Trends in Maine softshell clam landings.

ABSTRACT The soft-shell clam, Mya arenaria (L. 1758), has the highest landed value of Maine bivalves. Landings have been variable over the last century with current landings one third of their historical highs caused by low harvests in Eastern Maine. Diminished clam populations have been subjected to constant fishing pressure and heavy predation by green crabs apparently unchecked by winterkill. Clam stocks previously closed to harvesting because of pollution are now dug. Reduced breeding populations are tasked to produce a sufficient set to overcome offshore dispersal of larvae to repopulate flats on the coast of Maine. On-bottom and off-bottom sampling found few clam larvae in Eastern Maine. Recovery of the fishery will require reestablishment of breeding stocks.

KEY WORDS: clam landings, Gulf of Maine, green crabs, shellfish management.

INTRODUCTION

The softshell clam Mya arenaria supports the major bivalve industry in the State of Maine with a landed value of $16.61 million dollars in 2004. Since the early 1980s clam landings have declined by two-thirds. Washington County produced about two-thirds of the State landings in the 1970s but decreased to between 20% to 45% of landings in recent years (see Fig. 2 later).

[FIGURE 2 OMITTED]

Trends in landings seen in Figure 1 and Figure 2 comparing southern (S) versus eastern (E) Maine, do not show a regular cyclical trend in landings statewide, although Washington and Hancock counties have had two distinct peaks in landings over the past 50 years.

[FIGURE 1 OMITTED]

Management and Regulation

In 1901 Maine authorized towns to license diggers and regulate digging. Most soft-shells were canned. From 1901 to the 1930s, the State limited the canning season from September 15 to June 1. Shipping live clams out of state was prohibited. Summer digging was banned until 1937 when the three counties in the southwest lifted a ban because of the demand for live and shucked clams throughout the region (Fig. 1). In 1947 restrictions on summer digging and transporting clams were lifted in Lincoln County and restrictions in the rest of the State were removed in 1949. By 1958, only 10% of the catch was canned (Wallace 1997).

A minimum harvest size has been used as a clam conservation measure throughout the region. A two-inch minimum size law was passed by Maine in 1935 and repealed in 1963 (Fig. 1) when Maine authorized coastal municipalities to establish local ordinances to regulate digging and implement conservation measures. The number of towns with a local clamming ordinance increased to 45 in 1988 and 74 in 2003. In 1984, the 2-inch minimum harvest size was reestablished by Maine. Massachusetts has had a 2-inch softshell minimum for harvesting since the early 1900s (MacKenzie 1997). In the Bay of Fundy, New Brunswick and Nova Scotia both have a 44 mm (1.75-inch) minimum harvesting size (Jenkins et al.1997).

Fishing pressure on clams is limited by the number of harvesting licenses issued. The number of licensed diggers increased from 500 in 1888 to 2,600 in the late 1940s after World War II (Fig. 1). The number of licenses remained relatively constant until an in crease in the number of licenses in the 1970s corresponded with an increase in catch. Although catch had decreased by 1991 to levels not seen since 1959, the number of licenses decreased but remained above the number of licenses issued before the 1970s (Wallace 1997). Between 1990 and 2001, Cumberland and Lincoln counties issued the most licenses in S Maine with average clam harvest per license of 1,284 and 800.0 lb/license, respectively. In E Maine, despite the decline in landings in the early 1990s, the number of licenses remained relatively constant resulting in a lower catch per unit effort of 469 and 525 lbs per license in the same period. The recent increase in landings in Washington and Hancock counties (Fig. 1 & 2) in the mid 1990s has been matched by an increase in the number of licenses issued in these counties from 1,268 in 1996 to 1,940 in 2001. Consequently, catch per unit effort in these counties has remained low (Fig. 3).

[FIGURE 3 OMITTED]

Effort dedicated to clam management varies between municipalities. In 2002 Maine Department of Marine Resources data for appropriations for management and clam resource value for towns, the correlation between expenditures for management and value of clam landings was 0.41 (P < 0.01, n = 54). Of towns that listed expenditures for clam management, E Maine towns averaged $443 for management appropriations in 2002, whereas Southern and Midcoast Maine averaged $1206.

