- Email: [email protected]
The Iceland scallop, Chlamys islandica (O.F Müller), in Breidafjördur, west Iceland. III. Growth in suspended culture
Aquaculture 120 (1994) 295-303
The Iceland scallop, Chlamys islandica (0.F Miiller), in BreidafjSrdur, west Iceland. III. Growth in suspended culture Gudnin
G. Thorarinsdbttir
Marine Research Institute, P.O. Box 1390, 121 Reykjavik. Iceland
(Accepted 27 September 1993)
Abstract During two years, from September 1989 to September 199 1, a study was carried out to evaluate the potential of pearl net cultivation of Iceland scallop, Chlumys islandica (0.F Miiller ), in BreidafjSrdur, west Iceland. In September 1989 l-year-old spat (mean height 9.8 mm+ 2.0 s.d.) were transferred from collectors to pearl nets suspended from flotation at 6-8 m depth. Shell increment, chlorophyll-a, temperature and salinity were monitored each month at the experimental site. During the second year in the pearl net culture, growth of soft parts of the body was also measured. By September 1990 the scallops had an average shell height of 24.6 mm? 6.0 s.d. and in September 1991, three years after settlement, a height of 43 mm If:8.3 s.d. was reached. Growth rate was related to food availability (measured as chlorophyll-u). During both years of the investigation height-specific growth rate reached a maximum (0.7% day-’ and 0.3% day-‘, respectively) in early spring and again in summer, but it gradually decreased as the scallops grew older. The growth season lasted from March to October during both years of investigation. The results showed that the growth of Iceland scallops can be increased markedly by suspending the animals in hanging culture in more favourable environmental conditions than found on the natural scallop beds. The feasibility of cultivating Chlamys islandica in BreidaljBrdur is discussed.
1. Introduction A fishery for the Iceland scallop, Chlamys islandica, was established in west Iceland during 1969. Landings increased rapidly from 400 tonnes in the first year to a maximum 17400 tonnes in 1985, but decreased somewhat after that (Eiriksson, 1986). In 199 1 landings were 10300 tonnes of which about 9000 tonnes were caught in BreidafjBrdur (Anonymous, 1992 ). This project investigated the biological feasibility of using Japanese techniques 0044-8486/94/$07.00
0 1994 Elsevier Science B.V. All rights reserved
SSDI0044-8486(93)E0228-2
296
G.G. Thorarinsdbttir /Aquaculture 120 (1994) 295-303
to cultivate the Iceland scallop in BreidaIjBrdur. A previous paper (Thorarinsdottir, 199 1) reported that spat of Chlurn~~sislundica settled readily in spat collectors made of polyethylene bags containing 0.2 mm monofilament gill netting, placed 2-10 m above the seabed. This paper decribes further investigations of growth of l-year-old spat when transferred from spat collectors to pearl nets in hanging culture, and how temperature, salinity and food availability influence growth. Available information on the growth of natural populations of Chlumys islundica in Breidafjbrdur indicates that it takes about 6-7 years for scallops to reach marketable size ( 6 cm) (Eirfksson, 1986). From northern Norway investigations on the growth of Chlamys islundicu in hanging culture indicate that depth and food have marked influence on the growth rate (Wallace and Reinsnes, 1984, 1985).
2. Materials and methods The experimental locality was almost the same as previously described for the spat collection (Thorarinsdbttir, 1991), i.e. in Breidafjiirdur, west Iceland (65”02’N-22”49’W ), where there is a large natural population of Chlamys isZundicu (Fig. 1) . This site was chosen for the hanging culture because of limited fishing or local traffic as well low probability of damage to pearl nets caused by ice. The young scallops were transferred from the spat collectors to the pearl nets ( 30 x 30 x 30 cm, 4.5 mm mesh size) in the middle of September 1989, approximately 1 year after settlement. Each pearl net was stocked with 50 scallops of shell height of 9.8 mm ? 2.0 s.d. The density used was kept low since the scallops were to be in the pearl nets for the next 2 years. A total of 120 pearl nets was placed in groups of three on 40 ropes and suspended from two buoyed lines. The pearl nets were kept at a depth of 6-8 m, the bottom one being about 12 m above the seabed (Fig. 2). Each month, from October 1989 to September 199 1, one rope with three 22’ w
Fig. 1. A map of Iceland. The black square denotes the position of the experimental ern coast of Breidafjiirdur.
site on the south-
G.G. Thorarinsdcittir/Aquaculture 120 (1994) 295-303
297
Fig. 2. Diagram of culture cages.
pearl nets was sampled. In the laboratory the shell height of the juveniles, defined as the maximum distance between dorsal and ventral margin, was recorded to the nearest 0.1 mm using vernier calipers. Mean height was then calculated on each occasion and instantaneous growth rate (G) was estimated by the formula: G={(lnL2-lnL1)/(72-T1)}x100
(BalandJones,
1960).
