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The culture of the Iceland scallop, Chlamys islandica (O.F. Müller)
Aquaculture, 26 (1981/1982) 311-320 Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands
311
THE CULTURE OF THE ICELAND SCALLOP, CHLAMYS ISLANDICA (O.F. MULLER) I. SPAT COLLECTION AND GROWTH DURING THE FIRST YEAR
JEFF
C. WALLACE
Institute of Fisheries,
University of Tromsd, Postbox 488, N-9001 Tromsd (Norway)
(Accepted 16 April 1981)
ABSTRACT Wallace, J.C., 1982. The culture of the Iceland scallop, Chlamys islandica (O.F. Miiller). I. Spat collection and growth during the first year. Aquaculture, 26: 311-320. Spat of the Iceland scallop, Chlamys islandica (O.F. Miiller), was artificially collected using thin nylon monofilament as a substrate for settlement. Collectors placed at depths between 15 m and 40 m in Balsfjord, north Norway, collected most spat at depths of between 30 m and 40 m. After settlement in September the spat grew slowly until the following spring (April/May) when growth accelerated. One year after settlement a shell height of about 7 mm was attained. There was also some evidence that secondary migration in this species begins about 1 year after settlement.
INTRODUCTION
Of the European pectinid bivalves, or scallops, only two species, Pecten maximus and Chlamys opercularis, have been exploited to a significant degree by fishing. The interest in these species has in recent years been extended to include the possibility of artificial collection and rearing of spat, with a view to “scallop culture”. In Japan the culture of pectinid molluscs has been established for some years. The Iceland scallop, Clamys islandica, has a more northerly distribution than Chlamys opercularis, and the two species are rather similar in size and appearance. Chlamys isZandica occurs mainly within the subarctic transitional zone (Ekman, 1953), sometimes in quantities and concentrations sufficient to warrant a fishery. So far only Iceland has developed a fishery for the species, but some interest is now being shown in north Norway (Venvik and Vahl, 1979). The present investigation was undertaken with a view to establishing whether or not spat of Chlamys islandica could be artificially collected using methods similar to those proven for Chlamys opercularis and Pecten maximus and to determine the natural growth rate of newly settled spat.
0044-8486/82/0000-0000/$02.75
0 1982
Elsevier Scientific Publishing Company
312 MATERIALS
AND METHODS
Collec tars
The spat collectors consisted of “bags” made of plastic netting and containing quantities of nylon monofilament line. Fig. 1 shows a diagram of the spat collector system. Each collecting bag was made by sewing together
Subsurface
Netion
buoy
bag
Fig.1. Diagram of the spat collector system.
the edges of a 1 m2 square of “Netlon” plastic netting so that a cylinder was obtained. One end of the cylinder was then compressed and its edges sewn together. The resulting bag was filled with a “tangle” of nylon monofilament and sealed by compressing the open end at right angles to that of the closed end and sewing the edges together. The result was a closed, tetrahedron-shaped mesh bag, filled with nylon monofilament. The “Netlon” was of 4 mm mesh size, and the nylon monofilament was mainly 0.15 mm in diameter with some thicker (0.8 mm diameter) line in a few of the bags. The bags were fastened to nylon ropes, usually four bags to a rope, at 5 m intervals. The ropes were attached to a weight or anchor at one end and to an underwater float and a marker float at the other. Placement
of the collectors
The collectors were placed on the scallop bed at the entrace to Balsfjord near Trams@ (lat. 69’30’ N, long. 19’00’ E). Fig.2 shows the position of the bed. Previous investigations on this scallop population have shown that spawning occurs during a short period at the beginning of July (Skreset and
313
BALSFJORD
ARCTIC -__--___
CIRC
__---
Fig.2. Outline map of Norway showing the area mentioned in the text. The position the scallop beds in the region of TromsQ is shown in the inset.
