Cold Storage of Eastern Oysters, Crassostrea virginca, to Reduce Winter Mortality in an Increasingly Variable Environment

Progress report for FNE19-931

Project Type: Farmer
Funds awarded in 2019: $14,845.00
Projected End Date: 04/15/2021
Grant Recipient: Betsy's Cape Shore Salts
Region: Northeast
State: New Jersey
Project Leader:
Betsy Haskin
Betsy's Cape Shore Salts
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Project Information

Project Objectives:

This project seeks to determine if cold dry storage or subtidal high salinity winter handling methods can be used as a means to reduce Dermo oyster disease, eliminate polydorid worms and, most importantly, enhance overwinter and post-winter survival of intertidal farm-raised eastern oysters, Crassostrea virginica. The comparison is among three handling methods: intertidal, subtidal and cold storage. The oysters will be monitored for mortality, growth, condition, disease, and shell blisters in each treatment. If the project is successful oyster farmers on the intertidal flats of Delaware Bay and other similar regions will learn that winter cold storage is an option that improves winter and post-season oyster mortality and reduces losses to disease. They will have the incentive to invest in winter cold storage to improve profitability of their farms. In addition they may suffer fewer losses from fouling by polydorid worms.


Increasingly variable winter weather coincides with increasing winter and post winter oyster mortality on intertidal oyster farms in lower Delaware Bay. Currently local oyster growers say that their winter losses are in many thousands of dollars. If, as proposed, anomalously variable winter conditions, both directly and indirectly, lead to increased oyster mortality throughout the year, then a possible solution to the problem is to maintain oysters at a consistent cold temperature through the winter. Stable cold storage could resolve the problem of increasing winter and post-winter oyster mortality and provide significant savings for the farmers.

Cold storage during winter months has long been used in the Northeastern US where winter ice can crush oysters. Reportedly, Native Americans pulled oysters out of the water in late fall, buried them in the sand during the winter, and returned them to the water in the spring (ECSGA listserve 2018).  This method, called “pitting” has been adopted by modern day oyster farmers who use root cellars or similar underground storage in New England where winters are consistently cold.  Walton and Murphy (2005) report that these “pits” where oyster are stored for three months or more are typically 36 to 40 F and humid (90-100%). They compared survival of small seed oysters held in pits with those remaining on the intertidal or moved to subtidal waters below any ice. As expected, the intertidal oysters suffered large mortality from ice, while survival in both deep water and in pits was 80% or more. Seed grew slightly while overwintered subtidally, but subsequent growth in pitted oysters exceeded overall growth of subtidal oysters, possibly due to increased winter metabolic costs in cold water with little food.

In Maine, Hidu et al. (1988) demonstrated that small seed oysters held under refrigeration could survive well for six months.

Pitting has not been used in South Jersey where our winter soil temperatures are not consistently cold enough to safely use the “pitting” method of oyster storage, even though it is less costly than using mechanical storage. Therefore, I propose storing oysters in a climate-controlled chamber sensu Hidu et al. (1988). 

Dana Morse of Maine Cooperative Extension provides the following recommendations for using a commercial cooler to safely store oysters for several months. The temperature should be stable between 32-38 F and the oysters must be kept damp. Wet burlap which is periodically sprayed with water will keep the humidity high. His information does not include any follow up on post-winter oyster health.

Access to subtidal leases free of winter ice is not readily available or feasible for most intertidal shellfish farmers in South Jersey.  Overwintering oysters on the tidal flats has proven risky.  To overcome this liability, we have planned to use commercial cold storage for sub-market size oysters which are approximately 1.5 years old at the beginning of the winter because this is the size class where we experience the greatest loss with significant economic impact. We will monitor oyster mortality and growth every month starting at the end of winter to track their progress, or lack thereof, for the rest of the year.  In addition to the baseline of oyster mortality and growth we will sample for oyster meat condition, disease prevalence, and Polydora blisters at key points during the year. Dermo disease will be assessed in June and October following overwintering to capture the seasonal low point of remission and the peak infection intensity. MSX disease will also be assessed in May or June when infections might be expected to peak.  This sampling regimen should give us information on possible effects of our treatments on oyster health. If more oysters from the cold storage treatment survive and grow into market size by the end of the year we can link improved outcomes to stable cold storage in winter. Likewise outcomes for oysters overwintered subtidally will offer new insight on that treatment for those oyster farmers who do have access to deep water storage. The disease assays will provide more information about possible links to the progression and subsequent impact of Dermo disease and MSX. Additionally, Rawson et al. (2015) report that cold storage benefits oyster farmers by reducing the shell blisters caused by the fouling worm Polydora websteri . We may find that fouling from the worm Polydora cornuta significantly reduced in the spring. P.cornuta  biofouling has been a summer problem for oyster growers  for many years.

This study could provide valuable information to other oyster farmers about the effectiveness of several treatments for improving outcomes for intertidal oysters in South Jersey. Both treatments proposed are time-consuming but if survivability and improved health result, then these methods can be worth using. We are exploring the possible connection between winter stress and Dermo disease which has not yet been done, as well as adding an assay for MSX.