From September 1990, when the scallops had reached a height of 25 mm, weights of adductor muscle, gonad and other soft parts (mantle, gills, and digestive gland) were determined separately after drying at 60°C for 48 h. Each month from September 1989 to September 199 1, measurements of chlorophyll-u and salinity were made from samples taken at 8 m depth at the experimental site. Chlorophyll-u was measured by filtering 1 liter of water through a Whatman GF/C glassfilter. The filter with the sample was extracted in 90% acetone and the amount of chlorophyll-a determined spectrophotometrically as described by Parsons and Strickland ( 1963). Salinity was measured with an inductive salinometer. Temperature was measured at 8 m depth each hour during the experimental period by a self-registering thermometer. Mean temperature was then calculated for each month. 3. Results Of the 120 pearl nets placed in the sea, 72 were retrieved. Thirty-nine cages were lost or damaged because of a heavy storm during the winter of 1990. Mortality in the cages was about 10% during the first year and 15% in the second year. Seasonal changes in shell height? standard deviation of juveniles in the pearl nets are shown in Fig. 3. Almost no growth was observed from the time of transportation in September until March when the growth season started. In November when growth ceased again the juveniles had attained a mean shell height of
298
G.G. Thorarinsdbttir /Aquaculture 120 (1994) 295-303
Ssp
Nov
Jan
Mar
May
Jut
Sep
Nov
Jan
Mar
1990
lSl39
May
Jul
Sep
1991
Fig. 3. Mean height ( f s.d.) of juvenile Iceland scallop during 2 years in suspended culture. Table 1 Height-specific instantaneous growth rate (G) of juvenile scallop, mean monthly temperature and chlorophyll-a at 8 m depth at the culture site First year in suspended culture
Second year in suspended culture
Date 1989-1990
G (% day-‘)
Temp. (“C)
Chl-a (mgm-3)
Date 1990-1991
G (% day-‘)
Temp. (“C)
Chl-a (mgmm3)
September October
0.9 0.2 0.0 0.0 0.1 0.0 0.2 0.3 0.7 0.2 0.3 0.7
9.1 5.9 4.4 3.0 1.5 0.6 0.0 1.1 4.0 7.0 10.0 11.0
1.5 0.8 0.3 0.1 0.1 0.2 1.1 1.2 2.0 1.1 3.8 2.2
September October November December January February March April May June July August September
0.2 0.1 0.2 0.0 0.0 0.1 0.2 0.3 0.1 0.1 0.3 0.3 0.2
8.5 6.0 4.9 3.3 2.0 2.6 2.6 2.8 5.7 8.4 10.8 11.4 9.7
1.4 1.1 0.9 0.6 0.2 0.1 0.8 1.2 0.7 2.2 2.6 1.6 1.5
December January February March April May June July August
27 mm +- 5.7 s.d. The greatest monthly shell increment, the first year in the pearl nets, was from April to May (3.1 mm) and again from July to August (3.8 mm). A similar pattern of growth was recorded during the second year, with the greatest monthly shell increment registered from March to April (3.0 mm), and from June to July ( 3.9 mm). In September 199 1 the scallops had reached a mean height of 43 mm+ 8.3 s.d. The seasonal variation in instantaneous growth rate (% day-’ ) is shown in Table 1. In September 1989 when the spat was transferred to the pearl nets, the
299
G.G. Thorarinsdbttir /Aquaculture 120 (1994) 295-303
mean growth rate was 0.9% day-‘. After the transfer the growth rate declined and was minimal from November to February after which it increased again. Maximum growth rate was observed in May and again in August (0.7% day-‘) during this first year in the pearl nets. The instantaneous growth rate of the scallops decreased during the second year, but showed almost the same seasonal variations as the year before. A minimum was observed from December to February and a maximum in April, July and August ( 0.3% day- ’ ) . Fig. 4 shows the changes in the dry weight of gonads, adductor muscle, other soft parts (mantle+gills+digestive gland) and total soft parts of juvenile scallops in the size range 25-45 mm from September 1990 to September 199 1. From October there was a slight decrease in the dry weight of all soft parts until a minimum was reached in February, after which they slowly started increasing. The scallops had reached a mean height of 30-35 mm in June and by then most of them were sexually mature. At that time there was a great increase in the dry weight of all soft parts, which except for the gonads, increased further until a maximum was reached in August. In all individuals investigated the gonads were first visible in May 1991 at an age of 2.5 years and a height of 30-35 mm. The average dry weight of the gonads, measured as % total dry weight of the soft parts, was relatively low, 6.5, 10.8, 6.4, 3.8 and 4.1% from May to September. The seasonal variations in temperature and chlorophyll-u are shown in Table 1. During both years of investigation the mean temperature was highest in August ( 11.0 and 11.4”C, respectively) but during winter and spring the temperature was about l-2’ C higher in 199 1 than 1990. In 1990 the lowest mean temperature of - 0.2’ C was measured in March, while in 199 1 the lowest temperature of 2.0 oC occurred in January. Salinity varied little over the study period. A minimum (33.7-33.8%) was observed in May of both years while the maximum was in April 1991 (34.7%0).
.O- Mantle, .a-
0.7
Adductor
gllls,
dlgestlve
gland
muscle
A--*
l-+-w-*t~l-*+~++t~~: Sep
Ott
NOV
Dee
Jan
Feb
Mar
Apr
May
Jun
:
: Jul
: Aug
I SeP
Fig. 4. Seasonal variation in the dry weight of soft parts of scallop of 25-45 mm height during September 1990 to September 199 1.
300
G.G. Thorarinsdbftir /Aquaculture 120 (1994) 295-303
During both years of investigation the chlorophyll-u level was at a minimum through the winter but started increasing in March reaching a maximum in July, 3.8 mg rnM3and 2.6 mg mm3, respectively.