of
314
Brun 1969; Skreset 1973). The collectors were, therefore, placed early in September (1978) which was assumed to be around the time of maximum spatfall. Since the depths at which spat settlement occurs were unknown the collectors were placed at arbitarily chosen depths, both in shallower and in deeper water than the main concentration of adult scallops. The ropes were placed so that the lowest collectors were at a depth of 40 m, and the others at respectively 35 m, 30 m and 25 m. On one rope six collectors were attached, so that this rope could also collect spat at 20 m and at 15 m. Sampling The rope with six collectors was taken up 26 days after placement. Scallop spat were found to be present, the majority being taken from the lowest collectors. The ropes were thereafter taken up singly at roughly monthly intervals over a period of one year. Spat were obtained from the collectors by allowing the nylon monofilament to dry and then shaking it gently over a sheet of black plastic. The dried spat were detached by this action, and they could be picked up from the plastic sheet with a small moistened brush. For spat less than 2 mm in shell diameter a magnifying glass was necessary. The spat from each collector were counted and the largest “diameter” from umbo to opposite shell edge measured using a “Wild M 8” stereomicroscope fitted with a calibrated measuring occular. These measurements were made on a sub-sample of 30 spat, or on the total sample if this was less than 30. RESULTS
Fig.3 shows Chlamys islandica spat of various sizes, taken from the collectors. Depth of settlement Table I shows the numbers of spat recovered from the collectors and the depths at which they were recovered. With the exception of one collector, which collected very few spat, the collectors took most spat at depths of 30 m and greater. The numbers of spat per collector varied from a minimum of 16 to a maximum of 1791, but no collector failed to collect spat. Growth of spat Fig.4 shows the average size of the spat in the samples at each sampling date. Since there is no overlap in the size distributions of the spat collected in September and in November, it is likely that no further settlement occurred after the beginning of September. The growth can, therefore, be taken to
315
Fig.3.
Spat of differing
ages taken from the spat collectors.
indicate the rate of growth of the scallop spat in that area. It can be seen t,hat the spat grew slowly, but steadily, throughout the winter, frc3m an initial shell diameter of less than 0.5 mm at settlement in Septembc zr to about 2. 0 mm in April. Around April/May the growth rate started to a8ccelerate and a high growth rate was achieved in the summer months. There is an indicati on that the growth rate was beginning to fall by September. Attachment The spat were always found attached
to the thin, 0.15 mm nylon mono-
I
0.50 f 0.04
f
0.93 0.13
f
39 20 23 23
1.10 0.26
26
105
3/l
2
1.84 0.22
687
2748
435 1791 416 106
31/l
*Not counted because of small size and large numbers.
Mean shell height (mm)
812
Mean
228 1010 1289 720
2111
3247
23 36 16 36 Many* Many*
2719
Numbers of spat present at date of sampling
Total
15 20 25 30 35 40
Depth (ml
f
1.87 0.48
861
3443
431 988 646 1378
l/3
f
1.95 0.37
344
1377
72 329 421 555
1914
*
2.45 0.39
622
2488
571 455 867 595
2915
f
5.90 0.64
476
1904
185 448 603 668
31/7
+
6.96 0.81
312
1247
114 350 251 532
10/9
2091 5427 4516 4577
Total
232 603 565 572
Mean
Data obtained from the spat collectors showing the numbers of spat which settled at each of the collection depths, and their mean size + standard deviation
TABLE
317
/ / i
2-
A’
.A/
‘:
,,‘-bp 5
0
I 2.0 N
0 Diiii
F
M
&FTER SETTLEMENT
a
360
300
240
180 J
M
J
J
P
5
&ND TlME OF YEAR
Fig.4. Mean growth of the Iceland scallop spat during the first year after settlement.
