Spring awakening temperature and survival of sediment-covered eastern oysters Crassostrea virginica

Aquaculture 430 (2014) 188–194

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Spring awakening temperature and survival of sediment-covered eastern oysters Crassostrea virginica Luc A. Comeau ⁎ Department of Fisheries and Oceans, Gulf Fisheries Centre, Science Branch, P. O. Box 5030, Moncton, New Brunswick E1C 9B6, Canada

a r t i c l e

i n f o

Article history: Received 17 December 2013 Received in revised form 2 April 2014 Accepted 9 April 2014 Available online 18 April 2014 Keywords: Crassostrea virginica Oyster Siltation Burial Sedimentation Temperature

a b s t r a c t In the Gulf of St. Lawrence estuaries, mesh bags containing cultured oysters (Crassostrea virginica) are lowered onto the bottom in autumn prior to the formation of a thick ice cover. The oysters remain quiescent and unattended in near-freezing waters for four to five months, during which time they are susceptible to accidental burial by sedimenting particles. The objectives of this study were (1) to gain insight into the mechanism cueing the spring awakening of oysters, (2) to determine the approximate burial depth that oysters can withstand, and (3) to estimate the time it takes for mortality to occur under conditions of excessive siltation. Results indicate that water temperature is the primary factor controlling the timing of awakening, with the majority of oysters suddenly opening their valves when temperatures increased to 2.61 ± 0.66 °C. Supplementing the diet to mimic spring bloom conditions had no modulating influence on awakening behaviour. Oysters buried under 20 mm of sediment initially exhibited erratic valve movements and sometimes remained closed for days. Within 2 weeks, however, they did succeed in expulsing the overlying silt around their valve margins and in resuming normal valve movements, including circadian rhythmicity. By comparison, burial under 40 or 60 mm of silt invariably led to death within 11.7 ± 1.3 days. It is concluded that oysters should be re-suspended as soon as the ice cover breaks apart and moves offshore in the spring. Crown Copyright © 2014 Published by Elsevier B.V. All rights reserved.

1. Introduction The eastern oyster, Crassostrea virginica (Gmelin), has a remarkable distribution range extending approximately 4000 km in the Western Atlantic (Carriker and Gaffney, 1996). It can be found from the Gulf of Mexico northward through to the Gulf of St. Lawrence (GSL), Canada (Fig. 1). At its northernmost distribution limit in the GSL, it is inactive for four to five months during the winter when estuarine temperatures are stable over a narrow range between −1 °C and 0 °C (Comeau et al., 2012). The lack of pumping activity and valve movement during this extended period may render these animals susceptible to burial by sedimenting particles. Burial may be accelerated by heavy rainfall events in autumn which promote land runoff and soil erosion. Periods of high wind in October–November may also cause significant sediment re-suspension and localized siltation (Comeau et al., 2014; Mallet et al., 2006). In spring the melting of snow may aggravate matters by increasing land drainage and sediment loading in the estuaries. In addition, anthropogenic siltation may be a major contributor in regions with intensive agriculture, highway construction or dredging. Both natural burial and anthropogenic burial of oysters have long been recognised as widespread occurrences in Atlantic estuaries (Wilber et al., 2005).

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http://dx.doi.org/10.1016/j.aquaculture.2014.04.009 0044-8486/Crown Copyright © 2014 Published by Elsevier B.V. All rights reserved.

In Texas, for example, some reefs are found under more than 3 m of mud (Galtsoff, 1964). Sedimentation rates in cultured shellfish systems are generally higher than in the natural environment (Arakawa, 1980; Willemsen, 2005). Mesh Vexar bags tend to interfere with local hydrodynamics, reducing flow rates and promoting sedimentation, which may result in accidental burial, particularly when structures remain unattended for prolonged periods of time. For example, in GSL estuaries, mesh bags containing oysters are typically lowered onto the seabed in October–November and recovered five to six months later, i.e., after the thick (~1 m) winter ice cover breaks up and moves offshore, rendering the aquaculture farm site accessible again. Upon sinking to the bottom in the fall, bags tend to settle down into the soft sediment, thereby increasing the risk of burying the oysters. Such accidental burials may cause little stress to the oysters as long as they remain closed and reliant upon anaerobic metabolism (Bumett and Stickle, 2001; Gade, 1983; Greenway and Storey, 1999; Stickle et al., 1989). On the other hand, deleterious effects may arise when oysters resume their interaction with the environment in early spring. This postulate is consistent with industry members occasionally reporting oyster mortalities when bags are recovered from the seabed. The dead oysters have dark gaping shells containing black anoxic sediments and traces of still-decomposing tissues indicative of recent mortality. Few studies have quantitatively examined the effect of siltation on the survival of C. virginica. The earliest studies focussed on oyster

