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Effect of sorting on size-frequency distributions and growth of Arctic charr, Salvelinus alpinus L.
Aquaculture, 60 (1987) 27-31 Elsevier Science Publishers B.V., Amsterdam -
27 Printed in The Netherlands
Effect of Sorting on Size-Frequency Distributions and Growth of Arctic Charr, Salvelinus alpinus L. M. JOBLING and T.G. REINSNES Znstitutt for Fiskerifag, Universitetet i Troms0, Boks 3083, Guleng, 9001 Tromsra(Norway) (Accepted 9 September 1986)
ABSTRACT Jobling, M. and Reinsnes, T.G., 1987. Effect of sorting on size-frequency distributions and growth of Arctic charr, Salvelinus alpinus L. Aquaculture, 60: 27-31. Arctic charr, Salvelinus alpinus, were reared in either unsorted groups or in groups that had been sorted into ‘small’ and ‘large’ fish. At the start of the experiment the size-frequency distributions of unsorted and pooled ‘small’ and ‘large’ fish were similar, but after 5 months’ rearing distributions were markedly different. There was evidence that growth of ‘small’ fish improved in the absence of larger conspecifics, but size-sorting also led to reduced growth of some large fish, with the consequence that size-sorting did not lead to overall increases in biomass gain and rates of production.
INTRODUCTION
Grading and sorting of fish according to size is routinely carried out in commercial hatcheries in an attempt to restrict size ranges within rearing groups and thereby simplify feeding operations. It is also generally believed that small fish show improved growth when removed from the influence of larger conspecifics, with the implication that size-sorting of fish should lead to an overall increase in growth and production. In small-scale laboratory trials with brown trout, Salmo truttu, it was shown that growth of small fish did improve when large fish were removed, but the rearing of large fish together led to reductions in growth rates of some of the larger fish (Brown, 1946). This negative aspect of size-sorting seems to have been ignored and there is little systematic information about the effects of size-sorting on growth of fish under intensive culture conditions. Consequently, experiments were conducted with Arctic charr, Salvelinus alpinus, in order to answer the questions: (i) does size-sorting lead to changes in size-frequency distributions and growth rates of small and large fish? and, if so,
6044-8486/87/$03.50
0 1987 Elsevier Science Publishers B.V.
28
(ii) what are the influences of these changes on overall biomass increase and rates of production? MATERIALS AND METHODS
Arctic charr, Salvelinus alpinus, taken from a common pool of hatcheryreared fish were subjected to different size-sorting procedures and then ongrown for 5 months. Throughout the experiment commercially .available dry pellet food was provided to excess using automatic feeders. Water temperature was approximately 7°C and water flow rates were adjusted such that oxygen concentrations should not be limiting. Each tank (150 1) contained 100 fish and there were three replicate tanks per experimental treatment. For the purposes of analysis, data from the replicates were combined. At the start of the experiment fish were divided amongst the tanks as follows: (i) fish taken at random from hatchery stock, designated ‘unsorted’ fish; (ii) fish less than 30 g body weight, referred to as ‘small’ fish; (iii) fish weighing over 30 g, designated ‘large’ fish. Initial weights of fish ranged between 5 and 130 g, with the majority of the fish weighing 15-45 g (Fig. lA, B ) . Weight-frequency distributions were skewed and data were transformed by taking cube roots of weights since weight a length3. Harding-Cassie analysis (Harding, 1949; Cassie, 1954) confirmed that the transformation normalised the initial size-frequency distributions of both the ‘unsorted’ fish and the pooled ‘small’ and ‘large’ fish (Fig. 2A, B ) . At the end of the 5-month experiment fish were re-weighed, size-frequency distributions examined and growth assessed as rates of biomass increase calculated according to the formula G= (ln&-ln&)
X1oo
t
where, G is growth rate, B, is biomass at time t, B. is biomass at time 0 and t is time in days. RESULTS AND DISCUSSION
At the start of the experiment the weights of the ‘unsorted’ and pooled ‘small’ and ‘large’ fish spanned the same range (Fig. lA, B 1. Sizes ( cube roots of fish weight) of both groups were normally distributed (Fig. 2A, B) , and distributions of ‘small’ and ‘large’ fish, examined separately, were highly skewed (Fig. 2B). Weight ranges increased during the course of growth (Fig. lC, D) but, at the end of the experiment, size distribution of the ‘unsorted’ fish was still normal (Fig. 3A). By contrast, plotting the cumulative frequencies of the cube roots of weights of the pooled ‘small’ and ‘large’ fish produced a sigmoid curve (Fig. 3B), indicating that there had been a split into two distinct sub-groups.