4. Discussion In the middle of September 1988 spat of Chlamys islandica were artificially collected in Breidafjiirdur. The spat grew well during the whole of the first year in the collectors. The maximum growth rate was 3.6% day-‘, but it was somewhat reduced during the winter (Thorarinsdottir, 199 1) . A year after settlement, when the spat was transferred from the collectors to pearl nets for suspended culture, they had reached an average height of 9.8 mm. During both the years of investigation, the growth season of the scallops in the pearl nets lasted from March until October and was found to be related to availability of phytoplankton. Variations in salinity were only slight and hardly of importance. In March there was an increase in the phytoplankton biomass but the temperature was at the annual minimum. The maximum growth rate was observed in spring as phytoplankton biomass had increased while water temperature was still low and again in summer when coinciding with near maximum temperature and chlorophyll-a levels. The low temperature observed in March appeared not to inhibit growth, a fact that has also been noted for Chlamys islandica in northern Norway (Wallace and Reinsnes, 1985 ) , Chlamys opercularis in Denmark (Ursin, 1956) and England (Broom and Mason, 1978), and Pecten maximus off the Isle of Man (Mason, 1957). Sea temperature and food have often been discussed as the two main factors contributing towards favourable conditions for growth in bivalves. Some authors have been able to demonstrate a significant correlation between water temperature and shell growth (Kirby-Smith and Barber, 1974; Richardson et al., 1982) while others have not (Page and Hubbard, 1987). Food availability has often been found to exert a greater influence on growth rate than temperature in temperate scallop species (MacDonald and Thompson, 1985 ) as well as in species inhabiting regions of continually low temperatures, such as Chlumys islandica (Vahl, 1978). There was a marked reduction in growth rate during the second year in the pearl nets. This was mainly observed as a lower maximum growth rate in spring and summer, in spite of almost the same amount of phytoplankton food available (Table 1) . The reason for the observed difference was probably that the animals became bigger and therefore the ratio of water transport to oxygen consumption decreased. Further, Vahl ( I98 1) has pointed out that the fraction of ingested food available for growth declines with size, and alter a certain age, somatic growth declines in relation to the gonad output. Thus it was evident during the second year, that the animals, at an age of 2.5 years, started developing gonads (Fig. 4) and therefore a smaller fraction of the ingested food may have been used for
G.G. Thorarinsdbttir /Aquaculture 120 (1994) 295-303
301
growth. This was similarly found by Richardson et al. ( 1980, 1982) for the cockle, Cerastoderma edule, and the scallop, Chlamys opercularis, respectively. Increase in shell height has often been used as a measure of growth in bivalves, but since shell height is irreversible it is questionable whether changes in shell height reflect the patterns of growth of soft body tissues. In this study the growth of soft body parts in scallops of 25-45 mm height followed almost the same pattern as the shell growth, except in autumn when dry weight of the soft parts decreased but the shell height was still increasing. The seasonal changes in the dry weight of the muscle are believed mostly to be due to storage and utilization of protein and glycogen, where the muscle functions as an energy depot (Sundet and Vahl, 198 1). The seasonal variations in the combined group of mantle, gills and digestive glands are most probably due to varations in the digestive gland rather than in gills and mantle. Sundet and Vahl ( 198 1) found no consistent seasonal changes in dry weight of mantle and gills in Chlamys islandica but Sastry and Blake ( 197 1) reported a direct transport of stored lipids from the digestive gland to the gonad in Aequipecten irradians. Comparison of the shell growth of scallops on natural beds in Breidafjordur (Eirfksson, 1986) with that of the suspended culture at the same place during the first 3 years after settlement, shows a similar growth in the first year after settlement ( 10 mm). This may be explained by initial settlement of the natural spat on hydroids and bryozoans just above the bottom (Pickett and Franklin, 1975 ) where food supply and temperature were similar to that in the collectors. After the first year, however, the spat in nature migrate to the bottom with growth rate being reduced below that of the hanging culture. Eirfksson (1986) found the growth rate of Iceland scallop on natural beds in the second year after settlement to be about 90% and in the third year 60%. In this study the growth rate in the second year was 150% and in the third year 80%. The growth in shell height shown by the Iceland scallop after 5 years of natural growth was attained in 3 years in the suspended culture. The culture of the Iceland scallop in pearl nets situated up in the water column may have resulted in a longer growth season and considerably enhanced growth compared with that of natural beds. Scallops freely suspended in the water column have more access to phytoplankton food whereas the food supply for bottom dwelling scallop is composed of allochthonous detrital material, resuspended sediment plus phytoplankton (Sundet, 1988 ), a mixture which is not as energy rich as phytoplankton alone (Vahl, 1980). In this study the stocking density in the pearl nets was kept low in the beginning (Hovgaard, 1984)) but scallops on natural beds are possibly overcrowded and have to compete for space and food, and the density in Breidafjiirdur can reach 130 adults mm2 (Eirfksson, 1986 ). Chlamys islandica has a relatively slow natural growth rate, and even though it can be increased markedly in hanging culture in BreidaIjiirdur, the 4 years required to reach market size (6-7 cm) may prove to be too long to warrant the additional costs associated with the suspended culture. In order to fully evaluate the mariculture potential of this species in Iceland, additional research should include rigorous selection of sites to provide optimal survival and growth condi-
302
G.G. Thorarinsdbttir /Aquaculture I20 (1994) 295-303
tions. The area chosen for culture should be sheltered and protected from storm damage and should not be located where strong tidal currents can damage gear or disturb growth. Use of larger spat and still lower initial stocking density in the pearl nets than used here may yield larger scallops in 3 years, but extra cost of producing larger spat and additional pearl nets may be offsetting.
