We need oysters in our Bay and rivers. We don’t have enough. But one of the major impediments to restoring oysters is the availability of substrates. Substrates are what oyster larvae latch onto while they are mobile (e.g., other oyster shells, piers).
Oysters begin their lives as free-swimming larvae. Oyster parents release eggs and sperm into the water, and once the eggs are fertilized they become larvae. The larvae are mobile for two to three weeks before they attach onto a substrate (this attachment is called settlement), become immobile, and metamorphose into juvenile oysters which are referred to as spat.
If the larvae are unable to attach to a substrate they will not survive. So, finding the right substrate and enough of it is crucial to the success of oysters in the Chesapeake Bay and its rivers.
Oysters are essential to the health of the Bay because they can filter up to 50 gallons of water daily, cleaning the water and increasing the oxygen levels. Oysters clean the water by consuming micro algae and suspended particles, and they can also mediate nitrogen and carbon by removing and depositing any excess nutrients. Oyster reefs also serve as homes to fish and other filter feeders like mussels and barnacles.
Oyster larvae need a hard surface (e.g., shells and stones) to latch onto so that they can grow. In the wild, oyster larvae naturally attach to other oyster shells, which is why oyster shells are a preferred substrate. However, the availability of shells is limited and adding it is expensive due to decades of harvesting without shell replacement. There are attempts to recycle as many shells as possible, for example, the Oyster Recovery Partnership has a collection program for oyster shells.
Another way to get oyster shells is to dredge old oyster reefs buried in the silt. There are also fossilized shells that are available from the West Coast where there are literally mountains of shells that were discarded by shucking houses.
The demand for substrates will only increase as fisheries and aquaculture grow. Commercial fishermen, aquaculture lease holders, and oyster restoration partners are also interested in using alternative substrates, both to put out directly on the Bay bottom to catch wild larvae or to use in an oyster hatchery that produce “spat on shell” (which is larvae that are attached to oyster shells.)
Shells are limited and expensive, so more substrate is needed to expand the oyster industry. So, scientists have been evaluating alternatives to oyster shells for use in Maryland
Scientists must find other substrates that are as effective as shells that don’t leach dangerous chemicals into the water.
Potential substrates that have been used for oysters in this region and in other states can be grouped into three categories:
- Biogenic: oyster shell (fossilized, fresh, dredged), other species’ shells (whelk, clamshell)
- Geologic: granite, stone (river rock), amphibolite (used for paving or crushed stones), limestone marl
- Anthropogenic (human produced): concrete and recycled concrete.
In Havre de Grace, amphibolite rock is quarried, and restoration efforts have used the rock to build oyster reefs in sanctuaries (e.g., Harris Creek).
Other states also use alternative materials for oyster substrate. In Texas, limestone marl, river rock, and crushed concrete are common. North Carolina uses limestone marl, a state resource. Virginia uses granite chips in addition to fresh and dredged Eastern oyster shell. Often larger stones are used in sanctuaries and restoration sites, but smaller is better for harvest sites.
Engineered reefs are structures formed from molded concrete, such as reef balls, reef castles, or fish havens. These structures can be placed on the bottom of the river or bay to allow spat to settle naturally or can be placed in tanks at hatcheries to be settled with hatchery-reared oyster larvae. The composition of the concrete can be tailored to improve settlement (larvae attaching to the substrate) by adding calcium or pieces of shell to the mix. The majority of engineered reefs placed in our Bay are in oyster sanctuaries or on MARI (Maryland Artificial Reef Initiative) sites. The use of engineered reefs is not effective for harvesting because oysters can only be harvested by divers.
Large scale construction demolition projects can provide appropriate concrete materials. Crushed, recycled concrete is recovered from demolition sites and its toxic elements (e.g., rebar) removed.
The need for substrates is getting more attention. The DNR (Department of Natural Resources) submitted a permit to USACE (US Army Core of Engineers) and MDE (Maryland Department of the Environment) to plant hatchery reared spat on substrate other than shell.
Recently, the public fishery has expressed interest in utilizing alternative substrates on its bars. A permit was obtained from MDE and USACE to plant alternative substrates on a public fishery bar in Pocomoke Sound (Somerset County) and another site in Broad Creek (Talbot County). The Broad Creek site was just approved in late 2023 and will soon begin planting crushed, recycled concrete.
In July 2023, Governor Moore created a Shell and Substrate Task Force. The principal functions of the task force were to:
- Identify strategies and solutions to retain shell, increase its abundance and use more substrate across the Chesapeake Bay.
- Evaluate strategies to meet the demand from public fishery, aquaculture, and restoration.
- Evaluate the economic impacts of substrate and develop recommendations.
But it is important to know which substrates perform most effectively. Drs. Elizabeth North, Matt Gray, and Monica Fabra at University of Maryland Center for Environmental Science Horn Point Laboratory (HPL) are evaluating the efficacy of 9 different substrates. They are scientists with expertise in marine biology and experience with research on oysters.
The HPL team is evaluating the effectiveness of different substrates on (a) settlement (larvae latching onto the substrate and growing), (b) toxic leaching (verifying that there are no toxic chemicals leaching from the substrate), and (c) biofilm formation (determining the microorganisms that grow on the substrate that are important for larval settlement). It is part of a grant from the state of MD and is legislatively mandated by the 2023 senate bill SB830.
The 9 substrates are: (1) dredged Eastern oyster shells, (2) weathered Pacific oyster shells, (3) clam shell pieces, (4) whelk shells,(5) recycled concrete, (6) limestone marl, (7) granite, (8) river rocks, and (9) amphibolite. Fresh Eastern oyster shell (our Maryland oyster) was also used in the study.
To conduct the laboratory study, Dr. Fabra released oyster larvae into beakers and waited six weeks to count the number of spat that had latched onto each substrate. While results are preliminary and are from the laboratory only, the team found that spat successfully settled and grew on all of tested substrates.
Preliminary results suggest that, overall, shells did better slightly than rocks. By a small margin, the highest spats per beaker were dredged Eastern oyster shell and weathered Pacific oyster shell. Fresh Eastern oyster shell, whelk shell, concrete, and amphibolite also performed well.
She and her team are also evaluating these substrates to verify that they do not leach heavy metals. Another evaluation will be related to the formation of biofilms that are important for larval settlement on substrate.
In the late spring of 2025, the HPL team will test these substrates in our rivers using cages and trays at wild sites.
In evaluating the overall value of a substrate, it is important to consider the cost, weight, size, transportation costs, ease of handling as well as settlement and toxicity. Fresh oyster shells are expensive and the ability to find substrates that are local and cost-effective can be a leap for commercial fisheries, aquaculture and oyster restoration.
This is an exciting study, done right in our backyard, with benefits that will last for generations.
Dr. North will be discussing the HPL team’s research on Tuesday, March 4 at 5:30 at the Talbot Community Center. Check the Horn Point Laboratory website, www.umces.edu/hpl or their social media for more information about registration.
Angela Rieck, a Caroline County native, received her PhD in Mathematical Psychology from the University of Maryland and worked as a scientist at Bell Labs, and other high-tech companies in New Jersey before retiring as a corporate executive. Angela and her dogs divide their time between St Michaels and Key West Florida. Her daughter lives and works in New York City.