filament. No spat settled on either the “Netlon” outer bag or the thicker 0.8 mm diameter monofilament, and none were found on the ropes. However, when the last two samples were examined, a proportion of the scallop “seed”, now about 5-7 mm and around one year old, was found attached to the inside surface of the outer bag. In July 18% of the total seed was so attached and the proportion had increased to 35% by September. DISCUSSION
Growth The growth data obtained in this investigation confirm the assumptions of Johannessen (1973) who considered shells which were between 1.1 and 2.8 mm in spring (February-April) to be progeny of the previous summer’s spawning and those between 7.1 mm and 10.7 mm to be one year older. Determination of the earliest growth from annual rings on either the shell valves or the shell ligaments of older shells is almost impossible since no mark is laid down during the first winter. It is, therefore, probable that some workers have underestimated the age of older scallops by one year. Combining the growth data obtained in this investigation with data provided by Pettersen (1976) a growth curve for this population of Chlamys islundica can be drawn. Such a curve is shown in Fig.5 for scallops at a depth of 30 m. The scallops are almost 2 years old, taken from date of spawning, before they reach a shell height of more than 1 cm, and it takes 6 years for the shells to attain a height of 6 cm. According to Pettersen (1976) the average growth at 30 m on this scallop bed is higher than at 15 m, 40 m or 60 m.
1
2
3 YEARS
4 AFTER
5
6
7
8
SPATFALL
Fig.5. Curve showing the growth rate of Iceland scallops at a depth of 30 m in Balsfjord, north Norway. The data collected during the present study is represented in the first part of the curve, up to the large circle.
Depth
of settlement
The numbers of spat collected fell off sharply at between 30 m and 25 m (Table I) whilst fairly large numbers were collected at 40 m, the lowest depth sampled. It is therefore possible that setting occurs in even deeper water. One possible reason for the sparse settlement at depths above 30 m could be competition. The spat collectors at 25 m and in shallower water were always overgrown with brown algae which, being relatively fast growers in the summer season, might inhibit settling of scallop spat or kill by overgrowing any spat which settled. Comparisons of spatfall between the collectors should, of course, be treated with caution since the surface area of the settlement surface, the monofilament, although roughly the same, was not standardised in the different bags. Substrate
The spat was always found on the thinnest nylon monofilament which had a diameter of 0.15 mm. No spat had settled on either the “Netlon” bags (strand diameter 1.6 mm) or the thicker nylon monofilament (0.8 mm) which was mixed with the thinner material This type of substrate selection may explain why Pettersen (1976) did not have success when attempting to collect spat on more “solid” substrates, such as scallop shells. The natural settlement substrate for Chlamys islandica is unknown. Chlamys opercularis is known to settle on hydroids and bryozoans (Pickett and Franklin, 1975)
319
and it is possible that Chlamys islandica makes use of similar substrata. Brand et al. (1980) have recently reported moderate success at spat collection of Chlamys opercularis, using similar spat collecting bags which contained a variety of materials. The migration of 5-7 mm spat from the monofilament to the outer bags would seem to indicate that migration of scallop seed from the primary setting surface takes place about 1 year after settlement, when they have reached a shell height of over 5 mm. It is, of course, also possible that migration out of the “Netlon” bags occurred before the spat became too large to pass through the 4 mm mesh, in which case the attachment to the “Netlon” rather than to the monofilament might also indicate a change in substrate preference when a size of 5-7 mm had been attained. Sastry (1965) notes that there is no evidence for substrate selection by larval scallops, but this is certainly not the case for Chlamys islandica which favours a filamentous substrate for primary attachment. Possibilities for culture This investigation has shown that it is possible to collect spat of the Iceland scallop, Chlamys islandiqa, in relatively large quantities by artificial means. The possibility for “semiculture” of this species by spat collection and “on-growing” is therefore open. However, except during a short summer period, the natural growth rate at the collection depths is rather slow. A minimum market size of about 6 cm takes, in Balsfjord, at least 6 years to attain. In comparison, Chlamys opercularis spat from the west coast of Scotland can grow to around 14 mm by November of the year of settlement (Ventilla, 1976). The difference is probably due not only to species, but to the fact that Chlamys opercularis spawns earlier (April/May) so that settlement occurs around July. The spat can, therefore, grow during the autumn plankton bloom which occurs in the North Sea area. In north Norway spat settlement of Chlamys islandica occurs when planktonic production is declining. This, together with the lower temperatures can explain the relatively slow growth rate of newly settled spat. It is perhaps surprising that they grow at all during the arctic winter. Pettersen (1976) has found that growth of Chlamys islandica increases if the scallops are transplanted to shallower water and at the same time protected from predation by eider ducks, Somateria mokssima, (Brun, 1971). It is, therefore, possible that a commercially acceptable growth rate can be obtained by transplantion of spat for on-growing. ACKNOWLEDGEMENT
This investigation received financial support from the Norwegian Research Council for Science and the Humanities (NAVF-k.nr. D. 57.45-l).
