Recreational Clams

Evaluation: Some Action Needed/Closely Monitor
Issue Summary:

Populations of butter and gaper clams in some areas of lower South Slough and Coos estuaries have increased since a similar assessment was completed in the 1970’s. Populations of native littleneck clams and cockles have generally fallen since that time in the areas sampled.

Why do we care:

Recreational clamming is an important part of our area’s quality of life. Population shifts of any estuarine-dependent animals such as recreational clam species should be cause for further investigation to understand the issues associated with the changes.

What’s Happening?

Oregon Department of Fish and Wildlife’s (ODFW) Shellfish and Estuarine Assessment of Coastal Oregon (SEACOR) project documented the distribution of four recreationally important bay clam species: gaper clams (Tresus capax); butter clams (Saxidomus gigantea); native littleneck clams (Leukoma staminea); and cockles (Clinocardium nuttallii). Project scientists estimated the abundance of each species and described the habitat requirements for each species (ODFW Shellfish Program 2009a).

Both butter clams and gaper clams were found to be more abundant in some areas of the lower Coos and South Slough estuary study sites than they were inthe surveys done in the 1970’s.

Butter clam populations were the highest at Pigeon Point in the lower Coos estuary (Figure 1). The habitat where they were most frequently found were relatively high intertidal sandy bars (sometimes called “butter bars”) and in tide flats with little algae coverage. Gaper clams were most abundant at Clam Island (Figure 2). The habitat where gapers were mostly found was low intertidal flats with plenty of native eelgrass (Zostera marina) coverage (ODFW Shellfish Program 2009b).

In contrast, results of the SEACOR project showed that there were “considerably fewer” cockles in the study areas compared with gaper or butter clams, and fewer compared with the surveys done in the 1970’s (Figure 3). Likewise, native littleneck clams “were rarely found” during the SEACOR surveys. They report that “historical comparisons indicate that there has been a dramatic decrease in the littleneck population in Coos Bay” except for South Slough (Figure 4)(ODFW Shellfish Program 2009b).

The habitat where native littleneck clams were most frequently found were low tidal areas with eelgrass and oxygenated sediment.  When encountered, they were most abundant at Pigeon Point and South Slough study sites.   The habitat where cockles were most frequently found were algae-covered areas with oxygenated sediment.  When encountered, they were most abundant at Pigeon Point, South Slough and airport tide flat study sites.

Figure 3. Cockle abundance in the lower Coos and South Slough estuaries

Intertidal habitat attributes including, bed type, eelgrass cover, sediment type and intertidal elevation were measured and are summarized for the South Slough study sites in Figures 5-8. The distribution of butter, gaper and native little neck clams and cockles are summarized for the South Slough study sites in Figure 9.

Figure 4. Historic decline in native littleneck clam population in the lower Coos estuary.

SEACOR reports that the abundance and distribution of target bivalve species is closely related to habitat attributes (e.g., tidal
height- Figure 10) )(ODFW Shellfish Program 2009a, ODFW Shellfish Program 2009b). Since habitat characteristics vary in different regions of the estuary, species distribution and abundance varies by region (Figures 9 and 11). The relationships between bed type, eelgrass cover, sediment type, intertidal elevation and species abundance and distribution of target recreational clam species will be described in more detail in the SEACOR final report.

Figure 5. Intertidal bed type associated with the South Slough study sites.

Figure 6. Major sediment types associated with the South Slough study sites.

Figure 7. Eelgrass cover associated with the South Slough study sites.

Figure 8. Tidal height range associated with the South Slough study sites.

Figure 9. Distribution of butter clams, gaper clams, native little neck clams and cockles in the South Slough study sites. Graphic: ODFW SEACOR


From the SEACOR report summary (ODFW Shellfish Program 2009a): SEACOR is funded by Oregon Department of Fish and Wildlife recreational shellfish license fees and captures a snapshot of the status of Oregon’s estuarine resources for baseline data and for comparison to past (ODFW 1970’s “raccoon” report)(Bottom et al. 1979) and future studies. The SEACOR project initially targeted intertidal flats of the lower Coos Bay Estuary during 2008-2009. However, the state legislature has made the project permanent to allow future assessment of all of Oregon’s estuaries following this general sampling strategy. All results from the Coos Bay study will be presented in a formal ODFW report and further information can be found at Oregon Dept. of Fish and Wildlife.