Acknowledgements
Thanks are due to the National Research Council in Iceland and the Marine Research Institute for financial support. Valuable assistance has been given by the staff at the Marine Research Institute, Sigurdur Agustsson h/f, and PCtur Agustsson to all of whom I owe my sincere thanks. I thank Olafur S. Astthorsson for critical reading of the manuscript and linguistic help.
References Anonymous, 1992. State of Marine Stocks and Environmental Conditions in Icelandic waters 1992. Hafrannsoknastofnun, 29: 133 pp. Bal, J.N. and Jones, J.W., 1960. On the growth of brown trout of Llyn Tegid. Proc. Zool. Sot. London, 134: 1-41. Broom, M.J. and Mason, J., 1978. Growth and spawning in the pectinid Chlamys opercularis in relation to temperature and phytoplankton concentration. Mar. Biol., 47: 277-285. Eiriksson, H., 1986. Hiirpudiskurinn, Chlamys islandica, (Miiller). Hafrannsoknir, 35: 5-40 (in Icelandic with English summary). Hovgaard, P., 1984. Miljoets betydning for vekst hos haneskjell. Nor. Fiskeoppdr., 4: 33-36. Kirby-Smith, M.W. and Barber, R.T., 1974. Suspension-feeding aquaculture systems: effects of phytoplankton concentration and temperature on growth of the bay scallop. Aquaculture, 3: 135-145. MacDonald, B.A. and Thompson, R.J., 1985. Influence of temperature and food availability on the ecological energetics of the giant scallop Placopecten magellanicus. I. Growth rates of shell and somatic tissue. Mar. Ecol. Prog. Ser., 25: 279-294. Mason, J., 1957. The age and growth of the scallop Pecten maximus (L.) in Manx waters. J. Mar. Biol. Assoc. UK, 36: 473-492. Page, H.M. and Hubbard, D.M., 1987. Temporal and spatial patterns of growth in mussels Mytilus edulis on an offshore platform: relationships to water temperature and food availability. J. Exp. Mar. Biol. Ecol., 111: 159-179. Parsons, T.R. and Strickland, J.D.H., 1963. Discussion of spectrophotometric determination of marine plant pigments, with revised equations for ascertaining chlorophylls and carotenoids. J. Mar. Res., 21: 155-163. Pickett, G.D. and Franklin, A., 1975. The growth ofthe queen scallops (Chlamys opercularis) in cages off Plymouth, south-west of England. International Council for the Exploration of the Sea. Shellfish Committee, C.M. 1975/K: 24,4 pp. Richardson, C.A., Crisp, D.J., Runham, N.W. and Gruffydd, L., 1980. The use of tidal growth bands in the shell of Cerastoderma edule to measure seasonal growth rates under cool temperate and subarctic conditions. J. Mar. Biol. Assoc. UK, 60: 977-989. Richardson, C.A., Taylor, A.C. and Venn, T.J., 1982. Growth of the queen scallop, Chlamys opercularis in suspended cages in the Firth of Clyde. J. Mar. Biol. Assoc. UK, 62: 157-l 69. Sastry, A.N., and Blake, N.J., 197 1. Regulation of gonad development in the bay scallop Aequipecten irradians Lamarck. Biol. Bull. Mar. Biol. Lab., Woods Hole, Mass., 140: 48 pp.
G.G. Thorarinsdbttir / Aquaculture 120 (1994) 295-303
303
Sundet, J.H., 1988. Haneskjellets biology. Ottar, 2. Populrervitenskapelig tidsskrift fra Tromso Museum, 170: 3-8. Sundet, J.H. and Vahl, O., 1981. Seasonal changes in dry weight and biochemical composition of the tissues of sexually mature and immature Iceland scallops, Chlamys islandica. J. Mar. Biol. Assoc. UK, 61: 1001-1010. Thorarinsdottir, G.G., 199 1. The Iceland scallop, Chlamys islandica (O.F. Miiller ) in Breidafjordur, west Iceland. I. Spat collection and growth during the first year. Aquaculture, 97: 13-23. Ursin, E., 1956. Distribution and growth of the queen scallop, Chlamys opercularis (Lamellibranchiata). in Danish and Faroese waters. Meddr. Danm. Fisk. Havunders., 1: l-3 1. Vahl, O., 1978. Seasonal changes in oxygen consumption of the Iceland scallop (Chlamys islandica, O.F. Miiller) from 70”N. Ophelia, 17: 143-l 54. Vahl, O., 1980. Seasonal variations in seston and in the growth rate of the Iceland scallop, Chlamys islandica (O.F. Mtlller) from Balstjord, 70”N. J. Exp. Mar. Biol. Ecol., 48: 195-204. Vahl. O., 198 1. Energy transformations by the Iceland scallop, Chlamys islandica (O.F. Miiller) from 70”N. I. The age-specific energy budget and net growth efftciency. J. Exp. Mar. Biol. Ecol.. 53: 281-296. Wallace, J.C. and Reinsnes, T.G., 1984. Growth varation with age and water depth in the Iceland scallop ( Chlamys islandica, Pectinidae) . Aquaculture, 4 1: 14 1- 146. Wallace, J.C. and Reinsnes, T.G., 1985. The significance of various environmental parameters for growth of the Iceland scallop (Chlamys islandica, Pectinidae) in hanging culture. Aquaculture, 44: 229-242.
