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Age and growth model for the southern geoduck, Panopea abbreviata, off Puerto Lobos (Patagonia, Argentina)
Fisheries Research 69 (2004) 343–348
Age and growth model for the southern geoduck, Panopea abbreviata, off Puerto Lobos (Patagonia, Argentina) Enrique Mors´ana,∗ , N´estor F. Cioccob b
a Instituto de Biolog´ıa Marina y Pesquera Almirante Storni, San Antonio Oeste, R´ıo Negro, Argentina Centro Nacional Patag´onico (CENPAT-CONICET), Bvrd. Alte Brown s/n, 9120 Puerto Madryn, Chubut, Argentina
Received 5 January 2004; received in revised form 9 June 2004; accepted 9 June 2004
Abstract Panopea abbreviata is geoduck, endemic to the SW Atlantic, distributed from 23◦ S to 48◦ S in shallow to 75-m-depth sandy and muddy bottoms. The species is the target of an experimental fishery in San Mat´ıas Gulf (40◦ 50 –42◦ 05 S and 63◦ –65◦ 05 W). Studies on population dynamics and growth has not been reported. Age and growth parameters for in 100 individuals were determined using samples from Puerto Lobos (42◦ 00 S, 65◦ 05 W; 15 m depth). Patterns of growth bands observed in thin sections of valves allowed us to establish a longevity of 40 years. Basing on the degree of transparency of the shell margins of young (<8 years old) geoducks sampled in different months of the year, we found that deposition of internal growth bands is annual. Growth parameters were estimated using two versions of the von Bertalanffy (VB) model: classic, and double (a generalization that allows the rate at which an animal approaches the asymptotic size to change after some pivotal age (tp )). Southern geoducks grow rapidly during the first 9–12 years, acquiring 89–94% of its maximum size (L∞ = 106.5 mm, calculated with double VB model). Subsequent growth is slow. The use of classic versus double VB models to describe growth is discussed from the managerial context. © 2004 Elsevier B.V. All rights reserved. Keywords: Age; Growth models; Panopea; Geoduck
1. Introduction Some members of the family Hiatellidae, geoduks, are the largest burrowing bivalves in the word. One of its member species, Panopea abbreviata (Valenci∗
Corresponding author. Tel.: +54 293 442 1002. E-mail addresses: [email protected] (E. Mors´an), [email protected] (N.F. Ciocco). 0165-7836/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.fishres.2004.06.012
ennes, 1839), is endemic in the SW Atlantic, distributed from Rio de Janeiro (23◦ S) to Puerto Deseado (48◦ S) (Fig. 1). It burrows in sandy and muddy substrates to a depth of 40 cm, it is found in beds from shallow waters to sea depths of 75 m. Under an experimental status, the fishery of P. abbreviata from North Patagonia was launched in 1999. Three to ten metric tons of living clams were caught annually using artisanal fishing methods. During an
344
E. Mors´an, N.F. Ciocco / Fisheries Research 69 (2004) 343–348
Fig. 1. Location of the sampling site and geographic distribution of the southern geoduck, Panopea abbreviata.
exploratory phase of the fishery, high-density areas were located, some yielding individual catches per unit of effort up to 50 kg−1 diver h−1 (Ciocco, 2000). High-individual weight (up to 1.4 kg) and meat quality make this geoduck a product with high potential to be commercialized in international markets. However, lack of biological data (fecundity, reproductive cycle, individual growth, mortality) and uncertainty about the response of the resource to fishing pressures pose a risk for sustainable exploitation. Studies with Panopea abrupta, a species supporting an important fishery in the NW Pacific, have revealed that geoducks have potential life spans greater than 100 years, and that they can reach fresh weights of up to 3 kg (Goodwin and Pease, 1987). If P. abbreviata is also a long-lived species, age determination will provide a key element for management and will constitute the basis to understand growth and demographic structure of this resource.
The most traditional method used to estimate bivalve growth consists of examination and counting of rings in external-valve surfaces (Richardson and Walker, 1991). In many species, however, such rings may be difficult to distinguish, as is the case of the first-year growth marks and the close-packed rings laid down in the later period of the bivalve life (Richardson et al., 1993). The study of the internal bands in cross sections of shell and in acetate peel replicates has been explored as an alternative method to determine age because these are more clearly defined and are easier to count than external rings (Thompson et al., 1980). Thin sectioning is a laborious method but internal bands have been clearly observed (Palacios et al., 1994; Cerrato, 2000). Since reliable external growth-band readings are difficult to obtain in P. abbreviata and external rings may be absent in some years, we counted and determined the periodicity of internal growthbands by studying shell sections. We use these data
E. Mors´an, N.F. Ciocco / Fisheries Research 69 (2004) 343–348
to estimate size-at-age and determined growth patterns for the species.