L.A. Comeau / Aquaculture 430 (2014) 188–194

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Fig. 1. Map of the study area showing the Gulf of St. Lawrence, the northernmost distribution range of Crassostrea virginica.

survival near dredged material disposal sites. Following the dredging of the Intracoastal Waterway of South Carolina, Lunz (1938) concluded that the deposition of mud up to 25 mm thick causes no ill effects to wild oyster beds. Experimental work later demonstrated that excessive siltation can clog the gill apparatus with lethal consequences. Hsiao (1950) reported that oysters die when silt (amount not specified) deposited upon their shells remains for more than 3 days. Dunnington (1968) purposely buried oysters under conditions that did not permit recovery (76 mm of sediment) and reported survival times ranging between 2 days in summer (N 25 °C) to 3 weeks in winter (b 5 °C). The delayed onset of mortality in winter was attributed to a lower metabolic rate and slower consumption of energy reserves. It was also mentioned that oysters buried in less than 13 mm during preexperiment trials “could usually clear their bills of sediments if the water was warm enough for pumping.” From these reports, it appears that oysters can deal with moderate siltation levels, but that mortality occurs quite rapidly under excessive siltation. The first objective of this study was to gain insight into the mechanism triggering the spring awakening behaviour of C. virginica. Temperature was experimentally manipulated in order to verify whether this variable cues awakening. Food levels were also manipulated to gauge whether they can modulate the process, perhaps by stimulating oysters to awaken at lower temperatures than they would under low food conditions. The second objective was to determine the approximate burial depth that oysters can withstand. The third objective was to assess the time it takes for mortality to occur under excessive siltation.

between the two valves. The magnetic field in the form of output voltage (μV) was acquired by strain recording devices (DC 104R, Tokyo Sokki Kenkyujo Co., Japan). Output voltage was recorded once every minute. At the end of the experiments, voltage was converted into valve opening by applying conversion algorithms specific to each sensor assembly. More precisely, the adductor muscle was severed, and small calibration wedges were manoeuvred between the two valves at the point farthest from the

2. Methods 2.1. Valve opening In each experiment described below, oyster valve opening was measured using a valvometry system based on the Hall element principle (Nagai et al., 2006). The system allowed for the simultaneous monitoring of up to 32 individuals. A coated Hall element sensor (HW-300a, Asahi Kasei, Japan) was glued to one valve at the maximum distance from the hinge (Fig. 2). Then a small magnet (4.8 mm diameter × 0.8 mm height) was glued to the other valve, directly below the Hall sensor. The magnet and the Hall element weigh 0.1 g and 0.5 g, respectively. For comparison purposes, a 6-mm diameter live barnacle, a common epibiont on oyster shells (Doiron, 2008), weighs approximately 0.12 g. The magnetic field (flux density) between the sensor and magnet was a function of the gap

Fig. 2. Oyster wired with Hall magnetic sensor.

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hinge. Wedge height was 1–6 mm. The relationships between voltage and wedge height (i.e., valve opening in mm) were non-linear and strong (r2 N 0.94). To minimize the influence of oyster size, valve opening data were converted from millimetres to angles (θ in degrees) using the following equation (Wilson et al., 2005): θ ¼ 2 arcsin

  0:5W  100 SH

where W is the valve opening (mm) and SH (mm) is the oyster's shell height.