The Iceland scallop, Chlamys islandica (0.F Miiller), in BreidafjSrdur, west Iceland. III. Growth in suspended culture Gudnin
G. Thorarinsdbttir
Marine Research Institute, P.O. Box 1390, 121 Reykjavik. Iceland
(Accepted 27 September 1993)
Abstract During two years, from September 1989 to September 199 1, a study was carried out to evaluate the potential of pearl net cultivation of Iceland scallop, Chlumys islandica (0.F Miiller ), in BreidafjSrdur, west Iceland. In September 1989 l-year-old spat (mean height 9.8 mm+ 2.0 s.d.) were transferred from collectors to pearl nets suspended from flotation at 6-8 m depth. Shell increment, chlorophyll-a, temperature and salinity were monitored each month at the experimental site. During the second year in the pearl net culture, growth of soft parts of the body was also measured. By September 1990 the scallops had an average shell height of 24.6 mm? 6.0 s.d. and in September 1991, three years after settlement, a height of 43 mm If:8.3 s.d. was reached. Growth rate was related to food availability (measured as chlorophyll-u). During both years of the investigation height-specific growth rate reached a maximum (0.7% day-’ and 0.3% day-‘, respectively) in early spring and again in summer, but it gradually decreased as the scallops grew older. The growth season lasted from March to October during both years of investigation. The results showed that the growth of Iceland scallops can be increased markedly by suspending the animals in hanging culture in more favourable environmental conditions than found on the natural scallop beds. The feasibility of cultivating Chlamys islandica in BreidaljBrdur is discussed.
1. Introduction A fishery for the Iceland scallop, Chlamys islandica, was established in west Iceland during 1969. Landings increased rapidly from 400 tonnes in the first year to a maximum 17400 tonnes in 1985, but decreased somewhat after that (Eiriksson, 1986). In 199 1 landings were 10300 tonnes of which about 9000 tonnes were caught in BreidafjBrdur (Anonymous, 1992 ). This project investigated the biological feasibility of using Japanese techniques 0044-8486/94/$07.00
0 1994 Elsevier Science B.V. All rights reserved
SSDI0044-8486(93)E0228-2
296
G.G. Thorarinsdbttir /Aquaculture 120 (1994) 295-303
to cultivate the Iceland scallop in BreidaIjBrdur. A previous paper (Thorarinsdottir, 199 1) reported that spat of Chlurn~~sislundica settled readily in spat collectors made of polyethylene bags containing 0.2 mm monofilament gill netting, placed 2-10 m above the seabed. This paper decribes further investigations of growth of l-year-old spat when transferred from spat collectors to pearl nets in hanging culture, and how temperature, salinity and food availability influence growth. Available information on the growth of natural populations of Chlumys islundica in Breidafjbrdur indicates that it takes about 6-7 years for scallops to reach marketable size ( 6 cm) (Eirfksson, 1986). From northern Norway investigations on the growth of Chlamys islundicu in hanging culture indicate that depth and food have marked influence on the growth rate (Wallace and Reinsnes, 1984, 1985).
2. Materials and methods The experimental locality was almost the same as previously described for the spat collection (Thorarinsdbttir, 1991), i.e. in Breidafjiirdur, west Iceland (65”02’N-22”49’W ), where there is a large natural population of Chlamys isZundicu (Fig. 1) . This site was chosen for the hanging culture because of limited fishing or local traffic as well low probability of damage to pearl nets caused by ice. The young scallops were transferred from the spat collectors to the pearl nets ( 30 x 30 x 30 cm, 4.5 mm mesh size) in the middle of September 1989, approximately 1 year after settlement. Each pearl net was stocked with 50 scallops of shell height of 9.8 mm ? 2.0 s.d. The density used was kept low since the scallops were to be in the pearl nets for the next 2 years. A total of 120 pearl nets was placed in groups of three on 40 ropes and suspended from two buoyed lines. The pearl nets were kept at a depth of 6-8 m, the bottom one being about 12 m above the seabed (Fig. 2). Each month, from October 1989 to September 199 1, one rope with three 22’ w
Fig. 1. A map of Iceland. The black square denotes the position of the experimental ern coast of Breidafjiirdur.
site on the south-
G.G. Thorarinsdcittir/Aquaculture 120 (1994) 295-303
297
Fig. 2. Diagram of culture cages.
pearl nets was sampled. In the laboratory the shell height of the juveniles, defined as the maximum distance between dorsal and ventral margin, was recorded to the nearest 0.1 mm using vernier calipers. Mean height was then calculated on each occasion and instantaneous growth rate (G) was estimated by the formula: G={(lnL2-lnL1)/(72-T1)}x100
(BalandJones,
1960).