If there is an indication of reduced Polydora sp.fouling and also shell mud blisters, then that is another advantage of cold storage as the polydorid worms increase labor costs to remove bio-fouling and create unsightly mud blisters which reduce marketability.





Description of farm operation:

I have been working a small oyster farm on the intertidal flats in Delaware Bay about eight miles north of Cape May for 12 years. My farm uses the rack and bag method of growing oysters which is the one currently most used on these flats as it stands up to high winds and rough water. My main market is local restaurants and the busiest season is the summer when the Jersey shore is flooded with tourists wanting local seafood..
With some help from family members, I am able to keep the business running without employees. That means I am out in the water everyday watching what goes on and thinking about how to keep healthy oysters. My observations of changing weather conditions and oyster survivability led me to propose this SARE grant to answer a few questions.


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Materials and methods:

My project compares three methods of overwintering oysters in South Jersey: leaving them on the inter-tidal flats of Delaware Bay, exposed to cold snaps and warm spells, putting them into coolers and kept at stable temperatures (32-38 degrees), and keeping them underwater in a deeper basin. While my project was funded last February, it could not start until the beginning of the winter this past December, 2019. In early December we collected baseline data on sixty oysters for condition and size and Dermo disease. The presence and intensity of mud blisters (Polydora websteri) in the shells was also noted. We then filled fifteen oyster grow-out bags each with 225 sub-market oysters (year class 2018) and then fifteen more with 230 young seed oysters (year class 2019) in each. Five bags of each year class were placed in a cooler. Five more bags of each age group  were deployed underwater in a nearby harbor and the last set was secured on racks on the inter-tidal flats on my oyster farm. A total of thirty bags make up the set of samples: fifteen are older sub-market oysters which are one and a half years old and the other set of fifteen are approximately 6-8 months old.

Oyster condition, an indicator of the health of the oyster, is determined by a method suggested by Crosby and Gale (1990). Oysters are weighed, then shucked, dry empty shells are weighed, and meat is dried and weighed. The equation for Condition Index is

CI=dry soft tissue weight (g)x 1000/ internal shell cavity capacity(g)  The denominator is whole oyster weight-empty shell weight.

Oyster size is measured as height in mm.

When the Delaware Bay warmed up with water temperatures close to 50 degrees all the bags of oysters were returned to my farm on the inter-tidal flats. Generally it is the last half of March when waters are warm enough that there is food for oysters as they open up and start filtering water but this year with milder temperatures and Covid lurking, we moved faster. On March 11, 2020 oysters were removed from the cooler and placed back on the intertidal flats next to the oysters which had spent the winter there on the flats. The same day we retrieved oysters from the subtidal location where they had been stored for the winter and returned them to the flats.


We proposed that one week after oysters were returned to their place on the flats we would sample again for oyster condition, size, P. websteri mud blisters, and mortality. Our plan was take five oysters from each bag to measure so that we will have a total sample size of 75 oysters for each year class (2019 and (2018). The the number of dead oysters in every bag would be counted and recorded.

Because of Covid restrictions on travel and complications on working indoors to complete sampling we were not able to sample until two and a half weeks after oyster returned to the bay. We completed all measurements as proposed.

Every month after from April 2020 to October 2020 we counted the number of dead oysters in every bag and recorded that mortality. On June 20 we completed our second sampling for Condition Index, growth and disease. Again we were delayed by Covid restrictions on travel for a month. We took a sample of five oysters from each bag to measure condition and growth. Another 20 oysters  was taken to test for two diseases: MSX and Dermo. We had planned to sample in May for MSX because that is generally the month when it would be most prevalent if oysters have the disease but Dr. Bushek thought that the June sample was adequate.  Both Dermo and MSX are cumulative in oysters and would not be expected to have built up to high levels in younger oysters as compared to the older oysters.

In October 2020, we did the final sampling. Condition, growth, P. websteri,  and Dermo were measured along with the final mortality counts.

At this point the data is being analyzed to look for any differences among treatments.  There are several questions which we are asking. Is the condition of oysters coming out of cold storage different from the underwater storage or the condition of oysters remaining in the inter-tidal flats? Does a difference in condition correlate to a difference in disease or survivability? Are mortalities higher in some treatments at different times of the year? Does removing oysters from water for several months reduce the prevalence or intensity of P. websteri mud blisters in shells?




Research results and discussion:

As I explained above, my grant covers the effects of winter storage  I can say that several other oyster farmers who heard about this grant last winter decided to try winter storage themselves to see if it helped prevent winter losses of their oyster crop. Ironically, we had  a mild winter in South Jersey with mostly moderate temperatures. There was no Arctic vortex, nor a more sustained cold spell to freeze the bay, nor an extended warm spell to heat up the bay and so there did not appear to be a big advantage to taking the extra measures to move their oysters into cold storage. This current winter, I am not sure how many farmers are moving oyster to into a cooler.

Currently, it appears that there are no big differences among treatments for condition or disease.  We are thinking that a mild winter did not lead to a big stress on inter-tidal oysters this past winter.

We completed all the sampling and took all the measurements which was described above. Results of data analysis will be provided with my final report in April.

Research conclusions:

See final report coming in April.

Participation Summary
1 Farmer participating in research
Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture or SARE.