(cupped) valve down, and tilted forward such that their valve margins were close to the bottom of the chambers and hence to the deepest point in the sediment. The burial depth was measured from the bottom of the acrylic chamber to the top of the sediment. The sediment originated from the top seabed layer of Bedeque Bay. It had an organic content of 3.2 ± 0.3% and most of its particles (97.2 ± 0.5%) were smaller than 250 μm. Oysters in the 6 remaining chambers served as controls and received no sediment. Following the experimental setup, the valvometry system automatically monitored valve movements over a 30-day period. 2.4. Statistics

2.2. Awakening experiment In March 2013, a bag containing market-size oysters (shell height 89.5 ± 7.5 mm) was retrieved from the St. Simon Bay seabed through a hole in the ice. The oysters were transferred to the Étang Ruisseau Bar Ltd. wet holding facility on the shores of St. Simon Bay. Valvometry sensors were glued onto their valves. Oysters were then placed, with their cupped valve down, in six holding tanks (5 oysters per tank). These tanks were continuously supplied with unfiltered and near freezing (−0.5 °C) natural seawater pumped from the main channel entering St. Simon Bay. Water temperature was monitored continuously in each tank using Hobo UA-002-64 data loggers (Onset Computer Corporation, Massachusetts, USA). Following a one week acclimation period, temperature was manipulated in randomly selected tanks using digitally-controlled heating probes. In two replicate tanks temperature was slowly (~1.0 °C day−1) increased from −0.5 °C to 4.0 °C over a period of 5 days. This temperature rise simulated the early phase of the spring warming period (Department of Fisheries and Oceans Canada, unpublished data). In two other tanks temperature was similarly manipulated but a phytoplankton supplement was added to simulate a spring bloom. A mixed suspension of live Chaetoceros muelleri and Isochrysis galbana (50:50 of each algal species by cell number) was added to raise cell counts from ~ 5000 (natural baseline) to ~ 50,000 cells ml−1 (simulated bloom). C. muelleri occurs naturally in the area (Mather et al., 2010) and both species are widely used to feed oysters under experimental or aquaculture settings because they show adequate fatty acid profiles (Pernet et al., 2003, 2007). Counts were verified and adjusted twice daily. In the two remaining tanks neither temperature (− 0.5 °C) nor food (~ 5000 cells ml−1) was manipulated. At the end of the experiment, all tanks were returned to baseline conditions (− 0.5 °C, 5000 cells ml−1) for a one week period. Thereafter, the same manipulations were applied, with the exception that temperature rises occurred more rapidly and were repeated over time. The intent was to assess the robustness of the awakening response mechanism. Once a day, temperature increased rapidly (~2.2 °C h−1) from −0.5 to 13 °C over a period of approximately 6 h, and then dropped back to − 0.5 °C at approximately the same rate. This pattern was repeated daily over 5 consecutive days. 2.3. Siltation experiment In May 2012, 24 oysters (shell height 73.5 ± 4.3 mm) were collected in Bedeque Bay, Prince Edward Island (PEI), an important oyster fishing and aquaculture area with a history of sporadic overwintering mortality events tentatively attributed to siltation (Cosh et al., 2008). Oysters were brought to a nearby field laboratory in St. Peter's Bay, PEI, where valvometry sensors were glued onto their valves. Oysters were then transferred into 24 individual acrylic chambers continuously supplied with natural seawater (temperature ~ 12 °C). After a one-week acclimation period, a glass probe was used to prompt the oysters to close their valves prior to their burial. A 20, 40 or 60-mm layer of fine sediment was added, completely burying 6 oysters per treatment category (6 oysters × 3 siltation levels). Oysters were positioned with their left

All analyses were performed in SPSS v. 20 (IBM SPSS Inc., Chicago). A mixed model analysis of variance was used to test the effect of phytoplankton concentration (Food) on the awakening response, defined in terms of the temperature at awakening (TA) and the maximal valve opening (MO). The replicated tank (Rep) was set as a random factor. TA ðor MO Þij ¼ μ þ Foodi þ Rep j ðFoodi Þ þ εij where: TA MO μ Food Rep ε

temperature at the onset of the awakening event maximal valve opening following the awakening event overall mean of the population phytoplankton concentration, where i = 1 (natural), 2 (supplemented) replicated tank, where j = 1, 2 error.

A similar mixed model was developed to test the robustness of the awakening behaviour over time in the second experiment, where the temperature fluctuation treatment was repeated daily. Day of treatment (Day [i = 1 to 5]) and phytoplankton concentrations (Food [j = 1, 2]) were declared main fixed effects. The replicated tank (Rep [k = 1, 2]) was set as a random factor.   TA ðor MO Þijk ¼ μ þ Dayi þ Food j þ Day  Foodij þ Repk Dayi Food j þ εijk

