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3.3 RESULTS
Approximate substrate conditions are depicted in Figure 3-1. Although grain-size analysis was not conducted, descriptors for substrate characteristics generally conform with standard size categories, within the limits of visual observations. Boundaries between substrate types were more often subtle than distinct, with gradual shifts in substrate types. The nine categories presented on Figure 3-1 incorporate distinctions that could influence the infaunal and epifaunal benthic communities utilizing the area. Where the substrate was composed of a number of sediment types, the descriptors are listed in approximate decreasing order of occurrence (Figure 3-1). Sand was pervasive but was generally mixed with high proportions of gravel and/or cobble. The shallow “cove” south of the jetty included an expanse of sand with little coarser material. Low sand dunes occurred adjacent to the salt marsh. Boulders were present in most areas south of the salt marsh. Many of the boulders supported macroalgae (Fucus sp. or Ascophyllum nodosum) or barnacles, although many were bare of attached organisms. Silty sediments predominated only northeast of the salt marsh, although sandy silt was present in several other areas, including in the vicinity of the proposed terminal (Figure 3-1). A mussel bed (Mytilus edulis) was present seaward of the salt marsh. Clumps of mussels also occurred adjacent to boulders in many areas. Between the salt marsh and MLW were two small tide pools whose substrates were vegetated with eelgrass.
Few soft-shell clams were observed and these were found only in the areas of predominantly fine-grained (silt or sand) sediments (Figure 3-1). Siphon holes were occasionally apparent. No holes dug yielded more than one clam.
Polychaetes were evident in silty and sandy sediments and within mussel beds. Those observed were predominantly small-bodied taxa such as capitellids, spionids and maldanids. Glycerids (the polychaete family with which bloodworms are affiliated) were observed in silt and fine sand sediments and associated with mussel beds (Figure 3-1). Polychaetes appeared to be relatively more abundant in areas of primarily fine sediments than areas with a gravel component.
3.4 DISCUSSION
3.4.1 Historical Perspective - Soft-Shell Clams
The approximate locations of the boundaries of the intertidal sampling grid (zones) investigated in 1978 for soft-shell clams (Bertrand and Birge 1979) are overlain on the 1992 substrate map (Figure 3-2). Standing stock by zone estimated in 1978 is presented in Table 3-1. In 1978 soft-shell clams exhibited a patchy distribution along the Sears Island shoreline, ranging from 0 to 223.9 bushels/acre (0 to 95 bushels/ha). Across all zones surveyed in 1978, clams were present in 83 of the 283 samples (29%). Fifty-nine percent of the estimated total standing crop (calculated by multiplying acreage by bushels/acre) occurred in five zones (16, 18, 22, 23, 25), representing 26% of the acreage of the 1978 study area. In these five zones, clams were found in an average of 46% of the samples.
In the remaining 18 zones, clams occurred in an average of 22% of the samples. Zones 8-13, encompassing the proposed footprint of any of the alternative designs of the cargo terminal (D-1ID-1.1 and D2/D-2.l or D-3.5), contained 15% of the standing stock in 21% of the area surveyed. In the period when Bertrand and Birge performed their survey, a standing crop of 40 bushels/acre (99 bushels/hectare) was regarded as commercially attractive (USDOT and MDOT 1987). Using this criterion, areas 8 or 13 would have been considered desirable resources.
3.4.2 Soft-shell Clam Habitat
Distribution of clams in 1978 was loosely related to the substrate character. More than 60% of the clams collected occurred in sediments described as sand, sand-clay or sand-mud (Bertrand and Birge 1979). The remaining clams were collected from sediments that included some component of gravel or other “rock” material. Coarse sediments (>0.5 mm; gravel on the Wentworth scale) are difficult for clams to burrow in, although they may offer protection from predators (Newell and Hidu 1986). It is unlikely that intertidal substrate conditions have changed dramatically since 1978 with the exception of storm-related redistribution and localized changes caused by alterations of longshore currents near the jetty. As most substrate types yielded soft-shell clams in 1978, the shoreline examined in 1992 should be classified as potential clam habitat. Based on data developed by MDMR in 1978-1979 for USEPA, Fefer and Schetting (1980, Atlas Map 4) described the entire Searsport and Stockton Springs shorelines as soft-shell clam habitat. Areas where substrates are predominantly fine sand have the greatest potential to support high standing crops of soft-shell clams.
