Oyster Stock Assessment 2018
Science-based stock assessment for oysters in Maryland's portion of Chesapeake Bay
In the first assessment of the oysters in the Maryland portion of Chesapeake Bay in 135 years, scientists found that regions throughout the Chesapeake Bay were performing quite differently, and almost all regions have fewer oysters now than in 1999, the beginning of the assessment period.
“The bad news is the oyster abundance is less than in 1999. There is a real north-south gradient in how oysters are performing,” said Professor Mike Wilberg, who led the scientific team at the University of Maryland Center for Environmental Science that contributed to the assessment. “The good news is we have had population recovery since dark days of early 2000s, and there is potential for population and fishery to come back.”
The oyster stock assessment is a scientific study that uses available data to estimate the abundance of wild oysters in the Maryland portion of the Bay, the fraction of oysters that die each year from natural causes including disease, and the fraction of oysters that are harvested each year by fishing. The assessment develops biological reference points for fishery management plans in Maryland—benchmarks used to determine if abundance and harvest are at sustainable levels—and determines the overall health of the fishery.
“Our job was to give the best scientific estimates of the current status of the population to help inform future management decisions,” said Wilberg. “The oyster population is highly dynamic— it depends on reproductive success, natural mortality conditions, and fishing mortality rates that are quite variable from year to year and in different locations.”
The assessment is based on a 19-year period (1999-2000 through 2017-2018 seasons). The analysis was run for 36 regions within the Maryland portion of Chesapeake Bay and its tributaries.
The first modern stock assessment for oysters in Maryland was required by the Sustainable Oyster Population and Fishery Act of 2016 as a means toward achieving the goal of a more scientifically managed fishery. It provides guidance for the development of biological reference points for the management of the oyster population.
The University of Maryland Center for Environmental Science collaborated in the study at the request of the Maryland Department of Natural Resources, and the assessment was reviewed by an independent panel of fisheries stock assessment experts.
The status of Maryland's oyster population varies depending on the specific location. Some regions have shown substantial increases in oysters since the early 2000s, but others have continued to decline since that time. The regions with the best condition tend to be in the southern part of Maryland's Bay, while the poorest performing regions tended to be in the northern part. Recent harvest levels appear to be sustainable in some regions, but not sustainable in others.
No regions are considered depleted (otherwise known as overfished) based on the lowest abundance level within the assessment time frame (1999-2017), although a few areas were at their minimum abundance in the final year of the assessment (2017). In these areas, any future declines occurring without an interim increase in abundance would place them in the depleted category.
In the most recent completed fishing season (2017-2018), 19 regions had harvest fractions above the limit reference point (overfishing), three were between the target and limit reference points, and 14 were at or below the target reference point.
Maryland-wide, the estimated abundance of market-size oysters (3 inches or greater)—those targeted by the fishery—was highest in 1999 at 600 million, decreased to about 200 million individuals by 2002, and remained close to that level until 2010. After 2010, estimated market abundance increased through 2014 to more than 450 million and declined to about 300 million thereafter.
In 1999, estimated market abundance was highest in the Choptank River and Eastern Bay regions, but after 2006, estimated abundance was highest in the Choptank River and Tangier Sound regions.
Natural disease and mortality
Across areas, estimated natural mortality was generally higher and more variable in the beginning of the time series (1999 to 2002) and lower and less variable during 2003-2017. Natural mortality was extremely high in 2002 throughout most of Maryland, but has been at lower levels since then.
This fall, the report containing the results of this analysis as well as a list of potential management tools that could be used to manage the oyster population was submitted to the legislature. The stock assessment results combined with the list of management tools will provide the framework for future management decisions.