Soft-shell clams, Mya arenaria L., represent an iconic fishery in Maine, USA that typically ranks 2nd or 3rd annually in commercial value of all marine species harvested. The fishery is in a crisis as dockside landings have plummeted by 55% since 2000-2004, reaching an all-time low (5.2 million pounds) in 2023 with previous all-time lows occurring four times since 2017. Clams are co-managed by Maine’s Department of Marine Resources (DMR) working together with local communities. While adjacent communities may have different management practices, all are governed by the 2-inch clam law that regulates the taking of clams to those animals greater than or equal to 2 inches in shell length (SL). We wish to work with clammers and the clam industry to increase production by exploring specific aspects of the soft-shell clam reproductive cycle.
Beginning in 2023, we initiated a comparative field experiment in a community within three geographic regions of the Maine coast (southwest: Brunswick; midcoast: Bremen; downeast: Jonesport). Soft-shell clams ranging in SL from 30-50 mm were dug by us and those ranging from 51-100 mm were purchased directly from clammers or from clam buying stations located near (within 10 km) each study site. In March 2023, we placed 24 clams into 1-ft x 1-ft x 6-inch deep wooden boxes filled with ambient sediments (N = 42 boxes across each of three tidal heights: upper, mid, and lower intertidal) protected with a vinyl-coated wire (10.5mm aperture to exclude large crushing predators, birds, and fish) over a polypropylene mesh (3.2 mm to exclude smaller predators). Beginning the first week of May, clams were collected weekly for 19 weeks from two boxes at each tidal height at each study site, and within 24 hours were induced to spawn using a thermal shock technique. When this technique was successful, the eggs from a female were counted using a FlowCam. For animals not stimulated to release gametes, one group from each tidal height (N=6) was taken to estimate weekly condition and gonadal index, while another group (N=6) was taken every other week for histological analysis of gonadal tissue. In March 2024, we used 18 clams, and began the 19 weekly collections in mid-April.
We discovered region-specific spawning schedules, and determined that reproductive senility does not occur (the relationship between SL and number of eggs released was described by a power exponential function). Some commercial buying stations in Maine do not buy clams exceeding 3-inches (76.2 mm) in SL from harvesters. We sampled the commercial catch over 24 months at one station in the downeast and another in the southwest region. Approximately 5% of clams exceed 3.5 inches, which should be considered a coastwide upper size limit. Within five years of implementing this new size restriction, it would be possible to devise a sampling program to test whether more recruits settle and survive to the commercial fishery. If no improvement is forthcoming during that period, then the upper size limit would be revoked.