29
25r
20
80
140
20 FISH
80 WEIGHT
140
200
300
(g)
Fig. 1. Weight distribution of fish at the start (A, B) and end (C, D) of the experiment. A and C _ show weight distributions for ‘unsorted’ fish whereas B and D show pooled data for ‘small’ and ‘large’ fish. ‘Small’ fish are represented by the hatched area.
Size distributions of the ‘small’ and ‘large’ fish, examined separately, were not normal but were much less skewed than at the outset (Fig. 3B). These changes in size-frequency distributions imply not only that growth of some of the ‘small’ fish improved markedly in the absence of ‘large’ fish, but also that the process of size-sorting led to a depression of growth of a proportion of the ‘large’ fish. Consequently, it was of interest to compare overall biomass increases and mean growth rates of the unsorted and size-sorted groups of fish. At the start of the experiment, biomasses were 3.9 and 3.6 kg per 100 fish for the ‘unsorted and pooled ‘small’ and ‘large’ fish, respectively. After 5 months’ growth, biomasses had increased to 10.4 and-10.5 kg per 100 fish, showing that biomass increases and growth rates (0.68 and 0.72% d-’ for the ‘unsorted’ and pooled ‘small’ and ‘large’ fish, respectively) differed little betweeiiexperimental treatments. This shows that although growth of ‘small’ fish improved after sizeserting, the post-sorting depression of growth of a proportion of the Ilarge’ fish negated any gain made and size-sorting did not lead to an overall increase in biomass production. Thus, the results of these experiments with Arctic charr under intensive
30
99 95 r
I
2
I
3
I
I
4
I
5 CUBE
ROOT
FISH
2 WEIGHT
I
I 5
I
3
4
Fig. 2. Size-frequency distributions of fish at the start of the experiment analysed using the Harding-Cassie method. 0 --0, unsorted fish; x -- x , pooled data for ‘small’ and ‘large’ fish; 0 -- 0, ‘small’fish; n--n, ‘large’fish. QQ-
95-
l-
I 3 CUBE ROOT FISH WEIGHT
I 4
I 5
I 8
Fig. 3. Size-frequency distributions of fish at the end of the experiment analysed using the Harding-Cassie method. O--O, unsorted fish; X--X, pooled data for ‘small’ and ‘large’ fish; O--O, ‘small’ fish; 0 -- 0, ‘large’ fish.
31
culture are in agreement with those reported by Brown (1946) for brown trout reared in small groups in the laboratory. Results of both experiments provide evidence that growth of small fish is suppressed in the presence of larger conspecifics, as has been inferred from studies on a number of salmonid and nonsalmonid species (Yamagishi et al., 1974; Li and Brocksen, 1977; Jobling, 1985; Koebele, 1985; Saclauso, 1985; Sampath and Pandian, 1985). Several factors may determine the growth rates displayed by small fish (Magnuson, 1962; Rubenstein, 1981; Wickins, 1985; Jobling and Reinsnes, 1986 ) but behavioural interactions are hypothesized as being the major cause of growth depression of small individuals (Brown, 1946; Yamagishi et al., 1974; Li and Brockson, 1977; Jobling, 1985; Koebele, 1985; Saclauso, 1985; Sampath and Pandian, 1985). Since behavioural interactions may be an important determinant of growth of fish in intensive culture, and knowledge about such effects is limited, the topic is worthy of further attention.
