Mining Tampa Bay for a Glimpse of What Used to Be

By E.D. Estevez
Mote Marine Laboratory

No one alive today can truly appreciate how diverse, productive, and clean, places like Tampa Bay once were. As past conditions become even more remote with time we run the risk of setting our restoration and protection standards too low. This happens through an erosional process afflicting social values and resource management goals called the problem of sliding baselines. Historical ecology is an important tool in averting sliding baselines because it draws on multiple, often opportunistic lines of evidence to reconstruct resource conditions before the depredations of recent settlement and development began. A fine example of using written records to resurrect Tampa Bay’s past is the account by Michael Heerschap, of Hillsborough County’s Environmental Protection Commission, Some Historical Accounts of the Natural Conditions of Tampa Bay and Hillsborough County¹.

This essay’s title answers the question, “How large were Tampa Bay’s oyster resources?” by mining old reports and data about shell mining in Tampa Bay. Not much new is added to the geology, ecology, and archaeology I’ll review, but bringing those works back to the surface of 2010 to recast just what “immense” means might interest bay enthusiasts or inspire future bay scientists.

While organizing my office during the last holiday I came across a 1976 report entitled Environmental Impact of Oyster Shell Dredging in Tampa Bay, Florida, prepared by Drs. Joe Simon (University of South Florida Department of Biology) and Larry Doyle (USF College of Marine Science), with assistance by one of Simon’s graduate students, Bill Conner. Having spent the last decade or so studying live and dead mollusks and especially oysters in creeks and rivers of Tampa Bay and the Florida west coast generally, it was with some new interest that I sat down to revisit the story of Tampa Bay’s oyster shell resource, industry, and impacts.

While oyster deposits occur widely across the bay, the commercial mother-lode for the shell-mining industry was an area south of Interbay Peninsula (MacDill Air Force Base; Gadsden Point), eastward from the branching of the ship channel into Old Tampa and Hillsborough bays toward Gibsonton, Apollo Beach, and Ruskin. Dredging for oyster shell came into its own in the 1930s, with companies working from St. Petersburg and Tampa. Several companies provided shell for road base and rural roads throughout Pinellas and Hillsborough counties. The 1976 USF report described the process.

“Shell dredging is a relatively confined operation. Using a probing bar or vibracorer (or more recently seismic equipment), deposits of fossil oyster shell are located…. During dredging operations, bottom material (shell, sediment matrix, overburden, etc.) is removed by a dredge cutter head and hydraulically pumped to an above-surface hopper. There, the material is washed over a series of screens of varying mesh size. Shell and other larger particles retained by the screens are deposited onto barges for transport to landside stockpiles. Water and fine materials passing through the screens are discharged over the side to return to the Bay. This discharge of unusable material, plus material washing off the barges, contributes to a turbidity plume which has been of concern by state agencies.”

A dredge loads oyster shell in Tampa Bay. Reprinted with permission from the Florida Anthropological Society's publication "Submarine Shell Deposits of Tampa Bay."

Agency concern took the form of a grant by the Florida Department of Environmental Protection (then the Department of Environmental Regulation) to USF, to report on the long term environmental effects of shell dredging in Tampa Bay. The report found that shell dredging deepened local areas, changed sediment structure, and disturbed bottom-dwelling marine life for about a year, and concluded that lessons from shell dredging should not be used to inform larger dredging projects such as widening or deepening of ship channels. What caught my eye in this reading, though, were the empirical descriptions of the shell dredging process, and production rates.

Here are the basic facts from the 1976 report. Since 1946 the Benton Company dredged some 450,000 tons of oyster shell per year, with a 27-year total of 12,000,000 tons (12 MT). Another company, BayCon, dredged 6MT for a total historic yield of 18MT. The report quoted Dr. Jack Taylor, a respected invertebrate zoologist and benthic ecologist who had also studied the dredging. Taylor estimated a reserve resource of 15MT, making the total resource somewhere in the neighborhood of 35MT.

