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COMPARISON OF THREE METHODS FOR AGING THE ICELAND SCALLOP CHLAMYS ISLANDICA.

INTRODUCTION

Gilbert Bay (GB) on the southeastern coast of Labrador (Fig. 1) was the first Marine Protected Area (MPA) established in the subarctic coastal zone of eastern Canada (Government of Canada 2005). The bay is a shallow fjord, 28 km long and 1-2.5 km wide (Copeland et al. 2012). The management plan for the GB MPA (Fisheries and Oceans Canada 2007, 2013) protects the resident population of Atlantic cod Gadus morhua (Linnaeus) and supports conservation of the bay ecosystem (Copeland et al. 2013). Regulations (Government of Canada 2005) permit the commercial exploitation of Iceland scallop Chlamys islandica (Muller) in the Bay, but dredging is not allowed in regions of cod spawning. There was intensive commercial harvesting of scallops in GB for more than a decade before the MPA was established in 2005 (Morris et al. 2002).

State of the Scallop Resource in GB

There is little scientific information on the Iceland scallop stocks of coastal Labrador (Naidu et al. 2000, 2001). In eastern Canada, fisheries managers have set the shell size of recruitment to the Iceland scallop fishery at 60 mm (Fisheries and Oceans Canada 2009). Shell height (SH) is defined as the longest distance from shell hinge to ventral shell margin. The diameter of the metal rings of a dredge is set to retain scallops of commercial size (Morris et al. 2002).

The recruitment rate for Iceland scallop to the fishable stock is variable from year to year (Pedersen 1994, Naidu et al. 2000, Duncan et al. 2016). There can be recruitment failure if the environmental conditions deviate from the physical--biological oceanographic state required for successful recruitment to the stock (Orensanz et al. 2016). For poikilothermic marine animals such as the Iceland scallop, the growth rate of individuals in the fishable stock is regulated by seawater temperature, providing sufficient food is available (Jonasson et al. 2004, Brand 2006, Duncan et al. 2016). Predation on Iceland scallop by sea stars is substantial (Naidu et al. 2000, Duncan et al. 2016), and the rate of natural mortality in the field is variable (Naidu 1988).

One index of the state of a scallop resource is the size of scallops being harvested, relative to past catches or in comparison with scallops harvested from stocks elsewhere (Crawford 1992, Naidu et al. 2000, Fisheries and Oceans Canada 2009). Wroblewski et al. (2009) found that the mean SH of Iceland scallops (n = 5,273) harvested in GB during 2006 was significantly smaller than any of the mean sizes of Iceland scallops similarly measured by observers onboard commercial vessels fishing in the Strait of Belle Isle (SBI) of the Gulf of St. Lawrence. Local fishers have suggested this could be due to a slower growth rate of scallops in GB. Harvesters of scallops in GB voluntarily complete a logbook created by the local community-based MPA Advisory Committee specifically for monitoring the scallop resource. By sharing their catch data and local ecological knowledge, fisherpersons provide information for MPA management decisions (Morris et al. 2002, Fisheries and Oceans Canada 2013).

Several factors other than growth rate may explain the relatively small size of harvested scallops in GB. Recruitment variability (Arsenault & Himmelman 1996) or size-selective fishing mortality (i.e., repeated dredging of scallop beds), or a combination of these factors may be at play (Duncan et al. 2016). Given the absence of basic biological information on Iceland scallops in the MPA needed for fishery management purposes, Wroblewski et al. (2009) recommended that research be conducted on growth, recruitment, and natural mortality of scallops in the bay. This article focuses on growth of GB scallops. To test the hypothesis that Iceland scallops grow more slowly in GB than in the SBI, the von Bertalanffy growth model parameters (von Bertalanffy 1938) were determined for both regions.

MATERIALS AND METHODS

Sampling

A commercial scallop dredge towed behind the fishing vessel CFV Little Shell (hull length 10.5 m) was used to collect a total of 693 Iceland scallops from the GB MPA during late September 2007. The dredge had four buckets with 64-mm-diameter metal rings (Morris et al. 2002). Two of the four buckets were lined with 38-mm stretch nylon netting to retain scallops smaller than commercial size (60-mm SH).

