NSRWA and MassBays, in partnership with Mass Audubon, have been assessing mussel abundance at the mouth of the North and South Rivers. Fourth Cliff, a small barrier beach approximately a mile long, is located at the confluence of the North and South Rivers, and part of the North River Important Bird Area (IBA). Barrier beaches provide feeding and staging habitat for migrating shorebirds. Among the food that migratory shorebirds consume are small mussels (Mytilus edulis). The blue mussel beds in the North and South Rivers have significantly declined since the 1980s due to a number of factors. Food availability for migrating shorebirds is undoubtedly essential to a stopover site as they head south from their nesting territories. Terns rely on small fish such as sand lance and silversides, while most migrating shorebirds need the tidal flats for benthic organisms. Federally listed as “Threatened” in 2014, Rufa Red Knots (Calidris canutus rufa) rely heavily on blue mussel spat as a primary food source during their fall southward migration.
In addition to providing a crucial foraging stopover for Red Knot and other migrating shorebirds, blue mussel beds stabilize shorelines, positively impact water quality, and the ecology of the estuary. As storms increase in frequency and strength due to climate change and rising sea levels, protecting shorelines is becoming even more vital. The NSRWA and MassBays had previously piloted efforts to enhance blue mussels in the rivers in 2014 and 2015, including testing out spat collection systems, raising small groups of benthic mussels, and transferring larger adult mussels from docks to the benthic environment. Based on these past experiments, it had been determined that transferring larger blue mussels is more successful and efficient than raising spat for release. In 2022 and 2023 the goal was targeted primarily at A) identifying robust adult populations that could be relocated to the project site, and B) surveying the project site to identify the most suitable benthic locations at the project site.
In 2023, the surveys were initially targeted at identifying optimal habitat for mussel relocation. However, during the course of these surveys a newly established mussel bed was found on the site. The mussel bed was approximately 0.58 acres located on the northwest side of Fourth Cliff and contained an abundance of small mussels. The area was mapped and monitored over the course of the summer of 2023. Biologists also observed that the mussels were being preyed upon by gulls as well as by green crabs. This raised the concern that other predators in the summer could be depleting the supply before Red Knots arrive at the site.
Based on the 2023 findings, the 2024 approach was targeted at monitoring the success of the newly identified mussel beds. This included efforts to track changes in the overall area and location of the mussel bed(s). The density of mussels, distribution, and size of mussels were tracked throughout the year. Surveys were conducted at Third Cliff as well as Fourth Cliff to provide a ‘reference’ point and a more regional view.
The 2024 surveys started in March. NSRWA’s Watershed Ecologist Alex Mansfield was joined by volunteer Susan Gallagher. Susan was working towards her Mass Audubon Field Naturalist certification and chose these mussel beds for her field project. This added tremendous value to both programs and allowed us to complete the winter survey successfully. The primary goal of the March survey was to determine if the small mussel bed identified in 2023 had successfully ‘overwintered’ (i.e. survived the harsh storms and shifting sands of winter months). The survey did confirm that the bed was intact and healthy! The results of the March survey were exciting and set the tone for the rest of the 2024 program.
Surveys continued from March to October 2024. The surveys were conducted by a mix of team members including the NSRWA Watershed Ecologist, NSRWA interns, and volunteers. Surveys were generally conducted around the astronomically lowest tides each month in order to access the lowest parts of the intertidal zone.
Throughout the course of the 2024 surveys a series of distinct mussel beds were identified at both Third and Fourth Cliffs. This began with a revisiting of the area identified in July 2023 as having a large population of young-of-the-year mussel spat. Mussels in this area had survived the winter, and in fact the March 2024 survey revealed an even larger area than had previously been identified. Additional beds were later found at the rocky point of Fourth Cliff and along the banks at the mouth of the South River. An expansive bed of mussels was also identified at Third Cliff.
In addition to mapping areal coverage of mussel beds, the team also measured mussel size and growth through the season. Overall, the Fourth Cliff mussels showed consistent growth from April to August in terms of minimum, average, and maximum shell length. From August to September there was a slight decrease in the maximum shell length, which in turn reduced the average shell length. The cause of this decrease is unclear. It is possible that there was some mortality or predation on the largest mussels, although there was no evidence to support this. It is also possible that it was simply a product of the random sampling method and heterogeneity within the mussel bed. Minimum shell length continued to increase from August to September, suggesting continued growth of the overall population. From April to September the minimum shell length increased by 94%, average length by 44%, and maximum length by 44%.
The Third Cliff sampling was conducted as a comparison to the Fourth Cliff project area. The goal was to identify any differences in density, growth rate, and mortality between the two areas. In terms of size in July, the mussels at Third Cliff were substantially smaller than mussels at Fourth Cliff. By the end of the sampling period the Third Cliff mussels had grown rapidly but were still only ~80% the size of the average size of the mussels at Fourth Cliff. Given the very small size of the Third Cliff mussels in July, it is possible that this represented a 2024 set rather than an overwintered population.
Overall, the 2023 and 2024 surveys have been encouraging in terms of the ecology of the site. Areas that had previously been recommended for mussel enhancement now appear to be reestablishing naturally. Like other shellfish, mussel populations build upon their own success. More mussels mean more brood stock and more available substrate for recruitment. The growth seen throughout the 2024 surveys could be a good sign for coming years. If these larger mussels can successfully overwinter, they are likely to continue to expand in terms of area and density. At this point there does not appear to be a need to actively bring mussels to the site for enhancement.
While these past two years have shown a promising resurgence of mussels at the site, it is still unclear if this is a long-term trend or a short-lived anomaly. The widespread decline of mussels since the 1980s has reduced populations to an extent that is difficult to recover from. Additionally, the site itself is highly dynamic. Ocean storms frequently change the landscape in dramatic ways. Sand deposition can smother mussel beds (as observed at Third Cliff) while high energy waves can strip away even large boulders and everything attached to them.
We plan on continuing a similar sampling protocol in 2025. Conducting late winter surveys will confirm whether or not the identified mussel beds survive into another growing season. Using the overwintered condition as the baseline, summer to fall measurements will determine overall increases in mussel size and density. While predation did not appear to be an issue in 2024 it is still a significant concern for long-term reestablishment of stable mussel populations. Predation is typically most significant on newly settled spat. Developing mussel beds with a full suite of year classes is the best scenario for sustaining the population.
