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Journal of Shellfish Research, Vol. 25, No. 2, 461–474, 2006. BIOTIC AND ABIOTIC FACTORS INFLUENCING GROWTH AND SURVIVAL OF WILD AND CULTURED INDIVIDUALS OF THE SOFTSHELL CLAM (MYA ARENARIA L.) IN EASTERN MAINE BRIAN F. BEAL* University of Maine at Machias, 9 O’Brien Avenue, Machias, Maine 04654 ABSTRACT A series of intertidal field experiments was conducted from 1986–2003 in eastern Maine to examine biotic and abiotic factors influencing the growth and survival of wild and cultured individuals of the softshell clam, Mya arenaria L. Separate experi- ments examined: (1) the efficacy of transferring sublegal wild clams (<50.8 mm SL) from areas near the high intertidal zone where shell growth was slow to areas where growth was predicted to be faster; (2) effects of tidal height on wild and cultured clam growth; (3) effects of spatial variation on cultured clam growth; (4) dispersion and growth of cultured juveniles in small experimental units; (5) effects of the naticid gastropod, Euspira heros Say, predation on survival of wild and cultured clams and (6) the species composition of large, crustacean predators that forage intertidally during periods of tidal inundation. Protective netting (4.2 mm aperture) increased recovery rate of transferred clams by 120% and resulted in a 3-fold enhancement of wild recruits. Effects of tidal height on wild clam growth revealed complex behaviors in >0 y-class individuals. Clams growing near the upper intertidal take >8 y to attain a legal size of 50.8 mm SL, whereas animals near the mid intertidal generally take 4.5–6.5 y. Unexpectedly, clams initially 38–54 mm SL and growing near the extreme low tide mark at a mud flat in Eastport, Maine, added, on average, <2 mm of new shell in a year, which was 8–10 mm SL less than animals at higher shore levels. It is hypothesized that biological disturbance by moon snails, that consumed >90% of clams at the low shore levels, contributed to this slow growth. In another field trial from 1986–1987, moon snails and other consumers were allowed access to clams ranging in size from 15–51 mm. E. heros preyed on clams over the entire size range and attacked clams between 31–40 mm at a rate that was nearly double what had been expected. Mean snail size was estimated to range from 10–52 mm shell height (SH), based on a laboratory study that yielded information about the linear relationship between snail size and its borehole diameter. In an experiment from June to September 1993, moon snails consumed >70% of juvenile clams (ca. 10 mm SL) within a month after planting at each of three tidal heights. Snail sizes ranged from 15–20 mm SD with larger individuals occurring near the upper intertidal zone. Green crabs, Carcinus maenas (L.) also prey heavily on softshell clam populations, but most studies that use shell damage to assign a predator have assumed that all crushing and chipping predation is because of this invasive species. An intertidal trapping study demonstrated that both green crabs and rock crabs, Cancer irroratus Say, are present during periods of tidal inundation, with the latter species accounting for ca. 40% of large crustacean numbers. KEY WORDS: Mya arenaria, softshell clam, Maine, growth, predation, Euspira heros, Carcinus maenas INTRODUCTION dates. (Towns not interested in managing their clam stocks are not required to do so, and, therefore, the State must act alone to man- In Maine, USA, softshell clams, Mya arenaria L., are the third age those intertidal flats to the best of its ability.) However, the most important commercial marine species harvested behind lob- model ordinance is structured to permit sufficient flexibility to ster, Homarus americanus Milne Edwards, and cultured salmon, Salmo salar L. From 2001–2004, dockside clam landings in Maine enable a community to benefit from local knowledge, wants and averaged (±95% CI) 1113.9 ± 99.7 metric tons (worth $15.9 ± wishes by creating, adopting and regulating its unique manage- $1.68 million; Fig. 1), worth more than $50 million annually to the ment schemes that may exceed the basic provisions set out in the state economy (ME DMR 2005). model ordinance. These local modifications are enacted when a Each of Maine’s 105 coastal communities has the option to community requests them in writing to the DMR, and they are manage its intertidal clam stocks within its municipality using one granted by the DMR Commissioner. In this way, two adjacent of the oldest comanagement programs in the United States. Be- communities may have a common goal of increasing local clam ginning in 1962, Maine’s State Legislature created a community- stocks, but have very different looking shellfish ordinances. There based management structure between municipalities and the State are 3 common requirements of all communities with shellfish or- of Maine through the Department of Marine Resources (DMR). dinances: (1) the management plan be enforced by a warden paid