According to Schwind (1977) and Belding (1916), good soft-shell clam habitat characteristically is located in protected intertidal areas with limited erosion potentials, but where adequate tidal circulation will provide food, lime salts, and oxygen; while also eliminating waste materials. This generally does not include exposed beaches with surf Clams are found in various types of substrates, from rocky gravel to soft mud, but thrive in firm substrates of sand and mud.
The natural habitat of the soft-shell clam is between the tidelines, although it can be found subtidally. Subtidal beds are generally not extensive. Clam beds situated low in the intertidal zone, and subtidal zone are considered to have the potential to provide very significant spawns due to prolonged or permanent water coverage and greater water circulation.
The most important factor in clam habitat is a fair circulation of water. Growth of clams is in large measure determined by the proper availability of food (phytoplankton), and since the current or tide brings that food, sufficient tidal communication is necessary. Currents must not be so strong that they may cause erosion and/or silting of the resource. Suspended silt will affect clam feeding, and siltation of a bed will alter bottom elevations requiring the organisms to maintain proper depth. In extreme cases, silt can completely smother clam flats.
Soft-shell clams cannot establish themselves on exposed beaches where shifting sands will limit setting of seed. In the northern extent of the range, greater tidal heights and swifter tidal currents cause surface substrates to become rippled. Rippled sands are indicative of unproductive clam flats. Slimy bottoms and soft muds indicate limited water circulation, will also severely limit growth, and are not considered favorable to clams. Firmer substrates provide better clam habitat. Muds which contain hydrogen sulfide or other organic acids caused by decaying vegetation will prohibit growth and erode shells. Fine sands mixed with cementing muds provides the best substrate for clams.
Eelgrass beds are generally considered deleterious to a good soft-shell clam bed (Belding 1916, Schwind 1977). If eelgrass grows on, or in proximity to a clam flat it will tend to reduce tidal flows and may increase siltation to area flats. Mussel beds may act as spat collectors, but productive resources rarely become established in this conditions. If favorable mussel habitat exists, mussel beds will generally encroach upon and smother proximal softshell clam beds. While the danger of moving ice is less with soft-shell clams than with other bivalves (since they generally are deeper in the sediments), flowing ice and runoff erosion from melting ice cannot be entirely dismissed.
3.4.3 Factors Affecting Soft-shell Clam Populations
A number of factors interplay to control population sizes of soft-shell clams in the natural environment and could have contributed to the apparent decline in this species on the western shore of Sears Island between 1978 and 1992. These factors could include recruitment, substrate, food, predation, harvesting, pollutants, and disease. Although none of these factors was specifically investigated for this study, historical surveys documented long-term variability in standing crop.
Dow and Hurst (1975) identified the communities of Searsport and Stockton Springs as having a long lucrative history of soft-shell clam production interrupted by two substantial perturbations. Predation by an increasing population of green crabs (Carcinus maenas) and other species reduced soft-shell clam abundances locally by 90% between 1947 and 1963. Management practices, including predator control, were implemented to protect the resource until water quality conditions required closure of the resource for harvesting in 1966. Dow and Hurst (1975) noted an increase in the overall population of clams by the early 1970s. The second perturbation occurred in 1971 when an oil pipeline leaked fuel into Long Cove for more than three months. The fuel percolated through the sediment and was apparently toxic to clams for at least several years; by 1974, 86% of the standing crop of the “pre-spill Long Cove and western Sears Island population” had died (Dow and Hurst 1975). By 1977, 1971-1974 year-class clams had grown sufficiently to burrow into the deeper, oil-saturated sediments that continued to persist in parts of Long Cove; heavy mortality was then observed in these year classes (Dow 1978).