Pollution Closures

In 1946, Maine closed polluted waters under the National Shellfish Sanitation Program (Fig. 1). In 1974, 3,420 residential direct discharges of sewage had been identified. This number had decreased to 2,446 in 1992, but the number of prohibited acres closed to clamming peaked at about this time. Between 1985 and 1993, the total acreage closed to clamming increased (Fig. 4), whereas landings went down correspondingly perhaps because of a restricted fishing area (Wallace 1997). Before 1990, the DMR estimated that approximately one-third of all productive clam flats in Maine were closed because of pollution (Farrey et al. 1997). Opening flats resulted in an immediate increase in landings. Acreage closed to harvesting, decreased from 270,444 in 1993 to 153,847 acres in 2002; most of the closed acreage was in the S Maine. In New Brunswick, Canada about half of the flats were closed because of pollution in the mid 1990s and these closures were considered relatively permanent (Jenkins et al. 1997).

[FIGURE 4 OMITTED]

Gulf of Maine Tidal Dispersion

Tides and tidal currents in Gulf of Maine increase to the north and east. The tidal range is 3.4 m in Barnstable Harbor, on north side of Cape Cod, increasing to 6.5 m in Eastport in E Maine. Tidal flushing can be measured by determination of an exchange ratio and/or flushing time. The tidal exchange ratio is the ratio of bay volume at high tide to the total tidal flow. Flushing time is time required to replace the total volume of water. Flushing time decreases and exchange ratio increases with tidal amplitude.

Barnstable Harbor is at the southern end of the Gulf of Maine. In the 1940s and 1950s, the clam populations in the harbor decreased to negligible proportions and efforts to reestablish were largely unproductive (Ketchum 1954). Using a conservative exchange ratio for Barnstable Harbor of 0.3 (tidal volume 30% of bay volume), Ketchum (1954) estimated ~0.01% of the larvae spawned in the harbor would be retained to set. Offshore flushing was identified as the likely reason that "only rarely indeed is a substantial set obtained."

Tidal flushing would move clam larvae away from the mudflats where they are spawned. Edwards & Sharpies (1986) estimated flushing times for 1l0 Scottish lochs using tidal range, low water residual volume and the water surface area at low and high water. Flushing times varied from <0.5 to 14 days, with the shorter flushing times for small, shallow lochs and longer flushing times for deep-sea lochs. Using the same technique, flushing time of Quahog Bay on the mid Maine-coast (mean tidal range of 4.0 m and high water area of 4.3 [km.sup.2]) was estimated to be 1.25-1.5 days (Holte et al. 2003).

To determine the effect of a larger tidal range on flushing time, flushing times were determined for two bays in E Maine. Both bays had a tidal range of 5.2-5.3 m, but differed in size: high water area of Mason Bay is 3.7 [km.sup.2]; high water area of Englishman Bay is 131.7 [km.sup.2]. Using the technique of Edwards and Sharpies (1986) flushing times was determined as: 0.281 days for Mason bay and 1.8 days for Englishman bay.

A flushing time of <0.5 is less than one tidal cycle. Mason Bay, with an estimated flushing time of 0.281 days, drains nearly dry on spring tides. Low flushing times and high exchange ratios in Gulf of Maine bays, could be responsible for dispersing larvae offshore away from their native mudflats. Offshore dispersion of clam larvae would be expected to be greater where the tides are larger (i.e., E Maine relative to S Maine). Tidal processes would also be expected to have more effect on larval dispersal in the Gulf of Maine relative to bays and estuaries south of Cape Cod where tides are seldom greater than 1.5 m.

Data--Clam Sets Along the Maine Coast

Mudflats in different parts of Maine were surveyed for year 0 juveniles to compare clam sets in different parts of Maine. Core samples from mudflats were collected from E and S Maine in late fall and winter for 2 years and sieved. Juvenile clams (1.8-6.0 mm) from the set of the preceding summer were counted. Densities of juveniles averaged 16.9/[m.sup.2] from 120 samples from E Maine and 204.5/[m.sup.2] from 120 samples in S Maine (Vassiliev et al. 2000). Mya off-bottom settlement was also sampled with spat bags filled with monofilament line in locations in E and S Maine and retrieved monthly. Spat bags placed inside and outside the Scarborough River in S Maine had 484.4 [+ or -] 972.8 and 132.6 [+ or -] 189.5 juveniles/ bag, respectively, compared with 2.2 [+ or -] 6.4 and 1.6 [+ or -] 6.3 juveniles/ bag in the interior and outside of Mason Bay in E Maine. The differences between E and S Maine (P < 0.001) were statistically significant by t-test. The results of both of these samples suggest that the recent declines in clam harvests in E Maine may be at least partially caused by poor clam sets.