Dredge tows were made at seven locations (Fig. 1) along the main axis of GB: Middle Island (number of scallops collected, n = 100): Peckham Cove (n = 137); Kellys Point (n = 76); Rexons Point (n = 67); Coach Box Point (n = 54); Main Tickle (n= 121); and Leg Island (n = 138). The scallops were collected at depths of 5-30 m.

Growth Parameterized as Increase in SH with Age

Iceland scallop growth in GB was parameterized as increase in SH with age. The SH of each of the 693 scallops collected was determined in the laboratory. Shell height was measured to the nearest 0.1 mm with a vernier scale caliper. The age of each of the 693 scallops collected was estimated by interpreting external shell growth rings (Hart & Chute 2009, Chute et al. 2012) and also by counting shell hinge ligament growth increments (MacDonald & Bourne 1989, MacDonald et al. 1991). A subset of 30 scallops (systematically chosen to represent the entire size range of the 693 scallops collected) was aged by a third method, counting the internal shell layer growth zones in a sectioned shell of the scallop (Jones et al. 1978, Garcia-March & Marquez-Aliaga 2007).

Age Estimation

There is no single, universally accepted standard method for estimating the age of Iceland scallops. Three methods are commonly used for aging bivalve molluscs: counting external growth rings on the shell surface from the umbo to the ventral margin (Stevenson & Dickie 1954, MacDonald & Thompson 1988), counting growth increments from the peak to the base of the pyramid-shaped calcareous portion of the shell hinge ligament (Trueman 1953, Merrill et al. 1966, Johannessen 1973, MacDonald & Thompson 1988), and counting internal shell-layer growth zones of a sectioned shell (MacDonald & Thomas 1980, Arneri et al. 1998, Hua et al. 2001, Oshima et al. 2004, Garcia-March & Marquez-Aliaga 2007).

Naidu (1988) estimated the age of individual Iceland scallops collected in the SBI by interpreting the external growth rings. Pedersen (1994) demonstrated that the hinge ligament growth zones of Iceland scallops in West Greenland are laid down annually and counted ligament growth increments to determine the growth parameters.

In this study, three methods of aging Iceland scallops collected from GB are compared: interpreting external growth rings on the shell, counting growth increments of the shell hinge ligament, and counting internal shell layer growth zones in sectioned shells.

Interpreting External Growth Rings on the Shell

The external growth rings of all 693 scallops were assessed visually in the laboratory. The rings are radially distributed from the umbo to the shell margin. The upper valve was difficult to read because of its dark color and the attachment of barnacles. Counts were recorded based on reading the lower valve. Contrast could often be improved by immersing the specimen in water. Each pair of light and dark rings was considered as 1 y of growth. External growth rings of all 693 scallops were counted by the senior author (Liu 2009).

Counting Growth Increments of the Shell Hinge Ligament

The growth increments of the shell hinge ligament of all 693 scallops were counted using a binocular microscope. The inner layer of the ligament is a pyramid-shaped structure situated between the valves under the umbo. It consists of a calcified lateral region joined to each valve, with a soft central region between the laterals (Trueman 1953, Merrill et al. 1966). When the soft central region is removed, growth zones can be observed on the calcified lateral region. The lateral region was observed under a binocular microscope with magnifications of 10x for the larger shells and 40x for the smaller shells. For improved contrast, the specimen was immersed in water and then dried with tissue paper. Growth increments of the shell hinge ligament of all 693 scallops were counted by the senior author (Liu 2009).

Counting Internal Shell Layer Growth Zones in Sectioned Shells

The internal shell layer growth zones in a sectioned shell from each of the 30 scallops in the subset chosen to represent the collected size range were counted using a binocular microscope. The shells were cut with a precision sectioning saw (BUEHLER Ltd. IsoMet) at a speed of approximately 160 revolutions per minute. For shells larger than 30 mm SH, the valves were cut longitudinally from the umbo to the ventral shell margin along the axis of maximum growth. Shells less than 30 mm SH were embedded in epoxy resin for support during sawing. The transverse section of each half was roughly polished with 12, 9, and 6 micron alumina, then finely polished with 0.3 and then 0.05 micron alumina. After the section was polished, the growth zones were counted. The growth zones in a sectioned shell from each of the 30 scallops chosen to represent the selected size range were counted by the senior author (Liu 2009).

