Commercial harvest and population structure of a northern quahog (Mercenaria mercenaria linnaeus 1758) population in St. Mary's Bay, Nova Scotia, Canada.

ABSTRACT Innovative Fishery Products Inc. (IFP) has managed a 1682-ha northern quahog (Mercenaria mercenaria) lease in St. Mary's Bay, Nova Scotia, Canada, since 1997. This study describes the commercial harvest and age structure of the northern quahog population from St. Mary's Bay and provides estimates of total mortality and exploitation rates for the lease area. Overall, mean densities ranged from 48.3-88.4 individuals/[m.sup.2] for surveys conducted in June 2001 and 2002, and in May 2003. Catch and effort in the fishery increased from 1999 to 2001. The mean age to market was 7 y. Recruitment of spat (SL < 5 mm) was variable and age frequencies suggested immigration of juvenile quahogs (3-5 y old) onto the intertidal portion of the lease area. The abundance of large quahogs (SL > 60 mm or >8 y old) was low. Catch curve analyses resulted in a high estimate of total mortality (Z) for quahogs of ages 7-10. Commercial exploitation only represented 5% to 10% of the estimated standing stock of commercial size quahogs, which suggests that natural mortality may be high. However, field experiments conducted during 2003 suggested that summer survival (May to October) was high ranging from 93.8 [+ or -] 1.54 to 97.6 [+ or -] 2.14%. Causes of apparent high total mortality of adult quahogs are unclear, but winter-kill due to ice abrasion or scouring, predation, and the movement of quahogs from the lease area may be responsible.

KEY WORDS: northern quahog, population structure, St. Mary's Bay, commercial harvest, age structure

INTRODUCTION

The northern quahog, locally called the quahog, (also known as the hard clam in southern populations), is a bivalve mollusc found in shallow coastal waters from the Gulf of Mexico to its most northern limit in the southern Gulf of St. Lawrence. It is present either in small patches or large beds in both intertidal and subtidal reaches of coastal embayments (Grizzle et al. 2001). Geographic distribution in Atlantic Canada is limited to areas where summer water temperature exceeds 20[degrees]C (Landry & Sephton 1996) and therefore quahogs usually only occur in the southern portions of the Gulf of St. Lawrence. Two populations have been documented in the Bay of Fundy region of Atlantic Canada: in Sam Orr's Pond, near St. Andrews, New Brunswick, and in St. Mary's Bay, Nova Scotia (Whiteaves 1901, Dillon & Manzi 1992; Fig. 1). Details on these populations have never been described. Innovative Fishery Products (IFP) manages the St. Mary's Bay quahog population by way of a lease. This represents the only commercially viable quahog stock in the Bay of Fundy.

[FIGURE 1 OMITTED]

Quahogs are harvested in St. Mary's Bay from May to November with the peak harvest period occurring from June to September. Annual harvest has ranged from 95-370 tons since commercial exploitation began in 1997 (Fig. 2). The lease area is harvested by clam diggers with hand tools such as rakes, locally called clam hacks. The area has one of the largest tidal ranges (mean range = 25 m) in the world and clam diggers harvest quahogs for 4 h once a day during low tide. To date, the lease has been harvested by a maximum of 50 clam diggers per tide where the mean harvest per day for a clam digger has been as high as 167 kg.

Lease management is based on: (1) routine visual inspections for the presence of quahogs in the intertidal portion of the lease prior to the harvest season; (2) harvest rotation whereby the lease area is harvested in plots and plots may not be harvested every year; (3) a minimum shell length of [greater than or equal to] 50 mm although the harvest may include a small percentage of individuals between 45-49 mm; (4) daily harvest monitoring (i.e., weight and length frequency); (5) a harvest season from May to November; and (6) active lease surveillance throughout the year whereby IFP reports illegal harvesting activity to federal enforcement officers. In 2001, IFP and Fisheries and Oceans Canada entered a 4-y partnership to evaluate the use of biological information (i.e., abundance trends, population dynamics and biological characteristics) and population modeling to optimize quahog harvesting on the lease on a long-term basis. The St. Mary's Bay population was considered to be ideal to study population dynamics because it appears to be an isolated population (i.e., immigration and/or emigration are currently considered negligible), the population can be readily surveyed, the lease area is managed by one user group, and data on daily harvest and fishing effort (i.e., number of harvesters) are available. A precursor to population modeling is the requirement for a clear understanding of the population life cycle and basic population parameters. The objectives of this study are to describe the commercial harvest, age structure, and obtain some estimates of exploitation and total mortality rates for quahogs in St. Mary's Bay.

