Living along the coast is risky business, regardless of which side of the shoreline you call home. We make conscious, although not necessarily intelligent, decisions about where we settle. Other creatures, because of instinct or fate, do not.
The Atlantic Surf Clam, is one such animal. Thriving on the seaward side of this most dynamic of environments, a steady supply of unfortunate individuals is regularly cast up upon the shore. Although most abundant offshore, as their name implies, they range into shallow and even intertidal beach areas. Conspicuous because of their often great size, they are the first seashell that as children we discover, and without equal as an objet d'art or catchall for Neptune's other treasures that are brought home from the shore. Besides being the beachcomber's most utilitarian mollusk, surf clams are an important fishery resource and annually thousands of tons of adults are processed into clam strips, chowder and bait. They also are preyed upon at every stage in their life cycle by crabs, fishes, starfishes, gulls and other mollusks. Irregularly there are reports of great numbers of live clams being stranded. Typically they are of fairly uniform size and may be the bulk of an inshore population. While these events sometimes attract the attention of local newspapers, they are generally poorly documented scientifically.
My curiosity about these standings began on March 5, 1979, when the Coast Guard reported "enormous" numbers of clams washing ashore at the tip of Sandy Hook. Upon investigation it was noted that along a stretch of beach roughly 500 meters by 60 meters, several neat windrows of juvenile surf clams were accumulating. In places the heaps were over 50 cm deep.
A quick survey was done and specimens collected from twenty sites along seven transects. All appeared to be young-of-the-year and many were still alive. Those nearest the waterline were attempting to burrow into the sand, although exposure to the cold wind seemed to inhibit their activities. After several days a follow-up survey was done. The volume of decaying meat was sufficient enough that the larger piles were steaming in the cold spring air. A few stomach pellets from gulls indicated that some feeding was taking place, but there was no unusual concentration of the birds. Apparently ample food was available elsewhere.
The estimates of the numbers present are startling. I calculated that there were over 180 million clams on that one stretch of beach. While this at first sounded unreasonable, research into the subject uncovered some interesting information: four other mass strandings are reported here, including one not previously documented in the scientific literature. Also, surveys of some offshore beds have occasionally uncovered densities of hundreds and sometimes even thousands of live juveniles per square meter. This was some reassurance that I wasn't observing a molluscan mirage on the beach.
But the original question remained... how do these clams end up on the beach and why in such great numbers? While we should be alert to the possibility that human activities might be upsetting some delicate natural balance, we should also take care that we are not sometimes too quick to blame everything on "pollution."
The causes of the Sandy Hook stranding appear obvious enough: a nor'easter swept across the region the week before the stranding. Very heavy surf would be expected to cast ashore a great assortment of marine life; large, long-period swells, typical as these storms move offshore, would have provided ample and accelerated littoral drift to move the clams rapidly toward the tip of the Hook; opposing tidal currents from Raritan Bay and prevailing westerly winds (also typical after storms) tend to concentrate materials at the tip of the Hook as they are swept along our coast; and lastly, young clams can become slightly buoyant when desiccated. This would tend to increase their rate of drift along the beach and further concentration at the tip.
There are a huge number of surf clams in the ocean at any given time. But why doesn't a mass stranding accompany every storm along every beach? I see two possible reasons.
Surf clams vary greatly in number and size at different sites along our coast. This abundance at a particular location is related to reproductive success, which in surf clams is irregular and infrequent. Therefore, a biological factor, the abundance (or overabundance) of clams, is probably a prerequisite for a stranding.
The neat windrows offered another clue. The storm's passage coincided with the new moon and the exaggerated spring tidal ranges that accompany it when the earth, moon and sun are at syzygy. Rows of clams accumulated at the successively lower high-tide marks as tidal ranges moderated.
Additionally, during this storm, the moon was at the nearest point - "perigee" - in its elliptical orbit around the earth. On the relatively infrequent occasions when perigee and syzygy occur simultaneously, tidal ranges and currents are accentuated. The coincidence of this perigean-spring tide on February 26, during the nor'easter also may have contributed to the clams' demise.
Unfortunately, a search of past stranding reports does not indicate any correlation between these four factors at Sandy Hook and other mass strandings. Nevertheless, they make interesting reading and perhaps some conclusions can be drawn.
The 1969 "Irruption" and 1909 "Plague" of clams at Newport, R.I., occurred throughout the spring and summer, and seem to be attributable mainly to a huge set of young clams (although spring storms heralded the strandings).
These events were truly Mosaic in proportion, requiring the expenditures of considerable amounts of time and money for removal of the clams. (Something like 400 five-ton truckloads were removed in 1969). Some were transplanted to Narragansett Bay, many eaten by locals, but most were dumped and buried.
A 1960 Maryland stranding was associated with a nor'easter. The abundance of starfishes seems to indicate both species were experiencing a peak in populations at the time of the storm.
The earliest report of a mass stranding I was able to find is from the January 16, 1884, issue of the Red Bank Daily Register and is worth quoting: "One of the worst storms that ever swept the N.J. coast... [the ocean] made a clean breach to the Shrewsbury River at Sea Bright ... left great damage.... All along the beach from Sea Bright to Manasquan, sea clams were washed ashore in great numbers and hundreds of wagon loads were gathered by people and carted off." (One wonders how many new recipes for clam chowder were tried that winter.)
So, what causes these mass strandings? Like so much else in nature, there is no simple answer.
Certainly population density is important. Perhaps it is the key factor. Storms too. Although infrequent and unpredictable, they are an unavoidable risk to all coastal inhabitants. Their severity and possibly their coincidental arrival during periods of astronomically induced tidal extremes may provide the physical force needed to dislodge the clams.
In the long run these strandings may not necessarily be detrimental to the clam population, given their ability to repopulate depleted areas with one successful setting. There is also evidence that overcrowding severely inhibits the growth rate of all the clams in a bed.
So, mass strandings are probably another example of natural controls that manage animal populations within the vague boundaries that keep the environment running smoothly.
Something to think about:
- Define perigee and perihelion.
- Define syzygy.
- List two factors that might cause the stranding of marinelife at the shore.
- For more information on tides and storms, see: Fergus J. Wood (1978) The Strategic Role of Perigean Spring Tides in Nautical History and North American Coastla Flooding, 1635-1976 (U.S. Government Printing Office)
Dave Grant is the Society's chief naturalist and a contributing editor to Underwater Naturalist. He is on the faculty of Brookdale Community College and claims to have walked every beach from Mt. Desert, Maine, to Cape Hatteras, North Carolina.