Statistical Analyses

The von Bertalanffy model (von Bertalanffy 1938):

[mathematical expression not reproducible] (1)

was used for quantifying growth of Iceland scallops in GB. The dependent variable S[H.sub.t] is SH at time t, parameter S[H.sub.[infinity]] is the asymptotic SH, K is the growth coefficient determining the rate of change in SH, and [t.sub.0] is a scale correction giving the hypothetical age at zero SH. The von Bertalanffy function was fit to the data by iteration, using the least squares curve fitting procedures available in Sigma Plot 10 (Systat Software Inc. 2006).

The influence of aging method and sampling location on SH-at-age were examined by comparing von Bertalanffy growth parameters within the framework of the generalized linear model (GLM) (McCullagh & Nelder 1989). To accomplish this, the von Bertalanffy model was linearized as follows:

Ln(1 - S[H.sub.t]/S[H.sub.[infinity]]) = [K.sup.*](t - [t.sub.0]) (2a)

Growth coefficient K was estimated by reexpressing Eq. 2a as a GLM

Ln(1 - S[H.sub.t]/S[H.sub.[infinity]]) = [[beta].sub.0] + [[beta].sub.(t)][*.sup.](t - [t.sub.0]) (2b)

where [[beta].sub.0] is the mean value of the response variable and [[beta].sub.(t)] is the estimate of K.

Growth parameters were compared among aging methods with analysis of covariance (ANCOVA) according to a GLM with identity link:

[mathematical expression not reproducible] (3)

where Method is a categorical variable, and the interaction term tests for equality of slopes (equality of growth rates).

A one-way analysis of variance (ANOVA) was used to test whether the three methods produced different estimates of age. The GLM for the one-way ANOVA was

Age = [[beta].sub.0] + [[beta].sub.M][*.sup.]Method (4)

Growth parameters were also compared among the seven sampling locations in the bay. The GLM used in the analysis was

[mathematical expression not reproducible] (5)

where Loc is a categorical variable (seven locations), and the interaction term tests for differences in slopes (equality of growth rates).

To determine whether age determination differed in average among locations, a one-way ANOVA was conducted. The GLM for the one-way ANOVA was

Age = [[beta].sub.0] + [[beta].sub.Loc][*.sup.]Loc. (6)

All statistical analyses were done by iterative estimation, using the GLM procedure in S-PLUS release 8.0. Assumptions of homoscedasticity and consistency with a straight line model after linearization were evaluated with residual versus fit plots. The criterion for statistical significance was set at 5% type I error.

RESULTS

Comparison of Methods for Estimating Age of Iceland Scallops

Age as determined by the ligament growth increment method was similar to age by the internal shell layer method, with a tendency for the ligament method estimate to exceed the internal shell layer method by 10% ([A.sub.lingament]/[A.sub.internal] = 1.10 as estimated by regression, Fig. 2). Age as determined by the external growth ring method was about half that estimated by the internal shell layer method or the ligament method. The ratios as estimated by regression were [A.sub.external]/[A.sub.internal] = 0.52 (Fig. 3) and [A.sub.external]/[A.sub.lingament]= 0.47 (Fig. 4).

Growth rates differed significantly, as indicated by a statistically significant interaction term in an ANCOVA ([F.sub.2, 84] = 14.9, P < 0.001). To identify the source of heterogeneity, we carried out two a priori tests; ligament versus internal and external versus internal. As expected the growth rate estimates did not differ significantly for the ligament versus internal shell layer comparison ([F.sub.1, 56] = 0.99, P = 0.32). Growth estimated by the external growth ring method differed substantially and significantly from the internal shell layer method ([F.sub.1, 56] = 30.3, P < 0.001).