MATERIALS AND METHODS

Population Surveys

The lease area of St. Mary's Bay has a surface of 1682 ha with a maximum intertidal surface area of 628 ha or 6.28 [km.sup.2] Preharvest intertidal surveys were conducted in collaboration with IFP in June 2001 and 2002 and in May 2003. The surveys in June 2001 and 2002 consisted of one sampling station per 500 x 500 m sampling unit for a total of 45 stations. A sampling grid 500 m east by 250 m south was used during the 2003 intertidal survey for a total of 95 stations. Only the 45 traditional stations were used for survey comparisons among individual years. For the May 2003 survey, 10% of randomly selected stations were duplicated. At each sampling site, the upper sediment layer (depth = 25 mm) was collected from a 0.25-[m.sup.2] quadrat with garden tools and rinsed through a 2-mm mesh sieve for spat and juvenile collection. All remaining clams were then removed from the sediment by hand and garden tools to a maximum depth of 15 cm. All clams collected were bagged and frozen at -30[degrees]C until sample processing. Shell length and height were measured to the nearest 1 mm with Mitutoyo (Mitutoyo America Corporation, Aurora, Illinois) digital calipers. The whole weight of the frozen animals (shell and soft tissue) was measured to the nearest 0.1 g with a top loading digital balance.

Commercial Harvest Data

Daily harvest weight (kg) for the lease area was monitored by IFP during the 1997 to 2003 fishing seasons. Fishing effort was calculated by cumulating the number of diggers over the entire harvest season. Yearly catch rates were obtained by dividing the harvest weight by the fishing effort. Length frequency of the quahogs harvested (n = 200) was sampled twice weekly during 2003 to the nearest 1 mm, and total weight of the samples was measured to the nearest 0.01 kg. An estimate of the length frequency for the entire 2003 catch was obtained by multiplying the length frequencies by the ratio of catch weight to sample weight.

Age Determination

Shells from quahogs (n = 362) collected in the June 2002 preharvest survey were aged using an adaptation from the technique of Ropes and O'Brien (1979) for ageing surf clams, Spisula solidissima and further described by Sephton and Bryan (1990) and Jones et al. (1990). Thin sections were excised from the right-hand valve of specimens ranging from 25-110 mm in shell length. A valve was secured to the manipulative support of an Isomet (Buehler Ltd., Lake Bluff, Illinois) low speed geological saw where a 2-mm section was sliced between two diamond wafer cutting blades. One of the blades cut just anterior of the umbo yielding a highly polished thin section. The umbo side of the section was glued to a glass slide and viewed under a dissecting microscope at x25. The number of annuli was counted within the outer and middle shell layers in the radial section from the umbo to the ventral margin. Annuli were counted three times with two blind counts by the same observer and later validated by a second observer.

Growth curves are often used to describe the growth rate and estimate age at length for mollusc bivalve populations. Few quahogs older than 10 y were collected from the survey to properly estimate the growth curve. In addition, molluscan bivalves typically have highly variable growth rates whereby length frequency intervals of larger animals may encompass several age groups. In such cases, it is preferable to estimate the age composition using an age-length matrix coupled with the length frequency of the population rather than using a deterministic relationship between age and length (Hilborn & Walters 1992). Lengths for which no age reading were conducted were assigned to an unspecified group. These were large animals (69-111 mm).

The age-length matrix derived from the 2002 survey was used to calculate the proportion at age of quahogs for each 1-mm shell length interval. This age-length key was used in conjunction with the respective length frequencies for the June 2001 and 2002 and May 2003 population surveys. The numbers obtained for each age classes were expanded to the survey area by multiplying the numbers at age by the ratio of the total survey area to the sampled area. Similarly, the same age-length key was used to obtain an estimate of the age composition of the 2003 commercial harvest.