320 REFERENCES Brand, A.R., Paul, J.D. and Hoogsesteger, J.N., 1980. Spat settlement of the scallops Chlamys opercularis L. and Pecfen maximus L. on artificial collectors. J. Mar. Biol. Ass. U.K., 60: 379-390. Brun, E., 1971. Predation of Chlamys islandica (O.F. Miiller) by Eiders Somateria spp. Astarte, 4: 23-29. Ekman, S., 1953. Zoogeography of the Sea. Sidgwick and Jackson, London, 417 pp. Johannessen, O.H., 1973. Age determination in Chlamys islandica (O.F. Miiller). Astarte, 6: 15-20. Pettersen, I.H., 1976. Populasjonsstruktur, vekst og reproduksjon hos haneskjell (Chlamys islandica (O.F. Miiller)) ved Nordberg, Balsfjord. Cand. real. thesis in marine biology from the University of Tromsd. (In Norwegian). Pickett, G.D. and Franklin, A., 1975. The growth of queen scallops (Chlamys opercularis) in cages off Plymouth, southwest England. ICES, Shellfish and Benthos Committee, report CM 1975/K: 25, 4 pp. Sastry, A.N., 1965. The development and external morphology of pelagic larval and postlarval stages of the bay scallop Aeguipecten irradians concentricus Say, reared in the laboratory. Bull. Mar. Sci., 15: 417-435. Skreset, S., 1973. Spawning in Chlamys islandica (O.F. Miiller) in relation to temperature variations caused by vernal meltwater discharge. Astarte, 6: 9-14. Skreset, S., and Brun, E., 1969. On the reproduction of Chlamys islandica (O.F. Miiller) and its relation to depth and temperature. Astarte, 2: l-6. Ventilla, R.F., 1976. A scallop spat collector trial off the northern Ardnamurchan coast. Scallop Workshop, Baltimore, Ireland, May 1976. Paper AR. 1.15. 1.11.1. 7 pp. Venvik, T. and Vahl, O., 1979. Muligheter og begrensninger for fangst og produksjon av haneskjell. (Possibilities and limitations for fishery and production of the Iceland scallop). Report series E no. 2/79, Institute of Fisheries, University of Tromsd. (In Norwegian).
311
THE CULTURE OF THE ICELAND SCALLOP, CHLAMYS ISLANDICA (O.F. MULLER) I. SPAT COLLECTION AND GROWTH DURING THE FIRST YEAR
JEFF
C. WALLACE
Institute of Fisheries,
University of Tromsd, Postbox 488, N-9001 Tromsd (Norway)
(Accepted 16 April 1981)
ABSTRACT Wallace, J.C., 1982. The culture of the Iceland scallop, Chlamys islandica (O.F. Miiller). I. Spat collection and growth during the first year. Aquaculture, 26: 311-320. Spat of the Iceland scallop, Chlamys islandica (O.F. Miiller), was artificially collected using thin nylon monofilament as a substrate for settlement. Collectors placed at depths between 15 m and 40 m in Balsfjord, north Norway, collected most spat at depths of between 30 m and 40 m. After settlement in September the spat grew slowly until the following spring (April/May) when growth accelerated. One year after settlement a shell height of about 7 mm was attained. There was also some evidence that secondary migration in this species begins about 1 year after settlement.