The mean age for all age classes of the 30 individual scallops depended on method ([F.sub.2, 87] = 7.15, P = 0.001). Two a priori tests were carried out: ligament versus internal and external versus internal. Scallop ages estimated by counting internal shell layer growth zones were significantly higher than those determined by the external growth ring method ([F.sub.1, 58] = 11.8, P = 0.001). Similar mean ages were obtained from the internal shell layer method and the ligament growth increment method ([F.sub.1, 58] = 0.21, P = 0.65).

Spatial Variation in Growth of Iceland Scallops in GB

The mean SHs versus age (determined by the ligament method) for Iceland scallops sampled at seven locations within GB were similar at most locations (Fig. 5). The exceptions were Kellys Point and Rexons Point, where scallops less than age 10 were smaller than the same aged animals at the other locations. Consistent with the visual interpretation of Figure 5, the growth coefficient K as estimated from the linearized version of the von Bertalanffy function differed significantly among the seven locations ([F.sub.6, 122] = 7.97, P < 0.001); however, if the data from Kellys Point and Rexons Point are excluded, there is no significant difference among the five remaining locations ([F.sub.4, 84] = 1.55, P = 0.19). The differences in growth at Kellys Point and Rexons Point, although statistically significant, were not considered biologically significant because the differences were small and not evident above age 10.

The growth coefficient K of Iceland scallops in GB (data combined from all sampling areas) was 0.07 per year, and the mean asymptotic shell height S[H.sub.[infinity]] was 116.78 mm (Fig. 6). The data fit the von Bertalanffy curve well, except at ages 25 and 26. Back calculation from the model resulted in an estimate of 11.3 y of growth required for an Iceland scallop in GB to reach the harvestable SH of 60 mm (Fig. 6).

Comparison of Scallop Growth in GB with the SBI and West Greenland

The increase in SH with age was used to compare scallop growth in GB with scallop growth in the SBI. To be consistent with the methodology of Naidu (1988), we used the scallop ages estimated by the method of interpreting external growth rings. Results showed similar growth rate in both regions (Fig. 7). This visual interpretation of Figure 7 was confirmed by the analysis of covariance, which showed no significant difference in growth rate ([F.sub.1, 24] = 1.78, P = 0.19). The parameters of the von Bertalanffy function determined by iterative curve fitting to the GB data were similar to those determined for the SBI data (Fig. 8; Table 1, external ring method).

Increase in SH with age was also used to compare scallop growth in GB with scallop growth in the Nuuk area of West Greenland (NAWG). To be consistent with the methodology of Pedersen (1994), scallop ages estimated by the method of counting shell hinge ligament growth increments were used in the analysis. The mean SH versus age plot tor GB scallops was similar to the plot for scallops in the NAWG (Fig. 9). Pedersen (1994) scored individuals with more than 21 annual growth zones as age [21.sup.+], but in this study all ages were recorded as the number of growth zones counted. To remain consistent with the analysis by Pedersen (1994), all GB scallops older than 21 y were scored as [21.sup.+] in the statistical comparison. Analysis of covariance showed no significant difference in growth rates ([F.sub.1, 36] = 2.50, P = 0.12). Parameter estimates determined by iterative curve fitting to the GB data were similar to those determined for the West Greenland data (Fig. 10; Table 1, ligament method).

DISCUSSION

Estimating Age of Iceland Scallops

Determining the age of an Iceland scallop collected from GB by counting its shell hinge ligament growth increments and by counting its internal shell layer growth zones gave similar results. Compared with the other two methods, the external growth ring method underestimated the age of the scallop and overestimated the growth rate. External growth rings on the shell are not consistently formed in an annual pattern. Well-defined rings are not normally found in the first 5-7 y classes of Chlamys islandica (Johannessen 1973). Fewer discernible rings would result in an underestimate of the age and overestimate of the growth rate of an individual Iceland scallop by a researcher. Johannessen (1973) found that not all Icelandic scallops could be aged by counting the rings on the shell surface as the border of growth cessation in winter is not seen on the valves of all specimens. Contrary to the rings on the shell, the growth increments of the shell hinge ligament of an Iceland scallop can always be produced, making the age determination of every individual possible (Johannessen 1973).