SEACOR was tasked with examining, six goal-directed questions:
1) What is the (recreational clam) habitat of Coos Bay?
2) Where are clams located within Coos Bay
3) What are the densities of clams
4) What are the mechanism(s) driving clam distribution
5) What is the biomass of clams within Coos Bay
6) How do the clam populations differ between this (2008) and the 1970’s assessment.
These questions directed our sampling and statistical design. All of our sampling strategies are based on a general two-factor design of comparing across and among regions and tidal stratum for each target species.

Our sampling design was two tiered:
1) A superficial and extensive Rapid Assessment Method (RAM), with evenly spaced sampling to cover all extents of each tide flat; and
2) A thorough and limited Detailed Assessment Method (DAM), with randomly generated points within known bivalve habitat.
Both types of sampling designs were generated in ArcGIS v 9.3 using NPS AlaskaPak v 2.2 software.

The RAM sampling points were generated as a grid drawn over all of Coos Bay with a cell size of 100 m along shore by 50 m between points and then rotated 30° to make transects lie perpendicular to shore. Center points within each cell were used as waypoints and every other transect was sampled. Detailed assessment sampling points were created by drawing polygons over the RAM sampling area in each tide flat and then generating random points using NPS AlaskaPak within each boundary. Approximately 110 random points were generated for potential sites creating a coverage of 2.23 x10-4 possible points per m2. Points were then randomly selected using a random number table and ground truthed for suitability. For all points, latitude and longitude was generated by NPS AlaskaPak and transferred to Garmin GPSMap76 using the DNRGarmin GPS application software v 5.4.1.

For the RAM, six tidal flat regions were chosen and each sampling waypoint was categorized as suitable for sampling or excluded. Waypoints were excluded if 1) the waypoint was in a terrestrial habitat (dune grass, hummock, etc.), 2) the waypoint was on emergent bedrock (sediment layer over bedrock was 2 cm) were extracted, counted, and biometrics were measured.

The rapid assessment method (RAM) enabled us to collect habitat data and map variables using GIS to show distributional patterns. Topographic features and sediment classification vary from region to region with all regions containing various sediment types.

Examination of each region suggests that these tide flats are not homogenous across region or within region. There is a range of tidal heights at all regions and unique depressions, bays, channels and high spots at each region and within each tidal stratum. Distribution of algae was found to be in narrow strips in the high intertidal (all regions) and in swaths perpendicular to shore at Pigeon Point. While the highest abundance of eelgrass, Zostera marina occur in the low intertidal including the tide flat edge close to the shipping channel and in persistent depressions and channels within the tide flats (in all regions).

Average RAM and DAM environmental measurements and statistical analysis of these measurements suggest that the environmental structure and habitat is significantly different by region and by tidal stratum. Bivalves are the dominant bed type across all regions and tidal strata sampled.

The combined RAM and DAM survey allowed us to map the distribution and density of all four target species and other estuarine environmental factors. From our assessment we found a fundamental difference in the abundance and biomass calculations generated based on burrow hole counts (RAM) and based on the number of clams extracted (DAM). To account for the discrepancies between the two methods we established a mathematical relationship between RAM holes and DAM actual extracted clam count data. In this way we merged the depth of DAM sampling with the breadth of RAM sampling to generate population estimates for Coos Bay clam species. Burrow hole counts have been used historically and will likely be used in the future based on the efficiency of sampling.

Literature cited:

Bottom, Dan and B. Kreag, F. Ratti, C. Roye, R. Starr. 1979. Habitat classification and inventory methods for the management of Oregon estuaries. Oregon Department of Fish and Wildlife final estuary inventory project report. 84 pp.

ODFW Shellfish Program. 2009a. Shellfish and Estuarine Assessment of Coastal Oregon. Retrieved from
ODFW Shellfish Program. 2009b. SEACOR Results. Retrieved from