A pilot study was conducted in West Jonesport in 2022 to examine effects of: 1) tidal height; 2) stocking density; and, 3) predator exclusion using netting on the survival and growth of hatchery-reared (cultured) juvenile soft-shell clam. In addition, densities of wild juvenile clams (i.e., recruits) were estimated using experimental units containing planted hatchery seed (plant pots) as well as units that were initially empty (recruitment boxes). Detailed explanation of methods and results can be found in the report.
The experimental units were deployed 28 April 2022, and was completed after 180 days on October 25 2022.
Cultured soft-shell clams used in the study were reared at DEI during the spring/summer of 2021, and then overwintered (sensu Beal et al. 1995). This clam seed was planted in two stocking densities (60 and 120/ft2) and protected from predators using two types of flexible netting (4.2 mm square aperture; and a rectangular aperture of 0.9 mm x 1.7 mm). Experimental units (6-inch diameter x 6-inch deep plastic plant pots) were deployed in “blocks” at two tidal heights (upper vs. lower) that were 10 m apart.
Another estimate of wild clam recruitment was made using “clam recruitment boxes” (sensu Beal et al. 2018). Boxes were arrayed in groups of two (~1 m apart) on the surface of the flat in three upper (N = 6) and one lower intertidal (N = 2) locations.
At the end of the study, the contents of each experimental unit found was washed through a 1 mm sieve. All cultured and wild clam recruits were counted and measured. It is possible to discern a growth rate on each cultured clam as they mark themselves when they are transferred from the hatchery to field conditions (Beal et al. 1999). That is, clams leave a distinct disturbance line in their valves (Fig. 3) that coincides with deployment in sediments.
In addition, data was collected on the green crabs found in experimental units.
Some findings include:
- Statistical analysis of the survival of clams in any treatment revealed a lack of a statistically significant result on average survival for any of the eight hypothesis tests.
- Neither tidal height nor stocking density had a significant effect on clam growth; however, the type of netting used to protect clams from predators explained nearly 50% of the variability in observed shell growth. Animals in experimental units protected with PetScreen® added approximately 35% less shell than clams in units protected with the larger aperture (4.2 mm) netting.
- Wild clam recruits were recovered in only two plant pots in the high intertidal (the tidal gradient with the least amount of predation due to the length of time the area is exposed to air compared to the mid and low) – both protected with PetScreen®. Therefore, 91% of experimental units contained no wild soft-shell clam recruits.
- No significant difference in mean number of green crabs was observed in the plant pots between tidal heights, stocking densities, or types/sizes of netting.
- Mean number of green crabs in the plant pots was 1.7 ± 0.5, n = 23.
- Mean green crab size varied by netting (P = 0.0003), but the effect varied differently across tidal heights. That is, at the upper intertidal, larger crabs (13-20 mm CW) were observed in units protected with the larger aperture mesh whereas smaller crabs (mostly between 4-10 mm CW) were observed units protected with PetScreen®.
- Green crabs in recruitment ranged in size from 5.7 mm to 33.2 mm CW.
This short-term study (180 days) demonstrated that cultured soft-shell clam seed approximately a half-inch in shell length can survive and grow in this areas of West Jonesport, Maine. While attempts to demonstrate the effects of stocking density (60 vs. 120 individuals per ft2), tidal height (upper vs. lower), and type of predator deterrent netting failed to show anything statistically significant in terms of survival, the two types of protective netting did yield differences in clam growth. Using the smaller aperture netting (PetScreen®) resulted in a 35% reduction in shell growth compared with the larger aperture netting (Flexible – polypropylene).
Clams grew nearly an inch in length. It is likely that a two-inch clam is attainable at this site within 2 to 2.5 years of planting clams that are at least a half-inch in shell length.
The majority of intertidal flats along the Maine coast harbor green crabs at densities as high or higher than that of soft-shell clam recruits.
Read the full report.