Predation-Green Crabs

Green crabs are a major predator of shellfish from newly settled juveniles to adults. Predation by juvenile green crabs and fish was found to be the ultimate factor controlling abundance of Mya juveniles in Barnstable Harbor, Massachusetts (Hunt & Mullineaux 2002). Green crabs were scored as very serious shellfish predators (8.3 on a scale of 1-10 with 10 being the most damaging) in southern New England (Walton 2001).

Green crabs were not found north of Cape Cod (Fig. 1, Fig. 5) in 1872 (Smith 1879). By 1905, green crabs were found in Casco Bay in Eagle Harbor, Harpswell and the New Meadows River (Rathbun 1905). By 1930, samples had been collected as far east as Brooklin in Hancock County and by the 1930s, in Frenchmen's Bay and Winter Harbor in Hancock County, but they were not found in Washington County. By spring, 1951, their range extended eastward to Jonesport and Lubec. By the end of the summer of 1951, they were found throughout Passamaquoddy Bay (Scattergood 1952).

[FIGURE 5 OMITTED]

Traps especially designed to catch green crabs were used by the Maine Bureau of Commercial Fisheries to sample crab populations from 1953-1960, and 1965-1967 (Welch 1968). In S Maine, crabs were trapped in Love's Cove, Southport. In 1953, daily trap catches at Love's Cove ranged from 110-670 crabs. Tagging and recapture studies estimated a population of 10,000 crabs per acre (Spear 1955b). Trap catches of green crabs declined in all areas sampled (Table 1) between the mid 1950s and mid 1960's (Welch 1968).

Figure 6 shows surface water temperatures recorded at Booth-bay Harbor, Maine over the past 100 years. The years in which large numbers of green crabs were trapped on the Maine coast (Tab.1) were the years with extremely warm winter water temperatures (Fig. 6). Figure 1 and 2 show a decline in clam landings in the early 1950s when crab trap catches were high. In 1955, clam abundance in study areas was reported to have decreased by 50% over 4 years principally in places where green crabs were abundant (Glude 1955). As winter water temperature cooled in the 1960s, numbers of green crabs in E Maine declined sharply (Welch 1968). The fishery recovered in the mid 1960s (Fig. 1 & 2). In S Maine, green crab populations did not reach the low points observed in Eastern Maine and Canada (Welch 1968).

[FIGURE 6 OMITTED]

In recent years, sampling of green crabs in Maine has been limited. Green crab sampling was discontinued by the Maine Department of Marine resources (DMR) until 2001. In 1997 and 1998, green crabs were found in two locations sampled on Mt. Desert Island and two of three locations sampled in Casco Bay, but were not found in an estuary in Camden on Penobscot Bay (Whitlatch & Osman 1999). In the fall of 2002, 107 crabs per man-hour search were collected by DMR personnel at 12 sites along the Maine coast (Mercer 2003). This catch/man hour sampled is lower than the highest capture rate reported for Perry, Maine in 1953-1957 of 552/man hour, but higher than the low capture rate of 7/man hour in 1960-1965.

Data--Green Crab Observations 2003-2004

In fall 2003 and winter 2004, two locations in E Maine were sampled with commercial crab traps from October 2003 to January 2004 (Jacques et al. 2004). Trap catch/tide ranged from 11-0/trap/ tide at Tidal Falls from October to January and 76-0 trap/tide at Bunkers Harbor during the same period. Although the trap design was different from traps used by DMR in the 1950s, the crab catch at Bunkers Harbor in the fall adjusted to 1.9 tides/day was comparable to catch rates throughout Maine in the 1950s in Table 1. During the same fall--winter, four fishermen bottom dragging in E Maine, reported moderate to numerous live crabs in their traps in the fall, but four of five dragging in the spring found numerous dead green crabs in their hauls after a particularly severe winter with an average water temperature of 2.81[degrees]C, which is one of the lowest winter water temperatures in 60 years (See Fig. 8 later).

[FIGURE 8 OMITTED]

In laboratory, using crabs caught in the field trial, green crabs were immobilized at 1[degrees]C, mortality observed at -3[degrees]C with larger crabs dying sooner (-0.036 days survival/ram carapace width, P < 0.001) at this temperature (Jacques et al. 2004). The temperature threshold for immobility was lower than the 8[degrees]C observed by Atkinson & Parsons (1973), but the temperature threshold for mortality was similar to -2.3[degrees]C (Spear 1955a). Laboratory observations of immobility at 1[degrees]C corresponded with field observations because no crabs were caught when the water was below this temperature. On the Thames River, temperature was found to be the only physiochemical variable with a significant effect on green crab abundance (Attrill et al. 1999).

