Why are we sampling?
Intertidal bivalves, specifically mussels and clams, provide a critical prey resource for a variety of seabirds, marine mammals, fishes, and other invertebrates (e.g., sea stars). They also provide nutrients and energy to some terrestrial carnivores such as black and brown bears that seasonally forage in marine habitats. Bivalves also are important to humans for subsistence and recreational harvest. Mussels and clams constitute a large proportion of the diets of the black oystercatcher and the sea otter, two species that are central in the design of the Nearshore component of the Gulf Watch Alaska Program (see Sea Otters and Black Oystercatchers pages).
Two habitat types are widely recognized as supporting abundant bivalves. The first habitat type is the rocky intertidal that can have dense mussel beds. The second habitat type is soft-sediment (mud to cobble) intertidal beaches that support mussels and populations of several clam species. These clams are ecologically diverse in terms of habitat preferences (some species prefer more muddy/sandy habitats and others prefer cobbles), foraging strategies (e.g., filter-feeding or detritivores), and have a wide range of physiological tolerances. Measurements of bivalve diversity, density, size distribution, and contaminant levels are important for examining small (site to site) to large (among regions) scale variability and changes due to natural and anthropogenic forces. These metrics also provide useful inferences about changes in the environmental conditions as well as causes of change that can be detected in higher-level consumers that occupy nearshore food webs.
Where are we sampling?
We monitor bivalves at specific mussel beds and soft sediment beaches in close proximity to the sites where intertidal communities are monitored in western Prince William Sound (WPWS), Kenai Fjords National Park (KEFJ), Kachemak Bay (KBAY), and Katmai National Park and Preserve (KATM).
How are we sampling?
We sample mussel density, size class distribution, and mussel bed extent at specific mussel beds selected for their high abundance of mussels. We use measuring tapes to lay out transects and quadrats to define the area from which we are sampling. Our methods allow extrapolation of mussel abundance within the entire bed.
Clam species diversity, density, and size class are measured at the 0m tide level. We lay a tape along the 0m contour and then remove all sediment within a 0.5 X 0.5m quadrat, which is sifted to reveal the clams so that they can be identified, counted, and measured.
A SCIENTIST SAMPLES A QUADRAT FOR CLAMS AT A SOFT SEDIMENT SITE IN KATMAI NATIONAL PARK AND PRESERVE. PHOTO CREDIT: JIM PFEIFFENBERGER, NPS.
What are we finding?
Since we began sampling in 2008, mussel abundance has been dynamic across the Gulf of Alaska. Densities of large mussels (≥20 mm), a preferred prey size for many of the larger predators, were higher than average across all regions during the Pacific marine heatwave (2014-2018). This higher average followed below-average mussel densities across the Gulf of Alaska in years prior to the onset of the heatwave in 2014. These patterns are opposite those shown by mussel-consuming sea stars, due to their decline associated with warming water and sea star wasting syndrome. We speculate that mussel abundance may have increased at least partially due to decreased predation pressure (see Intertidal Communities).
DENSITY ANOMALIES (DEVIATION FROM THE AVERAGE) FOR LARGE MUSSELS (>20 MM) IN FOUR STUDY REGIONS SPANNING THE NORTHERN GULF OF ALASKA.
Clam densities and biomass vary by region over time, with KATM and KBAY tending to have higher clam densities and biomass than KEFJ or WPWS. Estimates of clam biomass, density, and species distribution indicate that Macoma species dominate the clam community across all four regions. However, because of their much larger size, the less abundant Saxidomous gigantea contributes a significant portion of the overall clam biomass in KATM and to a lesser degree, KBAY.
CLAM SPECIES BIOMASS (GRAMS OF WET WEIGHT PER QUARTER M2 OF EDIBLE TISSUE; UPPER PANEL) AND SPECIES DENSITY (NUMBER OF INDIVIDUALS PER QUARTER M2; LOWER PANEL) BY REGION: WPWS, KEFJ, KBAY AND KATM.