380 



A unique shell marker In juvenile, 

 hatchery-reared individuals of the 

 softshell clam, Mya arenaria L. 



Brian F. Beal 



University of Maine at Mactiias 



9 O'Brien Avenue, Machias, Maine 04654 



E-mail address bbeal aacad umm maine edu 



Robert Bayer 



University of Maine 



Hitchner tHall, Orono, Maine 04469 



M. Gayle Kraus 



University of Maine at Machias 



9 O'Brien Avenue, Mactiias, Maine 04654 



Samuel R. Chapman 



University of Maine 



Darling Marine Center, Walpole, Maine 04573 



The ability to identify individual 

 bivalve mollusks in field popula- 

 tions is fundamental to under- 

 standing potential population regu- 

 latory mechanisms (such as the in- 

 fluence of population density, tidal 

 height, and initial shell size on 

 growth and survival rates, and fe- 

 cundity schedules). Softshell clams, 

 Mya arenaria L., have been com- 

 mercially harvested from the soft- 

 bottom intertidal zone in Maine 

 since the mid-1800s and form the 

 basis of an extensive fishery along 

 its entire coast. Dramatic declines 

 in landings during the past decade 

 in Maine (Wallace, 1997 ), however, 

 have resulted in attempts to use 

 hatchery-reared juveniles to supple- 

 ment wild stocks (Beal, 1994). In the 

 past, distinguishing between cul- 

 tured and wild bivalves in the field 

 in order to follow their fate has been 

 performed by using alizarin stain- 

 ing techniques (Newell and Hidu, 

 1982), by tagging individuals 

 (Brousseau, 1979), or by applying 

 colored marks (e.g. paint dots) to 

 the valves (Peterson and Beal, 



1989). Here, we describe a natural 

 and unique shell marker for juve- 

 nile, hatchery-reared softshell 

 clams that forms on the outer 

 valves of individuals once they are 

 placed in the field. The distinctive 

 mark appearing on the surface of 

 each valve obviates the need to ap- 

 ply physical tags to individuals and 

 also eliminates the stress that 

 small clams otherwise undergo 

 when being tagged. 



Methods 



Clams were reared during the sum- 

 mer of 1983 at a 4-H clam hatch- 

 ery in Jonesboro, Maine, and were 

 held in running seawater in sedi- 

 ment-free trays at the Darling Ma- 

 rine Center, Walpole, Maine, until 

 25 May 1984. On that date, 5000 

 individuals (mean shell length [x^^ 

 ±1 SD1 = 10.4 ±1.52 mm; range= 

 from 6.7 to 14.6 mm; n=200) were 

 transported back to the Jonesboro 

 hatchery where the clams were di- 

 vided into five groups of 100, 200, 



300, and 400 individuals (i.e. a to- 

 tal of 20 groups ). All but ten individu- 

 als from each group (20 groupsx 

 10 individuals=200 clams) were 

 marked with a single, group-spe- 

 cific, color-coded dot (Mark-Tex 

 Corp. paint) on both valves near the 

 umbo. The remaining 200 clams 

 were painted with two dots to en- 

 sure individual recognition and 

 measured (greatest shell length) to 

 the nearest 0.1 mm by using ver- 

 nier calipers. Marking was per- 

 formed at this time because it was 

 not known whether a distinctive 

 mark would form naturally on the 

 surface of each valve once clams 

 began adding new shell in the field. 

 Clams were maintained overnight 

 without seawater at the University 

 of Maine at Machias in a walk-in 

 cooler (ca. 5°C). 



On 25 May 1984, a matrix con- 

 sisting of two rows of ten 0.25-m^ 

 plots was established near the 

 midtide level of an intertidal fiat 

 (the Narrows) located along the 

 western shore of the Chandler 

 River (44°39'04"N; 67°33'10"W) 

 near the town of Jonesboro, Maine. 

 Sediments consisted of poorly 

 sorted muds with a graphic mean 

 ±1 SD of 3.6 ±0.34(t) (N=2). The ma- 

 trix was located approximately 55 

 m from the extreme high water 

 mark and 35 m from the mean low 

 water mark. Both rows were par- 

 allel to the shore and were spaced 

 1 m apart. Black rigid-mesh enclo- 

 sures (DUPONT Vexar, 6.4 mm 

 aperture), approximately 0.25 m^ 

 and 15-cm wide, were installed 

 around each plot to restrict lateral 

 movements of the small clams 

 (Baptist, 1955). Because the enclo- 

 sures had no mesh roofs, they al- 

 lowed epibenthic and infaunal 

 predators isensu Commito and 

 Ambrose, 1985) access to the clams 

 (Peterson and Beal, 1989). The 

 mesh of each enclosure wall was 



Manuscript accepted ."j May 1998. 

 Fish. Bull. 97;380-.386 (1999). 