Spat recruitment to soft-shell clam populations is unpredictable, dependent upon the vagaries of spawning success, planktonic transport and mortality of the larvae and the settled spat. Size-class distribution of clams in 1978 suggested inter-annual variability in recruitment. Spat (_20 mm long) represented about 21% of the total clams collected in 1978 (note: spat abundance adjusted to standardize sampling effort), but were concentrated in Zone 3 (Figure 3-2) in an area with low densities of adult clams. Assuming growth rates similar to those observed in Hampton Harbor, NH (NAI 1985), these spat may actually represent the set from the two previous years.
In contrast, clams >50 mm long (three to six+ years old; Newell and Hidu 1986) comprised 50% of the population. This distribution suggests that spat recruitment was relatively low in 1978. Dow (1978) noted poor recruitment of 1975 and 1976 year classes and attributed this to observed increases in abundance of green crabs, a major predator on soft-shell clam spat. Green crabs and other soft-shell clam predators (moon snails, Polynices sp. and ribbon worms or rhynchocoels - Cerebratulus sp.), were observed in the project area in 1992. The combined natural effects of low spat settlement and high predation pressure between 1978 and 1992 could account for the apparently low standing crop observed.
Incidence of disease could also have influenced the soft-shell clam population. MDMR Area Biologist (Don Card, MDMR, pers. comm., 10/12/92) has observed a relatively high incidence of neoplasia, a fatal condition, in the Searsport area, compared to other areas in his jurisdiction. Neoplastic gonadal tumors were first recorded in the Searsport area in 1971 following the oil spill in Long Cove (Barry and Yevich 1975). Incidence of tumors in Long Cove ranged from 0 to 27%, averaging 6% between 1971 and 1974 and this phenomenon was initially attributed to the oil spill, although, experimental exposure of soft-shell clams to petroleum hydrocarbons did not corroborate this conclusion (Gardner et al. 1991). However, Long Cove was also exposed to herbicides, a common link with other areas in Maine where clams exhibited neoplasia (Gardner et al. 1991). Other investigators (reported in Hillman 1987) have indicated that sarcomatous neoplasia is actually caused by a viral infection but that environmental stresses, including low temperatures and pollutants, can enhance these effects. Neoplasia can reduce both the standing stock and the fecundity of a population by increasing the mortality of reproductive clams. An annual rate of 6% occurrence of neoplasia, coupled with poor spat recruitment, could reduce standing crop to <75% of its original value in 5 years.
3.4.4 Soft-Shell Clam Fishery
The entire Searsport-Stockton Springs shoreline was identified as commercial soft-shell clam beds by Fefer and Schettig (1980, Atlas Map 4). The whole area, including Sears Island, has been closed to shellfish harvesting since April 1987 due to bacterial contamination (Swan 1993). Between 1980 and 1987, harvesting was allowed during most winters (USDOT and MDOT 1987). About 80% of the clams collected in 1978 were of marketable size (_46 mm long). Although the intensity of harvesting is not documented, a combination of regular harvesting combined with low spat settlement could reduce the standing crop severely. Because harvesting has been restricted since 1987, successful recruitment and survival of any large year-class between 1987 and 1990 would have been evident even in a qualitative survey; 1990 clams would have reached about 20 mm in length and been easily visible by September 1992 (Newell and Hidu 1986). Few clams were observed in this survey (Section 3.3) even in areas that supported high standing crops in 1978. Therefore, either recruitment has been poor or mortality (from various sources, as described in Section 3.4.3) has been high.