2. Material and methods 2.1. Sampling Samples were collected from September 1999 to August 2000, in Puerto Lobos (San Mat´ıas Gulf, Argentina; 42◦ 00 S, 65◦ 05 W) (Fig. 1). Monthly, 30 geoduck samples were obtained by SCUBA divers using a water jet pump. Height (H) and length (L) of each valve were measured with a Vernier caliper to the nearest 0.1 mm. A sub-sample of 100 clams was separated for age studies, including a similar number of individuals from each of the size classes available. 2.2. Aging Right valves only were used for age determination. Thin sections were obtained by cutting the valves with a low-speed diamond saw across the hinge plate (Fig. 2). The cut surface of each valve was grounded and pol-
345
ished on a very fine grain (4000 grit) sandpaper platform of variable speed, and mounted on a microscope slide using cyanocrylate adhesive. A thin section of 0.5 mm was obtained by cutting the mounted valve through a plane parallel to that of the first cut. The second cut surface was grounded and polished with sandpaper of medium grain (1000 grit) and very fine grain (4000 grit) until adequate thickness and texture were obtained. Sections were then observed using a dissecting scope with transmitted light to establish the optical pattern of internal growth bands. Periodicity of deposition of internal growth increments was studied recording the degree of transparency of the border ring of geoducks shells younger than 8 years in each of the monthly samples obtained. 2.3. Growth Growth patterns were determined using size at age data from counts of inner shell layers. Since spawning occurs during summer, 1st January (early summer in the austral hemisphere) served as a reference date for aging. Growth parameters were described using two versions of von Bertalanffy (VB) growth model:
Fig. 2. Panopea abbreviata shell showing a standard cut plane and size dimensions. Shell length of the specimen is 92 mm.
346
E. Mors´an, N.F. Ciocco / Fisheries Research 69 (2004) 343–348
(1) classic
3. Results
Lt = L∞ (1 − e−k(t−t0 ) )
3.1. Aging
where L∞ is the asymptotic size (in mm); k is the rate at which L∞ is reached; t is the age (years); t0 is the age at size zero. (2) double This model is a generalization of the original model allowing the rate at which an animal approaches the asymptotic size to change after some pivotal age, tp : Lt =
tp =
Lt = L∞ (1 − e−k1 (t−t1 ) )
if t < tp
(1 − e−k2 (t−t2 ) )
if t > tp
Lt = L∞ k2 t2 − k1 t1 k2 − k 1
where k1 and k2 are instantaneous growth rate coefficients, and t1 and t2 are age intercepts parameters (Porch et al., 2002). The classic VB model assumes that growth rate varies linearly with size, so that the slope (k) is constant through the entire size range. Long-lived species may nearly attain their adult size during early years of their lives. The “double” VB growth curve allows modeling both phases (Craig, 1997), taking into account that smaller (younger) individuals might grow faster relative to size than larger (older) individuals. The parameters of both models were estimated by maximizing likelihood, using shell-length data. Comparisons between classic and double VB growth models were made using the likelihood ratio test (Kimura, 1980; Cerrato, 1990). The null hypothesis for the test was: H0 : k1 = 0 Under the null hypothesis, the double VB model becomes the classic three parameters model. The test 2 statistic −2 log(Λ) converges asymptotically to a χ(g) distribution, with g degrees of freedom, and equals the number of fixed parameters.