From September 1990, when the scallops had reached a height of 25 mm, weights of adductor muscle, gonad and other soft parts (mantle, gills, and digestive gland) were determined separately after drying at 60°C for 48 h. Each month from September 1989 to September 199 1, measurements of chlorophyll-u and salinity were made from samples taken at 8 m depth at the experimental site. Chlorophyll-u was measured by filtering 1 liter of water through a Whatman GF/C glassfilter. The filter with the sample was extracted in 90% acetone and the amount of chlorophyll-a determined spectrophotometrically as described by Parsons and Strickland ( 1963). Salinity was measured with an inductive salinometer. Temperature was measured at 8 m depth each hour during the experimental period by a self-registering thermometer. Mean temperature was then calculated for each month. 3. Results Of the 120 pearl nets placed in the sea, 72 were retrieved. Thirty-nine cages were lost or damaged because of a heavy storm during the winter of 1990. Mortality in the cages was about 10% during the first year and 15% in the second year. Seasonal changes in shell height? standard deviation of juveniles in the pearl nets are shown in Fig. 3. Almost no growth was observed from the time of transportation in September until March when the growth season started. In November when growth ceased again the juveniles had attained a mean shell height of
298
G.G. Thorarinsdbttir /Aquaculture 120 (1994) 295-303
Ssp
Nov
Jan
Mar
May
Jut
Sep
Nov
Jan
Mar
1990
lSl39
May
Jul
Sep
1991
Fig. 3. Mean height ( f s.d.) of juvenile Iceland scallop during 2 years in suspended culture. Table 1 Height-specific instantaneous growth rate (G) of juvenile scallop, mean monthly temperature and chlorophyll-a at 8 m depth at the culture site First year in suspended culture
Second year in suspended culture
Date 1989-1990
G (% day-‘)
Temp. (“C)
Chl-a (mgm-3)
Date 1990-1991
G (% day-‘)
Temp. (“C)
Chl-a (mgmm3)
September October
0.9 0.2 0.0 0.0 0.1 0.0 0.2 0.3 0.7 0.2 0.3 0.7
9.1 5.9 4.4 3.0 1.5 0.6 0.0 1.1 4.0 7.0 10.0 11.0
1.5 0.8 0.3 0.1 0.1 0.2 1.1 1.2 2.0 1.1 3.8 2.2
September October November December January February March April May June July August September
0.2 0.1 0.2 0.0 0.0 0.1 0.2 0.3 0.1 0.1 0.3 0.3 0.2
8.5 6.0 4.9 3.3 2.0 2.6 2.6 2.8 5.7 8.4 10.8 11.4 9.7
1.4 1.1 0.9 0.6 0.2 0.1 0.8 1.2 0.7 2.2 2.6 1.6 1.5
December January February March April May June July August
27 mm +- 5.7 s.d. The greatest monthly shell increment, the first year in the pearl nets, was from April to May (3.1 mm) and again from July to August (3.8 mm). A similar pattern of growth was recorded during the second year, with the greatest monthly shell increment registered from March to April (3.0 mm), and from June to July ( 3.9 mm). In September 199 1 the scallops had reached a mean height of 43 mm+ 8.3 s.d. The seasonal variation in instantaneous growth rate (% day-’ ) is shown in Table 1. In September 1989 when the spat was transferred to the pearl nets, the
299
G.G. Thorarinsdbttir /Aquaculture 120 (1994) 295-303
mean growth rate was 0.9% day-‘. After the transfer the growth rate declined and was minimal from November to February after which it increased again. Maximum growth rate was observed in May and again in August (0.7% day-‘) during this first year in the pearl nets. The instantaneous growth rate of the scallops decreased during the second year, but showed almost the same seasonal variations as the year before. A minimum was observed from December to February and a maximum in April, July and August ( 0.3% day- ’ ) . Fig. 4 shows the changes in the dry weight of gonads, adductor muscle, other soft parts (mantle+gills+digestive gland) and total soft parts of juvenile scallops in the size range 25-45 mm from September 1990 to September 199 1. From October there was a slight decrease in the dry weight of all soft parts until a minimum was reached in February, after which they slowly started increasing. The scallops had reached a mean height of 30-35 mm in June and by then most of them were sexually mature. At that time there was a great increase in the dry weight of all soft parts, which except for the gonads, increased further until a maximum was reached in August. In all individuals investigated the gonads were first visible in May 1991 at an age of 2.5 years and a height of 30-35 mm. The average dry weight of the gonads, measured as % total dry weight of the soft parts, was relatively low, 6.5, 10.8, 6.4, 3.8 and 4.1% from May to September. The seasonal variations in temperature and chlorophyll-u are shown in Table 1. During both years of investigation the mean temperature was highest in August ( 11.0 and 11.4”C, respectively) but during winter and spring the temperature was about l-2’ C higher in 199 1 than 1990. In 1990 the lowest mean temperature of - 0.2’ C was measured in March, while in 199 1 the lowest temperature of 2.0 oC occurred in January. Salinity varied little over the study period. A minimum (33.7-33.8%) was observed in May of both years while the maximum was in April 1991 (34.7%0).
.O- Mantle, .a-
0.7
Adductor
gllls,
dlgestlve
gland
muscle
A--*
l-+-w-*t~l-*+~++t~~: Sep
Ott
NOV
Dee
Jan
Feb
Mar
Apr
May
Jun
:
: Jul
: Aug
I SeP
Fig. 4. Seasonal variation in the dry weight of soft parts of scallop of 25-45 mm height during September 1990 to September 199 1.
300
G.G. Thorarinsdbftir /Aquaculture 120 (1994) 295-303
During both years of investigation the chlorophyll-u level was at a minimum through the winter but started increasing in March reaching a maximum in July, 3.8 mg rnM3and 2.6 mg mm3, respectively.
