384 



Fishery Bulletin 97(2), 1999 



managers wishing to assess the fate of hatchery seed 

 for stock enhancement purposes (Beal, 1994). In ad- 

 dition, it obviates the need to mark (physically or 

 chemically) individuals or a group. If initially un- 

 marked, juvenile, hatchery-reared softshell clams are 

 planted in the field and examined within an 18- 

 month period, the distinctive shell mark allows one 

 1) to determine initial planting size of individual 

 hatchery-reared clams (Fig. 2); 2) to determine rela- 

 tive or absolute growth rates (Fig. 3, A and B); and 

 3) to estimate the time (e.g. season) when death oc- 

 curred for those individuals that suffered mortality 

 after planting. Similar disturbance lines were ob- 

 served on juvenile, hatchery-reared northern qua- 

 hogs, Mercenaria mercenaria (L.), planted from the 

 laboratory to various subtidal and intertidal sites in 

 North Carolina (Beal, 1983). 



The origin of the shell mark may be related to con- 

 ditions within the hatchery environment. After the 

 field experiment described above, cultured softshell 

 clam juveniles from BIRSH were planted in inter- 

 tidal flats along the coast of Maine ( 1987-94) during 

 the months of April through October for both ma- 

 nipulative tests and community-based stock enhance- 

 ment efforts (Beal, 1994). In addition, cultured clam 

 stock produced in a commercial shellfish hatchery 

 (Mook Sea Farm, Inc., Walpole, Maine) were used in 

 stock enhancement programs in Gloucester and 

 Ipswich, Massachusetts (Whitten'). Regardless of 



20 40 60 80 100 120 140 160 



Relative grovirth index (x lOO) 



30- 



25- 



20- 



15 



10- 



B 



I—I Initial longths 

  Final lengths 



Jill 



Ik 



r-" 



10 15 20 25 



Shell length (mm) 



30 



Figure 3 



(A) Frequency distribution of relative growth rate index 

 for the 57 hatchery-reared softshell clams marked with two 

 dots (26 May to 1 September 1984). An index of lOO^-f indi- 

 cates a doubling of shell length. iBl Frequency distribu- 

 tion of initial (26 May 1984; n=200) and final shell lengths 

 il September 1984; n=57) of hatchery-reared softshell 

 clams. 



' Whitten, J. 1996. Merrimak Valley Planning Commission, 

 Haverhill, MA. Personal commun. 



hatchery origin, the same line appeared on the shells 

 of all survivors planted in the soft-bottom intertidal 

 zone at all locations, as well as on shells of those 

 that grew before dying. 



The mark has been observed in clams planted at 

 shell sizes as small as 3 mm and as large as 25 mm 

 (Beal, 1994). In addition, this mark appears regard- 

 less whether clams are transplanted to the field di- 

 rectly from the hatchery or are overwintered in sub- 

 merged, floating cages {sensu Beal et al., 1995) be- 

 fore transplanting to the field. 



