2008 NORTHEASTERN NATURALIST 15(2):209–214 Intertidal Oysters in Northern New England Mark Capone1,2,*, Ray Grizzle1,2, Arthur C. Mathieson1,3, and Jay Odell4 Abstract - Little is known about the distribution and ecology of intertidal oysters in northeastern North America. North of Chesapeake Bay, intertidal oysters have either been previously reported as non-existent or only occurring as single oysters or sparse clusters. In the present study, we report the occurrence of dense populations of intertidal oysters at several estuarine sites within New Hampshire and mid-coastal Maine, with these growing under dense canopies of the long-lived Ascophyllum nodosum (fucoid alga). The densities of these northern intertidal oysters rival subtidal populations in the same geography, and their sizes suggest a persistence of 5 or more years. Introduction Crassostrea virginica (Gmelin) (eastern oyster) inhabits coastal and estuarine waters from the Gulf of Saint Lawrence to the Gulf of Mexico (Carriker and Gaffney 1996), occurring subtidally, intertidally, or in both habitats. Intertidal populations are common in the South Atlantic (Bahr and Lanier 1981, Burrell 1986), limited in the mid-Atlantic (DeAlteris 1988), and are often described as non-existent in the Northeast (Kennedy and Sanford 1999, Whitlach 1982), presumably because extreme winter temperatures and ice-scour cause high mortalities of oyster sets. In one of the few studies of intertidal oysters in New England (i.e., Great Bay, NH), Hardwick-Witman and Mathieson (1983) found low densities (less than 1–5.6 individuals/ m2) of detached populations, which suggested to them a potential subtidal origin and ephemeral nature. By contrast, recent observations in New Hampshire and Maine have shown that intertidal oyster populations are more prevalent than previously suggested. In order to better characterize these intertidal populations, we sampled oyster assemblages from six New England estuarine sites, including three from Great Bay, NH and three others from the upper reaches of the Damariscotta River near Newcastle, ME (Fig. 1). We hope that enhanced knowledge regarding these persistent intertidal oyster assemblages will encourage future conservation, restoration, and a better understanding of their ecology. Methods Our initial studies at Weeks Point, NH (43°03'32"N, 70°51'42"W) were conducted during October 21, 2005 at which point we visually inspected a 1Jackson Estuarine Laboratory, 85 Adams Point, Durham, NH, 03824. 2Department of Zoology, Spaulding Life Sciences, University of New Hampshire, Durham, NH 03824. 3Plant Biology Department, Spaulding Life Sciences, University of New Hampshire, Durham, NH 03824. 4The Nature Conservancy, Virginia Program, Richmond, VA 22901. *Corresponding author - m.capone@unh.edu. 210 Northeastern Naturalist Vol. 15, No. 2 300-m length of shoreline for the occurrence of intertidal oysters, as well as for general ecological conditions. Ten haphazardly placed quadrats (0.1 m2) were sampled within the Ascophyllum nodosum (L.) Le Jolis (knobbed wrack) zone of the intertidal zone. The elevation of each quadrat was determined using a line level and a surveying rod (Dawes 1998, Mathieson et al. 1998), with vertical heights above or below mean low water (MLW) calculated based on predicted tidal levels (Harbor Master Program, Version 3, Zihua Software, Marlboro, CT). Non-destructive sampling methods were used to determine density, frond length (to nearest cm), annual growth rates, age, and amount of apical pruning for ten Ascophyllum plants within each quadrat. Annual (i.e., total) growth rates for A. nodosum during the past year were determined by measuring the mean length of intact apical tips that had originated from the last terminal bladder (cf. Baardseth 1970). Age was determined by counting the number of bladders per frond, with the formation of the initial bladder assumed to have taken 4 years and additional bladders added yearly thereafter (Baardseth 1970, Mathieson and Guo 1992). Canopies of Ascophyllum were lifted and all bivalves present within each quadrat were identified and counted, and their shell heights, beak to lip, were measured to the nearest millimeter with Vernier calipers . The remaining five sites included two locations within Great Bay, NH and three on the upper reaches of the Damariscotta River, in Newcastle, ME: Figure 1. Map of New England and Gulf of Maine. Arrows show the locations of Great Bay, NH and Damariscota River, ME study sites. 2008 M. Capone, R. Grizzle, A.C. Mathieson, and J. Odell 211 Nannie Island (43°4'8.98", 70°51'45.68"W) and Woodman Point, Greenland, NH (43°4'18.55"N, 70°51'37.00"W), plus Goose Rocks (44°00'51"N, 69°32'56"W), the shoreline just west of this location (44°00'46"N, 69°32'56"W), and Hog Island (44°00'46"N, 69°32'35"W) within Newcastle, ME. The presence of oysters at each of these sites was noted, plus their occurrence relative to fucoid algal coverage. Canopies of the fucoid alga A. nodosum were lifted, and the presence or absence of oysters noted, as well as any additional understory organisms present. In addition, at Goose Rocks only, haphazard samples of intertidal oysters were measured for shell height with Vernier calipers. Results Weeks Point Oysters were present on rock outcrops along the entire 300-m section of shoreline inspected, with nearly all live populations occurring under Ascophyllum canopies (Fig. 2B). Rare single oysters or small clusters grew on some cobbles at the base of Ascophyllum communities or on adjacent Figure 2. A) Hog Island, ME intertidal zone illustrating typical habitat where oysters were found; note extent of rockweed covering rocks. B) Weeks Point, NH intertidal oysters beneath rockweed. C) Ascophyllum and Fucus covering oysters at Hog Island, ME. D) Close-up of Goose Rock intertidal oyster community; note barnacles, mussels, and Ascophyllum. 