- Email: [email protected]
The effects of stocking density on early growth in Arctic charr, Salvelinus alpinus (L.)
Aquaculture, 73 (1988) 101-110 Elsevier Science Publishers B.V., Amsterdam -
101 Printed in The Netherlands
The Effects of Stocking Density on Early Growth in Arctic Charr, Salvelinus alpinus (L.) JEFF C. WALLACE, ARNE G. KOLBEINSHAVN and TROND G. REINSNES Norwegian College of Fisheries, University of Tromsn, P.O. Box 3083, Guleng, N-9001 Tromso (Norway) (Accepted 29 March 1988)
ABSTRACT Wallace, J.C., Kolbeinshavn, A.G. and Reinsnes, T.G., 1988. The effects of stocking density on early growth in Arctic than, Salvelinus alpinus (L.). Aquaculture, 73: 101-110. Fry of Arctic charr, Salvelinus alpinus (L.), were held at seven different stocking densities during and after the initial feeding period. Stocking density affected growth, the populations held at densities of 25 and 50 fry/l showing significantly slower growth and slightly higher mortality than the populations held at densities of 70-250 fry/l. In similar experiments using fish of initial weight 5.5 g and stocked at densities of 5.3,15.9 and 37 kg fish/m3 water, growth rate was observed to be positively correlated with stocking density. Arctic charr of initial weight 16 g grew extremely well after being stocked at an initial density of 110 kg/m3. It would appear that high population density affects young Arctic charr such that agonistic behaviour is inhibited and schooling behaviour stimulated.
INTRODUCTION
In intensive aquaculture, the density at which a fish species can be stocked is an important factor in the determination of production costs in relation to capital investment. The higher the stocking density, the lower will be the production cost per fish, assuming that satisfactory survival and growth are maintained. Earlier investigations have shown that growth of fish in culture is influenced by stocking density (Kalleberg, 1958; Keenleyside and Yamamoto, 1962; Refstie and Kittelsen, 1976; Refstie, 1977; Trzebiatowski et al., 1981; Knights, 1984,1985; Pickering and Stewart, 1984 ). As could be expected, optimal stocking densities vary from species to species. Age and/or size within a species, and exogenous factors such as temperature and feeding rate can also determine the stocking densities which give optimal production results. In salmonid fish culture there is, nevertheless, a remarkable uniformity of technology, at least as regards hatchery rearing, and little attention has been paid to possible interspecific differences in husbandry requirements. The following
0044-8486/88/$03.50
0 1988 Elsevier Science Publishers B.V.
102
investigations deal with the effect of stocking density on growth of Arctic charr fry and fingerlings. MATERIAL AND METHODS
Three series of experiments were carried out. The first involved charr fry in which exogenous feeding was becoming established; the second concerned fingerlings that had attained an average weight of 5.5 g, and the third involved fish of initial mean weight 16.0 g. In all cases the fish had hatched from eggs of cultured charr, the original wild stock being the population that spawns in Lake Storvatn, Hammerfest, northern Norway. The rearing containers used for the fry were modified 10-l capacity plastic buckets. They were placed, in duplicate series of seven, in hatchery troughs, which acted as common drainage channels. Water was supplied from a header tank. The depth in each container was regulated to 10 cm, found to be suitable for initial feeding of salmonids (Aulstad and Refstie, 1975)) which gave a volume of 4 1. Flow rate through the containers was regulated to 160 ml/min at the beginning of the experiment and it was gradually increased to 500 ml/min over a period of 4 months. Stocking densities were chosen to include and exceed those used in similar work on salmonid fry during initial feeding (Refstie and Kittelsen, 1976). The densities chosen were 1000,800,600,400, 280,200 and 100 charr fry per container. Water temperature rose from 6’ C to about 9’ C in the course of the experiment, which lasted 205 days. The fry were fed ad libitum automatically at 15-min intervals during 20 h each day, the food used being “Tess” salmon feed of particle sizes 0 and 1 (55% protein, 22% fat, 7% moisture ) , previously found to be suitable for initial feeding of charr (Wallace et al., 1988a). At monthly intervals a sample of about 30% of the fry in each container was taken and the fish individually weighed. Mortality was also noted. The containers used for the fingerlings were 70-cm diameter circular rearing tanks. Water volume was 104 1,and the flow was regulated initially to 7 1min-’ and increased to 10 1 min-’ during the second experiment, which lasted 74 days. Here the initial fingerling population consisted of fairly evenly sized fish of mean weight 5.5 _t 1.4 g. It was divided into three replicate groups of 100,300 and 700 fish, equivalent to stocking densities of 5.3, 15.9 and 37 kg fish/m3 water. The fish were fed ad libitum continuously for 8 h daily at a feeding level in excess of 3% body weight per day, using “EWOS” salmon feed. Samples of 200-300 fish were taken on days 23,38,50 and 74, and the fish were individually weighed. In the third experiment duplicate populations of 700 fish, of initial mean weight 16.0 + 3.2 g, were stocked at an initial density of 110 kg/m3 and raised for 95 days. The water flow was increased from 10 1min-’ to 15 1min-l during this period. Samples of 200-300 fish were weighed at approximately fortnightly intervals.