Preliminary Estimates of Mortality Rates and Survival

Instantaneous mortality rates (Z) can be estimated from catch curve analyses of the yearly age compositions for the surveys and commercial removal (see Ricker 1975). The slope of the descending limb of In numbers at age is an estimate of the total instantaneous mortality rate (Z). Estimates of Z can also be obtained from catch curve analyses conducted along individual cohorts. This is a better approach because it removes the assumption that cohorts are of similar abundance, but it does require data over several years. For the time series of preharvest surveys (2001 to 2003), a modified catch curve analysis was used. Sinclair (2001) used this approach to estimate total mortality rates of southern Gulf of St. Lawrence cod (Gadus morhua). The method is essentially an analysis of covariance and assumes that mortality rates in 2001 to 2002 and 2002 to 2003 were similar. Ages 7 to 10 were used in the analysis; the age-key only contained 5 specimens older than age 10. We excluded age classes 3-6 from the analysis, because both the length and age distribution for the surveys suggest that younger quahogs are less available to the survey than larger (and older) animals. Catch curve analysis assumes that all classes included in the analysis are fully selected, which is not the case for these age-classes. We note that fishing effort over the lease area in 2001 and 2002 was relatively constant (3145 and 3198 harvester days respectively). This would imply that at least the fishing portion of the mortality rate might have been constant. The statistical model used was:

ln [A.sub.ij] = [[beta].sub.0] + [[beta].sub.1]Y + [[beta].sub.2]I + [epsilon]

where [A.sub.ij] is the number of quahogs of age i in year j; Y is a class variable indicating year-class and 1 is the covariate age. [[beta].sub.1] are year-class effects and [[beta].sub.2] is the estimate of total mortality in the time period.

The exploitation rate (F) was calculated for the fishery up to September 15, 2003, and the proportion of the fishable biomass removed by the fishery was estimated for all 3 y by dividing commercial landings by the estimated biomass of commercial-size animals (shell length [greater than or equal to] 45 mm) from spring surveys.

The survival rate estimates were calculated using the standard equation:

S = [e.sup.-z]

where Z = [[beta].sub.2] from the analysis above. These analyses provide the first estimates of total mortality and fishery exploitation for this population.

Field experiments were also conducted from May to October 2003 (6-mo sampling period) to estimate summer survival of quahogs. Twelve stations were selected from high to low tide. At each station (0.12 [m.sub.2]), quahogs (6 quahogs per duplicate) from two size groups, 50-60 and 65-75 mm in shell length, were tagged and placed in the sediment. A small fence was placed around the area so that movements of the quahogs were restricted to the station. All quahogs were recovered at the end of the study period to estimate survival.

Statistical Analyses

A Kruskal-Wallis 1-way analysis of variance was used to determine significant differences in catch rates from 1997 to 2003 because the data was not normally distributed (Sokal & Rohlf 1981). A Kruskal-Wallis 1-way analysis of variance was also used to determine significant differences in mean densities from 2001 to 2003. A Tukey multiple comparison post hoc test was used for classifying the differences among catch rates and densities (Sokal & Rohlf 1981). A Kolmogorov-Smirnov two-sample test was used to compare densities without spat between 2002 and 2003 (Sokal & Rohlf 1981). Alpha level was 0.05 for all analyses. All statistics are presented as mean (X) [+ or -] SE (SE) and were estimated using SYSTAT version 10 (SPSS Inc. 2000). SAS (SAS Institute 1999-2000) was used for the modified catch curve analysis.

RESULTS

Population Surveys

Mean total quahog densities for the 2001, 2002, and 2003 surveys from traditional sampling stations (n = 45) were 54.8, 88.4, and 48.3 individuals/[m.sup.2] respectively (Table 1). A comparison of mean total densities and mean densities excluding spat (shell length [greater than or equal to] 5 mm/[m.sup.2]) suggested variable recruitment in 2002 and 2003 (Table 1). Few juveniles of shell length between 5 and 30 mm (<3-4 y old) and larger adult quahogs (shell length [greater than or equal to] 60 mm, [greater than or equal to] 8 y old) were collected during the 2001 to 2003 surveys (Table 2 and Table 3; Fig. 3 and Fig. 4). Based on the age-length key of aged animals, the quahogs of unspecified ages would likely be 11 y and older.