MacDonald (1984) concluded that for the sea scallop (Placopecten magellanicus), ligament growth increments more accurately revealed the true age (sea scallops of known-age were examined) and gave more consistent results than counting external growth rings on the shell. Jones et al. (1978) concluded that internal shell layer growth zones observed in shell cross-sections of the Atlantic surf clam (Spisula solidissima) are formed annually, and there is a corresponding growth line on the exterior shell. However, Jones et al. (1978) noted there are also extra growth lines on the exterior shell of the Atlantic surf clam which can be misleading when age is estimated from these external lines. MacDonald and Thomas (1980) found that counting the internal shell layer growth zones of the softshell clam (Mya arenaria) visible in thin sections of the shell can accurately reveal the age of an individual softshell clam of known-age. The accuracy of the internal shell layer method has been confirmed by Hua et al. (2001) for freshwater mussels (Cyprogenia stegaria and Lexingtonia dolabelloides).

Aging bivalve molluscs by counting internal shell layer growth zones is time consuming. Counting shell hinge ligament growth increments can provide an age estimate similar to the internal shell layer method and is much more convenient because it does not require sectioning the shell.

Growth of Iceland Scallops in the GB MPA

This study provides the first data on SH-at-age for the Iceland scallops in the GB MPA (Fig. 6). Although individual Iceland scallops measuring up to 140 mm have been found in northwest Iceland (Garcia 2006), the maximum SH is generally around 120 mm (Crawford 1992). The largest shell measured in this study was 104 mm, collected at Kellys Point in GB. The estimated asymptotic SH in GB was 117 mm (Fig. 6; Table 1, ligament method), close to the maximum SH reported for the species.

Growth of Iceland scallops is influenced by environmental factors, including seawater temperature and salinity, sea bottom substrate suitability, and planktonic food availability (Wallace & Reinsnes 1984, Pedersen 1994, Thorarinsdottir 1994). In GB, mean SH versus age did not vary substantially among the seven locations sampled (Fig. 5), although there were small and statistically detectable differences at young ages at two locations. Wroblewski et al. (2007) found that seawater temperature in GB decreased with depth, but varied little with location along the main axis of the bay. Bottom substrate type was similar at the seven sampled locations in GB (Copeland et al. 2012). The influence of food (plankton) availability on scallop growth in GB is unknown. There is no published research on plankton production in GB, but plankton concentration is likely to vary spatially because of the estuarine circulation in the bay (Wroblewski et al. 2007). This study found scallop growth rates differed little (although statistically significant) in GB. This suggests the environmental conditions where scallop beds occur in the bay are similar.

Scallop Growth in GB in comparison with Scallop Growth in the SBI and in the NAWG

Scallops living in GB, the SBI, and the NAWG grow similarly, i.e., they exhibited similar mean SH versus age curves (Figs. 7 and 9). Pedersen (1994) concluded that West Greenland scallops grew more slowly than scallops in Canada (SBI), Iceland, and Norway, but also noted that the growth rate data were based on different age estimation methods. This study found that growth rates in GB were similar to those in West Greenland, when the same age estimation method was used.

Growth rates were overestimated by the external growth ring method compared with the consistency of results by the other two methods. Consequently, K estimated for the SBI (data of Naidu 1988) was about twice that for GB and West Greenland (Table 1), likely biased upward by a method that underestimates age of older individuals.

The similarity in growth of Iceland scallops in GB, the SBI, and the NAWG is apparently due to comparable environmental conditions (Pedersen 1994, Fisheries and Oceans Canada 2009, Copeland et al. 2012). Both GB and the SBI are influenced by the Labrador Current, a continuation of the West Greenland Current and the Baffin Island Current (Buch 1984, Best et al. 2011).

The small size of scallops harvested in GB, relative to the average size of scallops harvested in the SBI (Wroblewski et al. 2009), cannot be attributed to differences in growth rate. Because the growth of Iceland scallops in these regions is similar, the smaller size must be due to other factors such as recruitment failure, size-selective fishing mortality, or a combination thereof. The difference is likely due to intensive fishing in GB in the past (Morris et al. 2002), when repeated dredging of scallop grounds may have removed the larger individuals.