A thin shell section showing the surface of a cut through the umbo and hinge is given (Fig. 3). The inner shell layer pattern consists of alternating translucent and opaque regions. During early years, translucent an opaque bands are broad. Analysis of the external margin translucence of the sections, combined with data on date of collection, allow for establishing the periodicity of deposition of layers. Over 90% of geoducks collected between September and February (spring to late summer in the Southern hemisphere) displayed opaque borders. This dominance gradually shifted back during fall months (March–May). By June and July (winter), 100% of the border rings were translucent. Each band composed by a wide-opaque region and the narrowtranslucent region forms a yearly cycle of shell growth. Then, 32 age classes, 2+ to 40+ years, were identified by reading growth rings. 3.2. Growth Table 1 summarizes the parameters of the classic and double VB models for length and height data, while Fig. 4 shows the resulting growth curves. Size at age data reveals that shell growth of southern geoduck is rapid during years 8–10, and then decreases abruptly, Table 1 Growth parameter estimations for Panopea abbreviata from Puerto Lobos, using the classic and double V on Bertalanffy models VB
Length (mm)
Classic VB L∞ k t0
101.32 0.211 −1.50
Double VB L∞ k1 k2 t1 t2 tp P (k1 = 0)
106.5 0.183 0.030 −1.76 −65.97 11.0 0.015
L∞ : mm; P: probability of null hypothesis (equality of both models, tested using the likelihood ratio test).
E. Mors´an, N.F. Ciocco / Fisheries Research 69 (2004) 343–348
347
Fig. 3. Shell section of a Panopea abbreviata individual older than 30 years.
Fig. 4. Panopea abbreviata growth curves fitted to shell-length data.
being very slow in older individuals. The double VB curve allows such a two-phase growth patterns for the length data better than the classic model (P = 0.015, Table 1).
4. Discussion In Panopea abbreviata, shell growth rings are shown by a translucent region formed during winter, and a broad-opaque region deposited spring to fall. Studying the intertidal clam Mya arenaria, Lewis and Cerrato (1997), suggested that the translucence pattern is determined by metabolic activity in association
with seasonal changes in bottom-water temperature. Shaul and Goodwin (1982) explained that a “growth line” is formed in Panopea abrupta by dissolution of calcium carbonate due to anaerobic metabolism during winter inactivity. Like P. abrupta, the southern geoduck retracts its siphons, remaining in an apparently inactive condition during winter (personal observation). Such behavior is a consequence of the reduced temperatures and food availability affecting the metabolic rate. The relationship between shell layer formation and period of spawning is being studied. Life-span (up to 40 years) and maximum size (110 mm; shell length) of P. abbreviata from Puerto Lobos are moderate as compared with those of estimated for P. abrupta (104–146 years, and 250 mm shell length) (Sloan and Robinson, 1984; Harbo et al., 1983). The southern geoduck attains 89–94% of its maximum size during the first 9–12 years, after which growth is much slow. As P. abrupta, the best fit of the growth models to data depends on the demographic structure of the studied population. Higher estimations of k are expected when target populations are composed of younger clams (Hoffmann et al., 2000). This fact may lead to management of populations as a fast-growing species. Growth patterns of P. abbreviata were better explained by the double VB function for length data, but the resulting two-phase curve was similar in appearance to
348
E. Mors´an, N.F. Ciocco / Fisheries Research 69 (2004) 343–348
that obtained by the classic VB model. For precautionary approach management purposes, the double VB model would be more adequate, especially considering that the k estimation (k1 and k2 ) is lower than that estimated with the classic model, and because it is linked with a more conservative criteria under a yield per recruit perspective. However, the southern geoduck fishery is at an experimental stage and no definitive management measures has been stated, it is not yet possible to evaluate the relevance of the growth parameters variability. Many factors like depth, productivity, substrate or density surely affect the geoduck growth, adding sources of variability that must be studied while the fishery develops. References Cerrato, R.M., 1990. Interpretable statistical test for growth comparison using parameters in the von Bertalanffy equation. Can. J. Fish. Aquat. Sci. 47, 1416–1426. Cerrato, R.M., 2000. What fish biologists should know about shells. Fish. Res. 46, 39–49. Ciocco, N.F., 2000. Almeja panopea, un nuevo recurso pesquero para el mar argentino. Infopesca Int. 6, 36–39. Craig, P.C., 1997. The von Bertalanffy growth curve: when a good fit is not enough. Naga, The ICLARM Quarterly 22 (4), 28–29. Goodwin, L., Pease, B.C., 1987. The distribution of geoduck (Panopea abrupta) size, density, and quality in relation to habitat characteristics such as geographic area, water depth, sediment type, and associated flora and fauna in Puget Sound, Washington. State of Washington, Department of Fisheries, Technical Report 102, p. 44.