REFERENCES Brown, M.E., 1946. The growth of brown trout (Salmo truth Linn.) . I. Factors influencing the growth of trout&y. J. Exp. Biol., 22: 118129. Cassie, R.M., 1954. Some uses of probability paper in the analysis of size-frequency distributions. Aust. J. Mar. Freshwater Res., 5: 513-522. Harding, J.P., 1949. The use of probability paper for the graphical analysis of polymodal frequency distributions. J. Mar. Biol. Assoc. U.K., 28: 141-153. 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. and Reinsnes, T.G., 1986. Physiological and social constraints on growth of Arctic charr, Salvelinus alpinus L.: an investigation of factors leading to stunting. J. Fish Biol., 28: 379-384. Koebele, B.P., 1985. Growth and the size hierarchy effect: an experimental assessment of three proposed mechanisms; activity differences, disproportional food acquisition, physiological stress. Environ. Biol. Fish., 12: 181-188. Li, H.W. and Brocksen, R.W., 1977. Approaches to the analysis of energetic costs of intraspecific competition for space by rainbow trout (Salmo guirdneri) . J. Fish Biol., 11: 329-341. Magnuson, J.J., 1962. An analysis of aggressive behaviour, growth and competition for food and space in medaka [ Oryzias latipes (Pisces; Cyprinodontidae) 1. Can. J. Zool., 40: 313-363. Rubenstein, D.I., 1981. Individual variation and competition in the Everglades pygmy sunfish. J. Anim. Ecol., 50: 337-350. Saclauso, CA., 1985. Interaction of growth with social behaviour in Tilapia zilli raised at three different temperatures. J. Fish Biol., 26: 331-337. Sampath, K. and Pandian, T.J., 1985. Effects of size hierarchy on surfacing behaviour and conversion rate in an air-breathing fish, Channa striates. Physiol. Behav., 34: 51-56. Wickins, J.P., 1985. Growth variability in individually confined elvers, Anguilla anguilla (L. ) . J. Fish Biol., 27: 469-478. Yamagishi, H., Maruyama, T. and Mashiko, K., 1974. Social relation in a small experimental population of Odontobutis obscures (Temminck et Schlegel) as related to individual growth and food intake. Oecologia, 17: 187-202.
27 Printed in The Netherlands
Effect of Sorting on Size-Frequency Distributions and Growth of Arctic Charr, Salvelinus alpinus L. M. JOBLING and T.G. REINSNES Znstitutt for Fiskerifag, Universitetet i Troms0, Boks 3083, Guleng, 9001 Tromsra(Norway) (Accepted 9 September 1986)
ABSTRACT Jobling, M. and Reinsnes, T.G., 1987. Effect of sorting on size-frequency distributions and growth of Arctic charr, Salvelinus alpinus L. Aquaculture, 60: 27-31. Arctic charr, Salvelinus alpinus, were reared in either unsorted groups or in groups that had been sorted into ‘small’ and ‘large’ fish. At the start of the experiment the size-frequency distributions of unsorted and pooled ‘small’ and ‘large’ fish were similar, but after 5 months’ rearing distributions were markedly different. There was evidence that growth of ‘small’ fish improved in the absence of larger conspecifics, but size-sorting also led to reduced growth of some large fish, with the consequence that size-sorting did not lead to overall increases in biomass gain and rates of production.
INTRODUCTION
Grading and sorting of fish according to size is routinely carried out in commercial hatcheries in an attempt to restrict size ranges within rearing groups and thereby simplify feeding operations. It is also generally believed that small fish show improved growth when removed from the influence of larger conspecifics, with the implication that size-sorting of fish should lead to an overall increase in growth and production. In small-scale laboratory trials with brown trout, Salmo truttu, it was shown that growth of small fish did improve when large fish were removed, but the rearing of large fish together led to reductions in growth rates of some of the larger fish (Brown, 1946). This negative aspect of size-sorting seems to have been ignored and there is little systematic information about the effects of size-sorting on growth of fish under intensive culture conditions. Consequently, experiments were conducted with Arctic charr, Salvelinus alpinus, in order to answer the questions: (i) does size-sorting lead to changes in size-frequency distributions and growth rates of small and large fish? and, if so,
6044-8486/87/$03.50
0 1987 Elsevier Science Publishers B.V.