103
In the experiments using fingerlings, water temperature was maintained at 9°C. Dissolved oxygen concentration never fell below 7 mg 1-l. The amount of food fed to the fish was gradually increased throughout the study, to allow for fish growth. The fact that both fry and fingerlings always received food in excess of requirements was ensured by the small amounts of excess food which were removed daily during routine inspection. Arctic charr will readily take food from the tank bottom, which is “cleaned” by the fish except under conditions of excess feeding. Unlike young salmon and trout, Arctic charr do not rest on the tank bottom, but distribute themselves throughout the water column immediately after swim-up. Accumulation of small amounts of excess food, which was never allowed to remain in the tanks for more than 24 h, was not considered in any way detrimental to the fish. All tanks drained from central outlet pipes of the “monk” construction, which ensured a horizontal drainage current across the bottom. Water passing over excess food left the tanks immediately. From the weight data obtained from all experiments, population mean specific growth rates and changes in stocking densities were calculated and compared. Means were compared using Student’s_& and differences at the 5% level were considered significant. Mean specific growth rates were calculated using the formula G=lOO(ln
IV,-ln
lV,)/(T-t)
where G is specific growth rate, Wr is weight at time T, W, is weight at time t and (T- t) is time in days between weighings. RESULTS
No significant differences were found between duplicate groups of fry and the results from duplicates were pooled. The growth rates of the fry held at the five highest densities were not significantly different from each other. The same was true of the growth rates at the two lowest densities. Each of these two groups, i.e., five highest and two lowest densities, could therefore be regarded as one group. The growth rates of the two pooled groups differed significantly (Fig. 1). Differences became obvious after about 130 days, when the fry were around 1 g in weight. After 200 days the fry held at high densities had attained a mean weight of almost 2.5 g while those held at low stocking densities averaged less than 2.0 g in weight. The specific growth rates for the two groups during five periods of the experiment are shown in Table 1. Mortality was low in all groups. It was, however, slightly lower in the high density groups (average mortality 3.2% ) than in the low density groups (4.8% mortality).
20
100
60
TIME
140
IN
180
220
DAYS
Fig. 1. Growth in groups of charr fry. Salvelinus alpinus (L.), held at high and low stocking densities during the first 220 days of exogenous feeding. TABLE 1 Mean specific growth rates of Arctic charr fry held at high and low stocking densities during and after initial exogenous feeding, calculated for five intervals during the experiment Fish density (no./I)
Specific growth rate (% weight/day) Interval (days) 1 (14-40)
2 (40-80)
3 (80-135)
4 (135-180)
5 (180-205)
High
0.54
3.05
2.03
1.28
0.64
(>70/1) Low
0.78
2.45
2.26
0.92
0.21
(<50/l)
Fingerlings Mortality during the experiments with fingerlings was negligible, six fish dying during weight measurements. No significant differences between replicate samples were found. The highest mean weight was achieved by the population held at the highest stocking density, and the poorest growth was that of the population with the lowest fish density (Fig. 2). After 74 days the highest stocking density had increased from 37 kg/m3 to 90 kg/m3, and growth did not appear to be slowing down in any of the experimental groups. Indeed, all mean
105
Fig. 2. Changes in the mean fish weights and stocking densities as a result of fish growth during 50 days in populations of Arctic charr fingerlings held at three different stocking densities. TABLE 2 Mean specific growth rates of Arctic charr fingerlings calculated for each measurement interval Fish density (no./tank)
stocking densites,
Specific growth rate (% weight/day) Interval
(days)
(O-23)
2 (23-38)
3 (38-50)
4 (50-74)
1.14 0.82 0.76
1.50 1.02 0.62
0.63 1.11 1.30
1.10 1.64 1.35
1
700 300 100
held at three different
specific growth rates for the last measurement period showed an increase over those of the previous period. Table 2 gives the mean specific growth rates for each consecutive measurement interval. The third experiment was a continuation of the monitoring of the highest density groups, after a pause during which mean fish weight increased from
106
$80 w El60 540 ki &120 100 g
5o
2
40
5
30
i
2 20 ZE i 101
f 0
’ 20 TIME
I 40
I 60
I 80
I 100
IN DAYS
Fig. 3. Changes in mean fish weight (standard error shown) and stocking density as a result of growth during 95 days in Arctic charr fingerlings stocked at an initial density of 110 kg/m3 and with an initial mean weight of 16 g.
13.4 g to 16.0 g. During the 95 days of the experiment, stocking density increased from 110 kg/m3 to 225 kg/ m3. The highest mean specific growth rate of 1.47% body weight per day was registered during the 20 days the fish took to increase their mean weight from 19 g to 25.5 g, while stocking density increased from 130 kg/m3 to around 170 kg/m” (Fig. 3). Inspection of all fish at the experiment’s conclusion revealed no evidence of fin damage or dermal injury.
