaneous rate of fishing mortality
Z the instantaneous rate of total mortality
The values in the table are annual values, so that the mortality for 0-year old fish must be multiplied by 4 in order to give the mortality of the first quarter of the year, i.e., the Autumn of the year in which they were stocked
Table 5 Development of the size (number of individuals at the beginning of the year) of the Lake Rahtijarvi C. Peled population and the Lake Valkea-Mustajarvi C. muksun population, calculated using population analysis and the estimated size based on marking-recapture
|Lake||1974||1975||1976||1977||1978||1979||1980||1981||Marking-recapture summer 1981|
|Rahtijarvi||-||846||1 243||1 949||2 172||1 931||1 727||1 726||1 455|
|Valkea-Mustajarvi||441||340||887||1 616||2 259||3 353||4 019||4 454||4 108|
Fig. 1 Location of the lakes used in the study (Evo)
Fig. 2 Lake Rahtijärvi (Lake R) and Lake Valkea-Mustajärvi (Lake V-M)
Fig. 3 The average weight at various ages of selected year-classes of C. peled and C. muksun whitefish stocked in Lake Rahtijärvi. Calculated on the basis of von Bertalanffy's growth equations and length-weight relationships. C. muksun were stocked only once, in 1975 (see Table 1)
Fig. 4 The average weight at various ages of selected year-classes of C. peled and C. muksun stocked in Lake Valkea-Mustajärvi. Calculated on the basis on von Bertalanffy's growth equations and length-weight relationships
Fig. 5 The annual incremental growth of C. peled and C. muksun stocked in Lakes Rahtijärvi and Valkea-Mustajärvi as a function of the age of fish, calculated on the basis of von Bertalanffy's growth equations, for those year-classes in which year both C. peled and C. muksun were stocked in the same lake
Fig. 6 The development in size and biomass of a year-class for C. peled and C. muksun in Lake Rahtijärvi when the size of the stocking was 1 300 individuals. The instantaneous rate of natural mortality used is the average for 1974–81. The maximum biomass, calculated on the basis of various calculations, is marked with an arrow on the biomass curve
Fig. 7 The development of the size and biomass of a year-class for C. peled and C. muksun in Lake Valkea-Mustajärvi when the size of the stocking was 1 400 individuals. The instantaneous rate of natural mortality used is the average for 1973–81. The maximum biomass is marked with an arrow on the biomass curve