Periodogram analysis was used to ascertain whether significant periodic components existed in valve opening time series derived from the siltation experiment, which was carried out over an extended period of time (30 days). Linear trends were removed using the ordinary least squares (OLS) method prior to performing the analysis. Fourier spectral analyses were then performed on the residuals from the OLS trend analysis (Warner, 1998). Periodogram values were calculated for each Fourier frequency, thus providing a numerical representation of the magnitude of the periodicity present in the data at each periodic cycle. The Fisher test and critical values tabled by Russell (1985) were applied to test the significance of each periodic cycle. The Fisher test required the calculation of the g-value, which in turn provided the proportion of the total variance that was accounted for by each periodic component. 3. Results 3.1. Awakening None of the oysters (0/10) in the control group (− 0.5 °C) opened their valves, whereas the majority of oysters (14/20) awakened when subjected to a slow (~ 1.0 °C day− 1) temperature rise (− 0.5 to 4.0 °C). A close inspection of the data for the remaining oysters (6/20) revealed a technical issue warranting the exclusion of these particular

L.A. Comeau / Aquaculture 430 (2014) 188–194

individuals. Water penetrated these Hall sensors and short-circuited communications to the data loggers, resulting in voltage values that were continually aberrant and inconsistent with voltage-valve opening algorithms. On average the temperature at awakening was 2.61 ± 0.66 °C; a typical awakening response is shown in Fig. 3a. Note that oysters quickly closed their valves during the cooling phase of the experiment. A mixed ANOVA (Table 1a) indicated that increasing the phytoplankton concentration in the tank had no significant effect on the temperature at which the oysters awakened (p = 0.64). However, significant behavioural differences were detected between these two treatment groups following awakening. Maximum valve gape was significantly higher (p b 0.01) for naturally-fed oysters (3.20 ± 1.61°, n = 8) than those receiving the algae supplement (2.33 ± 1.04°, n = 6). Similar responses were recorded when challenging the oysters to a series of rapid temperature fluctuations (Fig. 3b). None of the oysters (0/10) in the control group (−0.5 °C) opened their valves, whereas all (20/20) of those subjected to temperature rises (−0.5 to 13.0 °C) awakened each time the treatment was applied. The awakening responses were robust over time: temperature at awakening and magnitude of opening were statistically similar between days (Table 1b, see Day). Increased phytoplankton levels had no significant effect on the temperature at which the oysters awakened (p = 0.33). A significant tank replicate effect (p b 0.01) suggests different awakening temperatures across experimental units. As in the previous experiment, maximum valve opening was significantly higher in (p b 0.01) naturally-fed

Temperature Valve opening

4

2

3

2

1

1

0 quiescence

awakening

Temperature (°C)

3

a

-1

0 0

14

Valve opening (°)

5

2

4

6

8

18

b

Table 1 Mixed model ANOVAs testing for the effect of a supplemented diet (Food), time (Day), and tank replicates (Rep) on the awakening response. Models were built using data from (a) the slow temperature increase experiment over a 5-day period and (b) the rapid temperature increase treatment repeated daily over a 5-day period. Source

a) Food Rep(Food) Error b) Day Food Food × Day Rep(Food × Day) Error

df

Temperature at awakening

Maximum valve opening

MS

Pr N F

1 2 10

0.21 0.68 0.39

0.64 0.22

2.61 0.11 2.32

0.01 0.95

4 1 4 10 80

1.50 1.00 0.80 7.09 0.93

0.93 0.71 0.98 b0.01

3.27 45.05 1.64 7.16 3.97

0.77 0.03 0.92 0.07

MS

Pr N F

Bold values highlight statistical significance (p b 0.05).

oysters (6.43 ± 1.77°, n = 10) than in those receiving a daily dietary supplement (4.97 ± 2.50°, n = 10). 3.2. Siltation Silt-free oysters remained open most of the time (96.6 ± 2.0%) and exhibited a prominent shell opening rhythm (Fig. 4a). Spectral analysis and the Fisher test indicated that the 24-h periodicity was dominant and highly significant (p b 0.001) for each individual in the control group. The proportion of the variance accounted for by the 24-h periodicity averaged 7.9 ± 2.2% (n = 6). Oysters buried under 20 mm of sediment were able to expulse the overlying silt around their valve margins (Fig. 5a). They initially exhibited erratic valve movements lacking rhythmicity and closed their valves for several days (Fig. 4b). However, within 14 days of the burial event and for the remainder of the experiment, they behaved like siltfree oysters. Their valves were open most of the time (98.4 ± 1.0%) and they exhibited a dominant 24-h oscillation period, which accounted for 11.1 ± 3.6% (n = 6) of the variance. Fig. 5b shows the similar 24-h rhythms of two oysters, one silt-free and the other buried under 20 mm of sediment. None of the oysters buried under 40 or 60 mm of sediment survived (Fig. 4c–d). Time of death was inferred from the last detection of a valve micro-closure (Fig. 4c). The timing of this event was similar between the two groups (p = 0.27, Mann–Whitney test), averaging 11.7 ± 1.3 days (n = 11). 4. Discussion