DISCUSSION
These results contrast with many others concerning the relationship between growth and stocking density in fish culture, where the conclusion generally is reached that growth rate and stocking density are inversely related (Kincaid et al., 1976; Backiel and Le Cren, 1978). Atlantic salmon (Salmo sahr) and rainbow trout (S. guirdneri) have been shown to experience highest mortalities at lowest stocking densities during the initial feeding period (Refstie and Kittelsen, 1976; Refstie, 1977), and this is in agreement with the tendency noted here. However, the same investigations also showed that stocking density had no effect on growth rate in salmon fry,
107
and that individual growth in rainbow trout was higher at low stocking density. This is not the case in charr fry. The specific growth rates in both groups of fry were low at the beginning of the experiment and rose sharply after about 1 month (Table 1). The initial low growth may have been due to stress caused by the initial counting and stocking. It may also have been a result of complete establishment of exogenous feeding during this period. Specific growth rates of charr fry are greatly enhanced by the transfer from yolk remains to exogenous food (Wallace et al., 1988b). The decline in growth rates in both groups after an early maximum is to be expected. The difference between the groups was that the growth rates of the fry held at low stocking densities decreased more rapidly after about 130 days than those of the high density groups. The data obtained from the fingerlings are also in contrast with available data on salmonids, growth rate in this case increasing with increasing stocking density. Mean specific growth rates for growth to 10.8 g, the highest mean weight attained by the slowest growing population, were 0.9%, 1.1% and 1.2% per day for the fish held at densities of 100,300 and 700 per tank, respectively. The highest growth was therefore achieved while stocking density rose from 37 kg/m3 to 72 kg/m3 and the lowest growth rate occurred while stocking density increased from 5 kg/m3 to about 10 kg/m3. The changes in the specific growth rates during the experiment (Table 2) indicate that the increases in fish densities and size were influencing growth. The fish stocked at 300 per tank, for example, increased their growth rates to a maximum of 1.64% per day, while the stocking density rose to about the same as the initial stocking density of the highest density group. One can speculate that growth in the middle density group would later have followed the same course as that of the highest density group. However, it would also appear from these data that growth rates can be maximized at higher stocking densities for smaller/younger fish than for larger/older fish. The highest growth rate during period 4 was achieved by the fish stocked at 300 individuals per tank. The inexplicably low growth rate achieved by the highest density group during period 3 may have been due to sampling variance, since the mean weight value for this group at the third weighing appears rather high (Fig. 2 ). A positive correlation between fish size and specific growth rate, as has been noted here, is not to be expected; it has, however, recently been reported from other experiments with Arctic charr (Jobling, 1985, 1987). Frequent or continuous feeding, as was done here, has been suggested as being a necessary condition for its occurrence, the hypothesis being that frequent feeding allows a proportion of the population which otherwise would experience less than optimal growth, due to subversion by dominant individuals, to feed more and increase their growth rates, thus raising the population mean specific growth rate. This hypothesis is supported by the observation that size grading of small Arctic charr is unnecessary under conditions of frequent feeding (Wallace and
108
Kolbeinshavn, 1988 ), while infrequent feeding (twice per day) leads to an increase in size variance within the population (Jobling and Wandsvik, 1983). The same connection between feeding frequency and differential growth in cultured eels (Arzguilh an.guiZZa)has been suggested (Wickens, 1983). In contrast, Knights (1987) points out that small eels fail to feed in the presence of larger fish, even when food is available after the larger eels are satiated. An element of learning is suggested as being operative here. The eels were also studied under conditions of low stocking density and infrequent feeding. It is difficult from these data to estimate an optimal stocking density for the rearing of charr fingerlings. It is apparent that very good growth can be achieved at stocking densities in excess of 100 kg/m3 and that low stocking densities are to be avoided. In contrast, young rainbow trout appear to grow best at lower stocking denLities. Trout of initial weight 22 g, stocked at four densities ranging initially from 3.3 to 13.2 kg/m3, grew best at the lower densities, attaining weights of 246 g and 194 g at the lowest and highest densities, respectively (Trzebiatowski et al., 1981). Densities no higher than 13-61 kg/m3 have been recommended as a means of minimizing stress in rainbow trout after handling, and posthandling stress in coho salmon (Oncorhynchus kisutch) is caused by stocking densities of 16 kg/m3 or higher (Wedemeyer, 1976). The fact that optimal stocking density for Arctic charr appears to be high compared to those reported for other salmonids remains unexplained. Although Arctic charr is perhaps more of a schooling species than salmon and trout, young charr, like the other species, exhibit aggressive behaviour towards each other in their natural environment. This aggression is reported to develop “at the end of the first summer of life” (Fabricius, 1953), which indicates that it is perhaps a factor to be considered during the initial feeding period. Territorial behaviour in Arctic charr of about 1 year of age and l-2 g in weight has been observed (Noakes, 1980). A hypothesis which could explain the results obtained here is that high stocking densities stimulate the development of schooling behaviour while simultaneously inhibiting the development of aggressive behaviour. There is evidence that dominance hierarchies in fish are broken down by high population densities (Kawanabe, 1969; Refstie and Kittelsen, 1976; Seymour, 1984). In addition, schooling behaviour may affect feeding positively (Frost, 1977). On the other hand, low stocking densities and/or restricted feeding may allow territorial or other aggressive behaviour to develop, resulting in stress and reduced growth. An increase in agonistic behaviour in eels has been observed at stocking densities below 25 kg/m3 (Seymour, 1984). The limiting factors for growth of all sizes of fish at extremely high stocking densities are probably of a physical rather than behavioural nature, e.g., elevated ammonium levels, inadequate oxygen supply or, as has been suggested by Wedemeyer (1976)) limited food availability because of restricted freedom of movement.