Harbo, R.M., Adkins, B.E., Breen, P.A., Hobbs, K., 1983. Age and size in market samples of geoduck clams (Panopea generosa). Can. MS Rep. Fish. Aquat. Sci. 1174, 77. Hoffmann, A., Bradbury, A., Goodwin, C.L., 2000. Modelling geoduck, Panopea abrupta (Conrad 1849) population dynamics. J. Shellfish Res. 19, 57–62. Kimura, D.K., 1980. Likelihood methods for the estimation the von Bertalanffy growth curve. Fish. Bull. 77 (4), 765–776. Lewis, D.E., Cerrato, R.M., 1997. Growth uncoupling and the relationship between shell growth and metabolism in the soft shell clam Mya arenaria. Mar. Ecol. Prog. Ser. 158, 177– 189. Palacios, R., Orensanz, J.M., Armstrong, D.A., 1994. Seasonal and life-long variation of Sr/Ca ratio in shells of Mya arenaria from Grays Harbor (Wasington)—an ancillary criterion in demographic studies. Estuar. Coast. Shellfish Sci. 39, 313– 327. Porch, C.E., Wilson, C.A., Nieland, D.L., 2002. A new growth model for red drum (Sciaenops ocellatus) that accommodates seasonal and ontogenic changes in growth rates. Fish. Bull. 100, 149–152. Richardson, C.A., Walker, P., 1991. An analysis of the age structure of the hard shell clam Mercenaria mercenaria from acetate peel replicas of shell sections. ICES J. Mar. Sci. 48, 229–236. Richardson, C.A., Collis, S.A., Ekaratne, K., Dare, P., Key, D., 1993. The age determination and growth rate of the European flat oyster, Ostrea edulis in British waters determined from acetate peels of umbo growth lines. ICES J. Mar. Sci. 50, 493–500. Shaul, W., Goodwin, L., 1982. Geoduck (Panope generosa: Bivalvia) age as determined by internal growth lines in the shell. Can. J. Fish. Aquat. Sci. 39, 632–636. Sloan, N.A., Robinson, S.M., 1984. Age and gonad development in the geoduck clam Panope abrupta (Conrad) from Southern British Columbia. J. Shellfish Res. 4, 131–137. Thompson, I., Jones, D.S., Dreibelbis, D., 1980. Annual internal growth banding and life history of the ocean quahog Arctica islandica (Mollusca: Bivalvia). Mar. Biol. 57, 25–34.
Age and growth model for the southern geoduck, Panopea abbreviata, off Puerto Lobos (Patagonia, Argentina) Enrique Mors´ana,∗ , N´estor F. Cioccob b
a Instituto de Biolog´ıa Marina y Pesquera Almirante Storni, San Antonio Oeste, R´ıo Negro, Argentina Centro Nacional Patag´onico (CENPAT-CONICET), Bvrd. Alte Brown s/n, 9120 Puerto Madryn, Chubut, Argentina
Received 5 January 2004; received in revised form 9 June 2004; accepted 9 June 2004
Abstract Panopea abbreviata is geoduck, endemic to the SW Atlantic, distributed from 23◦ S to 48◦ S in shallow to 75-m-depth sandy and muddy bottoms. The species is the target of an experimental fishery in San Mat´ıas Gulf (40◦ 50 –42◦ 05 S and 63◦ –65◦ 05 W). Studies on population dynamics and growth has not been reported. Age and growth parameters for in 100 individuals were determined using samples from Puerto Lobos (42◦ 00 S, 65◦ 05 W; 15 m depth). Patterns of growth bands observed in thin sections of valves allowed us to establish a longevity of 40 years. Basing on the degree of transparency of the shell margins of young (<8 years old) geoducks sampled in different months of the year, we found that deposition of internal growth bands is annual. Growth parameters were estimated using two versions of the von Bertalanffy (VB) model: classic, and double (a generalization that allows the rate at which an animal approaches the asymptotic size to change after some pivotal age (tp )). Southern geoducks grow rapidly during the first 9–12 years, acquiring 89–94% of its maximum size (L∞ = 106.5 mm, calculated with double VB model). Subsequent growth is slow. The use of classic versus double VB models to describe growth is discussed from the managerial context. © 2004 Elsevier B.V. All rights reserved. Keywords: Age; Growth models; Panopea; Geoduck
1. Introduction Some members of the family Hiatellidae, geoduks, are the largest burrowing bivalves in the word. One of its member species, Panopea abbreviata (Valenci∗
Corresponding author. Tel.: +54 293 442 1002. E-mail addresses: [email protected] (E. Mors´an), [email protected] (N.F. Ciocco). 0165-7836/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.fishres.2004.06.012
ennes, 1839), is endemic in the SW Atlantic, distributed from Rio de Janeiro (23◦ S) to Puerto Deseado (48◦ S) (Fig. 1). It burrows in sandy and muddy substrates to a depth of 40 cm, it is found in beds from shallow waters to sea depths of 75 m. Under an experimental status, the fishery of P. abbreviata from North Patagonia was launched in 1999. Three to ten metric tons of living clams were caught annually using artisanal fishing methods. During an
344
E. Mors´an, N.F. Ciocco / Fisheries Research 69 (2004) 343–348
Fig. 1. Location of the sampling site and geographic distribution of the southern geoduck, Panopea abbreviata.