Maine Coastal Water Temperature and Green Crab Abundance

Because S Maine and E Maine clam populations have shown different trends through time (Fig. 2), separate time series of local water temperatures were obtained for each region. Water temperatures for the Eastern Maine Coast/Bay of Fundy were from the Canadian Atlantic Zonal Monitoring Program (AZMP, 2004), which collects data through a network of sampling locations to provide basic information on the physical chemical and biological properties of the Northwest Atlantic continental shelf. The Prince 5 Station is at 44.9[degrees]N latitude and 66.8[degrees]E longitude, east of Eastport, Maine, records continuous daily average water temperatures and salinity at a depth of 50 m. Average daily temperatures and salinities for monthly time series were obtained for 1924 through the present from the Bedford Marine Institute.

Records of surface water temperature (1905 to the present) in S Maine were obtained from the Maine DMR Laboratory, Boothbay Harbor (BBHR). From 1905 through 1949, three thermometer readings taken from the station wharf were averaged. From 1950, measurements were taken 1.68 m below mean low water with infrequent measurements taken from the inlets of the flowing water tanks. Salinity was not measured. The correlation between the mean annual water temperature at Prince 5 and BBHR between 1941 and 2001 was 0.437 (P < 0.01) This correlation between annual water temperature at these two stations is similar to the correlations both within and between years between different regions of the Gulf of Maine surface waters with 15 significant correlations between 0.47 and 0.62 and 30 correlations <0.46 (Mountain & Manning 1994).

The average of water temperatures for the preceding 4 years was examined to predict clam landings because a temperature effect on either clam growth or clam predators would affect landings for four subsequent years as individuals that are presently juveniles grow to market size. Green crabs preferentially prey on small clams and the effects of green crab predation may not be seen until the older age classes are not replaced (Floyd & Williams 2004). Annual clam landings in S Maine (York, Cumberland, Sagadahoc, Lincoln, Knox and Waldo counties) and average winter water temperature for the preceding 4 years for the months of January, February and March measured at BBHR are in Figure 7. The minimum water temperature generally occurs between calendar days 70 and 80 (Mountain & Manning 1994), which would be when most of the winterkill of green crabs would occur. Figure 8 contains annual clam landings in E Maine (Hancock and Washington Counties) and preceding 4-year winter water temperature measured at Prince 5.

[FIGURE 7 OMITTED]

Temperature time series for southern and eastern Maine display a warm period in the early 1950s as previously noted, a cold period through the early to mid 1960s followed by a warming period in the late 1970s (Fig. 7, 8). These variations in water temperature at two locations agree with the decadal scale temperature variations observed for the shelf-wide volume-averaged eastern Scotia Shelf and central Gulf of Maine data for the upper 300 m (Loder et al. 2001). The relatively abrupt winter temperature, decreases observed in late 1970s at BBHR and in the late 1980s at Prince 5 were not seen in averaged data for the entire region.

The relationship between winter water temperature and clam abundance differ in E Maine and S Maine (Fig. 7, 8). The correlation between preceding 4-year water and clam abundance is -0.53 in S Maine and +0.54 in E Maine. The colder winter waters in S Maine could cause sufficient green crab winterkill to decrease green crab population size, decrease clam predation and increase subsequent clam landings. As a result, the correlation between winter water temperature (lag up to 4) and clam landings is negative in S Maine (r = -0.53).

Eastern Gulf waters do not get as cold as S Gulf waters in the winter because the southwesterly Gulf is less stratified than the eastern Gulf in the winter (Mountain & Manning 1994). Surface waters in winter in E Maine may not usually be cold enough to cause wide spread crab winterkill. The observed effect of warmer winter temperature in E Maine has been to increase landings (r = +0.54), possibly caused by a longer growing season for clams and early spring algal bloom. However, extremely cold winters such as that observed in the late 1950s and early 1960s (Fig. 8) could cause high green crab mortality even in eastern Maine. Decreased green crab densities were sampled in eastern Maine during this period (Table 1).