109 REFERENCES Aulstad, D. and Refstie, T., 1975. The effect of water depth in rearing tanks on growth and mortality in salmon and rainbow trout fingerlings. Prog. Fish Cult., 37: 113-114. Backiel, T. and Le Cren, E.D., 1978. Some density relationships for fish population parameters. In: S.D. Gerking (Editor), The Biological Basis of Freshwater Fish Production. Blackwell, Oxford, pp. 279-302. Fabricius, E., 1953. Aquarium observations on the spawning behaviour of the charr, Salmo alpinus. Rep. Inst. Freshwater Res. Drottningholm, 34: 14-48. Frost, W.E., 1977. The food of charr, Salvelinus willughbii, in Windermere. J. Fish Biol., 11: 531547. Jobling, M., 1985. Physiological and social constraints on growth of fish with special reference to Arctic charr, Salvelinus alpinus (L.). Aquaculture, 44: 83-90. Jobling, M., 1987. Growth of Arctic charr (Saluelinus alpinus L.) under conditions of constant light and temperature. Aquaculture, 60: 243-249. Jobling, M. and Wandsvik, A., 1983. Effect of social interactions on growth rates and conversion efficiency of Arctic charr, Salvelinus alpinus L. J. Fish Biol., 22: 577-584. Kalleberg, H., 1958. Observations in stream tank of territoriality and competition in juvenile salmon and trout, Salmo salar L. and Salmo trutta L. Rep. Inst. Freshwater Res. Drottningholm, 39: 55-98. Kawanabe, H., 1969. The significance of social structure in production of the ayu (Plecoglossus altiualis). In: T.G. Northcote (Editor), Symposium on Salmon and Trout in Streams. University of British Columbia, Vancouver, B.C., pp. 243-251. Keenleyside, M.A.H. and Yamamoto, F.T., 1962. Territorial behaviour of juvenile Atlantic salmon, Salmo salar L. Behaviour, 19: 139-169. Kincaid, H.L., Bridges, W.R., Thomas, A.E. andDonahoo, M.J., 1976. Rearing capacity of circular containers of different sizes for fry and fingerling rainbow trout. Prog. Fish Cult., 38: 11-17. Knights, B., 1984. Energetics and fish farming. In: P. Tytler and P. Calow (Editors), Fish Energetics: New Perspectives. Croom Helm, Beckenham, Kent, pp. 309-340. Knights, B., 1985. Feeding behaviour and fish culture. In: C.B. Cowey, A.M. Mackie and J.G. Bull (Editors), Nutrition and Feeding of Fish. Academic Press, London, pp. 223-241. Knights, B., 1987. Agonistic behaviour and growth in the European eel, Anguilla anguilla L., in relation to warm-water aquaculture. J. Fish Biol., 31: 265-276. Noakes, D.L.G., 1980. Social behaviour in young charrs. In: E.K. Balon (Editor), Charrs, Salmonid Fishes of the Genus Saluelinus. Junk, The Hague, pp. 683-702. Pickering, A.D. and Stewart, A., 1984. Acclimation of the interrenal tissue of the brown trout, Salmo trutta L., to chronic crowding stress. J. Fish Biol., 24: 731-740. Refstie, T., 1977. Effect of density on growth and survival of rainbow trout. Aquaculture, 11: 329334. Refstie, T. and Kittelsen, A., 1976. Effect of density on growth and survival of artificially reared Atlantic salmon. Aquaculture, 8: 319-326. Seymour, E.A., 1984. High stocking rates and moving water solve the grading problem. Fish Farmer, 7(5): 12-14. Trzebiatowski, R., Filipiak, J. and Jakubowski, R., 1981. Effect of stock density on growth and survival of rainbow trout (Salmo gairdneri Rich.). Aquaculture, 22: 289-295. Wallace, J.C. and Kolbeinshavn, A., 1988. The effect of size grading on subsequent growth in fingerling Arctic charr, Salvelinus alpinus (L.). Aquaculture, 73: 97-100. Wallace, J.C., Kolbeinshavn, A.G. and Aasjord, D., 1988a. On egg size, food particle size and initial feeding in Arctic charr, Salvelinus alpinus (L.). Proc. EAS Int. Conf., Aquaculture Europe ‘87, Amsterdam, 1987. Wallace, J.C., Amin, A. and Gudmundsson, A., 1988b. The histology andphysiology of the transfer
110
from yolk absorption to exogenous feeding in Arctic charr, Salvelinus alpinus (L.). ICES Symposium, Early Life History of Fish, Bergen, Oct. 1988. Wedemeyer, G.A., 1976. Physiological response of juvenile coho salmon (Oncorhynchus kisutch) and rainbow trout (Salmo gairdneri) to handling and crowding stress in intensive fish culture. J. Fish. Res. Board Can., 33: 2699-2702. Wickens, J., 1983. Speed-up for slow eels. Fish Farmer, 6 (2): 24-25.