[FIGURES 3-4 OMITTED]

The age composition of quahogs larger than 25 mm sampled in the preharvest surveys showed a similar age structure in the 3 y of the surveys (Fig. 4, Table 2). Age 7 was the dominant age-class.

Commercial Harvest and Catch Rate

Age 7 was also the dominant age-class of the commercial landings in 2003 (Fig. 4, Table 3). Mean age to market was 7 y (i.e., 50 mm SL, 25 mm shell hinge) and most of the harvest were animals between 6 and 10 y of age. In total, 4.2 million quahogs accounted for the landings of 258 tons in 2003.

Catch and effort increased from 1999 to 2001. The mean catch rate for the period 1997 to 2003 was 98.3 [+ or -] 1.29 kg/days fished. Catch rates increased from 1999 to 2003 and catch rates for 2002 and 2003 were significantly higher (P < 0.001) than in previous years (Fig. 5). The 3-fold increase in harvest from 1997 to 2001 is concurrent with an increase in fishing effort expressed in harvester days during that period (Fig. 5).

[FIGURE 5 OMITTED]

Preliminary Estimates of Mortality Rates and Survival

The modified catch curve analysis (analysis of covariance) of survey numbers indicated no significant difference (P = 0.98) in year-class abundance for the 1992 to 1995 cohorts (ages 7-10 in 2001 to 2003) and the common slope also suggested a high rate of mortality (Z = 1.32). As a result, a catch curve analysis was conducted using the pooled data which gave an estimate of Z = 1.40 equivalent to a survival rate of about 25% (Fig. 6).

[FIGURE 6 OMITTED]

Because total mortality was estimated to be high, mortality attributed to commercial harvest of the lease (usually referred to as fishing mortality--F) appears to be low. The exploitation rate (in numbers) for 2003 was calculated (number removed in the fishery divided by the estimate of quahogs present at the beginning of the harvest season from the surveys) to be 3.0% for quahogs [greater than or equal to] 45 mm and 4.5% for quahogs [greater than or equal to] 50 mm. Estimates of the proportion of the fishable biomass taken in the fishery for all 3 y indicated that IFP exploited approximately 5% and 10% of the standing biomass of commercial size ([greater than or equal to] 50 mm). The survival estimates from the field experiments with tagged quahogs conducted during the summer of 2003 suggested that survival was high at 93.8 [+ or -] 1.54% to 97.6 [+ or -] 2.14%, for small and commercial size quahogs respectively. This would imply a natural mortality of 2.4% to 6.2% for the 6-mo (May to October) study trial.

DISCUSSION

Quahog densities in St. Mary's Bay were higher than those of commercially harvested populations of the Gulf of St. Lawrence, where densities have been estimated between 4.6-16.4 individuals/ [m.sup.2] (Landry et al. 1993). Fegley (2001) found that 80% of quahog densities (individuals of SL >30 mm) reported throughout North America and England ranged from 1-15 individuals/[m.sup.2], whereas the remaining 20% of the studies he examined described densities >500 individuals/[m.sup.2]. Thus, mean quahog densities for animals >30 mm in St. Mary's Bay from 2001 to 2003 surveys were greater than densities usually encountered in natural quahog populations (Table 1). Populations with densities >500 individuals/[m.sup.2] are characteristic of intensive shellfish aquaculture areas and rarely exist in nature (Castagna 1984, Fegley 2001). The higher densities observed give some indication of the potential stocking density of this species in an aquaculture setting. Because the mean density of quahogs [greater than or equal to] 30 mm showed some decline, density estimates were not significantly different over the 3 y of the population surveys.

Mean age to market for quahogs from St. Mary's Bay was 7 y and IFP appears to be exploiting quahogs of 6 y and older. Age to market is younger than that for populations documented in the Gulf of St. Lawrence. Quahog populations of the Gulf of St. Lawrence reach commercial market size (SL [greater than or equal to] 50 mm) at ages ranging from 9 to 13 y (Landry et al. 1993). Growth of these populations is much lower than southern United States populations, which take 2 to 5 y to reach market size (Grizzle et al. 2001). Differences in growth are attributed to many factors and include temperature, food quality and quantity, and salinity. Studies are planned to examine environmental and genetic differences between quahogs from St. Mary's Bay and populations from the Gulf of St. Lawrence.