INTRODUCTION
Of the European pectinid bivalves, or scallops, only two species, Pecten maximus and Chlamys opercularis, have been exploited to a significant degree by fishing. The interest in these species has in recent years been extended to include the possibility of artificial collection and rearing of spat, with a view to “scallop culture”. In Japan the culture of pectinid molluscs has been established for some years. The Iceland scallop, Clamys islandica, has a more northerly distribution than Chlamys opercularis, and the two species are rather similar in size and appearance. Chlamys isZandica occurs mainly within the subarctic transitional zone (Ekman, 1953), sometimes in quantities and concentrations sufficient to warrant a fishery. So far only Iceland has developed a fishery for the species, but some interest is now being shown in north Norway (Venvik and Vahl, 1979). The present investigation was undertaken with a view to establishing whether or not spat of Chlamys islandica could be artificially collected using methods similar to those proven for Chlamys opercularis and Pecten maximus and to determine the natural growth rate of newly settled spat.
0044-8486/82/0000-0000/$02.75
0 1982
Elsevier Scientific Publishing Company
312 MATERIALS
AND METHODS
Collec tars
The spat collectors consisted of “bags” made of plastic netting and containing quantities of nylon monofilament line. Fig. 1 shows a diagram of the spat collector system. Each collecting bag was made by sewing together
Subsurface
Netion
buoy
bag
Fig.1. Diagram of the spat collector system.
the edges of a 1 m2 square of “Netlon” plastic netting so that a cylinder was obtained. One end of the cylinder was then compressed and its edges sewn together. The resulting bag was filled with a “tangle” of nylon monofilament and sealed by compressing the open end at right angles to that of the closed end and sewing the edges together. The result was a closed, tetrahedron-shaped mesh bag, filled with nylon monofilament. The “Netlon” was of 4 mm mesh size, and the nylon monofilament was mainly 0.15 mm in diameter with some thicker (0.8 mm diameter) line in a few of the bags. The bags were fastened to nylon ropes, usually four bags to a rope, at 5 m intervals. The ropes were attached to a weight or anchor at one end and to an underwater float and a marker float at the other. Placement
of the collectors
The collectors were placed on the scallop bed at the entrace to Balsfjord near Trams@ (lat. 69’30’ N, long. 19’00’ E). Fig.2 shows the position of the bed. Previous investigations on this scallop population have shown that spawning occurs during a short period at the beginning of July (Skreset and
313
BALSFJORD
ARCTIC -__--___
CIRC
__---
Fig.2. Outline map of Norway showing the area mentioned in the text. The position the scallop beds in the region of TromsQ is shown in the inset.
of
314
Brun 1969; Skreset 1973). The collectors were, therefore, placed early in September (1978) which was assumed to be around the time of maximum spatfall. Since the depths at which spat settlement occurs were unknown the collectors were placed at arbitarily chosen depths, both in shallower and in deeper water than the main concentration of adult scallops. The ropes were placed so that the lowest collectors were at a depth of 40 m, and the others at respectively 35 m, 30 m and 25 m. On one rope six collectors were attached, so that this rope could also collect spat at 20 m and at 15 m. Sampling The rope with six collectors was taken up 26 days after placement. Scallop spat were found to be present, the majority being taken from the lowest collectors. The ropes were thereafter taken up singly at roughly monthly intervals over a period of one year. Spat were obtained from the collectors by allowing the nylon monofilament to dry and then shaking it gently over a sheet of black plastic. The dried spat were detached by this action, and they could be picked up from the plastic sheet with a small moistened brush. For spat less than 2 mm in shell diameter a magnifying glass was necessary. The spat from each collector were counted and the largest “diameter” from umbo to opposite shell edge measured using a “Wild M 8” stereomicroscope fitted with a calibrated measuring occular. These measurements were made on a sub-sample of 30 spat, or on the total sample if this was less than 30. RESULTS
Fig.3 shows Chlamys islandica spat of various sizes, taken from the collectors. Depth of settlement Table I shows the numbers of spat recovered from the collectors and the depths at which they were recovered. With the exception of one collector, which collected very few spat, the collectors took most spat at depths of 30 m and greater. The numbers of spat per collector varied from a minimum of 16 to a maximum of 1791, but no collector failed to collect spat. Growth of spat Fig.4 shows the average size of the spat in the samples at each sampling date. Since there is no overlap in the size distributions of the spat collected in September and in November, it is likely that no further settlement occurred after the beginning of September. The growth can, therefore, be taken to
315
Fig.3.