This structure, or agreement, is called a “shellfish ordinance,” and through the town; (2) no clam less than two-inches (50.8 mm) in it allows towns to adopt, amend or repeal a set of shellfish con- shell length (SL) can be harvested legally and (3) 10% of licenses servation measures that regulate the taking of shellfish within the sold in a community must be made available to Maine citizens intertidal zone of the municipality. The ordinance is a broadly outside the particular municipality. In 2005, 73 communities (or defined document that includes language about conservation ac- 70%) with intertidal habitat chose to manage those areas for M. tivities, qualifications of a licensee, license fees, limiting effort, arenaria production. harvesting methods and tools, enforcement, the organization and Since the inception of the shellfish ordinance, local stewardship rights of a local stewardship committee and other measures. The committees have adopted a number of shellfish management tools first step a town must take once it declares its intention to manage in an attempt to maintain high yields of clams and keep their its local clam assets is to adopt a “model ordinance,” or general clamming habitat productive. The following list includes manage- management plan, that outlines basic requirements the state man- ment activities that have been used in Maine since 1962—no com- munity has adopted all of the activities: (1) limiting the number of licenses sold; (2) restricting harvest volumes; (3) limiting all har- *Corresponding author: E-mail: [email protected] vesting to recreational diggers; (4) limiting when harvesting can 461 462 BEAL the biotic and abiotic factors affecting growth and survival of wild and cultured softshell clams. METHODS Experiment I Clam Transfers In 2004, approximately 44% of the 73 Maine communities that actively managed their shellfish stocks participated in some form of transfer program (ME DMR 2005). These activities are very labor intensive because they involve harvesting and transporting wild “seed clams” from areas where growth is slow or retarded (i.e., near the upper intertidal where clam growth can be 95% Figure 1. Commercial landings of softshell clams in Maine from 1950 slower than growth of the similar animals at the low intertidal to 2004. Data from: http://www.maine.gov/dmr/commercialfishing/ [Beal et al., 2001]) to areas where growth is faster. Most commu- softshellclam.htm. nities transfer clam seed that is approximately 38–45 mm SL (pers. obs.). To the best of my knowledge, there has been only one occur (e.g., no Sunday digging; no night digging); (5) restricting attempt to quantify the efficacy of transfers in Maine. Here, infor- the areas where harvesting can occur (i.e., flat rotations); (6) ap- mation is presented that examines short-term survival, growth and plying tree brush or snow-fencing to intertidal areas to encourage recruitment patterns of transferred clams. juvenile clams settlement; (7) applying wire fencing or plastic On May 2, 1998, approximately 165 kg of soft-shell “seed” netting to deter green crab predation and encourage juvenile clam clams were dug by commercial harvesters from the city of East- settlement; (8) assessing stock volume and size frequency distri- port, Maine in the high intertidal zone of a flat near Carlow Island butions (i.e., clam surveys); (9) transferring clams from high den- in Passamaquoddy Bay (44°56.34ЈN; 67°01.77ЈW). Animals were sity/slow growth areas to low density/fast growth areas; (10) en- slow growing, and annual ring counts (Newcombe 1935) indicated hancing flats with hatchery-reared, or cultured clam seed; (11) that the oldest animals were >15 y old. Clams were sieved, installing municipal sewage treatment systems and reducing over- washed, and stored overnight in a walk-in cooler at 4°C. A random demonstrated that the ranges of (96 ס board discharges (e.g., water quality monitoring programs) and sample of these clams (n ± 39.3 ס municipal leasing of flats (each community has the right to set SL’s varied from 23.2 mm to 58.5 mm with a mean SL (12) aside up to 25% of its productive clamming habitat for private 9.5 mm. A relationship between clam mass and clam number was clam farming operations). developed for those clams by counting five separate lots of indi- × 107.21 + 0.03 ס Shellfish committee members, in concert with State resource viduals at 0.45, 1.36, 2.27 and 3.18 kg (Count P < 0.001). This relationship was used ;0.969 ס r2 ;20 ס managers, must make decisions concerning the status and health of Mass; n soft-shell clam populations for a variety of applications, the most to estimate number of clams seeded into plots (9.3 m2) at a nearby common being whether the population is abundant enough to be intertidal flat, Carrying Place Cove, Eastport, Maine (44°54.04ЈN; harvested in a sustainable fashion and what level of harvesting will 67°01.28ЈW), at low tide on 3 May 1998.

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