4. Discussion In the middle of September 1988 spat of Chlamys islandica were artificially collected in Breidafjiirdur. The spat grew well during the whole of the first year in the collectors. The maximum growth rate was 3.6% day-‘, but it was somewhat reduced during the winter (Thorarinsdottir, 199 1) . A year after settlement, when the spat was transferred from the collectors to pearl nets for suspended culture, they had reached an average height of 9.8 mm. During both the years of investigation, the growth season of the scallops in the pearl nets lasted from March until October and was found to be related to availability of phytoplankton. Variations in salinity were only slight and hardly of importance. In March there was an increase in the phytoplankton biomass but the temperature was at the annual minimum. The maximum growth rate was observed in spring as phytoplankton biomass had increased while water temperature was still low and again in summer when coinciding with near maximum temperature and chlorophyll-a levels. The low temperature observed in March appeared not to inhibit growth, a fact that has also been noted for Chlamys islandica in northern Norway (Wallace and Reinsnes, 1985 ) , Chlamys opercularis in Denmark (Ursin, 1956) and England (Broom and Mason, 1978), and Pecten maximus off the Isle of Man (Mason, 1957). Sea temperature and food have often been discussed as the two main factors contributing towards favourable conditions for growth in bivalves. Some authors have been able to demonstrate a significant correlation between water temperature and shell growth (Kirby-Smith and Barber, 1974; Richardson et al., 1982) while others have not (Page and Hubbard, 1987). Food availability has often been found to exert a greater influence on growth rate than temperature in temperate scallop species (MacDonald and Thompson, 1985 ) as well as in species inhabiting regions of continually low temperatures, such as Chlumys islandica (Vahl, 1978). There was a marked reduction in growth rate during the second year in the pearl nets. This was mainly observed as a lower maximum growth rate in spring and summer, in spite of almost the same amount of phytoplankton food available (Table 1) . The reason for the observed difference was probably that the animals became bigger and therefore the ratio of water transport to oxygen consumption decreased. Further, Vahl ( I98 1) has pointed out that the fraction of ingested food available for growth declines with size, and alter a certain age, somatic growth declines in relation to the gonad output. Thus it was evident during the second year, that the animals, at an age of 2.5 years, started developing gonads (Fig. 4) and therefore a smaller fraction of the ingested food may have been used for
G.G. Thorarinsdbttir /Aquaculture 120 (1994) 295-303
301
growth. This was similarly found by Richardson et al. ( 1980, 1982) for the cockle, Cerastoderma edule, and the scallop, Chlamys opercularis, respectively. Increase in shell height has often been used as a measure of growth in bivalves, but since shell height is irreversible it is questionable whether changes in shell height reflect the patterns of growth of soft body tissues. In this study the growth of soft body parts in scallops of 25-45 mm height followed almost the same pattern as the shell growth, except in autumn when dry weight of the soft parts decreased but the shell height was still increasing. The seasonal changes in the dry weight of the muscle are believed mostly to be due to storage and utilization of protein and glycogen, where the muscle functions as an energy depot (Sundet and Vahl, 198 1). The seasonal variations in the combined group of mantle, gills and digestive glands are most probably due to varations in the digestive gland rather than in gills and mantle. Sundet and Vahl ( 198 1) found no consistent seasonal changes in dry weight of mantle and gills in Chlamys islandica but Sastry and Blake ( 197 1) reported a direct transport of stored lipids from the digestive gland to the gonad in Aequipecten irradians. Comparison of the shell growth of scallops on natural beds in Breidafjordur (Eirfksson, 1986) with that of the suspended culture at the same place during the first 3 years after settlement, shows a similar growth in the first year after settlement ( 10 mm). This may be explained by initial settlement of the natural spat on hydroids and bryozoans just above the bottom (Pickett and Franklin, 1975 ) where food supply and temperature were similar to that in the collectors. After the first year, however, the spat in nature migrate to the bottom with growth rate being reduced below that of the hanging culture. Eirfksson (1986) found the growth rate of Iceland scallop on natural beds in the second year after settlement to be about 90% and in the third year 60%. In this study the growth rate in the second year was 150% and in the third year 80%. The growth in shell height shown by the Iceland scallop after 5 years of natural growth was attained in 3 years in the suspended culture. The culture of the Iceland scallop in pearl nets situated up in the water column may have resulted in a longer growth season and considerably enhanced growth compared with that of natural beds. Scallops freely suspended in the water column have more access to phytoplankton food whereas the food supply for bottom dwelling scallop is composed of allochthonous detrital material, resuspended sediment plus phytoplankton (Sundet, 1988 ), a mixture which is not as energy rich as phytoplankton alone (Vahl, 1980). In this study the stocking density in the pearl nets was kept low in the beginning (Hovgaard, 1984)) but scallops on natural beds are possibly overcrowded and have to compete for space and food, and the density in Breidafjiirdur can reach 130 adults mm2 (Eirfksson, 1986 ). Chlamys islandica has a relatively slow natural growth rate, and even though it can be increased markedly in hanging culture in BreidaIjiirdur, the 4 years required to reach market size (6-7 cm) may prove to be too long to warrant the additional costs associated with the suspended culture. In order to fully evaluate the mariculture potential of this species in Iceland, additional research should include rigorous selection of sites to provide optimal survival and growth condi-
302
G.G. Thorarinsdbttir /Aquaculture I20 (1994) 295-303
tions. The area chosen for culture should be sheltered and protected from storm damage and should not be located where strong tidal currents can damage gear or disturb growth. Use of larger spat and still lower initial stocking density in the pearl nets than used here may yield larger scallops in 3 years, but extra cost of producing larger spat and additional pearl nets may be offsetting.