The increase in harvest until 2001 seems to be caused primarily by an increase in fishing effort. However, catch rates increased significantly in 2002 and 2003 whereas effort remained stable in 2002 and declined in 2003 (Fig. 5). Since 2001, distribution charts from the intertidal surveys showing where market-sized animals are located have been available to IFP. These, combined with harvesters becoming more knowledgeable of the lease area have likely increased fishing success. Thus, the increase in catch rates in 2002 and 2003 was likely caused by an increase in harvest efficiency rather than by an increase in the standing stock. We observe that the population surveys do not suggest an increase in population during that period.

It is important to note that the age at length matrix for 2002 was used to calculate the age composition of the 2001 to 2003 surveys, as well as the commercial harvest in 2003. This assumes that during this short period, there have not been large changes in growth. The age at length matrix used for the calculations contained few large individuals, which might have affected the estimation of the age composition of commercial harvest in 2003. There is usually little difficulty in identifying annuli for these age classes (Jones et al. 1990). Age determination and growth rate estimates will be conducted annually in the future.

The interannual increase in numbers of quahogs from age 3 to 7 in the preharvest surveys suggested that younger quahogs have a distribution that is wider than the survey area and that quahogs may progressively "recruit" to the survey area and therefore prevented us from calculating mortality rates for juveniles. The origin of these quahogs and their movements are currently unknown. Eversole et al. (2000) suggested that shifts in age distribution from different sites at Two Sisters Creek in South Carolina were partly due to the resuspension and distribution of post larval quahogs. The byssus of a quahog can be released or broken whereby the juvenile quahog could be resuspended within the water column (Carriker 2001). Eversole et al. (2000) further point out that this would still require the suitable hydrodynamic features for transport and survival of post larval quahogs. St. Mary's Bay is an area of high tidal surge with two complete tidal cycles daily of up to an average tidal height of 25 m in regions where mean low water are 0-15 m (Greenburg 1984, Pearce & Sucsy 1986). Winds are prevalent throughout the year (Seibert & Reddy 1969, Greenburg 1984) and would intensify water movement over the entire lease. The redistribution of post larval quahogs and more mature juvenile quahogs is possible in such an area.

Few studies have discussed potential geospatial exclusion among spat, juvenile and adult quahogs. Rice et al. (1989) suggested that large concentrations of adult quahogs may affect the recruitment of juvenile quahogs and chemical cues were suggested to have a role in this behavior. The actual relationship between adult spatial patterns and differential larval settlement, postlarval survival, or redistribution is still unclear (Armonies 1996, Peterson 1986, Wilson 1990). A subtidal survey of the lease area (n = 49) was conducted in July 2003 to determine the presence or absence of quahogs in this area. Only five quahogs were found in the survey (Fig. 7). Results suggest that quahogs were almost exclusively found in the intertidal portion of the lease (Fig. 7).

[FIGURE 7 OMITTED]

The catch curve analysis indicated that the total mortality rate (Z) of quahogs from ages 7-10 is high and that only 25% of animals of these age groups survive from 1 y to the next. This estimate includes natural mortality (i.e., due to predation, disease, low temperatures, or other environmentally induced mortalities) and removals in the commercial fishery. However, both the estimates of the exploitation rate for 2003 and the fraction of the fishable biomass taken by the fishery for all 3 y were low. This implies that natural mortality on these age groups was high. Predation may be the cause of a portion of this mortality, because predation by sea gulls is common over the lease area. Hibbert (1977) observed high predation rates on adult quahogs in the intertidal mudflat at Hamble Spit in Southampton, England, estimated at 5-10 individuals/[m.sup.2] per year. Preliminary indications of high mortalities of adult quahogs in the subtidal lease area by the moon snail (Lunatia heros), observations from the tagging experiment in 2003 suggested that mortalities due to natural causes were low during summer in the lease area. Therefore, we suggest that the high natural mortality indicated likely occurs at other times of the year than summer.

