In 2012, clammers from the Maine Clammers Association sounded the alarm about the dramatic increase in invasive green crab populations in Casco Bay and the corresponding decrease in soft-shell (steamer) clams. Immediate action was needed to safeguard this economically and culturally important food source. In 2013, the Town of Freeport took the initiative to begin efforts to address the problem, investing in applied marine research conducted by the DEI, other science professionals, and clammers, to investigate how to restore clam populations.
Then, in 2014 DEI began its historic, large-scale field research to determine the cause of the clam decline, obtain information to sustain the fishery and enhance clam populations. Working directly with clammers from the Maine Clammers Association, from 2014 through 2017 DEI conducted 27 field experiments at 78 intertidal locations. For a summary of these experiments, click here. In 2018 we conducted an additional two experiments at three intertidal sites.
To determine the results of the experiments we quite a few sediment core samples and then process them by placing them in a sieve and washing out the mud. Then we count and measure all soft-shell clams, green crabs, and other shellfish. We also record how many milky ribbon worms there are. From 2013 to 2018, we processed over 36.5 TONS of intertidal mud in order to determine our results!
The results from these experiments show conclusively that predation by invasive and native species is the most important factor in the clam decline. Predation rates have increased with warming ocean temperatures, and are expected to continue to rise.
Background on the importance of clamming and the decline of landings
Over the past thirty years, Maine’s soft-shell clam landings have ranked second or third in commercial value of all marine species harvested in Maine waters, with dockside values in 2015 reaching $22.54 million (Fig. 1).
From 2009-2012, the fifty-three (53) licensed commercial clammers in Freeport harvested more soft-shell clams from the 36-miles of shoreline in Freeport than any other coastal town in Maine (DMR, 2013). Dockside revenues from clamming in that community alone in 2012 were nearly 1/10th of the value from the entire state of Maine; however, since 2012, the Town of Freeport has seen a nearly 70% decline in landings and a nearly 50% decline in revenues to clammers (Fig. 2).
This decline in landings represents significant losses in the standing stock of clams from flats in that community, which coincides with a gloomy forecast that State of Maine shellfish managers and clammers predicted would occur in 2013. The relatively sudden problem has occurred at the same time that dramatic increases in population numbers of the invasive European green crab, Carcinus maenas, have been observed (Whitlow and Grabowski, 2012). The increase in green crab numbers is associated with recent ocean warming trends (de Rivera et al., 2007), but also could be a result of adaptation to cold-temperature regimes in the northern part of its range (Audet et al., 2003; Roman, 2006). In addition, research on Freeport flats in 2014 and 2015 has shown that another predator, a nemertean worm, Cerebratulus lacteus, has increased in numbers and has exacerbated the problem of declining clam stocks (see Final Report for 2015). Some of this information is contained in the most recent newsletter from the Maine Clammer’s Association (click here for a copy of the Winter 2016 MCA newsletter).
Clammers and Shellfish Committees in Freeport and in other coastal towns in Casco Bay, including Brunswick, Harpswell, West Bath, and Yarmouth, noticed during the past few years that commercial densities of soft-shell clams have shifted from the mid- and lower intertidal to the upper shore where the harvest now occurs almost exclusively (R. Tozier, Chebeauge Island shellfish warden, pers. comm; Clint Goodenow, Freeport clammer, pers. comm.; B. Beal, pers. obs.).
Clams along the upper shore grow more slowly than at lower tidal levels (Beal et al., 2001), and are significantly older than clams found lower on the shore (Powers et al. 2006). Therefore, clammers now are benefiting from a cumulative subsidy of years of commercial inactivity at upper shore levels. Because natural recruitment of Mya is highly variable throughout its range (Hunt et al., 2003; Bowen and Hunt, 2009; Vassiliev et al. 2010; Morse and Hunt, 2013), at current exploitation rates it is unclear how long soft-shell clams in Freeport and other southern Maine communities will remain a viable commercial industry.
In 2013, using only local municipal funds, the town of Freeport initiated an historic pilot-scale shellfish management program to examine the interactive effects of predation and ocean acidification on the dynamics of wild clam populations. In April 2013, Freeport’s Town Council approved $65,000 for its Shellfish Commission to undertake a three-pronged study to quantify population numbers of green crabs at selected intertidal and subtidal sites, to examine effects of green crab fencing on soft-shell clam recruitment, and to sample sediment and overlying water column pH to quantify potential threats of an increasingly acidic environment on settling clam post-larvae (sensu Green et al., 2004, 2009).
Please click on the links below to follow how the work proceeded and what results were obtained.
Audet, D., Davis, D.S., Miron, G., Moriyasu, M., Benhalima, K. & Campbell, R. 2003. Geographic expansion of a nonindigenous crab, Carcinus maenas (L.), along the Nova Scotian shore into the southeast Gulf of St. Lawrence, Canada. J. Shellfish Res. 22:255-262.
Beal, B.F., Parker, M.R. & Vencile, K.W. 2001. Seasonal effects of intraspecific density and predator exclusion along a shore-level gradient on survival and growth of juveniles of the soft-shell clam, Mya arenaria L., in Maine, USA. J. Exp. Mar. Biol. Ecol. 264:133-169.
Bowen, J.E. & Hunt, H.L. 2009. Settlement and recruitment patterns of the soft-shell clam, Mya arenaria, on the northern shore of the Bay of Fundy, Canada. Estuaries Coasts 32:758-772.
deRivera, C.E., Hitchcock, N.G., Teck, S.J., Steves, B.P., Hines, A.H. & Ruiz, G.M. 2007. Larval development rate predicts range expansion of an introduced crab. Mar. Biol. 150:1275-1288.
DMR. 2013. Maine Department of Marine Resources Commercial Fisheries Landings.
Green, M.A., Jones, M.E., Boudreau, C.L., Moore, R.L. & Westman, B.A. 2004. Dissolution mortality of juvenile bivalves in coastal marine deposits. Limnol. Oceanogr. 49:727-734.
Green, M.A., Waldbusser, G.G., Reilly, S.L., Emerson, K. & O’Donnell, S. 2009. Death by dissolution: Sediment saturation state as a mortality factor for juvenile bivalves. Limnol. Oceanogr. 54:1037-1047.
Hunt, H.L., McLean, A.D. & Mullineaux, L.S. 2003. Post-settlement alteration of spatial patterns of soft clam (Mya arenaria) recruits. Estuaries 26:72-81.
Morse, B.L. & Hunt, H.L. 2013. Impact of settlement and early post-settlement events on the spatial distribution of juvenile Mya arenaria on an intertidal shore. J. Exp. Mar. Biol. Ecol. 448:57-65.
Powers, S.P., Bishop, M.A., Grabowski, J.H. & Peterson, C.H. 2006. Distribution of the invasive bivalve Mya arenaria L. on intertidal flats of southcentral Alaska. J. Sea Res. 55:207-216.
Roman, J. 2006. Diluting the founder effect: cryptic invasions expand a marine invader’s range. Proc. Royal Soc. B: Biol. Sci. 273:2453-2459.
Vassiliev, T., Fegley, S.R. & Congelton, Jr., W.R. 2010. Regional differences in initial settlement and juvenile recruitment of Mya arenaria L. (soft-shell clam) in Maine. J. Shellfish Res. 29:337-346.