28
(ii) what are the influences of these changes on overall biomass increase and rates of production? MATERIALS AND METHODS
Arctic charr, Salvelinus alpinus, taken from a common pool of hatcheryreared fish were subjected to different size-sorting procedures and then ongrown for 5 months. Throughout the experiment commercially .available dry pellet food was provided to excess using automatic feeders. Water temperature was approximately 7°C and water flow rates were adjusted such that oxygen concentrations should not be limiting. Each tank (150 1) contained 100 fish and there were three replicate tanks per experimental treatment. For the purposes of analysis, data from the replicates were combined. At the start of the experiment fish were divided amongst the tanks as follows: (i) fish taken at random from hatchery stock, designated ‘unsorted’ fish; (ii) fish less than 30 g body weight, referred to as ‘small’ fish; (iii) fish weighing over 30 g, designated ‘large’ fish. Initial weights of fish ranged between 5 and 130 g, with the majority of the fish weighing 15-45 g (Fig. lA, B ) . Weight-frequency distributions were skewed and data were transformed by taking cube roots of weights since weight a length3. Harding-Cassie analysis (Harding, 1949; Cassie, 1954) confirmed that the transformation normalised the initial size-frequency distributions of both the ‘unsorted’ fish and the pooled ‘small’ and ‘large’ fish (Fig. 2A, B ) . At the end of the 5-month experiment fish were re-weighed, size-frequency distributions examined and growth assessed as rates of biomass increase calculated according to the formula G= (ln&-ln&)
X1oo
t
where, G is growth rate, B, is biomass at time t, B. is biomass at time 0 and t is time in days. RESULTS AND DISCUSSION
At the start of the experiment the weights of the ‘unsorted’ and pooled ‘small’ and ‘large’ fish spanned the same range (Fig. lA, B 1. Sizes ( cube roots of fish weight) of both groups were normally distributed (Fig. 2A, B) , and distributions of ‘small’ and ‘large’ fish, examined separately, were highly skewed (Fig. 2B). Weight ranges increased during the course of growth (Fig. lC, D) but, at the end of the experiment, size distribution of the ‘unsorted’ fish was still normal (Fig. 3A). By contrast, plotting the cumulative frequencies of the cube roots of weights of the pooled ‘small’ and ‘large’ fish produced a sigmoid curve (Fig. 3B), indicating that there had been a split into two distinct sub-groups.
29
25r
20
80
140
20 FISH
80 WEIGHT
140
200
300
(g)
Fig. 1. Weight distribution of fish at the start (A, B) and end (C, D) of the experiment. A and C _ show weight distributions for ‘unsorted’ fish whereas B and D show pooled data for ‘small’ and ‘large’ fish. ‘Small’ fish are represented by the hatched area.
Size distributions of the ‘small’ and ‘large’ fish, examined separately, were not normal but were much less skewed than at the outset (Fig. 3B). These changes in size-frequency distributions imply not only that growth of some of the ‘small’ fish improved markedly in the absence of ‘large’ fish, but also that the process of size-sorting led to a depression of growth of a proportion of the ‘large’ fish. Consequently, it was of interest to compare overall biomass increases and mean growth rates of the unsorted and size-sorted groups of fish. At the start of the experiment, biomasses were 3.9 and 3.6 kg per 100 fish for the ‘unsorted and pooled ‘small’ and ‘large’ fish, respectively. After 5 months’ growth, biomasses had increased to 10.4 and-10.5 kg per 100 fish, showing that biomass increases and growth rates (0.68 and 0.72% d-’ for the ‘unsorted’ and pooled ‘small’ and ‘large’ fish, respectively) differed little betweeiiexperimental treatments. This shows that although growth of ‘small’ fish improved after sizeserting, the post-sorting depression of growth of a proportion of the Ilarge’ fish negated any gain made and size-sorting did not lead to an overall increase in biomass production. Thus, the results of these experiments with Arctic charr under intensive
30
99 95 r
I
2
I
3
I
I
4
I
5 CUBE
ROOT
FISH
2 WEIGHT
I
I 5
I
3
4
Fig. 2. Size-frequency distributions of fish at the start of the experiment analysed using the Harding-Cassie method. 0 --0, unsorted fish; x -- x , pooled data for ‘small’ and ‘large’ fish; 0 -- 0, ‘small’fish; n--n, ‘large’fish. QQ-
95-
l-
I 3 CUBE ROOT FISH WEIGHT
I 4
I 5
I 8
Fig. 3. Size-frequency distributions of fish at the end of the experiment analysed using the Harding-Cassie method. O--O, unsorted fish; X--X, pooled data for ‘small’ and ‘large’ fish; O--O, ‘small’ fish; 0 -- 0, ‘large’ fish.
