Commercial harvest and population structure of a northern quahog population in St. Mary's Bay, Nova Scotia, Canada

Journal of Shellfisheries Research, Jan, 2005 by Kevin Leblanc, Marc Ouellette, Ghislain A. Chouinard, Thomas Landry

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.