There are many other documented sources of natural mortality in quahogs. Kraeuter (2001) provided a detailed overview on quahog predation, including that of crabs. A variety of crab species, including the green crab (Cancinus maenas) and the nemertean worm (Cerebratulus lacteus) are also present on the lease area, but their actual impact on this quahog population is poorly understood. Whetstone and Eversole (1978) suggested that crabs are the principle predator affecting quahog populations. An early study of quahog predation by the green crab in the Gulf of Maine suggested that they are an important factor in reducing quahog seedbeds (Moulton & Gustafson 1955). Green crab surveys have been conducted since 2002 to estimate the size distribution and abundance of the population. Gut analysis of specimens captured from the lease in 2003 are being conducted.

Mortality rates for adult quahog populations from New England States are usually low and uniform throughout the year and rarely exceed 50% for age classes between 6-10 y old (Kennish 1978). Large losses of adult quahogs may be attributed to winter kill caused by ice scouring on the lease. Photographs taken of the lease area in December 2002 showed the presence of large ice cakes of 1.5 x 2 x 2 m (height x length x width). Ice cakes covered the intertidal region from January to April 2002 to 2004. Winter kill has been suggested for losses of large amounts of oysters (Crassostrea virginica) and quahogs throughout much of the Gulf of St. Lawrence Region in 2002 to 2003 (Landry, pers. comm. 2003), and the phenomenon has never been quantified in Atlantic Canada to date. Our study suggests that it may be an important factor in the population dynamics of quahogs in this area as well.

Exploitation rates assume that this is a closed population (there is no emigration and/or immigration). These estimates would be suspect if there were substantial seasonal movements (i.e., emigration from or immigration to the lease area over the summer) or if there were age-dependent movements of quahogs (i.e., net movement of older quahogs outside the area). Tagging experiments may help assess passive movement of quahogs on the lease.

In conclusion, this study describes some of the characteristics of a unique quahog population in the Bay of Fundy, and provides a detailed description of the current harvest activities of the quahog, including size and age composition of the fishery removals. Such information is rarely available for this type of fishery. Continued sampling of the population and the harvest activities of IFP could lead to the use of age-structured population models (i.e., virtual population analysis) to gain a better understanding of the dynamics of this population, and in turn, to optimize lease production. In the longer term, these and other analyses, such as yield and spawner-per-recruit analyses will assist in establishing reference points for this quahog lease. Finally, the location of this population is intermediate in relation to the latitudinal range of quahog on the east coast of North America and as such constitutes an interesting reference site.

Figure 2. Annual landings of northern quahogs from the lease area in
St. Mary's Bay, Nova Scotia, Canada, from 1997 to 2003.

Year    Landings (t)

1997      95
1998     108
1999     149
2000     248
2001     314
2002     370
2003     285

Note: Table made from bar graph.

TABLE 1.
Mean northern quahog densities (individuals/[m.sup.2],
n = 45) for St. Mary's Bay, Nova Scotia, Canada.

                    Total density      Density (1)
                     (P = 0.010)       (P = 0.850)
                                      (without spat)

Survey data    n      X        se      x        se
   2001       45   54.8 (b)   13.1    n/a      n/a
   2002       45   88.4 (a)   15.9    49.0     14.5
   2003       45   48.3 (b)   10.5    42.0     10.4

                      Density (2)
                      (P = 0.828)
               ((Shell length [greater than
                  or equal to] 30 mm)

Survey data         x          se
   2001            46.8       12.2
   2002            43.7       13.6
   2003            36.7       9.58

(1) Spat were animals with a shell length [less than or equal to]
5 mm.

(2) Densities for individuals with shell length [greater than or
equal to] 30 mm are provided to compare with data from Fegley (2001).

Letters represent significant differences among the means.

TABLE 2.
Length frequency (shell length in mm) of northern quahogs from
population surveys (45 stations) conducted in June 2001 and 2002
and in May 2003 in the lease area in St. Mary's Bay, Nova
Scotia, Canada.