The 1976 report mentioned that a typical dredge’s yearly take was 1.25MT from 75 acres of bay bottom, implying a 15-year run until the reserve was depleted. A little math shows that the original resource covered some 1,980 acres, or about 3.1 square miles of bay bottom. The annual harvest rate works out to 16,666 tons per acre per year, which is 0.38 tons or 765 pounds of shell per square foot per year. In other words, there were 765 pounds of oyster shell under each square foot of bay bottom where shell was present.

If we only knew the weight of a cubic foot of washed oyster shell we could calculate the average thickness of the shell deposit! With a calibrated bucket and a set of bathroom scales in hand, a weekend trip to a cooperative landscape supply yard produced an average weight of 64 pounds for a cubic foot of half-inch washed shell². Then a wintry low-tide foray into the Braden River yielded an average weight for whole, 2 to 4 inch, washed and air-dried dead oyster shells of 32 pounds per cubic foot. Not knowing the original mix of shell sizes in the resource, the two weights were averaged. (Because the shell was washed before being reported as product weight, any contributions of material finer than a half-inch were ignored.) At 48 pounds per cubic foot, the 765 pounds under a square foot of bay bottom would have extended downward some 16 feet.

To recap, the 33MT oyster shell resource that occupied some 3.1 square miles of bay bottom was, on average, 16 feet thick. To put the resource in a more tangible light, that quantity would put a one-foot layer of shell over 49 square miles of land. That’s equal to covering 10 Tampa International Airports in a foot of oyster shell from fence to fence to fence. It’s also roughly equal to a foot of shell over all of the land in the City of St. Petersburg!

A number of assumptions and decisions went into the foregoing calculations that of course can tip the results up or down. For example, a 1975 Florida Department of Natural Resources (now Fish & Wildlife Conservation Commission) report³ established for lease purposes that a cubic yard of shell product weighed approximately 1500 pounds, or 55 pounds per cubic foot. The higher weight reduces the amount of shell estimated here by about 12 percent⁴. But the overall impact of the exercise for me is an awe-inspiring glimpse of how productive the bay had been, and for how long it must have been so. Questions pour forth. How old were the oysters? Were the oysters intertidal, subtidal, or both? How large were they? Did they guard the entrance to Hillsborough Bay or grow along the banks of an ancient river? Did they, as oysters commonly do, shape their environment by retarding circulation, creating anastomizing channels, and freshening their backwaters? What was their affect on water quality? How did they respond to changing sea level? And were they used by bayshore settlements of early human populations?

The rest of this story picks up at the last question, precisely, because paleontological and archeological studies of dredged oyster shell deposits answer several questions, at least in part, and add a fascinating dimension to our appreciation of this immense resource. Around 1960, Dr. Lyman Warren, a physician at the Veterans Administration Center at Bay Pines, learned but could not substantiate the finding of an artifact from a Dunedin driveway where a load of oyster shell had been dumped (the artifact showed up later). Motivated by the report, Dr. Warren began a decade-long survey of freshly mined shell at shell yards, and shell spread for roadways around the bay, looking for their paleontological and archaeological contents. In 1964, his brief note in the Florida Anthropologist, Possibly Submerged Oyster Shell Middens of Upper Tampa Bay, set forth evidence that the buried shell deposits “may well be submerged oyster shell middens from the “Paleo-Indian” or Archaic Periods, or both, of Tampa Bay’s prehistory.”

Warren and Albert C. Goodyear, assisted by several others, became familiar and welcomed faces at shell yards owned by the Pinellas County Road Department, Benton Company (St. Petersburg and Clearwater), Tampa Sand Corporation, Tampa, and Bay Dredging, Inc. (also of Tampa). The men also went to sea, working closely with Thomas Sikes, Captain of Benton’s Dredge Three. Many hours were spent hiking bay area roads, inspecting road base, rural roads, and driveways for material. “The distribution of artifacts on spread shelled roads,” wrote Goodyear and Warren, “is characteristically spotty; one can walk for miles and find nothing, and then in a single driveway or stretch of road find a concentration of… material.” Special effort was made examining Starkey Road and streets connecting to Belcher Road in Pinellas County, and along Sligh Avenue near Temple Terrace in Hillsborough County.