With the caveat that specimens of known-age have not been examined, results presented here suggest that Iceland scallops in GB require ~11 y to reach a SH of 60 mm (Fig. 6), the size at which the scallops recruit to the fishery. Canadian fisheries management assumes an earlier recruitment age of 7 y (Fisheries and Oceans Canada 2009), based on the research on SBI scallops for which age was estimated from external growth rings, and for which growth rate may have been overestimated.

Benefits of the MPA to the Iceland Scallop Fishery in GB

The management plan for the GB MPA closed cod spawning areas in the inner bay (Fisheries and Oceans Canada 2007, 2013) The seabed (sand with coralline algae-encrusted gravel) in these protected areas are suitable habitat for Iceland scallop (Cope-land et al. 2012). These pristine scallop habitats in the inner bay have not been dredged as narrow seaway passages and shallow sills make them inaccessible to scallop fishing vessels (Morris et al. 2002, Wroblewski et al. 2009). The export of planktonic scallop larvae from these protected shallow areas may lead to recruitment in deeper scallop beds in the outer bay (Arsenault et al. 2000, Copeland et al. 2012). Marine scallop resources are ideally suited to spatial management (Duncan et al. 2016, Stewart & Howarth 2016). Dramatic recoveries of scallop fisheries have occurred after large-scale closures (Hart & Rago 2006), complemented by the development of marine-protected areas (Howarth et al. 2015). Monitoring the average size of Iceland scallops harvested in GB should continue, and future research in the MPA should include a field study on the effect of the closed areas on recruitment.

ACKNOWLEDGMENTS

The authors thank Dr. Evan Edinger and Dr. Ray Thompson for their advice on aging scallops, and Glenn Piercey, Renita Aranha, and Dr. Owen Sherwood for their assistance in sectioning shells. This research was conducted as part of the GB MPA monitoring program under a Collaborative Research Agreement between Fisheries and Oceans Canada and Memorial University of Newfoundland. Capt. Redgeway Russell of the CFV Little Shell, MPA Community Coordinator Marilyn Penney and James Russell of Williams Harbour assisted in field sampling. Alison Copeland helped prepare the figures. Shanshan Liu held a Graduate Studies Fellowship from Memorial University of Newfoundland.

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SHANSHAN LIU, (1) JOSEPH S. WROBLEWSKI (2*) AND DAVID C. SCHNEIDER (2)

(1) Environmental Science Program, Memorial University of Newfoundland, St. John's, Newfoundland and Labrador. NL A1B 3X7. Canada; (2) Department of Ocean Sciences, Memorial University of Newfoundland, St. Johns, Newfoundland and Labrador, NL A1C 5S7, Canada

(*) Corresponding author. E-mail: jwroblew@mun.ca

DOI: 10.2983/35.036.0309
TABLE 1.
Von Bertalanffy growth equation fitted parameters for Iceland scallops
in GB, the NAWG, and the SBI.

Methods                GB          GB             NAWG       SBI
                       Ligament    External       Ligament   External

S[H.sub.[infinity]]    116.8       101.4          105.2      111.0
(mm)
K ([year.sup.-1])        0.07        0.17           0.08       0.14
[t.sub.0] (years)        0.02        0.19           0.05       0.12
[r.sup.2]                0.99        1.00           0.99       0.99
Mean SH range (mm)      13-102.9    15.4-93.7      11-87       6.8-90.8
Age range (years)        2-26        2-15           2-21       1-14
n                      693         693          1,041        284

Methods used for aging scallops in the NAWG and the SBI were counting
ligament growth increments and interpreting external growth rings,
respectively. Both methods were used in estimating ages of scallops
from GB.
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Author:Liu, Shanshan; Wroblewski, Joseph S.; Schneider, David C.
Publication:Journal of Shellfish Research
Article Type:Report
Geographic Code:1CNEW
Date:Dec 1, 2017
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