Shell length            Year
    (mm)        2001    2002    2003

    1-5          66     447      71
    6-10          4      21      34
   11-15          1       3       9
   16-20          0       5       6
   21-25          4      11       8
   26-30         25      25       5
   31-35         67      61      29
   36-40        103     115      74
   41-45        124     129     110
   46-50        107      92     113
   51-55         55      44      90
   56-60         34      23      42
   61-65          8       6      15
   66-70          6       5       6
   71-75          2       3       1
   76-80          3       2       0
   81-85          4       1       0
   86-90          0       1       4
   91-95          2       0       0
   96-100         2       1       0
  101-105         0       0       0
  106-110         0       0       0
  111-115         0       0       1
   Total        617     995     618

TABLE 3.
Age composition of quahogs of 25 mm and larger from population
surveys (numbers expanded to the survey area) conducted in June
2001 and 2002 and May 2003 and from the 2003 commercial
landings in the lease area in St. Mary's Bay, Nova Scotia, Canada.
The unspecified group is composed of large individuals (>70 mm,
older than age 11) for which no age assignment could be made.

                   Population surveys ('000)      Landings
                                                   ('000)

                                  Year

    Age          2001       2002       2003        2003

     3           2,425      2,039      1,996         0.1
     4           4,369      3,949      1,955         0.2
     5          22,119     24,061     12,392        19.2
     6          59,755     56,786     52,854       692.8
     7         107,617    101,312    104,149     1,965.2
     8          46,912     44,341     46,625       807.9
     9           4,523      3,256      5,442       206.5
    10           2,061      3,178      1,861       110.5
    11               0          0          0         0.0
    12             472      1,417        630        43.4
    13             472        472        157         8.0
    14               0          0          0         0.0
    15               0        472          0        10.1
Unspecified      6,138        472      3,305       344.8
   Total       256,865    241,755    231,367     4,208.6

ACKNOWLEDGMENTS

The authors thank Rachel Caissie, Andre Drapeau, Remi Sonier, Denise Muise, Keenan Melanson, Scott Bertram, and Jean-Francois Mallet. Funding for this four-year joint project was provided by the Aquaculture Collaborative Research and Development Program of Fisheries and Oceans Canada. Additional in-kind support for this project was received from IFP.

LITERATURE CITED

Armonies, W. 1996. Changes in distribution patterns of O-group bivalves in the Wadden Sea: byssus-drifting releases juveniles from the constraints of hydrography. J. Sea Res. 35:323-334.

Castagna, M. 1984. Methods of growing Mercenaria mercenaria from post-larval to preferred-size speed for field planting. Aquaculture 39: 355-359.

Carriker, M. R. 2001. Functional morphology and behavior of shelled veligers and early juveniles. In: J. N. Kraeuter & M. Castagna, editors. Biology of the quahog. New York: Elsevier. pp. 283-303.

Dillon, R. T., Jr. & J. J. Manzi. 1992. Population genetics of the quahog, Mercenaria mercenaria, at the northern limit of its range. Can. J. Fish. Aquat. Sci. 49:2574-2578.

Eversole, A. G., N. Devillers & W. D. Anderson. 2000. Age and size of Mercenaria mercenaria in Two Sisters Creek, South Carolina. J. Shellfish Res. 19(1):51-56.

Fegley, S. R. 2001. Demography and dynamics of quahog populations. In: J. N. Kraeuter & M. Castagna, editors. Biology of the quahog. New York: Elsevier. pp. 383-422.

Greenburg, S. A. 1984. A review of the physical oceanography of the Bay of Fundy. In: D. C. Gordon & M. J. Dadswell, editors. Update on the marine environmental consequences of tidal power development in the upper reaches of the Bay of Fundy. NB, Canada: Moncton. pp. 9-30.

Grizzle, R. E., V. M. Bricelj & S. E. Shumway. 2001. Physiological ecology of Mercenaria mercenaria. In: J.N. Kraeuter & M. Castagna, editors. Biology of the quahog. New York: Elsevier. pp. 305-382.

Hibbert, C. J. 1977. Growth and survivorship in a tidal-flat population of the bivalve Mercenaria mercenaria from Southampton water. Mar. Biol. 44:71-76.

Hilborn, R. & C. J. Walters. 1992. Quantitative fisheries stock assessment: choice, dynamics and uncertainty. New York: Chapman and Hall. 570 pp.

Jones, D. S., I. R. Quitmyer, W. S. Arnold & D. C. Marelli. 1990. Annual shell banding, age and growth rate of quahogs (Mercenaria spp.) from Florida. J. Shellfish Res. 9(1):215-225.