101 Printed in The Netherlands
The Effects of Stocking Density on Early Growth in Arctic Charr, Salvelinus alpinus (L.) JEFF C. WALLACE, ARNE G. KOLBEINSHAVN and TROND G. REINSNES Norwegian College of Fisheries, University of Tromsn, P.O. Box 3083, Guleng, N-9001 Tromso (Norway) (Accepted 29 March 1988)
ABSTRACT Wallace, J.C., Kolbeinshavn, A.G. and Reinsnes, T.G., 1988. The effects of stocking density on early growth in Arctic than, Salvelinus alpinus (L.). Aquaculture, 73: 101-110. Fry of Arctic charr, Salvelinus alpinus (L.), were held at seven different stocking densities during and after the initial feeding period. Stocking density affected growth, the populations held at densities of 25 and 50 fry/l showing significantly slower growth and slightly higher mortality than the populations held at densities of 70-250 fry/l. In similar experiments using fish of initial weight 5.5 g and stocked at densities of 5.3,15.9 and 37 kg fish/m3 water, growth rate was observed to be positively correlated with stocking density. Arctic charr of initial weight 16 g grew extremely well after being stocked at an initial density of 110 kg/m3. It would appear that high population density affects young Arctic charr such that agonistic behaviour is inhibited and schooling behaviour stimulated.
INTRODUCTION
In intensive aquaculture, the density at which a fish species can be stocked is an important factor in the determination of production costs in relation to capital investment. The higher the stocking density, the lower will be the production cost per fish, assuming that satisfactory survival and growth are maintained. Earlier investigations have shown that growth of fish in culture is influenced by stocking density (Kalleberg, 1958; Keenleyside and Yamamoto, 1962; Refstie and Kittelsen, 1976; Refstie, 1977; Trzebiatowski et al., 1981; Knights, 1984,1985; Pickering and Stewart, 1984 ). As could be expected, optimal stocking densities vary from species to species. Age and/or size within a species, and exogenous factors such as temperature and feeding rate can also determine the stocking densities which give optimal production results. In salmonid fish culture there is, nevertheless, a remarkable uniformity of technology, at least as regards hatchery rearing, and little attention has been paid to possible interspecific differences in husbandry requirements. The following
0044-8486/88/$03.50
0 1988 Elsevier Science Publishers B.V.
102
investigations deal with the effect of stocking density on growth of Arctic charr fry and fingerlings. MATERIAL AND METHODS
Three series of experiments were carried out. The first involved charr fry in which exogenous feeding was becoming established; the second concerned fingerlings that had attained an average weight of 5.5 g, and the third involved fish of initial mean weight 16.0 g. In all cases the fish had hatched from eggs of cultured charr, the original wild stock being the population that spawns in Lake Storvatn, Hammerfest, northern Norway. The rearing containers used for the fry were modified 10-l capacity plastic buckets. They were placed, in duplicate series of seven, in hatchery troughs, which acted as common drainage channels. Water was supplied from a header tank. The depth in each container was regulated to 10 cm, found to be suitable for initial feeding of salmonids (Aulstad and Refstie, 1975)) which gave a volume of 4 1. Flow rate through the containers was regulated to 160 ml/min at the beginning of the experiment and it was gradually increased to 500 ml/min over a period of 4 months. Stocking densities were chosen to include and exceed those used in similar work on salmonid fry during initial feeding (Refstie and Kittelsen, 1976). The densities chosen were 1000,800,600,400, 280,200 and 100 charr fry per container. Water temperature rose from 6’ C to about 9’ C in the course of the experiment, which lasted 205 days. The fry were fed ad libitum automatically at 15-min intervals during 20 h each day, the food used being “Tess” salmon feed of particle sizes 0 and 1 (55% protein, 22% fat, 7% moisture ) , previously found to be suitable for initial feeding of charr (Wallace et al., 1988a). At monthly intervals a sample of about 30% of the fry in each container was taken and the fish individually weighed. Mortality was also noted. The containers used for the fingerlings were 70-cm diameter circular rearing tanks. Water volume was 104 1,and the flow was regulated initially to 7 1min-’ and increased to 10 1 min-’ during the second experiment, which lasted 74 days. Here the initial fingerling population consisted of fairly evenly sized fish of mean weight 5.5 _t 1.4 g. It was divided into three replicate groups of 100,300 and 700 fish, equivalent to stocking densities of 5.3, 15.9 and 37 kg fish/m3 water. The fish were fed ad libitum continuously for 8 h daily at a feeding level in excess of 3% body weight per day, using “EWOS” salmon feed. Samples of 200-300 fish were taken on days 23,38,50 and 74, and the fish were individually weighed. In the third experiment duplicate populations of 700 fish, of initial mean weight 16.0 + 3.2 g, were stocked at an initial density of 110 kg/m3 and raised for 95 days. The water flow was increased from 10 1min-’ to 15 1min-l during this period. Samples of 200-300 fish were weighed at approximately fortnightly intervals.