212 Northeastern Naturalist Vol. 15, No. 2 mudflats. Oyster densities ranged from 10 to 150 individuals/m2, with a mean density of 57 ± 40.8 individuals/m2 (standard deviation); they extended from 0.0 to +1.2 m. The mean oyster shell height was 60.1 ± 8.3 mm, with a maximum height of 89.5 mm. The ribbed mussel, Geukensia demissa (Dillwyn), was also common amongst oysters, occurring with a mean density and shell height of 11 ± 11.0 individuals/m2 and 35.1 ± 11.7 mm, respectively. The mean age of Ascophyllum plants was 7.2 ± 1.2 years, with a maximum age of 13 years. Ascophyllum densities ranged from 60 to 180 individuals/m2, with a mean density of 96 ± 35.5 individuals/m2; their mean frond length was 55.4 ± 16.6 cm. As determined by simple linear regression, oyster shell height was inversely related to tidal height (r2 = 0.62, p = 0.007). No significant relationships were found between oyster metrics and plant metrics. Oyster size was not related to Ascophyllum length (r2 = 0.035, p = 0.29) nor density (r2 = 0.0011, p = 0.93), while oyster density was also not related to Ascophyllum length (r2 = 0.17, p = 0.24) or density (r2 = 0.029, p = 0.63). Other New Hampshire/Maine sites Similar intertidal oyster populations were found under Ascophyllum canopies at the other five sites, with their densities being consistent with those at Weeks Point. At Nannie Island and Woodman Point (i.e., New Hampshire) oysters were rarely found on bare substrata, while newly settled oyster spat (<30 mm shell height) were present in some areas. Ribbed mussels were present at all three New Hampshire sites. Mytilus edulis (Linnaeus) (blue mussels), Semibalanus balanoides (Linnaeus) (acorn barnacles), and Littorina littorea (Linnaeus) (periwinkles) were present with oysters at all Maine sites. Attached oysters were present at 0.0 MLW on exposed bare substrata on the shoreline west of Goose Rock, while above this elevation, they were restricted to subcanopy fucoid habitats. The mean shell height for oysters at the Goose Rock site was 63 ± 21.5 mm, with a maximum shell height of 109.5 mm. Discussion We are unaware of any previous reports of relatively high densities of persistent intertidal oysters in the northeastern North America. This absence suggests that historically they either did not persist or that their abundances were so low that they did not warrant attention or detection. Anecdotal stories from local New Hampshire oystermen support the first hypothesis, as they are unable to recall the occurrence of intertidal oysters prior to the past ten years. Commercial oystermen in Maine also believe that the intertidal expansion of oysters reported here is a recent phenomenon. It is possible, given the cryptic nature of algal-covered intertidal oysters that they may have escaped detection; however, the alternative hypothesis (i.e., recent expansion) deserves further attention. 2008 M. Capone, R. Grizzle, A.C. Mathieson, and J. Odell 213 High densities of intertidal oysters, up to 150 individuals/m2, were found in New Hampshire and Maine under canopies of long-lived Ascophyllum nodosum and attached to rocky substrata. Intertidal oysters provide a complex structure for the attachment of additional epifauna, such as ribbed mussels and barnacles. At low tide, the extensive rockweed cover in these geographies can completely cover oysters (Fig. 2), protecting them from environmental extremes (Bertness et al. 1999). As noted above, oysters were extremely rare on bare rocky substrata at all study sites. Bare rock outcrops of equal size and tidal height compared to those covered by Ascophyllum typically had no attached oysters. Small oysters (<30 mm) were present on bare substrata at Nannie Island and Woodman Point, showing that larval supply did not necessarily limit intertidal oyster distribution. While they can reach 109.5 mm under extensive Ascophyllum coverage, the absence of large oysters on bare substrata suggests that postsettlement mortality limits their distribution. The intertidal zone where Ascophyllum and Fucus vesiculosus L. (bladder wrack) occur within the Great Bay estuarine system is estimated to be 130,000 m2 or 32.12 acres (Josselyn 1978). If the entirety of this zone were inhabited by oysters at densities found in this study, there could be 7.4 x 106 ± 5.3 x 106 intertidal oysters or roughly 36% of Great Bay’s known subtidal oyster population (Trowbridge 2005) presently unaccounted for in the management of this resource. In summary, northern intertidal oysters require further examination. Have these populations recently expanded due to climate-related shift in habitat utilization caused by global warming (Sagarin et al. 1999) or have they simply been missed during previous studies? Further, why do oysters not occur in high densities on bare substrata in the northeastern United States as found in the Southeast? These and other questions require a broader understanding of oyster ecology in the Northeast, as the extension of oysters into the intertidal has implications on oyster harvesting, restoration, and management, especially relative to climate change. Acknowledgments We thank the Weeks family for allowing us access to the shoreline at Weeks Point and for providing a variety of ancillary information regarding oyster populations at this site. We also thank Chris Davis for helping us sample intertidal oyster populations near Newcastle, ME, and Dr. Roger Mann and an anonymous reviewer for valuable comments on the manuscript. 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