Oyster Update

January 14, 2015

A bit of news for Restore Rock Creek’s Oysters Rock Volunteers — this is a much belated update from DNR on the status of the oyster population at Ft. Carroll.

DNR visited the oyster reef at For Carroll on 10/3/14 to asses the health of the reef and our effort to seed that reef with new oysters.

The quick summary is mortality amongst spat was higher than for older oysters, which is totally normal. Spat are most vulnerable and larger oysters are more durable.

The older oysters looked good – nice clumps, large oysters and low mortality. Some of these oysters are really large (120 mm [~5 inches]).

Young oysters and spat were growing fine but scars (dead spat) were evident – this is normal as long as the numbers aren’t too high, which was the case.

The Fort Carroll oyster reef has oysters planted by Maryland Grows Oysters (MGO) participants on Rock, Stoney, and Bodkin Creeks and also has hatchery oysters planted by the Chesapeake Bay Foundation and Living Classroom Foundation.

DNR surveys the Fort Carroll reef to track the performance of MGO oysters. DNR scientists drag a handscrape over the reef to collect samples to analyze and count oysters on the reef. The data below are the combined results of five samples taken that day.

Live oysters collected             343
Dead oysters collected           113
Percentage mortality               25%
Natural oysters                        9
Percentage natural oysters     2.6%

This level of mortality (25%) is quite normal because hatchery spat are vulnerable to predation from crabs and flatworms because their shells do not afford the protection larger oysters enjoy from their shells. Focusing now on how the spat and young oysters are performing in the same sample of 343 live oysters we find evidence of spat planted in the summer of 2014 by MGO growers:

Total live oyster collected         343
2014 spat                                 181
Percent mortality of spat          33%
Older oysters                           162
Percent mortality of oysters     12%

The really good news is both mortality rates are low (33% and 12%). DNR says that mortality rates could exceed 50% if serious issues existed on the reef.

Focusing now on observations derived from the shells gives DNR a sense of how well the population and its habitat are performing. Clumps provide good habitat. The largest clump was made up of 13 individuals and the smallest clump was a single individual.

Average number of spat per shell            5.3
Average number older oysters per shell  3.3
Overall average across the sample         4.1

The fact natural oysters were also collected is good to see. This means the reef is reproducing itself, but at a low rate (only 9 natural oysters were collected). This level of reproduction is very low and likely not enough to build a strong population on the reef. The obvious conclusion is that seeding is improving the reef’s population on the order of 30 times the population density that would be there without planting.

How does this compare to other sites around the bay?

DNR expresses its data from these annual surveys in terms of numbers of oysters per bushel. The sample size collected at Fort Carroll is designed to provide one half bushel. So, a bushel of oysters collected from Fort Carroll should produce about 686 oysters, which is a very high result!

In summary, the Fort Carroll reef is doing well. It is experiencing normal mortality levels and is providing a living reef community. This is our objective! It is the living reef that adds value to the bay, not just the oysters. It is important to note this reef is located on the edge of the salinity tolerance of oysters. Should a storm, or series of storms with extremely high rainfall occur, then mortality could spike due to low salinity and change this situation.

DNR’s conclusion is that our efforts to grow oysters is enhancing the oyster population on the site and enriching the benthic community.

Oysters Rock!!

Allan Straughan
Volunteer Oyster Grower and
Executive Director
Koolhof Earth


Figure 1 — This is an example of young spat growing up in cages on Rock Creek.


Figure 2 — These are examples of “clusters” of oysters growing around Fort Carroll.