16

12

191

4.1. Awakening

14 12 8 10 6

8

4

6

2

4

Valve opening (°)

Temperature (°C)

10

2 0 0 0

2

4

6

8

Elapsed time (days) Fig. 3. Valve opening of an 86-mm (shell height) Crassostrea virginica exposed to (a) slow and (b) rapid changes in temperature.

Temperature and food availability in cold temperature ecosystems tend to be positively correlated, thus making it difficult to distinguish the relative impact of these two variables on seasonal behaviours such as winter dormancy (Coma et al., 2000). The present study provides compelling evidence that temperature is the main factor inducing winter quiescence and cueing spring awakening in C. virginica. A close monitoring of valve movements under controlled conditions revealed that all oysters with functioning Hall sensors awakened when the temperature rose from −0.5 to 4.0 °C, which was intended to mimic the period immediately following the breakup and retreat of the winter ice cover. On average, awakenings occurred when the water temperature reached 2.61 ± 0.66 °C, consistent with a recently reported 2.20 ± 1.09 °C threshold (Comeau et al., 2012). The latter was based on the modelling of valve opening series over two consecutive springs. These activity thresholds for GSL oysters are lower than those inferred from the pioneering work on populations from the east coast of the United

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10

a

8 6 4 2 0 10

b

8 6

Valve opening (°)

4 2 0 10

c

8 6

6 4

4

2

last micro-closure

2 13.3

13.4

13.5

13.6

13.7

13.8

0 10

d

8 6 4 2 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30

Elapsed time (days) Fig. 4. Valve opening of Crassostrea virginica (a) in a silt-free environment, (b) buried under 20 mm, (c) buried under 40 mm and (d) buried under 60 mm of sediment.

States, where water temperature is seldom b5 °C. The landmark papers (Galtsoff, 1926, 1928; Loosanoff, 1958; Nelson, 1921) suggested that feeding, ventilation and valve opening activity are the exception below 5.0–7.6 °C. The lower activity threshold reported in the present study may reflect adaptability at the northernmost range limit of the species, consistent with the latitudinal-compensation theory (Dittman, 1997; Kokita, 2004; Pörtner, 2002; Sukhotin et al., 2006). The lower threshold may also be due to methodological differences between studies, specifically the advantage of being able to detect fine valve movements in the present study. There was no evidence that enhanced food availability can modulate the timing of spring awakening under rising temperature conditions. Increasing the phytoplankton cell counts from ~5000 (natural baseline) to 50,000 cells ml−1 (simulated bloom) had no significant effect on the temperature at which the oysters awakened (p = 0.64). However, the increased cell counts lowered the extent of valve opening once the oysters had awakened: estimates of maximum valve opening were curtailed in two experiments by 23% (4.97 vs 6.43°) and 27% (2.33 vs 3.20°). Although speculative, it is posible that the higher particle concentration and/or limited digestion processes reduced the need to open. This would be consistent with the view that feeding is subject to physiological regulation in accordance with food resources and nutritional needs (Bayne, 1998; Cranford, 2001; Morton, 1973). It is also likely that oysters have difficulty processing abundant food particles at low

temperatures; few individuals reportedly produce faeces (Loosanoff, 1958) or clear particles (Comeau et al., 2008) below 4 °C. 4.2. Siltation A wide range of burial depths has been observed under natural (Wilber and Clarke, 2010) and dredging-related conditions (Galtsoff, 1964; Ingle, 1952; Rose, 1973). However, determining the depth of sediment coverage that induces mortality in oysters is complicated by several other environmental factors (Wilber et al., 2005). Laboratory work on the topic of burial is dated and surprisingly scarce. Dunnington (1968) noted, from preliminary observations during the setup of an experiment, that oysters buried under b 13 mm “could usually clear their bills of sediments if the water was warm enough for pumping.” The present study corroborates Dunnington's observation; it was found that oysters could withstand burial depths of 20 mm at 12 °C (shell height 89.5 ± 7.5 mm). However, their normal rhythmic behaviour was temporarily disturbed: they exhibited erratic valve movements and remained closed for days. Within 14 days they succeeded in expulsing the overlying silt around their valve margins and resuming normal valve movements, including circadian rhythmicity. In contrast, none of the oysters buried under 40 or 60 mm of sediment survived the experiment. When they were exhumed and examined at the end of the trial, it was apparent that their valves had remained open for a considerable