31
culture are in agreement with those reported by Brown (1946) for brown trout reared in small groups in the laboratory. Results of both experiments provide evidence that growth of small fish is suppressed in the presence of larger conspecifics, as has been inferred from studies on a number of salmonid and nonsalmonid species (Yamagishi et al., 1974; Li and Brocksen, 1977; Jobling, 1985; Koebele, 1985; Saclauso, 1985; Sampath and Pandian, 1985). Several factors may determine the growth rates displayed by small fish (Magnuson, 1962; Rubenstein, 1981; Wickins, 1985; Jobling and Reinsnes, 1986 ) but behavioural interactions are hypothesized as being the major cause of growth depression of small individuals (Brown, 1946; Yamagishi et al., 1974; Li and Brockson, 1977; Jobling, 1985; Koebele, 1985; Saclauso, 1985; Sampath and Pandian, 1985). Since behavioural interactions may be an important determinant of growth of fish in intensive culture, and knowledge about such effects is limited, the topic is worthy of further attention.
REFERENCES Brown, M.E., 1946. The growth of brown trout (Salmo truth Linn.) . I. Factors influencing the growth of trout&y. J. Exp. Biol., 22: 118129. Cassie, R.M., 1954. Some uses of probability paper in the analysis of size-frequency distributions. Aust. J. Mar. Freshwater Res., 5: 513-522. Harding, J.P., 1949. The use of probability paper for the graphical analysis of polymodal frequency distributions. J. Mar. Biol. Assoc. U.K., 28: 141-153. 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. and Reinsnes, T.G., 1986. Physiological and social constraints on growth of Arctic charr, Salvelinus alpinus L.: an investigation of factors leading to stunting. J. Fish Biol., 28: 379-384. Koebele, B.P., 1985. Growth and the size hierarchy effect: an experimental assessment of three proposed mechanisms; activity differences, disproportional food acquisition, physiological stress. Environ. Biol. Fish., 12: 181-188. Li, H.W. and Brocksen, R.W., 1977. Approaches to the analysis of energetic costs of intraspecific competition for space by rainbow trout (Salmo guirdneri) . J. Fish Biol., 11: 329-341. Magnuson, J.J., 1962. An analysis of aggressive behaviour, growth and competition for food and space in medaka [ Oryzias latipes (Pisces; Cyprinodontidae) 1. Can. J. Zool., 40: 313-363. Rubenstein, D.I., 1981. Individual variation and competition in the Everglades pygmy sunfish. J. Anim. Ecol., 50: 337-350. Saclauso, CA., 1985. Interaction of growth with social behaviour in Tilapia zilli raised at three different temperatures. J. Fish Biol., 26: 331-337. Sampath, K. and Pandian, T.J., 1985. Effects of size hierarchy on surfacing behaviour and conversion rate in an air-breathing fish, Channa striates. Physiol. Behav., 34: 51-56. Wickins, J.P., 1985. Growth variability in individually confined elvers, Anguilla anguilla (L. ) . J. Fish Biol., 27: 469-478. Yamagishi, H., Maruyama, T. and Mashiko, K., 1974. Social relation in a small experimental population of Odontobutis obscures (Temminck et Schlegel) as related to individual growth and food intake. Oecologia, 17: 187-202.