exploratory phase of the fishery, high-density areas were located, some yielding individual catches per unit of effort up to 50 kg−1 diver h−1 (Ciocco, 2000). High-individual weight (up to 1.4 kg) and meat quality make this geoduck a product with high potential to be commercialized in international markets. However, lack of biological data (fecundity, reproductive cycle, individual growth, mortality) and uncertainty about the response of the resource to fishing pressures pose a risk for sustainable exploitation. Studies with Panopea abrupta, a species supporting an important fishery in the NW Pacific, have revealed that geoducks have potential life spans greater than 100 years, and that they can reach fresh weights of up to 3 kg (Goodwin and Pease, 1987). If P. abbreviata is also a long-lived species, age determination will provide a key element for management and will constitute the basis to understand growth and demographic structure of this resource.
The most traditional method used to estimate bivalve growth consists of examination and counting of rings in external-valve surfaces (Richardson and Walker, 1991). In many species, however, such rings may be difficult to distinguish, as is the case of the first-year growth marks and the close-packed rings laid down in the later period of the bivalve life (Richardson et al., 1993). The study of the internal bands in cross sections of shell and in acetate peel replicates has been explored as an alternative method to determine age because these are more clearly defined and are easier to count than external rings (Thompson et al., 1980). Thin sectioning is a laborious method but internal bands have been clearly observed (Palacios et al., 1994; Cerrato, 2000). Since reliable external growth-band readings are difficult to obtain in P. abbreviata and external rings may be absent in some years, we counted and determined the periodicity of internal growthbands by studying shell sections. We use these data
E. Mors´an, N.F. Ciocco / Fisheries Research 69 (2004) 343–348
to estimate size-at-age and determined growth patterns for the species.
2. Material and methods 2.1. Sampling Samples were collected from September 1999 to August 2000, in Puerto Lobos (San Mat´ıas Gulf, Argentina; 42◦ 00 S, 65◦ 05 W) (Fig. 1). Monthly, 30 geoduck samples were obtained by SCUBA divers using a water jet pump. Height (H) and length (L) of each valve were measured with a Vernier caliper to the nearest 0.1 mm. A sub-sample of 100 clams was separated for age studies, including a similar number of individuals from each of the size classes available. 2.2. Aging Right valves only were used for age determination. Thin sections were obtained by cutting the valves with a low-speed diamond saw across the hinge plate (Fig. 2). The cut surface of each valve was grounded and pol-
345
ished on a very fine grain (4000 grit) sandpaper platform of variable speed, and mounted on a microscope slide using cyanocrylate adhesive. A thin section of 0.5 mm was obtained by cutting the mounted valve through a plane parallel to that of the first cut. The second cut surface was grounded and polished with sandpaper of medium grain (1000 grit) and very fine grain (4000 grit) until adequate thickness and texture were obtained. Sections were then observed using a dissecting scope with transmitted light to establish the optical pattern of internal growth bands. Periodicity of deposition of internal growth increments was studied recording the degree of transparency of the border ring of geoducks shells younger than 8 years in each of the monthly samples obtained. 2.3. Growth Growth patterns were determined using size at age data from counts of inner shell layers. Since spawning occurs during summer, 1st January (early summer in the austral hemisphere) served as a reference date for aging. Growth parameters were described using two versions of von Bertalanffy (VB) growth model:
Fig. 2. Panopea abbreviata shell showing a standard cut plane and size dimensions. Shell length of the specimen is 92 mm.