Acknowledgements
Thanks are due to the National Research Council in Iceland and the Marine Research Institute for financial support. Valuable assistance has been given by the staff at the Marine Research Institute, Sigurdur Agustsson h/f, and PCtur Agustsson to all of whom I owe my sincere thanks. I thank Olafur S. Astthorsson for critical reading of the manuscript and linguistic help.
References Anonymous, 1992. State of Marine Stocks and Environmental Conditions in Icelandic waters 1992. Hafrannsoknastofnun, 29: 133 pp. Bal, J.N. and Jones, J.W., 1960. On the growth of brown trout of Llyn Tegid. Proc. Zool. Sot. London, 134: 1-41. Broom, M.J. and Mason, J., 1978. Growth and spawning in the pectinid Chlamys opercularis in relation to temperature and phytoplankton concentration. Mar. Biol., 47: 277-285. Eiriksson, H., 1986. Hiirpudiskurinn, Chlamys islandica, (Miiller). Hafrannsoknir, 35: 5-40 (in Icelandic with English summary). Hovgaard, P., 1984. Miljoets betydning for vekst hos haneskjell. Nor. Fiskeoppdr., 4: 33-36. Kirby-Smith, M.W. and Barber, R.T., 1974. Suspension-feeding aquaculture systems: effects of phytoplankton concentration and temperature on growth of the bay scallop. Aquaculture, 3: 135-145. MacDonald, B.A. and Thompson, R.J., 1985. Influence of temperature and food availability on the ecological energetics of the giant scallop Placopecten magellanicus. I. Growth rates of shell and somatic tissue. Mar. Ecol. Prog. Ser., 25: 279-294. Mason, J., 1957. The age and growth of the scallop Pecten maximus (L.) in Manx waters. J. Mar. Biol. Assoc. UK, 36: 473-492. Page, H.M. and Hubbard, D.M., 1987. Temporal and spatial patterns of growth in mussels Mytilus edulis on an offshore platform: relationships to water temperature and food availability. J. Exp. Mar. Biol. Ecol., 111: 159-179. Parsons, T.R. and Strickland, J.D.H., 1963. Discussion of spectrophotometric determination of marine plant pigments, with revised equations for ascertaining chlorophylls and carotenoids. J. Mar. Res., 21: 155-163. Pickett, G.D. and Franklin, A., 1975. The growth ofthe queen scallops (Chlamys opercularis) in cages off Plymouth, south-west of England. International Council for the Exploration of the Sea. Shellfish Committee, C.M. 1975/K: 24,4 pp. Richardson, C.A., Crisp, D.J., Runham, N.W. and Gruffydd, L., 1980. The use of tidal growth bands in the shell of Cerastoderma edule to measure seasonal growth rates under cool temperate and subarctic conditions. J. Mar. Biol. Assoc. UK, 60: 977-989. Richardson, C.A., Taylor, A.C. and Venn, T.J., 1982. Growth of the queen scallop, Chlamys opercularis in suspended cages in the Firth of Clyde. J. Mar. Biol. Assoc. UK, 62: 157-l 69. Sastry, A.N., and Blake, N.J., 197 1. Regulation of gonad development in the bay scallop Aequipecten irradians Lamarck. Biol. Bull. Mar. Biol. Lab., Woods Hole, Mass., 140: 48 pp.
G.G. Thorarinsdbttir / Aquaculture 120 (1994) 295-303
303
Sundet, J.H., 1988. Haneskjellets biology. Ottar, 2. Populrervitenskapelig tidsskrift fra Tromso Museum, 170: 3-8. Sundet, J.H. and Vahl, O., 1981. Seasonal changes in dry weight and biochemical composition of the tissues of sexually mature and immature Iceland scallops, Chlamys islandica. J. Mar. Biol. Assoc. UK, 61: 1001-1010. Thorarinsdottir, G.G., 199 1. The Iceland scallop, Chlamys islandica (O.F. Miiller ) in Breidafjordur, west Iceland. I. Spat collection and growth during the first year. Aquaculture, 97: 13-23. Ursin, E., 1956. Distribution and growth of the queen scallop, Chlamys opercularis (Lamellibranchiata). in Danish and Faroese waters. Meddr. Danm. Fisk. Havunders., 1: l-3 1. Vahl, O., 1978. Seasonal changes in oxygen consumption of the Iceland scallop (Chlamys islandica, O.F. Miiller) from 70”N. Ophelia, 17: 143-l 54. Vahl, O., 1980. Seasonal variations in seston and in the growth rate of the Iceland scallop, Chlamys islandica (O.F. Mtlller) from Balstjord, 70”N. J. Exp. Mar. Biol. Ecol., 48: 195-204. Vahl. O., 198 1. Energy transformations by the Iceland scallop, Chlamys islandica (O.F. Miiller) from 70”N. I. The age-specific energy budget and net growth efftciency. J. Exp. Mar. Biol. Ecol.. 53: 281-296. Wallace, J.C. and Reinsnes, T.G., 1984. Growth varation with age and water depth in the Iceland scallop ( Chlamys islandica, Pectinidae) . Aquaculture, 4 1: 14 1- 146. Wallace, J.C. and Reinsnes, T.G., 1985. The significance of various environmental parameters for growth of the Iceland scallop (Chlamys islandica, Pectinidae) in hanging culture. Aquaculture, 44: 229-242.