Clam Recruitment is Measured by Using a Simple Tool
Settling clams fall through apertures in the top mesh of the recruitment box. The box protects the clams from most predators, enabling them to survive and grow. At the end of the clam growing season the boxes’ contents, along with core samples of mudflat sediment collected adjacent to the boxes, are analyzed to determine the number of clams that settled onto the mudflats, as well as the number that survived the season. Survivors are important indicators of commercial soft-shell clam populations in subsequent years.
Methods
At each of the 31 sites, an array of 12 recruitment boxes is deployed in the lower mid intertidal. The boxes are deployed prior to soft-shell clam spawning season, and sediment core samples are taken to assess baseline densities of clams. Boxes are retrieved at the end of the clam growing season in October or November, and sediment core samples from outside the boxes are collected again. These samples from the adjacent, unprotected mud, compared to samples from the box, reveal the rate of predator success that season.
Results
2024 Clam Recruitment Monitoring Network Final Report
Results from the 2024 Clam Recruitment Monitoring Network can be found in the Technical Report #6: Clam Recruitment Monitoring Network Results. The report summarizes data from spring and fall mudflat surveys, information about clam recruitment and growth during that time frame, seawater temperatures, and green crab density and size across 12 towns in Maine.
Overall, the average densities of soft-shell clams recovered from recruitment boxes in 2024 as a whole were higher than in 2021-2023, but were lower on average in the Midcoast region than in previous years. The location with the highest recruit densities was in Beals with an average recruit density of almost 200 clams per square foot. In 2024, there were on average 38 recruits per square foot compared to 35 recruits in 2023, 25 recruits in 2022, and 19 clam recruits per square foot in 2021. The average density of green crabs across the Network declined in 2024 with an average density of 2.5 green crabs per square foot in recruitment boxes compared to 3.7 green crabs in 2023, 5.5 green crabs in 2022, and 4.5 per square foot in 2021. The Downeast region had particularly low densities of green crabs in the boxes with an average of only 0.06 crabs per square foot and the majority of towns with zero green crabs found in all of the boxes.
2023 Clam Recruitment Monitoring Network Final Report
Results from the 2023 Clam Recruitment Monitoring Network can be found in the Technical Report #5: Clam Recruitment Monitoring Network Results . The report summarizes data from spring and fall mudflat surveys, information about clam recruitment and growth during that time frame, seawater temperatures, and green crab density and size across 12 towns in Maine.
Overall, the average densities of soft-shell clams recovered from recruitment boxes in 2023 were higher than in 2022 and 2021. The top three locations with the highest recruit densities were all located in the downeast region, with densities of over 100 clams per square foot. In 2023 we found, on average, 35 recruits per square foot compared to 25 per square foot in 2022 and 19 clams per square foot in 2021. In 2023, there was an average of 3.7 green crabs per square foot in recruitment boxes across all sites compared to 5.5 per square foot in 2022 and 4.5 per square foot in 2021.
2022 Clam Recruitment Monitoring Network Final Report
Results from the 2022 Clam Recruitment Monitoring Network can be found in the upcoming Technical Report #4: Clam Recruitment Monitoring Results. The report summarizes and shares data from the fall mudflat survey, information about clam recruitment, temperatures, clam recruit growth, recruitment and growth of other commercially important shellfish species, along with presence and size of green crabs on a town and statewide basis.
Overall, average densities of clams recovered from recruitment boxes in 2022 were higher than in 2021. Seven of the top ten locations for average recruit density were from the downeast region whereas five of the bottom ten locations were located in towns either in or west of Phippsburg.
In 2022 we found, on average, 25 recruits per square ft. compared to 19 clams per square ft. in 2022 and 95 clams per square ft. in 2020. In 2022 there were 5.5 green crabs per sq. ft across the 26 sites, while in 2021 there were 4.5 green crabs per square ft. across 21 sites, while in 2020 we found 2 crabs on average across all 18 sites.
2021 Clam Recruitment Monitoring Network Final Report
Results from the 2021 Clam Recruitment Monitoring Network can be found in the upcoming Technical Report #3: Clam Recruitment Monitoring Results. The report summarizes and shares data from the fall mudflat survey, information about clam recruitment, temperatures, clam recruit growth, recruitment and growth of other commercially important shellfish species, along with presence and size of green crabs on a town and statewide basis.
Across all sites average clam recruitment density in 2021 was substantially lower than it was in 2020. Last year we found only 19 clams per square ft. in our predator protected boxes vs. 95 clams per square ft. in 2020. The low number of recruits found was likely due to higher numbers of green crabs. In 2021 there were 4.5 green crabs per square ft. across all 21 sites, while in 2020 we found only 2 crabs on average across all 18 sites.
2020 Clam Recruitment Monitoring Network Final Report
Re
sults from the 2020 Clam Recruitment Monitoring Network can be found in the Technical Report #2: Clam Recruitment Monitoring Results. The report summarizes and shares data from the fall mudflat survey, information about clam recruitment, temperatures, clam recruit growth, and presence and size of green crabs on a town and statewide basis.
Across the coast, clam recruitment densities were lower than expected based on data from previous years. Eight of the eighteen sites had average clam densities of less than 10 clams per sq. ft. Only three sites recorded average clam recruit densities of over 100 clams per sq. ft. (Winnock Neck in Scarborough, Dobbins Island in Beals, and Gleason Cove in Sipayik). At Gleason Cove we found a single recruitment box with 4,331 recruits, the second highest number of clams ever recorded in a single box. Gleason Cove received more than 1,000 recruits per sq. ft.
May 2020 Baseline Clam Survey Results
In May of 2020, DEI scientists, along with community members, conducted clam surveys in the each of the areas where the clam recruitment monitoring stations were deployed in order to establish baseline densities of clams and their sizes.
The results of the survey, along with information about the Soft-Shell Clam Recruitment Monitoring Network, can be found in Technical Report #1: 2020 Spring Baseline Clam Survey Results. Click on the report to the left to read it.
Green Crab Specific Results (2020 & 2021)
Associate Director Sara Randall presented the green crab specific results during a 2022 Downeast Discussion Climate Change Seminar. 2021 was warmer on the mudflats than 2020, and as a result we found more green crabs at our monitoring stations and less clam recruits. Watch below to learn more.
Funding
From 2020-2021 this project was funded by Maine Sea Grant. Luke’s Lobster Keeper Fund through The Ocean Foundation supported the work of documenting recruitment and growth of other commercial shellfish species. In 2022, funding is provided by Maine Economic Improvement Fund Small Campus Initiative and Maine Outdoor Heritage Fund.