346
E. Mors´an, N.F. Ciocco / Fisheries Research 69 (2004) 343–348
(1) classic
3. Results
Lt = L∞ (1 − e−k(t−t0 ) )
3.1. Aging
where L∞ is the asymptotic size (in mm); k is the rate at which L∞ is reached; t is the age (years); t0 is the age at size zero. (2) double This model is a generalization of the original model allowing the rate at which an animal approaches the asymptotic size to change after some pivotal age, tp : Lt =
tp =
Lt = L∞ (1 − e−k1 (t−t1 ) )
if t < tp
(1 − e−k2 (t−t2 ) )
if t > tp
Lt = L∞ k2 t2 − k1 t1 k2 − k 1
where k1 and k2 are instantaneous growth rate coefficients, and t1 and t2 are age intercepts parameters (Porch et al., 2002). The classic VB model assumes that growth rate varies linearly with size, so that the slope (k) is constant through the entire size range. Long-lived species may nearly attain their adult size during early years of their lives. The “double” VB growth curve allows modeling both phases (Craig, 1997), taking into account that smaller (younger) individuals might grow faster relative to size than larger (older) individuals. The parameters of both models were estimated by maximizing likelihood, using shell-length data. Comparisons between classic and double VB growth models were made using the likelihood ratio test (Kimura, 1980; Cerrato, 1990). The null hypothesis for the test was: H0 : k1 = 0 Under the null hypothesis, the double VB model becomes the classic three parameters model. The test 2 statistic −2 log(Λ) converges asymptotically to a χ(g) distribution, with g degrees of freedom, and equals the number of fixed parameters.
A thin shell section showing the surface of a cut through the umbo and hinge is given (Fig. 3). The inner shell layer pattern consists of alternating translucent and opaque regions. During early years, translucent an opaque bands are broad. Analysis of the external margin translucence of the sections, combined with data on date of collection, allow for establishing the periodicity of deposition of layers. Over 90% of geoducks collected between September and February (spring to late summer in the Southern hemisphere) displayed opaque borders. This dominance gradually shifted back during fall months (March–May). By June and July (winter), 100% of the border rings were translucent. Each band composed by a wide-opaque region and the narrowtranslucent region forms a yearly cycle of shell growth. Then, 32 age classes, 2+ to 40+ years, were identified by reading growth rings. 3.2. Growth Table 1 summarizes the parameters of the classic and double VB models for length and height data, while Fig. 4 shows the resulting growth curves. Size at age data reveals that shell growth of southern geoduck is rapid during years 8–10, and then decreases abruptly, Table 1 Growth parameter estimations for Panopea abbreviata from Puerto Lobos, using the classic and double V on Bertalanffy models VB
Length (mm)
Classic VB L∞ k t0
101.32 0.211 −1.50
Double VB L∞ k1 k2 t1 t2 tp P (k1 = 0)
106.5 0.183 0.030 −1.76 −65.97 11.0 0.015
L∞ : mm; P: probability of null hypothesis (equality of both models, tested using the likelihood ratio test).
E. Mors´an, N.F. Ciocco / Fisheries Research 69 (2004) 343–348
347
Fig. 3. Shell section of a Panopea abbreviata individual older than 30 years.
Fig. 4. Panopea abbreviata growth curves fitted to shell-length data.
being very slow in older individuals. The double VB curve allows such a two-phase growth patterns for the length data better than the classic model (P = 0.015, Table 1).