103
In the experiments using fingerlings, water temperature was maintained at 9°C. Dissolved oxygen concentration never fell below 7 mg 1-l. The amount of food fed to the fish was gradually increased throughout the study, to allow for fish growth. The fact that both fry and fingerlings always received food in excess of requirements was ensured by the small amounts of excess food which were removed daily during routine inspection. Arctic charr will readily take food from the tank bottom, which is “cleaned” by the fish except under conditions of excess feeding. Unlike young salmon and trout, Arctic charr do not rest on the tank bottom, but distribute themselves throughout the water column immediately after swim-up. Accumulation of small amounts of excess food, which was never allowed to remain in the tanks for more than 24 h, was not considered in any way detrimental to the fish. All tanks drained from central outlet pipes of the “monk” construction, which ensured a horizontal drainage current across the bottom. Water passing over excess food left the tanks immediately. From the weight data obtained from all experiments, population mean specific growth rates and changes in stocking densities were calculated and compared. Means were compared using Student’s_& and differences at the 5% level were considered significant. Mean specific growth rates were calculated using the formula G=lOO(ln
IV,-ln
lV,)/(T-t)
where G is specific growth rate, Wr is weight at time T, W, is weight at time t and (T- t) is time in days between weighings. RESULTS
No significant differences were found between duplicate groups of fry and the results from duplicates were pooled. The growth rates of the fry held at the five highest densities were not significantly different from each other. The same was true of the growth rates at the two lowest densities. Each of these two groups, i.e., five highest and two lowest densities, could therefore be regarded as one group. The growth rates of the two pooled groups differed significantly (Fig. 1). Differences became obvious after about 130 days, when the fry were around 1 g in weight. After 200 days the fry held at high densities had attained a mean weight of almost 2.5 g while those held at low stocking densities averaged less than 2.0 g in weight. The specific growth rates for the two groups during five periods of the experiment are shown in Table 1. Mortality was low in all groups. It was, however, slightly lower in the high density groups (average mortality 3.2% ) than in the low density groups (4.8% mortality).
20
100
60
TIME
140
IN
180
220
DAYS
Fig. 1. Growth in groups of charr fry. Salvelinus alpinus (L.), held at high and low stocking densities during the first 220 days of exogenous feeding. TABLE 1 Mean specific growth rates of Arctic charr fry held at high and low stocking densities during and after initial exogenous feeding, calculated for five intervals during the experiment Fish density (no./I)
Specific growth rate (% weight/day) Interval (days) 1 (14-40)
2 (40-80)
3 (80-135)
4 (135-180)
5 (180-205)
High
0.54
3.05
2.03
1.28
0.64
(>70/1) Low
0.78
2.45
2.26
0.92
0.21
(<50/l)
Fingerlings Mortality during the experiments with fingerlings was negligible, six fish dying during weight measurements. No significant differences between replicate samples were found. The highest mean weight was achieved by the population held at the highest stocking density, and the poorest growth was that of the population with the lowest fish density (Fig. 2). After 74 days the highest stocking density had increased from 37 kg/m3 to 90 kg/m3, and growth did not appear to be slowing down in any of the experimental groups. Indeed, all mean
105
Fig. 2. Changes in the mean fish weights and stocking densities as a result of fish growth during 50 days in populations of Arctic charr fingerlings held at three different stocking densities. TABLE 2 Mean specific growth rates of Arctic charr fingerlings calculated for each measurement interval Fish density (no./tank)
stocking densites,
Specific growth rate (% weight/day) Interval
(days)
(O-23)
2 (23-38)
3 (38-50)
4 (50-74)
1.14 0.82 0.76
1.50 1.02 0.62
0.63 1.11 1.30
1.10 1.64 1.35
1
700 300 100
held at three different
specific growth rates for the last measurement period showed an increase over those of the previous period. Table 2 gives the mean specific growth rates for each consecutive measurement interval. The third experiment was a continuation of the monitoring of the highest density groups, after a pause during which mean fish weight increased from
106
$80 w El60 540 ki &120 100 g
5o
2
40
5
30
i
2 20 ZE i 101
f 0
’ 20 TIME
I 40
I 60
I 80
I 100
IN DAYS
Fig. 3. Changes in mean fish weight (standard error shown) and stocking density as a result of growth during 95 days in Arctic charr fingerlings stocked at an initial density of 110 kg/m3 and with an initial mean weight of 16 g.
13.4 g to 16.0 g. During the 95 days of the experiment, stocking density increased from 110 kg/m3 to 225 kg/ m3. The highest mean specific growth rate of 1.47% body weight per day was registered during the 20 days the fish took to increase their mean weight from 19 g to 25.5 g, while stocking density increased from 130 kg/m3 to around 170 kg/m” (Fig. 3). Inspection of all fish at the experiment’s conclusion revealed no evidence of fin damage or dermal injury.