Winter water temperature has been found to impact green crab populations and bivalve predation in other areas. A relationship between winter temperatures and clam sets has been observed in Europe in areas with green crabs. High recruitment of bivalves after severe winters in temperate shallow waters occurs in some areas of the Wadden Sea (Strasser et al. 2003). Although there are numerous possible explanations for increased larval recruitment after severe winters, reduced numbers of larvae of both Mya and Carcinus were observed after the winter of 1995/6 "supporting the hypothesis that reduced epibenthic predation is an important factor in high bivalve recruitment after severe winters" rather than enhanced Mya larval supply (Strasser & Guenther 2001).

CONCLUSION

Softshell landings have dramatically declined in Maine over the past 25 y because of low productivity of clam flats in Eastern Maine. Low productivity of clam flats would be affected by:

* high fishing pressure on diminished stocks;

* heavy predation by green crab populations not limited by winterkill in the E Gulf.

Density of clam sets would be decreased by:

* harvesting stocks previously protected by pollution closures.

* Dispersion of clam larvae away from their flats of origin by strong tidal currents.

Sporadic heavy sets of juvenile clams do occur. Epifanio and Garvine (2001) describe north winds, Ekman flow and the resulting shoreward downwelling moving the blue crab, Callinectes sapidus (Rathbun) megalopae larvae inshore toward settlement sites in bays along midAtlantic states. Clam pediveligers that move to the surface waters could likewise be moved inshore. The prevailing wind direction in August through November in Portland, ME is from the NNW to NW (National Climatic Data Center, 1998; Fefer & Schettig 1980) and would produce favorable shoreward Ekman flows. The unpredictable occurrence of favorable winds on surface patches of pediveliger larvae would result in settlement events.

The extension of the range of the green crab up the coast of New England has negatively impacted clam populations particularly in areas such as E Maine where warmer winter water temperatures may have limited winterkill of green crabs. Change in the Gulf of Maine Sea Surface temperature (SST) over 120 years was estimated from a NOAA Extended Reconstructed SST data provided by the NOAA-CIRES Climate Diagnostics Center (Clean Air-Cool Planet 2005). The change in mean annual SST between 1880 and 2001 was estimated to be 0.6[degrees]C and the change in mean winter temperature over the same period was 0.2[degrees]C. Green crab sampling should be used to determine effect of crab populations on clam stocks. If crab populations are determined to be one of the factors limiting reestablishment of clam stocks, techniques for crab control should be developed and implemented (Walton 2000).

Management practices over the past century have not maintained a consistent clam fishery (Fig. 1) and present management practices such as a minimum harvest size have not been successful at reestablishing clam populations in E Maine (Fig. 2).

Management strategies should be implemented to increase harvests and sets:

* Clam mariculture could be used to produce both marketable clams and sets to repopulate local flats. Coastal areas could be managed to produce harvests from both cultivated and wild clams while producing sufficient larvae for a natural set.

* Rotational digging of flats should be evaluated to maintain breeding populations while allowing adults to grow without breakage and reburial from premature digging. Areas that are dug in the rotation could be repopulated with a natural set, hatchery produced seed or seed from areas with high density sets.

Regional coordination of restoration effort may be required because local spawn may not become the local set.

ACKNOWLEDGMENTS

This project was supported by the Maine Agricultural Experiment Station (MAES Pub. No. 2856) and the Lobster Institute. The use of data from Maine Department of Marine Resources and Canadian Atlantic Zonal Monitoring Program is acknowledged and appreciated. Comments from W.C. Walton on the manuscript were very useful.

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WILLIAM R. CONGLETON, JR., (1) * TRACY VASSILIEV, (1) ROBERT C. BAYER, (1) BRYAN R. PEARCE, (2) JENNIFER JACQUES (1) AND CAROLYN GILLMAN (l)

(1) Department of Animal and Veterinary Science, University of Maine, Orono, Maine 04469; (2) Department of Civil and Environmental Engineering, University of Maine, Orono, Maine 04469

* Corresponding author. E-mail: [e-mail]

TABLE 1.
Green crab sampling between 1953 and 1965 with a standard green
crab trap (Welch 1968) except for Perry, Maine where samples are
crabs found per man-hour search.

                              Year of sampling

Location                      1953-7       1958-9   1960-5

Bocabec River New Brunswick   343          53-41    7.5/trap day
Perry                         552                   7/man-hour
Lubec                         13           16       0/trap day
Jonesport, Cummins Beach      235-500      50       50-6/trap day
Mount Desert I.               255-8        0        0/trap day
Southport, ME                 110-670/
                                trap day