4. Discussion In Panopea abbreviata, shell growth rings are shown by a translucent region formed during winter, and a broad-opaque region deposited spring to fall. Studying the intertidal clam Mya arenaria, Lewis and Cerrato (1997), suggested that the translucence pattern is determined by metabolic activity in association
with seasonal changes in bottom-water temperature. Shaul and Goodwin (1982) explained that a “growth line” is formed in Panopea abrupta by dissolution of calcium carbonate due to anaerobic metabolism during winter inactivity. Like P. abrupta, the southern geoduck retracts its siphons, remaining in an apparently inactive condition during winter (personal observation). Such behavior is a consequence of the reduced temperatures and food availability affecting the metabolic rate. The relationship between shell layer formation and period of spawning is being studied. Life-span (up to 40 years) and maximum size (110 mm; shell length) of P. abbreviata from Puerto Lobos are moderate as compared with those of estimated for P. abrupta (104–146 years, and 250 mm shell length) (Sloan and Robinson, 1984; Harbo et al., 1983). The southern geoduck attains 89–94% of its maximum size during the first 9–12 years, after which growth is much slow. As P. abrupta, the best fit of the growth models to data depends on the demographic structure of the studied population. Higher estimations of k are expected when target populations are composed of younger clams (Hoffmann et al., 2000). This fact may lead to management of populations as a fast-growing species. Growth patterns of P. abbreviata were better explained by the double VB function for length data, but the resulting two-phase curve was similar in appearance to
348
E. Mors´an, N.F. Ciocco / Fisheries Research 69 (2004) 343–348
that obtained by the classic VB model. For precautionary approach management purposes, the double VB model would be more adequate, especially considering that the k estimation (k1 and k2 ) is lower than that estimated with the classic model, and because it is linked with a more conservative criteria under a yield per recruit perspective. However, the southern geoduck fishery is at an experimental stage and no definitive management measures has been stated, it is not yet possible to evaluate the relevance of the growth parameters variability. Many factors like depth, productivity, substrate or density surely affect the geoduck growth, adding sources of variability that must be studied while the fishery develops. References Cerrato, R.M., 1990. Interpretable statistical test for growth comparison using parameters in the von Bertalanffy equation. Can. J. Fish. Aquat. Sci. 47, 1416–1426. Cerrato, R.M., 2000. What fish biologists should know about shells. Fish. Res. 46, 39–49. Ciocco, N.F., 2000. Almeja panopea, un nuevo recurso pesquero para el mar argentino. Infopesca Int. 6, 36–39. Craig, P.C., 1997. The von Bertalanffy growth curve: when a good fit is not enough. Naga, The ICLARM Quarterly 22 (4), 28–29. Goodwin, L., Pease, B.C., 1987. The distribution of geoduck (Panopea abrupta) size, density, and quality in relation to habitat characteristics such as geographic area, water depth, sediment type, and associated flora and fauna in Puget Sound, Washington. State of Washington, Department of Fisheries, Technical Report 102, p. 44.
Harbo, R.M., Adkins, B.E., Breen, P.A., Hobbs, K., 1983. Age and size in market samples of geoduck clams (Panopea generosa). Can. MS Rep. Fish. Aquat. Sci. 1174, 77. Hoffmann, A., Bradbury, A., Goodwin, C.L., 2000. Modelling geoduck, Panopea abrupta (Conrad 1849) population dynamics. J. Shellfish Res. 19, 57–62. Kimura, D.K., 1980. Likelihood methods for the estimation the von Bertalanffy growth curve. Fish. Bull. 77 (4), 765–776. Lewis, D.E., Cerrato, R.M., 1997. Growth uncoupling and the relationship between shell growth and metabolism in the soft shell clam Mya arenaria. Mar. Ecol. Prog. Ser. 158, 177– 189. Palacios, R., Orensanz, J.M., Armstrong, D.A., 1994. Seasonal and life-long variation of Sr/Ca ratio in shells of Mya arenaria from Grays Harbor (Wasington)—an ancillary criterion in demographic studies. Estuar. Coast. Shellfish Sci. 39, 313– 327. Porch, C.E., Wilson, C.A., Nieland, D.L., 2002. A new growth model for red drum (Sciaenops ocellatus) that accommodates seasonal and ontogenic changes in growth rates. Fish. Bull. 100, 149–152. Richardson, C.A., Walker, P., 1991. An analysis of the age structure of the hard shell clam Mercenaria mercenaria from acetate peel replicas of shell sections. ICES J. Mar. Sci. 48, 229–236. Richardson, C.A., Collis, S.A., Ekaratne, K., Dare, P., Key, D., 1993. The age determination and growth rate of the European flat oyster, Ostrea edulis in British waters determined from acetate peels of umbo growth lines. ICES J. Mar. Sci. 50, 493–500. Shaul, W., Goodwin, L., 1982. Geoduck (Panope generosa: Bivalvia) age as determined by internal growth lines in the shell. Can. J. Fish. Aquat. Sci. 39, 632–636. Sloan, N.A., Robinson, S.M., 1984. Age and gonad development in the geoduck clam Panope abrupta (Conrad) from Southern British Columbia. J. Shellfish Res. 4, 131–137. Thompson, I., Jones, D.S., Dreibelbis, D., 1980. Annual internal growth banding and life history of the ocean quahog Arctica islandica (Mollusca: Bivalvia). Mar. Biol. 57, 25–34.