L.A. Comeau / Aquaculture 430 (2014) 188–194

a

b

193

found dead or dying in Bedeque Bay (PEI), the exact proportion depending on the area fished within the bay (Cosh et al., 2008). Such naturallyoccurring events may be caused by a combination of factors, perhaps including siltation and depletion of energy reserves over the course of a long winter. On the other hand, siltation represents a consistent threat in aquaculture settings. It is accepted that the mesh bags containing cultured oysters increase sedimentation rates which may contribute to the risk of burial, particularly when these bags remain unattended for prolonged periods of time, as is the case for the winter period. The exact burial depth of oysters in these bags has not been documented and presumably varies considerably depending on the timing of sinking in the fall, the hardness of the bottom sediment, the ice cover contact with the bottom, and the localized sedimentation rates. Based on the present investigation, it would be advisable to re-suspend cultured oysters prior to the onset of their spring awakening, and more precisely before the water temperature increases to 2.61 ± 0.66 °C. It is estimated that this threshold temperature is attained in 14.9 ± 5.2 days following the departure of ice (n = 8 years, Department of Fisheries and Oceans Canada, unpublished data). Once the oysters have awakened, burialinduced mortality is expected within 11.7 ± 1.3 days assuming they cannot expulse the overlying sediment (this study). At lower temperatures, they could presumably last longer than at 12 °C, but their pumping activity would also be reduced. 4.4. Conclusion Temperature, and not food, is the main factor cueing the spring awakening of C. virginica. Awakening occurred when the experimental temperatures reached 2.61 ± 0.66 °C. Burial depths of 20 mm led to behaviour disturbance, but only for a short period of time (≤ 14 days) while oysters were expulsing overlying silt around their valve margins. Burial depths greater than 40 mm led to death within 11.7 ± 1.3 days. Given these results it is advisable to re-suspend cultivated stock as quickly as possible come spring, preferably within 14.9 ± 5.2 days following the departure of the ice cover. The present work was limited to large oysters (shell height 73.5 ± 4.3 mm) and juvenile oysters may respond differently to sedimentation. Acknowledgments

Fig. 5. (a) Photograph of buried oyster that successfully managed to expulse the overlying silt away from its shell edge and (b) periodogram values showing the similar 24-h rhythms of two oysters, one silt-free and the other buried under 20 mm of sediment.

amount of time; their tissues were decomposed and their inter-valve cavity contained silt. Death, as inferred from the last detected valve movement, occurred 11.7 ± 1.3 days after burial. These results apply to naturally-oriented oysters (cup down).

Thomas Landry, André Mallet and Jeffrey Davidson provided access to their field laboratories and stimulating discussions on the topic of siltation. Special thanks are extended to Claire Carver who kindly agreed to read an early draft of this paper and offered very helpful comments. This study was funded by the Department of Fisheries and Oceans of Canada (Aquaculture Collaborative Research and Development Program, project MG-09-03-002) in partnership with the oyster company L'Étang Ruisseau Bar Ltd. References

4.3. Implications At their northernmost distribution limit in the GSL, oysters are inactive for the four to five winter months when estuarine temperatures are stable over a narrow range between − 1 °C and 0 °C (Comeau et al., 2012). An intriguing question is whether a lack of valve movement over such an extended period renders these animals susceptible to burial by sedimenting particles. Data on sediment deposition in the coastal estuaries of eastern Canada are scarce. In PEI, where estuaries act as sediment traps in highly turbid environments, sediment deposition reportedly ranges between 4 (Bartlett, 1977) and 9 mm year −1 (Davidson et al., 2009). Such rates are well within the oyster's tolerance range based on the present study. Therefore, it seems unlikely that naturally-occurring siltation is responsible for massive mortality events, such as that in the spring of 2003, when 30–80% of the wild oysters were

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