Spat of differing
ages taken from the spat collectors.
indicate the rate of growth of the scallop spat in that area. It can be seen t,hat the spat grew slowly, but steadily, throughout the winter, frc3m an initial shell diameter of less than 0.5 mm at settlement in Septembc zr to about 2. 0 mm in April. Around April/May the growth rate started to a8ccelerate and a high growth rate was achieved in the summer months. There is an indicati on that the growth rate was beginning to fall by September. Attachment The spat were always found attached
to the thin, 0.15 mm nylon mono-
I
0.50 f 0.04
f
0.93 0.13
f
39 20 23 23
1.10 0.26
26
105
3/l
2
1.84 0.22
687
2748
435 1791 416 106
31/l
*Not counted because of small size and large numbers.
Mean shell height (mm)
812
Mean
228 1010 1289 720
2111
3247
23 36 16 36 Many* Many*
2719
Numbers of spat present at date of sampling
Total
15 20 25 30 35 40
Depth (ml
f
1.87 0.48
861
3443
431 988 646 1378
l/3
f
1.95 0.37
344
1377
72 329 421 555
1914
*
2.45 0.39
622
2488
571 455 867 595
2915
f
5.90 0.64
476
1904
185 448 603 668
31/7
+
6.96 0.81
312
1247
114 350 251 532
10/9
2091 5427 4516 4577
Total
232 603 565 572
Mean
Data obtained from the spat collectors showing the numbers of spat which settled at each of the collection depths, and their mean size + standard deviation
TABLE
317
/ / i
2-
A’
.A/
‘:
,,‘-bp 5
0
I 2.0 N
0 Diiii
F
M
&FTER SETTLEMENT
a
360
300
240
180 J
M
J
J
P
5
&ND TlME OF YEAR
Fig.4. Mean growth of the Iceland scallop spat during the first year after settlement.
filament. No spat settled on either the “Netlon” outer bag or the thicker 0.8 mm diameter monofilament, and none were found on the ropes. However, when the last two samples were examined, a proportion of the scallop “seed”, now about 5-7 mm and around one year old, was found attached to the inside surface of the outer bag. In July 18% of the total seed was so attached and the proportion had increased to 35% by September. DISCUSSION
Growth The growth data obtained in this investigation confirm the assumptions of Johannessen (1973) who considered shells which were between 1.1 and 2.8 mm in spring (February-April) to be progeny of the previous summer’s spawning and those between 7.1 mm and 10.7 mm to be one year older. Determination of the earliest growth from annual rings on either the shell valves or the shell ligaments of older shells is almost impossible since no mark is laid down during the first winter. It is, therefore, probable that some workers have underestimated the age of older scallops by one year. Combining the growth data obtained in this investigation with data provided by Pettersen (1976) a growth curve for this population of Chlamys islundica can be drawn. Such a curve is shown in Fig.5 for scallops at a depth of 30 m. The scallops are almost 2 years old, taken from date of spawning, before they reach a shell height of more than 1 cm, and it takes 6 years for the shells to attain a height of 6 cm. According to Pettersen (1976) the average growth at 30 m on this scallop bed is higher than at 15 m, 40 m or 60 m.