DISCUSSION
These results contrast with many others concerning the relationship between growth and stocking density in fish culture, where the conclusion generally is reached that growth rate and stocking density are inversely related (Kincaid et al., 1976; Backiel and Le Cren, 1978). Atlantic salmon (Salmo sahr) and rainbow trout (S. guirdneri) have been shown to experience highest mortalities at lowest stocking densities during the initial feeding period (Refstie and Kittelsen, 1976; Refstie, 1977), and this is in agreement with the tendency noted here. However, the same investigations also showed that stocking density had no effect on growth rate in salmon fry,
107
and that individual growth in rainbow trout was higher at low stocking density. This is not the case in charr fry. The specific growth rates in both groups of fry were low at the beginning of the experiment and rose sharply after about 1 month (Table 1). The initial low growth may have been due to stress caused by the initial counting and stocking. It may also have been a result of complete establishment of exogenous feeding during this period. Specific growth rates of charr fry are greatly enhanced by the transfer from yolk remains to exogenous food (Wallace et al., 1988b). The decline in growth rates in both groups after an early maximum is to be expected. The difference between the groups was that the growth rates of the fry held at low stocking densities decreased more rapidly after about 130 days than those of the high density groups. The data obtained from the fingerlings are also in contrast with available data on salmonids, growth rate in this case increasing with increasing stocking density. Mean specific growth rates for growth to 10.8 g, the highest mean weight attained by the slowest growing population, were 0.9%, 1.1% and 1.2% per day for the fish held at densities of 100,300 and 700 per tank, respectively. The highest growth was therefore achieved while stocking density rose from 37 kg/m3 to 72 kg/m3 and the lowest growth rate occurred while stocking density increased from 5 kg/m3 to about 10 kg/m3. The changes in the specific growth rates during the experiment (Table 2) indicate that the increases in fish densities and size were influencing growth. The fish stocked at 300 per tank, for example, increased their growth rates to a maximum of 1.64% per day, while the stocking density rose to about the same as the initial stocking density of the highest density group. One can speculate that growth in the middle density group would later have followed the same course as that of the highest density group. However, it would also appear from these data that growth rates can be maximized at higher stocking densities for smaller/younger fish than for larger/older fish. The highest growth rate during period 4 was achieved by the fish stocked at 300 individuals per tank. The inexplicably low growth rate achieved by the highest density group during period 3 may have been due to sampling variance, since the mean weight value for this group at the third weighing appears rather high (Fig. 2 ). A positive correlation between fish size and specific growth rate, as has been noted here, is not to be expected; it has, however, recently been reported from other experiments with Arctic charr (Jobling, 1985, 1987). Frequent or continuous feeding, as was done here, has been suggested as being a necessary condition for its occurrence, the hypothesis being that frequent feeding allows a proportion of the population which otherwise would experience less than optimal growth, due to subversion by dominant individuals, to feed more and increase their growth rates, thus raising the population mean specific growth rate. This hypothesis is supported by the observation that size grading of small Arctic charr is unnecessary under conditions of frequent feeding (Wallace and
108
Kolbeinshavn, 1988 ), while infrequent feeding (twice per day) leads to an increase in size variance within the population (Jobling and Wandsvik, 1983). The same connection between feeding frequency and differential growth in cultured eels (Arzguilh an.guiZZa)has been suggested (Wickens, 1983). In contrast, Knights (1987) points out that small eels fail to feed in the presence of larger fish, even when food is available after the larger eels are satiated. An element of learning is suggested as being operative here. The eels were also studied under conditions of low stocking density and infrequent feeding. It is difficult from these data to estimate an optimal stocking density for the rearing of charr fingerlings. It is apparent that very good growth can be achieved at stocking densities in excess of 100 kg/m3 and that low stocking densities are to be avoided. In contrast, young rainbow trout appear to grow best at lower stocking denLities. Trout of initial weight 22 g, stocked at four densities ranging initially from 3.3 to 13.2 kg/m3, grew best at the lower densities, attaining weights of 246 g and 194 g at the lowest and highest densities, respectively (Trzebiatowski et al., 1981). Densities no higher than 13-61 kg/m3 have been recommended as a means of minimizing stress in rainbow trout after handling, and posthandling stress in coho salmon (Oncorhynchus kisutch) is caused by stocking densities of 16 kg/m3 or higher (Wedemeyer, 1976). The fact that optimal stocking density for Arctic charr appears to be high compared to those reported for other salmonids remains unexplained. Although Arctic charr is perhaps more of a schooling species than salmon and trout, young charr, like the other species, exhibit aggressive behaviour towards each other in their natural environment. This aggression is reported to develop “at the end of the first summer of life” (Fabricius, 1953), which indicates that it is perhaps a factor to be considered during the initial feeding period. Territorial behaviour in Arctic charr of about 1 year of age and l-2 g in weight has been observed (Noakes, 1980). A hypothesis which could explain the results obtained here is that high stocking densities stimulate the development of schooling behaviour while simultaneously inhibiting the development of aggressive behaviour. There is evidence that dominance hierarchies in fish are broken down by high population densities (Kawanabe, 1969; Refstie and Kittelsen, 1976; Seymour, 1984). In addition, schooling behaviour may affect feeding positively (Frost, 1977). On the other hand, low stocking densities and/or restricted feeding may allow territorial or other aggressive behaviour to develop, resulting in stress and reduced growth. An increase in agonistic behaviour in eels has been observed at stocking densities below 25 kg/m3 (Seymour, 1984). The limiting factors for growth of all sizes of fish at extremely high stocking densities are probably of a physical rather than behavioural nature, e.g., elevated ammonium levels, inadequate oxygen supply or, as has been suggested by Wedemeyer (1976)) limited food availability because of restricted freedom of movement.