Warren published a second paper in 1972, and a third in the same volume of Florida Anthropologist, with Goodyear as the first author⁵. Their joint paper in particular marshaled all of the paleontological and archaeological evidence collected over the prior decade (see below). Their findings ranged from conclusive to speculative but from the body of their work it may reasonably be concluded that human beings had indeed co-existed with the mid-Bay’s oyster communities; depended upon the oysters as food and raw material for tools, and probably lived and died among the natural reefs and middens. Were all of the massive shell resources buried beneath the bay actually built by humans? Probably not. The evidence buried beneath the Bay is gone. But it is provocative to imagine, as we speed along the bay-area’s older highways, that just below us are innumerable remnants of early humans including their tools, their prey, and their very bones.

How, then, does a better knowledge of the immensity and value of Tampa Bay’s ancient oyster resource help us to try harder to understand and restore the Bay? One could point out the obvious, that Tampa Bay oysters were clean enough for humans to eat — for millennia. Might not success in limiting nitrogen loads to the bay inspire the next great challenge, to open more waters to shellfish harvesting? A less obvious but familiar note could be struck respecting the filtration of bay waters lost to oyster declines. It can easily be shown that billions of oysters filtered particulates from bay water continuously, again, for millennia. Has the loss of such valuable ecological services affected the ecology of Tampa Bay, and should we be developing plans to maintain or even expand oyster communities, especially if and when climate and other global changes begin to affect the Bay? A geological axiom holds that the present is the key to the past. I believe the past can be the key to the future of Tampa Bay. Back to the Future!

Thanks to Lucas Boat Works and Big Earth Landscape Supply Company, both of Bradenton; Bill Burger of Terra Ceia Island, Mote Marine librarian Susan Stover, and Donna Estevez for their assistance.

FEATURES OF THE SHELL DEPOSITS AND THEIR CONTENTS

Gleaned from publications by Lyman Warren and Albert Goodyear

The dredged shell was 99% Crassostrea virginica (American oyster).

Shells were generally present as single valves, suggesting the oysters were shucked.

The deposits abutted old natural and modern dredged channels. They laid next to but not in old natural channels.

Deposits laid with their tops about 10 feet below high tide, and with their bases 20-50 feet below, giving them thicknesses of 10-40 feet.⁶

Pleistocene fossils were only found at the lowest levels of the deposits (depths of 45-50 feet).

Fossils included remains of prehistoric horse, mammoth, alligator, deer, and tortoises.

There were no associations of Pleistocene fossils and early man; no mammalian fossils were found where artifacts were found.

No small or medium sized animal skeletal remains were found.

Only tools of chipped stone were found — partial and whole projectile points, horse’s hoof scrapers, chert knives, hammer stones, cores, and choppers⁷.

No shell or bone artifacts were found. Potsherds were few, sand tempered and limestone tempered types.

One midshaft of a human femur was found.

3000 – 2750 BP may have been the final period of occupation with sea level rise and/or sedimentation isolating the deposits until their discovery by dredgers.

1 www.sarasota.wateratlas.usf.edu/upload/documents/HistoricalAccountsNaturalConditionsTampa.pdf

2 Today, much commercial shell is mined from inland deposits and contains the fossilized remains of many species of marine gastropods and bivalves, not just oysters. The industry called such product “pit shell” to distinguish it from oyster shell. Numerous lines of evidence make it clear that most of the shell dredged from Tampa Bay was oyster.

3 http://aquacomm.fcla.edu/776/1/FMRP011.pdf

4 On the other hand, the FDNR report tabulated 25 MT of shell taken from the bay, roughly 40% more than the 18 MT used here.

5 http://ufdcweb1.uflib.ufl.edu/ufdc/?m=hd4J&i=60653

6 For comparison, the mound at Cockroach Key has an elevation of 35 feet above sea level.

7 One preserved wood artifact was also found.