Kennish, M. J. 1978. Effects of thermal discharges on mortality of Mercenaria mercenaria in Barnegat Bay, New Jersey. Envion. Geol. 2: 223-254.

Kraeuter, J.N. 2001. Predators and predation. In: J.N. Kraeuter & M. Castagna, editors. Biology of the quahog. New York: Elsevier. pp. 441-589.

Landry, T., T. W. Sephton & D. A. Jones. 1993. Growth and mortality of Northern quahaug, (Linnaeus, 1758) Mercenaria mercenaria, in Prince Edward Island. J. Shellfish Res. 12(2):321-327.

Landry, T. & T.W. Sephton. 1996. Southern Gulf Northern quahaug. Fisheries and oceans Canada Atlantic fisheries stock status report, 96/ 102E. 3pp.

Moulton, J. M. & A. H. Gustafson. 1955. Green crabs and the redistribution of quahogs. Science 123:992.

Pearce, B. R. & P. Sucsy. 1986. Bay of Fundy Tides. In: B. B. Parker, editor. Applications of real-time oceanographic circulation modeling: symposium proceedings. Laurel, Maryland. pp. 257-268.

Peterson, C. H. 1986. Enhancement of Mercenaria mercenaria densities in sea grass beds: is pattern fixed during settlement season or altered by subsequent differential survival? Limnol. Oceanogr. 31(1):200-205.

Rice, M. A., C. Hickox & I. Zehra. 1989. Effects of intensive fishing effort on the population structure of quahogs, Mercenaria mercenaria (Linnaeus 1758), in Narragansett Bay. J. Shellfish Res. 8(2):345-354.

Ricker, W. E. 1975. Computation and interpretation of biological statistics of fish populations. Bull. Fish. Res. Board Can. 191:xviii+. 382 pp.

Ropes, J. W. & L. O'Brien. 1979. A unique method of aging surf clams. Bull. Am. Malacol., Union Inc. pp. 58-61.

SAS Institute. 1999-2000. SAS online document, version 8: for all systems. February 2000. Cary, North Carolina.

Seibert, G.H. & M. P. M. Reddy. 1969. Petit Passage tide and current survey, 1966. unpublished manuscript. Bedford Institute, Dartmouth, NS, Canada, data series 1968-7-D. 47 pp.

Sephton, T. W. & C. F. Bryan. 1990. Age and growth rate determinations for the Atlantic surf clam, Spisula solidissima (Dillwyn, 1817), in Prince Edward Island, Canada. J. Shellfish Res. 9(1):177-185.

Sinclair, A. F. 2001. Natural mortality of cod (Gadus morhua) in the southern Gulf of St. Lawrence. ICES J. Mar. Sci. 58:1-10.

SPSS Inc. 2000. Systat user's guide, ver. 10, vol. 1. Chicago, Illinois. 663 pp.

Sokal, R. R & F. J. Rohlf. 1981. Biometry: the principles and practice of statistics in biological research, 2nd ed. New York: W. H. Freeman and Company. pp. 383-422.

Whetstone, J. M. & A. G. Eversole. 1978. Predation on quahogs, Mercenaria mercenaria, by mud crabs, Panopeus hebstii. Proceed. Natl Shellfish. Assoc. 68:4248.

Wilson, F. S. 1990. Temporal and spatial patterns of settlement: a field study of mollusks in Bogue Sound, North Carolina. J. of Mar. Biol Ecol. 139:201-220.

Whiteaves, J. F. 1901. Catalogue of the marine invertebrates of eastern Canada. S. E. Dawson, Ottawa, Ontario. 271 pp.

KEVIN LEBLANC, (1) * MARC OUELLETTE, (2) GHISLAIN A. CHOUINARD (2) AND THOMAS LANDRY (2)

(1) Transport Canada, Heritage Court, 95 Foundry Street, P.O. Box 42, Moncton, New Brunswick, Canada EIC 8K6; (2) Gulf Fisheries Centre, Fisheries and Oceans Canada, 343 Universite Avenue, P.O. Box 5030, Moncton, New Brunswick, Canada E1C 9B6

* Corresponding Author: E-mail: [e-mail]; Fax: [phone]