109 REFERENCES Aulstad, D. and Refstie, T., 1975. The effect of water depth in rearing tanks on growth and mortality in salmon and rainbow trout fingerlings. Prog. Fish Cult., 37: 113-114. Backiel, T. and Le Cren, E.D., 1978. Some density relationships for fish population parameters. In: S.D. Gerking (Editor), The Biological Basis of Freshwater Fish Production. Blackwell, Oxford, pp. 279-302. Fabricius, E., 1953. Aquarium observations on the spawning behaviour of the charr, Salmo alpinus. Rep. Inst. Freshwater Res. Drottningholm, 34: 14-48. Frost, W.E., 1977. The food of charr, Salvelinus willughbii, in Windermere. J. Fish Biol., 11: 531547. Jobling, M., 1985. Physiological and social constraints on growth of fish with special reference to Arctic charr, Salvelinus alpinus (L.). Aquaculture, 44: 83-90. Jobling, M., 1987. Growth of Arctic charr (Saluelinus alpinus L.) under conditions of constant light and temperature. Aquaculture, 60: 243-249. Jobling, M. and Wandsvik, A., 1983. Effect of social interactions on growth rates and conversion efficiency of Arctic charr, Salvelinus alpinus L. J. Fish Biol., 22: 577-584. Kalleberg, H., 1958. Observations in stream tank of territoriality and competition in juvenile salmon and trout, Salmo salar L. and Salmo trutta L. Rep. Inst. Freshwater Res. Drottningholm, 39: 55-98. Kawanabe, H., 1969. The significance of social structure in production of the ayu (Plecoglossus altiualis). In: T.G. Northcote (Editor), Symposium on Salmon and Trout in Streams. University of British Columbia, Vancouver, B.C., pp. 243-251. Keenleyside, M.A.H. and Yamamoto, F.T., 1962. Territorial behaviour of juvenile Atlantic salmon, Salmo salar L. Behaviour, 19: 139-169. Kincaid, H.L., Bridges, W.R., Thomas, A.E. andDonahoo, M.J., 1976. Rearing capacity of circular containers of different sizes for fry and fingerling rainbow trout. Prog. Fish Cult., 38: 11-17. Knights, B., 1984. Energetics and fish farming. In: P. Tytler and P. Calow (Editors), Fish Energetics: New Perspectives. Croom Helm, Beckenham, Kent, pp. 309-340. Knights, B., 1985. Feeding behaviour and fish culture. In: C.B. Cowey, A.M. Mackie and J.G. Bull (Editors), Nutrition and Feeding of Fish. Academic Press, London, pp. 223-241. Knights, B., 1987. Agonistic behaviour and growth in the European eel, Anguilla anguilla L., in relation to warm-water aquaculture. J. Fish Biol., 31: 265-276. Noakes, D.L.G., 1980. Social behaviour in young charrs. In: E.K. Balon (Editor), Charrs, Salmonid Fishes of the Genus Saluelinus. Junk, The Hague, pp. 683-702. Pickering, A.D. and Stewart, A., 1984. Acclimation of the interrenal tissue of the brown trout, Salmo trutta L., to chronic crowding stress. J. Fish Biol., 24: 731-740. Refstie, T., 1977. Effect of density on growth and survival of rainbow trout. Aquaculture, 11: 329334. Refstie, T. and Kittelsen, A., 1976. Effect of density on growth and survival of artificially reared Atlantic salmon. Aquaculture, 8: 319-326. Seymour, E.A., 1984. High stocking rates and moving water solve the grading problem. Fish Farmer, 7(5): 12-14. Trzebiatowski, R., Filipiak, J. and Jakubowski, R., 1981. Effect of stock density on growth and survival of rainbow trout (Salmo gairdneri Rich.). Aquaculture, 22: 289-295. Wallace, J.C. and Kolbeinshavn, A., 1988. The effect of size grading on subsequent growth in fingerling Arctic charr, Salvelinus alpinus (L.). Aquaculture, 73: 97-100. Wallace, J.C., Kolbeinshavn, A.G. and Aasjord, D., 1988a. On egg size, food particle size and initial feeding in Arctic charr, Salvelinus alpinus (L.). Proc. EAS Int. Conf., Aquaculture Europe ‘87, Amsterdam, 1987. Wallace, J.C., Amin, A. and Gudmundsson, A., 1988b. The histology andphysiology of the transfer
110
from yolk absorption to exogenous feeding in Arctic charr, Salvelinus alpinus (L.). ICES Symposium, Early Life History of Fish, Bergen, Oct. 1988. Wedemeyer, G.A., 1976. Physiological response of juvenile coho salmon (Oncorhynchus kisutch) and rainbow trout (Salmo gairdneri) to handling and crowding stress in intensive fish culture. J. Fish. Res. Board Can., 33: 2699-2702. Wickens, J., 1983. Speed-up for slow eels. Fish Farmer, 6 (2): 24-25.