1
2
3 YEARS
4 AFTER
5
6
7
8
SPATFALL
Fig.5. Curve showing the growth rate of Iceland scallops at a depth of 30 m in Balsfjord, north Norway. The data collected during the present study is represented in the first part of the curve, up to the large circle.
Depth
of settlement
The numbers of spat collected fell off sharply at between 30 m and 25 m (Table I) whilst fairly large numbers were collected at 40 m, the lowest depth sampled. It is therefore possible that setting occurs in even deeper water. One possible reason for the sparse settlement at depths above 30 m could be competition. The spat collectors at 25 m and in shallower water were always overgrown with brown algae which, being relatively fast growers in the summer season, might inhibit settling of scallop spat or kill by overgrowing any spat which settled. Comparisons of spatfall between the collectors should, of course, be treated with caution since the surface area of the settlement surface, the monofilament, although roughly the same, was not standardised in the different bags. Substrate
The spat was always found on the thinnest nylon monofilament which had a diameter of 0.15 mm. No spat had settled on either the “Netlon” bags (strand diameter 1.6 mm) or the thicker nylon monofilament (0.8 mm) which was mixed with the thinner material This type of substrate selection may explain why Pettersen (1976) did not have success when attempting to collect spat on more “solid” substrates, such as scallop shells. The natural settlement substrate for Chlamys islandica is unknown. Chlamys opercularis is known to settle on hydroids and bryozoans (Pickett and Franklin, 1975)
319
and it is possible that Chlamys islandica makes use of similar substrata. Brand et al. (1980) have recently reported moderate success at spat collection of Chlamys opercularis, using similar spat collecting bags which contained a variety of materials. The migration of 5-7 mm spat from the monofilament to the outer bags would seem to indicate that migration of scallop seed from the primary setting surface takes place about 1 year after settlement, when they have reached a shell height of over 5 mm. It is, of course, also possible that migration out of the “Netlon” bags occurred before the spat became too large to pass through the 4 mm mesh, in which case the attachment to the “Netlon” rather than to the monofilament might also indicate a change in substrate preference when a size of 5-7 mm had been attained. Sastry (1965) notes that there is no evidence for substrate selection by larval scallops, but this is certainly not the case for Chlamys islandica which favours a filamentous substrate for primary attachment. Possibilities for culture This investigation has shown that it is possible to collect spat of the Iceland scallop, Chlamys islandiqa, in relatively large quantities by artificial means. The possibility for “semiculture” of this species by spat collection and “on-growing” is therefore open. However, except during a short summer period, the natural growth rate at the collection depths is rather slow. A minimum market size of about 6 cm takes, in Balsfjord, at least 6 years to attain. In comparison, Chlamys opercularis spat from the west coast of Scotland can grow to around 14 mm by November of the year of settlement (Ventilla, 1976). The difference is probably due not only to species, but to the fact that Chlamys opercularis spawns earlier (April/May) so that settlement occurs around July. The spat can, therefore, grow during the autumn plankton bloom which occurs in the North Sea area. In north Norway spat settlement of Chlamys islandica occurs when planktonic production is declining. This, together with the lower temperatures can explain the relatively slow growth rate of newly settled spat. It is perhaps surprising that they grow at all during the arctic winter. Pettersen (1976) has found that growth of Chlamys islandica increases if the scallops are transplanted to shallower water and at the same time protected from predation by eider ducks, Somateria mokssima, (Brun, 1971). It is, therefore, possible that a commercially acceptable growth rate can be obtained by transplantion of spat for on-growing. ACKNOWLEDGEMENT
This investigation received financial support from the Norwegian Research Council for Science and the Humanities (NAVF-k.nr. D. 57.45-l).
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