Harvesting and standing crop of soft-shell clams east of Casco Bay has declined dramatically since the late 1970s (Widoff 1988; W. Foster, H. Winters, MDMR, 11/18/93, pers. comm.). Beal (UM Machias, reported by Kyle [1994] and pers. comm., 1994) reported that clam stocks have been decimated in Hancock and Washington Counties in recent years. Card (MDMR, Regional Biologist, 1/18/94, pers. comm.) confirmed that in Islesboro (the Penobscot Bay community nearest Sears Island with a soft-shell clam management program), natural settlement of spat has been very limited in recent years. There has been a severe decline in marketable stocks in Islesboro in recent years (E. Bachelor, Shellfish Constable, Islesboro ME, 1/24/94, pers. comm.). High variability in soft-shell clam populations among years has been observed elsewhere in New England. The soft-shell clam population in Hampton Harbor, NH, where standing crop has ranged from 6 to 152 bushels/acre (2 to 62 bushels/ha), has been examined for 20 years (NAI I 992b). These studies have demonstrated that standing crop is not easily linked to either larval abundance or spat settlement. In that area, one strong year class (1976) dominated the population structure for several years while high spat settlement in 1984 disappeared from the population. Factors such as harvesting pressure, predator abundance (and fluctuations) and disease directly affected standing crop.
3.4.5 Created Clam Flats
The Maine Department of Transportation consulted with the Maine Department of Marine Resources (MDMR) as well as federal resource and regulatory agencies (NMFS, USFWS, EPA, and ACOE) to determine desirable and appropriate means for mitigating the loss of intertidal soft-shell clam habitat associated with the Sears Island causeway and cargo pier (NAI 1987). In 1988 three clam flats were constructed in Stockton Harbor under MDMR guidance (12 MRSA 6675). Terrestrial sand was imported to create two 175m x 15m beds along the causeway (Flats #1, near Sears Island, and #2, near Kidder Point) and one 250m x 40m bed in the cove adjacent to Kidder Point (Flat #3). A portion of Flat #1 (12m x 6m) was seeded in December 1988 with marked Mya arenaria ranging from 15-50 mm in length (mean length 33.0 mm), transplanted from Thomas Point Beach in Brunswick, Maine. The other two beds were left to seed naturally.
Spat cores were not collected in the 1992 survey because no clams smaller than 25 mm had been found in the adult cores. Spat were, however, collected in 1993, with densities ranging from 17/rn2 on Flat #3 to 50/in2 on Flats #1 and #2 (Table 3-2). Spat densities on Flat #3 were the lowest of the monitoring effort, but densities on Flats #1 and #2 were higher than in at least one other year. Adult densities have declined steadily since 1989 or 1990, and reached their lowest levels of the monitoring period on each of the three flats in 1993 (1-2/in2). The few adults collected were, however, among the largest observed (49-54 mm) over the five year period (Table 3-3).
Observations made during the 1992 clam flat survey noted several green crab (Carcinus maenas) pits in the study area, and crabs were found in fucoid algae near the clam flats. Green crabs are known to consume spat and juvenile Mya arenaria (Ropes 1969) and may have contributed to the absence of clams. In 1993, field staff noted that many clams were dead in place, with valves intact and soft tissues in a state of decay, indicating a possible cause of death other than predation. Because so many dead clams were present in the spat and adult samples, numbers of individuals encountered and their lengths were enumerated (Table 3-4).
Average density of dead clams was highest on Flat #1(89/rn2), while average length was lowest (26 mm). Flats #2 and #3 had similar average densities (53 and 55/rn2, respectively), although clams from Flat #3 were, on average, larger than those on Flat #2 (41 mm compared to 29 mm).
Currently, the reason for the decline in abundances of soft-shell clams in the created clam flats is unknown. As discussed in Section 3.4.3, there are several factors that could affect standing crop that are unrelated to the quality of the substrate. To help determine whether standing crop on the created flats has been a natural occurrence or is related to physical characteristics of the constructed flats, several components are suggested for addition to the monitoring program:
• Obtain spat and adult population data from a control site - Islesboro might be a suitable location because it is the closest town where there is a shellfish management program, hence historical perspective;
• continue annual survey of spat and adult populations on created flats;
• test live and recently deceased clams for neoplasia;
• transplant known quantities of seed clams over specific portions of the flats and conduct predator exclusion experiments;
• document cause of death when possible (e.g., evidence of drill or crab predation);
• compare substrate of created flats to natural clam flats and to other created flats in Maine where recruitment has been successful.